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diff --git a/8700-h/8700-h.htm b/8700-h/8700-h.htm new file mode 100644 index 0000000..c279347 --- /dev/null +++ b/8700-h/8700-h.htm @@ -0,0 +1,26973 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" +"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> +<head> +<meta http-equiv="Content-Type" content="text/html;charset=utf-8" /> +<meta http-equiv="Content-Style-Type" content="text/css" /> +<title>The Evolution of Man, by Ernst Haeckel</title> + +<style type="text/css"> + +body { margin-left: 20%; + margin-right: 20%; + text-align: justify; } + +h1, h2, h3, h4, h5 {text-align: center; font-style: normal; font-weight: +normal; line-height: 1.5; margin-top: .5em; margin-bottom: .5em;} + +h1 {font-size: 300%; + margin-top: 0.6em; + margin-bottom: 0.6em; + letter-spacing: 0.12em; + word-spacing: 0.2em; + text-indent: 0em;} +h2 {font-size: 150%; margin-top: 2em; margin-bottom: 1em;} +h3 {font-size: 150%; margin-top: 2em;} +h4 {font-size: 120%;} +h5 {font-size: 110%;} + +hr {width: 80%; margin-top: 2em; margin-bottom: 2em;} + +div.chapter {page-break-before: always; margin-top: 4em;} + +p {text-indent: 1em; + margin-top: 0.25em; + margin-bottom: 0.25em; } + +.p2 {margin-top: 2em;} + +p.poem {text-indent: 0%; + margin-left: 10%; + font-size: 90%; + margin-top: 1em; + margin-bottom: 1em; } + +p.letter {text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +p.noindent {text-indent: 0% } + +p.center {text-align: center; + text-indent: 0em; + margin-top: 1em; + margin-bottom: 1em; } + +p.right {text-align: right; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +p.footnote {font-size: 90%; + text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +sup { vertical-align: top; font-size: 0.6em; } + +div.fig { display:block; + margin:0 auto; + text-align:center; + margin-top: 1em; + margin-bottom: 1em;} + +a:link {color:blue; text-decoration:none} +a:visited {color:blue; text-decoration:none} +a:hover {color:red} + +</style> + +</head> + +<body> + +<pre> +The Project Gutenberg EBook of The Evolution of Man, by Ernst Haeckel + +This eBook is for the use of anyone anywhere in the United States and most +other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms of +the Project Gutenberg License included with this eBook or online at +www.gutenberg.org. If you are not located in the United States, you'll have +to check the laws of the country where you are located before using this ebook. + +Title: The Evolution of Man + +Author: Ernst Haeckel + +Release Date: August 1, 2003 [EBook #8700] +[Most recently updated: April 5, 2020] + +Language: English + +Character set encoding: UTF-8 + +*** START OF THIS PROJECT GUTENBERG EBOOK THE EVOLUTION OF MAN *** + + + + +Produced by Produced by Sue Asscher and Derek Thompson + + + + + + +</pre> + +<h1>The Evolution of Man</h1> + +<h4>A POPULAR SCIENTIFIC STUDY</h4> + +<h2>by Ernst Haeckel</h2> + +<p class="center"> +Translated from the Fifth (enlarged) Edition by Joseph McCabe<br/> +<br/> +[Issued for the Rationalist Press Association, Limited] +<br/><br/><br/> + +WATTS & CO.<br/> +17 Johnson’s Court, Fleet Street, London, E.C.<br/> +1912<br/> +</p> + +<hr /> + +<div class="fig" style="width:55%;"> +<img src="images/front.gif" width="204" height="303" alt="From the painting by Franz von Lenbach, 1899" /> +</div> + +<h2>Contents</h2> + +<table summary="" style=""> + +<tr> +<td> <a href="#pref01">LIST OF ILLUSTRATIONS</a></td> +</tr> + +<tr> +<td> <a href="#pref02">GLOSSARY</a></td> +</tr> + +<tr> +<td> <a href="#pref03">TRANSLATOR’S PREFACE</a></td> +</tr> + +<tr> +<td> <a href="#pref04">TABLE: CLASSIFICATION OF THE ANIMAL WORLD</a><br/><br/></td> +</tr> + +<tr> +<td> <a href="#chap01">Chapter I. THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION</a></td> +</tr> + +<tr> +<td> <a href="#chap02">Chapter II. THE OLDER EMBRYOLOGY</a></td> +</tr> + +<tr> +<td> <a href="#chap03">Chapter III. MODERN EMBRYOLOGY</a></td> +</tr> + +<tr> +<td> <a href="#chap04">Chapter IV. THE OLDER PHYLOGENY</a></td> +</tr> + +<tr> +<td> <a href="#chap05">Chapter V. THE MODERN SCIENCE OF EVOLUTION</a></td> +</tr> + +<tr> +<td> <a href="#chap06">Chapter VI. THE OVUM–THE AMŒBA</a></td> +</tr> + +<tr> +<td> <a href="#chap07">Chapter VII. CONCEPTION</a></td> +</tr> + +<tr> +<td> <a href="#chap08">Chapter VIII. THE GASTRÆA THEORY</a></td> +</tr> + +<tr> +<td> <a href="#chap09">Chapter IX. THE GASTRULATION OF THE VERTEBRATE</a></td> +</tr> + +<tr> +<td> <a href="#chap10">Chapter X. THE CŒLOM THEORY</a></td> +</tr> + +<tr> +<td> <a href="#chap11">Chapter XI. THE VERTEBRATE CHARACTER OF MAN</a></td> +</tr> + +<tr> +<td> <a href="#chap12">Chapter XII. THE EMBRYONIC SHIELD–GERMINATIVE AREA</a></td> +</tr> + +<tr> +<td> <a href="#chap13">Chapter XIII. DORSAL BODY–VENTRAL BODY</a></td> +</tr> + +<tr> +<td> <a href="#chap14">Chapter XIV. THE ARTICULATION OF THE BODY</a></td> +</tr> + +<tr> +<td> <a href="#chap15">Chapter XV. FŒTAL MEMBRANES AND CIRCULATION</a></td> +</tr> + +<tr> +<td> <a href="#chap16">Chapter XVI. STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT</a></td> +</tr> + +<tr> +<td> <a href="#chap17">Chapter XVII. EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT</a></td> +</tr> + +<tr> +<td> <a href="#chap18">Chapter XVIII. DURATION OF THE HISTORY OF OUR STEM</a></td> +</tr> + +<tr> +<td> <a href="#chap19">Chapter XIX. OUR PROTIST ANCESTORS</a></td> +</tr> + +<tr> +<td> <a href="#chap20">Chapter XX. OUR WORM-LIKE ANCESTORS</a></td> +</tr> + +<tr> +<td> <a href="#chap21">Chapter XXI. OUR FISH-LIKE ANCESTORS</a></td> +</tr> + +<tr> +<td> <a href="#chap22">Chapter XXII. OUR FIVE-TOED ANCESTORS</a></td> +</tr> + +<tr> +<td> <a href="#chap23">Chapter XXIII. OUR APE ANCESTORS</a></td> +</tr> + +<tr> +<td> <a href="#chap24">Chapter XXIV. EVOLUTION OF THE NERVOUS SYSTEM</a></td> +</tr> + +<tr> +<td> <a href="#chap25">Chapter XXV. EVOLUTION OF THE SENSE-ORGANS</a></td> +</tr> + +<tr> +<td> <a href="#chap26">Chapter XXVI. EVOLUTION OF THE ORGANS OF MOVEMENT</a></td> +</tr> + +<tr> +<td> <a href="#chap27">Chapter XXVII. EVOLUTION OF THE ALIMENTARY SYSTEM</a></td> +</tr> + +<tr> +<td> <a href="#chap28">Chapter XXVIII. EVOLUTION OF THE VASCULAR SYSTEM</a></td> +</tr> + +<tr> +<td> <a href="#chap29">Chapter XXIX. EVOLUTION OF THE SEXUAL ORGANS</a></td> +</tr> + +<tr> +<td> <a href="#chap30">Chapter XXX. RESULTS OF ANTHROPOGENY</a><br/><br/></td> +</tr> + +<tr> +<td> <a href="#chap31">INDEX</a></td> +</tr> + +</table> + +<div class="chapter"> + +<h2><a name="pref01"></a>LIST OF ILLUSTRATIONS</h2> + +<a href="#illus01">Fig. 1.</a> The human ovum<br/> +<a href="#illus02">Fig. 2.</a> Stem-cell of an echinoderm<br/> +<a href="#illus03">Fig. 3.</a> Three epithelial cells<br/> +<a href="#illus04">Fig. 4.</a> Five spiny or grooved cells<br/> +<a href="#illus05">Fig. 5.</a> Ten liver-cells<br/> +<a href="#illus06">Fig. 6.</a> Nine star-shaped bone-cells<br/> +<a href="#illus07">Fig. 7.</a> Eleven star-shaped cells<br/> +<a href="#illus08">Fig. 8.</a> Unfertilised ovum of an echinoderm<br/> +<a href="#illus09">Fig. 9.</a> A large branching nerve-cell<br/> +<a href="#illus10">Fig. 10.</a> Blood-cells<br/> +<a href="#illus11">Fig. 11.</a> Indirect or mitotic cell-division<br/> +<a href="#illus12">Fig. 12.</a> Mobile cells<br/> +<a href="#illus13">Fig. 13.</a> Ova of various animals<br/> +<a href="#illus14">Fig. 14.</a> The human ovum<br/> +<a href="#illus15">Fig. 15.</a> Fertilised ovum of hen<br/> +<a href="#illus16">Fig. 16.</a> A creeping amœba<br/> +<a href="#illus17">Fig. 17.</a> Division of an amœba<br/> +<a href="#illus18">Fig. 18.</a> Ovum of a sponge<br/> +<a href="#illus19">Fig. 19.</a> Blood-cells, or phagocytes<br/> +<a href="#illus20">Fig. 20.</a> Spermia or spermatozoa<br/> +<a href="#illus21">Fig. 21.</a> Spermatozoa of various animals<br/> +<a href="#illus22">Fig. 22.</a> A single human spermatozoon<br/> +<a href="#illus23">Fig. 23.</a> Fertilisation of the ovum<br/> +<a href="#illus24">Fig. 24.</a> Impregnated echinoderm ovum<br/> +<a href="#illus25">Fig. 25.</a> Impregnation of the star-fish ovum<br/> +<a href="#illus26">Figs. 26–27.</a> Impregnation of sea-urchin ovum<br/> +<a href="#illus28">Fig. 28.</a> Stem-cell of a rabbit<br/> +<a href="#illus29">Fig. 29.</a> Gastrulation of a coral<br/> +<a href="#illus30">Fig. 30.</a> Gastrula of a gastræad<br/> +<a href="#illus31">Fig. 31.</a> Gastrula of a worm<br/> +<a href="#illus32">Fig. 32.</a> Gastrula of an echinoderm<br/> +<a href="#illus33">Fig. 33.</a> Gastrula of an arthropod<br/> +<a href="#illus34">Fig. 34.</a> Gastrula of a mollusc<br/> +<a href="#illus35">Fig. 35.</a> Gastrula of a vertebrate<br/> +<a href="#illus36">Fig. 36.</a> Gastrula of a lower sponge<br/> +<a href="#illus37">Fig. 37.</a> Cells from the primary germinal layers<br/> +<a href="#illus38">Fig. 38.</a> Gastrulation of the amphioxus<br/> +<a href="#illus39">Fig. 39.</a> Gastrula of the amphioxus<br/> +<a href="#illus40">Fig. 40.</a> Cleavage of the frog’s ovum<br/> +<a href="#illus41">Figs. 41–44.</a> Sections of fertilised toad ovum<br/> +<a href="#illus45">Figs. 45–48.</a> Gastrulation of the salamander<br/> +<a href="#illus49">Fig. 49.</a> Segmentation of the lamprey<br/> +<a href="#illus50">Fig. 50.</a> Gastrulation of the lamprey<br/> +<a href="#illus51">Fig. 51.</a> Gastrulation of ceratodus<br/> +<a href="#illus52">Fig. 52.</a> Ovum of a deep-sea bony fish<br/> +<a href="#illus53">Fig. 53.</a> Segmentation of a bony fish<br/> +<a href="#illus54">Fig. 54.</a> Discoid gastrula of a bony fish<br/> +<a href="#illus55">Figs. 55–56.</a> Sections of blastula of shark<br/> +<a href="#illus57">Fig. 57.</a> Discoid segmentation of bird’s ovum<br/> +<a href="#illus58">Figs. 58–61.</a> Gastrulation of the bird<br/> +<a href="#illus62">Fig. 62.</a> Germinal disk of the lizard<br/> +<a href="#illus63">Figs. 63–64.</a> Gastrulation of the opossum<br/> +<a href="#illus65">Figs. 65–67.</a> Gastrulation of the opossum<br/> +<a href="#illus68">Figs. 68–71.</a> Gastrulation of the rabbit<br/> +<a href="#illus72">Fig. 72.</a> Gastrula of the placental mammal<br/> +<a href="#illus73">Fig. 73.</a> Gastrula of the rabbit<br/> +<a href="#illus74">Figs. 74–75.</a> Diagram of the four secondary germinal layers<br/> +<a href="#illus76">Figs. 76–77.</a> Cœlomula of sagitta<br/> +<a href="#illus78">Fig. 78.</a> Section of young sagitta<br/> +<a href="#illus79">Figs. 79–80.</a> Section of amphioxus-larvæ<br/> +<a href="#illus81">Figs. 81–82.</a> Section of amphioxus-larvæ<br/> +<a href="#illus83">Figs. 83–84.</a> Chordula of the amphioxus<br/> +<a href="#illus85">Figs. 85–86.</a> Chordula of the amphibia<br/> +<a href="#illus87">Figs. 87–88.</a> Section of cœlomula-embryos of vertebrates<br/> +<a href="#illus89">Figs. 89–90.</a> Section of cœlomula-embryo of triton<br/> +<a href="#illus91">Fig. 91.</a> Dorsal part of three triton-embryos<br/> +<a href="#illus92">Fig. 92.</a> Chordula-embryo of a bird<br/> +<a href="#illus93">Fig. 93.</a> Vertebrate-embryo of a bird<br/> +<a href="#illus94">Figs. 94–95.</a> Section of the primitive streak of a chick<br/> +<a href="#illus96">Fig. 96.</a> Section of the primitive groove of a rabbit<br/> +<a href="#illus97">Fig. 97.</a> Section of primitive mouth of a human embryo<br/> +<a href="#illus98">Figs. 98–102.</a> The ideal primitive vertebrate<br/> +<a href="#illus103">Fig. 103.</a> Redundant mammary glands<br/> +<a href="#illus104">Fig. 104.</a> A Greek gynecomast<br/> +<a href="#illus105">Fig. 105.</a> Severance of the discoid mammal embryo<br/> +<a href="#illus106">Figs. 106–107.</a> The visceral embryonic vesicle<br/> +<a href="#illus108">Fig. 108.</a> Four entodermic cells<br/> +<a href="#illus109">Fig. 109.</a> Two entodermic cells<br/> +<a href="#illus110">Figs. 110–114.</a> Ovum of a rabbit<br/> +<a href="#illus115">Figs. 115–118.</a> Embryonic vesicle of a rabbit<br/> +<a href="#illus119">Fig. 119.</a> Section of the gastrula of four vertebrates<br/> +<a href="#illus120">Figs 120.</a> Embryonic shield of a rabbit<br/> +<a href="#illus121">Figs. 121–123.</a> Dorsal shield and embryonic shield of a rabbit.<br/> +<a href="#illus124">Fig. 124.</a> Cœlomula of the amphioxus<br/> +<a href="#illus125">Fig. 125.</a> Chordula of a frog<br/> +<a href="#illus126">Fig. 126.</a> Section of frog-embryo<br/> +<a href="#illus127">Figs. 127–128.</a> Dorsal shield of a chick<br/> +<a href="#illus129">Fig. 129.</a> Section of hind end of a chick<br/> +<a href="#illus130">Fig. 130.</a> Germinal area of the rabbit<br/> +<a href="#illus131">Fig. 131.</a> Embryo of the opossum<br/> +<a href="#illus132">Fig. 132.</a> Embryonic shield of the rabbit<br/> +<a href="#illus133">Fig. 133.</a> Human embryo at the sandal-stage<br/> +<a href="#illus134">Fig. 134.</a> Embryonic shield of rabbit<br/> +<a href="#illus135">Fig. 135.</a> Embryonic shield of opossum<br/> +<a href="#illus136">Fig. 136.</a> Embryonic disk of a chick<br/> +<a href="#illus137">Fig. 137.</a> Embryonic disk of a higher vertebrate<br/> +<a href="#illus138">Figs. 138–142.</a> Sections of maturing mammal embryo<br/> +<a href="#illus143">Figs. 143–146.</a> Sections of embryonic chicks<br/> +<a href="#illus147">Fig. 147.</a> Section of embryonic chick<br/> +<a href="#illus148">Fig. 148.</a> Section of fore-half of chick-embryo<br/> +<a href="#illus149">Figs. 149–150.</a> Sections of human embryos<br/> +<a href="#illus151">Fig. 151.</a> Section of a shark-embryo<br/> +<a href="#illus152">Fig. 152.</a> Section of a duck-embryo<br/> +<a href="#illus153">Figs. 153–155.</a> Sole-shaped embryonic disk of chick<br/> +<a href="#illus156">Figs. 156–157.</a> Embryo of the amphioxus<br/> +<a href="#illus158">Figs. 158–160.</a> Embryo of the amphioxus<br/> +<a href="#illus161">Figs. 161–162.</a> Sections of shark-embryos<br/> +<a href="#illus163">Fig. 163.</a> Section of a Triton-embryo<br/> +<a href="#illus164">Figs. 164–166.</a> Vertebræ<br/> +<a href="#illus167">Fig. 167.</a> Head of a shark-embryo<br/> +<a href="#illus168">Figs. 168–169.</a> Head of a chick-embryo<br/> +<a href="#illus170">Fig. 170.</a> Head of a dog-embryo<br/> +<a href="#illus171">Fig. 171.</a> Human embryo of the fourth week<br/> +<a href="#illus172">Fig. 172.</a> Section of shoulder of chick-embryo<br/> +<a href="#illus173">Fig. 173.</a> Section of pelvic region of chick-embryo<br/> +<a href="#illus174">Fig. 174.</a> Development of the lizard’s legs<br/> +<a href="#illus175">Fig. 175.</a> Human-embryo five weeks old<br/> +<a href="#illus176">Figs. 176–178.</a> Embryos of the bat<br/> +<a href="#illus179">Fig. 179.</a> Human embryos<br/> +<a href="#illus180">Fig. 180.</a> Human embryo of the fourth week<br/> +<a href="#illus181">Fig. 181.</a> Human embryo of the fifth week<br/> +<a href="#illus182">Fig. 182.</a> Section of tail of human embryo<br/> +<a href="#illus183">Figs. 183–184.</a> Human embryo dissected<br/> +<a href="#illus185">Fig. 185.</a> Miss Julia Pastrana<br/> +<a href="#illus186">Figs. 186–190.</a> Human embryos<br/> +<a href="#illus191">Fig. 191.</a> Human embryos of sixteen to eighteen ays<br/> +<a href="#illus192">Figs. 192–193.</a> Human embryo of fourth week<br/> +<a href="#illus194">Fig. 194.</a> Human embryo with its membranes<br/> +<a href="#illus195">Fig. 195.</a> Diagram of the embryonic organs<br/> +<a href="#illus196">Fig. 196.</a> Section of the pregnant womb<br/> +<a href="#illus197">Fig. 197.</a> Embryo of siamang-gibbon<br/> +<a href="#illus198">Fig. 198.</a> Section of pregnant womb<br/> +<a href="#illus199">Figs. 199–200.</a> Human fœtus–placenta<br/> +<a href="#illus201">Fig. 201.</a> Vitelline vessels in germinative area<br/> +<a href="#illus202">Fig. 202.</a> Boat-shaped embryo of the dog<br/> +<a href="#illus203">Fig. 203.</a> Lar or white-handed gibbon<br/> +<a href="#illus204">Fig. 204.</a> Young orang<br/> +<a href="#illus205">Fig. 205.</a> Wild orang<br/> +<a href="#illus206">Fig. 206.</a> Bald-headed chimpanzee<br/> +<a href="#illus207">Fig. 207.</a> Female chimpanzee<br/> +<a href="#illus208">Fig. 208.</a> Female gorilla<br/> +<a href="#illus209">Fig. 209.</a> Male giant-gorilla<br/> +<a href="#illus210">Fig. 210.</a> The lancelet<br/> +<a href="#illus211">Fig. 211.</a> Section of the head of the lancelet<br/> +<a href="#illus212">Fig. 212.</a> Section of an amphioxus-larva<br/> +<a href="#illus213">Fig. 213.</a> Diagram of preceding<br/> +<a href="#illus214">Fig. 214.</a> Section of a young amphioxus<br/> +<a href="#illus215">Fig. 215.</a> Diagram of a young amphioxus<br/> +<a href="#illus216">Fig. 216.</a> Transverse section of lancelet<br/> +<a href="#illus217">Fig. 217.</a> Section through the middle of the lancelet<br/> +<a href="#illus218">Fig. 218.</a> Section of a primitive-fish embryo<br/> +<a href="#illus219">Fig. 219.</a> Section of the head of the lancelet<br/> +<a href="#illus220">Figs. 220.</a> Organisation of an ascidia<br/> +<a href="#illus221">Figs. 221.</a> Organisation of an ascidia<br/> +<a href="#illus222">Figs. 222–224.</a> Sections of young amphioxus-larvæ<br/> +<a href="#illus225">Fig. 225.</a> An appendicaria<br/> +<a href="#illus226">Fig. 226.</a> Chroococcus minor<br/> +<a href="#illus227">Fig. 227.</a> Aphanocapsa primordialis<br/> +<a href="#illus228">Fig. 228.</a> Protamœba<br/> +<a href="#illus229">Fig. 229.</a> Original ovum-cleavage<br/> +<a href="#illus230">Fig. 230.</a> Morula<br/> +<a href="#illus231">Figs. 231–232.</a> Magosphæra planula<br/> +<a href="#illus233">Fig. 233.</a> Modern gastræads<br/> +<a href="#illus234">Figs. 234–235.</a> Prophysema primordiale<br/> +<a href="#illus236">Figs. 236–237.</a> Ascula of gastrophysema<br/> +<a href="#illus238">Fig. 238.</a> Olynthus<br/> +<a href="#illus239">Fig. 239.</a> Aphanostomum langii<br/> +<a href="#illus240">Figs. 240–241.</a> A turbellarian<br/> +<a href="#illus242">Figs. 242–243.</a> Chætonotus<br/> +<a href="#illus244">Fig. 244.</a> A nemertine worm<br/> +<a href="#illus245">Fig. 245.</a> An enteropneust<br/> +<a href="#illus246">Fig. 246.</a> Section of the branchial gut<br/> +<a href="#illus247">Fig. 247.</a> The marine lamprey<br/> +<a href="#illus248">Fig. 248.</a> Fossil primitive fish<br/> +<a href="#illus249">Fig. 249.</a> Embryo of a shark<br/> +<a href="#illus250">Fig. 250.</a> Man-eating shark<br/> +<a href="#illus251">Fig. 251.</a> Fossil angel-shark<br/> +<a href="#illus252">Fig. 252.</a> Tooth of a gigantic shark<br/> +<a href="#illus253">Figs. 253–255.</a> Crossopterygii<br/> +<a href="#illus256">Fig. 256.</a> Fossil dipneust<br/> +<a href="#illus257">Fig. 257.</a> The Australian dipneust<br/> +<a href="#illus258">Figs. 258–259.</a> Young ceratodus<br/> +<a href="#illus260">Fig. 260.</a> Fossil amphibian<br/> +<a href="#illus261">Fig. 261.</a> Larva of the spotted salamander<br/> +<a href="#illus262">Fig. 262.</a> Larva of common frog<br/> +<a href="#illus263">Fig. 263.</a> Fossil mailed amphibian<br/> +<a href="#illus264">Fig. 264.</a> The new zealand lizard<br/> +<a href="#illus265">Fig. 265.</a> Homœosaurus pulchellus<br/> +<a href="#illus266">Fig. 266.</a> Skull of a permian lizard<br/> +<a href="#illus267">Fig. 267.</a> Skull of a theromorphum<br/> +<a href="#illus268">Fig. 268.</a> Lower jaw of a primitive mammal<br/> +<a href="#illus269">Figs. 269–270.</a> The ornithorhyncus<br/> +<a href="#illus271">Fig. 271.</a> Lower jaw of a promammal<br/> +<a href="#illus272">Fig. 272.</a> The crab-eating opossum<br/> +<a href="#illus273">Fig. 273.</a> Fœtal membranes of the human embryo<br/> +<a href="#illus274">Fig. 274.</a> Skull of a fossil lemur<br/> +<a href="#illus275">Fig. 275.</a> The slender lori<br/> +<a href="#illus276">Fig. 276.</a> The white-nosed ape<br/> +<a href="#illus277">Fig. 277.</a> The drill-baboon<br/> +<a href="#illus278">Figs. 278–282.</a> Skeletons of man and the anthropoid apes<br/> +<a href="#illus283">Fig. 283.</a> Skull of the java ape-man<br/> +<a href="#illus284">Fig. 284.</a> Section of the human skin<br/> +<a href="#illus285">Fig. 285.</a> Epidermic cells<br/> +<a href="#illus286">Fig. 286.</a> Rudimentary lachrymal glands<br/> +<a href="#illus287">Fig. 287.</a> The female breast<br/> +<a href="#illus288">Fig. 288.</a> Mammary gland of a new-born infant<br/> +<a href="#illus289">Fig. 289.</a> Embryo of a bear<br/> +<a href="#illus290">Fig. 290.</a> Human embryo<br/> +<a href="#illus291">Fig. 291.</a> Central marrow of a human embryo<br/> +<a href="#illus292">Figs. 292–293.</a> The human brain<br/> +<a href="#illus294">Figs. 294–296.</a> Central marrow of human embryo<br/> +<a href="#illus297">Fig. 297.</a> Head of a chick embryo<br/> +<a href="#illus298">Fig. 298.</a> Brain of three craniote embryos<br/> +<a href="#illus299">Fig. 299.</a> Brain of a shark<br/> +<a href="#illus300">Fig. 300.</a> Brain and spinal cord of a frog<br/> +<a href="#illus301">Fig. 301.</a> Brain of an ox-embryo<br/> +<a href="#illus302">Fig. 302.</a> Brain of a human embryo<br/> +<a href="#illus303">Fig. 303.</a> Brain of a human embryo<br/> +<a href="#illus304">Fig. 304.</a> Brain of the rabbit<br/> +<a href="#illus305">Fig. 305.</a> Bead of a shark<br/> +<a href="#illus306">Figs. 306–310.</a> Heads of chick-embryos<br/> +<a href="#illus311">Fig. 311.</a> Section of mouth of human embryo<br/> +<a href="#illus312">Fig. 312.</a> Diagram of mouth-nose cavity<br/> +<a href="#illus313">Figs. 313–314.</a> Heads of human embryo<br/> +<a href="#illus315">Figs. 315–316.</a> Face of human embryo<br/> +<a href="#illus317">Fig. 317.</a> The human eye<br/> +<a href="#illus318">Fig. 318.</a> Eye of the chick embryo<br/> +<a href="#illus319">Fig. 319.</a> Section of eye of a human embryo<br/> +<a href="#illus320">Fig. 320.</a> The human ear<br/> +<a href="#illus321">Fig. 321.</a> The bony labyrinth<br/> +<a href="#illus322">Fig. 322.</a> Development of the labyrinth<br/> +<a href="#illus323">Fig. 323.</a> Primitive skull of human embryo<br/> +<a href="#illus324">Fig. 324.</a> Rudimentary muscles of the ear<br/> +<a href="#illus325">Figs. 325–326.</a> The human skeleton<br/> +<a href="#illus327">Fig. 327.</a> The human vertebral column<br/> +<a href="#illus328">Fig. 328.</a> Piece of the dorsal cord<br/> +<a href="#illus329">Figs. 329–330.</a> Dorsal vertebræ<br/> +<a href="#illus331">Fig. 331.</a> Intervertebral disk<br/> +<a href="#illus332">Fig. 332.</a> Human skull<br/> +<a href="#illus333">Fig. 333.</a> Skull of new-born child<br/> +<a href="#illus334">Fig. 334.</a> Head-skeleton of a primitive fish<br/> +<a href="#illus335">Fig. 335.</a> Skulls of nine primates<br/> +<a href="#illus336">Figs. 336–338.</a> Evolution of the fin<br/> +<a href="#illus339">Fig. 339.</a> Skeleton of the fore-leg of an amphibian<br/> +<a href="#illus340">Fig. 340.</a> Skeleton of gorilla’s hand<br/> +<a href="#illus341">Fig. 341.</a> Skeleton of human hand<br/> +<a href="#illus342">Fig. 342.</a> Skeleton of hand of six mammals<br/> +<a href="#illus343">Figs. 343–345.</a> Arm and hand of three anthropoids<br/> +<a href="#illus346">Fig. 346.</a> Section of fish’s tail<br/> +<a href="#illus347">Fig. 347.</a> Human skeleton<br/> +<a href="#illus348">Fig. 348.</a> Skeleton of the giant gorilla<br/> +<a href="#illus349">Fig. 349.</a> The human stomach<br/> +<a href="#illus350">Fig. 350.</a> Section of the head of a rabbit-embryo<br/> +<a href="#illus351">Fig. 351.</a> Shark’s teeth<br/> +<a href="#illus352">Fig. 352.</a> Gut of a human embryo<br/> +<a href="#illus353">Figs. 353–354.</a> Gut of a dog embryo<br/> +<a href="#illus355">Figs. 355–356.</a> Sections of head of lamprey<br/> +<a href="#illus357">Fig. 357.</a> Viscera of a human embryo<br/> +<a href="#illus358">Fig. 358.</a> Red blood-cells<br/> +<a href="#illus359">Fig. 359.</a> Vascular tissue<br/> +<a href="#illus360">Fig. 360.</a> Section of trunk of a chick-embryo<br/> +<a href="#illus361">Fig. 361.</a> Merocytes<br/> +<a href="#illus362">Fig. 362.</a> Vascular system of an annelid<br/> +<a href="#illus363">Fig. 363.</a> Head of a fish-embryo<br/> +<a href="#illus364">Figs. 364–366.</a> The five arterial arches<br/> +<a href="#illus367">Figs. 367–370.</a> The five arterial arches<br/> +<a href="#illus371">Figs. 371–372.</a> Heart of a rabbit-embryo<br/> +<a href="#illus373">Figs. 373–374.</a> Heart of a dog-embryo<br/> +<a href="#illus375">Figs. 375–377.</a> Heart of a human embryo<br/> +<a href="#illus378">Fig. 378.</a> Heart of adult man<br/> +<a href="#illus379">Fig. 379.</a> Section of head of a chick-embryo<br/> +<a href="#illus380">Fig. 380.</a> Section of a human embryo<br/> +<a href="#illus381">Figs. 381–382.</a> Sections of a chick-embryo<br/> +<a href="#illus383">Fig. 383.</a> Embryos of sagitta<br/> +<a href="#illus384">Fig. 384.</a> Kidneys of bdellostoma<br/> +<a href="#illus385">Fig. 385.</a> Section of embryonic shield<br/> +<a href="#illus386">Figs. 386–387.</a> Primitive kidneys<br/> +<a href="#illus388">Fig. 388.</a> Pig-embryo<br/> +<a href="#illus389">Fig. 389.</a> Human embryo<br/> +<a href="#illus390">Figs. 390–392.</a> Rudimentary kidneys and sexual organs<br/> +<a href="#illus393">Figs. 393–394.</a> Urinary and sexual organs of salamander<br/> +<a href="#illus395">Fig. 395.</a> Primitive kidneys of human embryo<br/> +<a href="#illus396">Figs. 396–398.</a> Urinary organs of ox-embryos<br/> +<a href="#illus399">Fig. 399.</a> Sexual organs of water-mole<br/> +<a href="#illus400">Figs. 400–401.</a> Original position of sexual glands<br/> +<a href="#illus402">Fig. 402.</a> Urogenital system of human embryo<br/> +<a href="#illus403">Fig. 403.</a> Section of ovary<br/> +<a href="#illus404">Figs. 404–406.</a> Graafian follicles<br/> +<a href="#illus407">Fig. 407.</a> A ripe graafian follicle<br/> +<a href="#illus408">Fig. 408.</a> The human ovum<br/> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="pref02"></a>GLOSSARY</h2> + +<p class="noindent"> +<b>Acrania:</b> animals without skull (<i>cranium</i>).<br/> + +<b>Anthropogeny:</b> the evolution (<i>genesis</i>) of man (<i>anthropos</i>).<br/> + +<b>Anthropology:</b> the science of man.<br/> + +<b>Archi-:</b> (in compounds) the first or typical—as, archi-cytula, +archi-gastrula, etc.<br/> + +<b>Biogeny:</b> the science of the genesis of life (<i>bios</i>).<br/> + +<b>Blast-:</b> (in compounds) pertaining to the early embryo (<i>blastos</i> = a +bud); hence:—<br/> + Blastoderm: skin (<i>derma</i>) or enclosing layer of the embryo.<br/> + Blastosphere: the embryo in the hollow sphere stage.<br/> + Blastula: same as preceding.<br/> + Epiblast: the outer layer of the embryo (ectoderm).<br/> + Hypoblast: the inner layer of the embryo (endoderm).<br/> + +<b>Branchial:</b> pertaining to the gills (<i>branchia</i>).<br/> + +<b>Caryo-:</b> (in compounds) pertaining to the nucleus (<i>caryon</i>); hence:—<br/> + Caryokineses: the movement of the nucleus.<br/> + Caryolysis: dissolution of the nucleus.<br/> + Caryoplasm: the matter of the nucleus.<br/> + +<b>Centrolecithal:</b> see under <b>Lecith-.</b><br/> + +<b>Chordaria</b> and <b>Chordonia:</b> animals with a dorsal chord or back-bone.<br/> + +<b>Cœlom</b> or <b>Cœloma:</b> the body-cavity in the embryo; hence:—<br/> + Cœlenterata: animals without a body-cavity.<br/> + Cœlomaria: animals with a body-cavity.<br/> + Cœlomation: formation of the body-cavity.<br/> + +<b>Cyto-:</b> (in compounds) pertaining to the cell (<i>cytos</i>); hence:—<br/> + Cytoblast: the nucleus of the cell.<br/> + Cytodes: cell-like bodies, imperfect cells.<br/> + Cytoplasm: the matter of the body of the cell.<br/> + Cytosoma: the body (<i>soma</i>) of the cell.<br/> + +<b>Cryptorchism:</b> abnormal retention of the testicles in the body.<br/> + +<b>Deutoplasm:</b> see <b>Plasm.</b><br/> + +<b>Dualism:</b> the belief in the existence of two entirely distinct +principles (such as matter and spirit).<br/> + +<b>Dysteleology:</b> the science of those features in organisms which refute +the “design-argument”.<br/> + +<b>Ectoderm:</b> the outer (<i>ekto</i>) layer of the embryo.<br/> + +<b>Entoderm:</b> the inner (<i>ento</i>) layer of the embryo.<br/> + +<b>Epiderm:</b> the outer layer of the skin.<br/> + +<b>Epigenesis:</b> the theory of gradual development of organs in the embryo.<br/> + +<b>Epiphysis:</b> the third or central eye in the early vertebrates.<br/> + +<b>Episoma:</b> see <b>Soma.</b><br/> + +<b>Epithelia:</b> tissues covering the surface of parts of the body (such as +the mouth, etc.)<br/> + +<b>Gonads:</b> the sexual glands.<br/> + +<b>Gonochorism:</b> separation of the male and female sexes.<br/> + +<b>Gonotomes:</b> sections of the sexual glands.<br/> + +<b>Gynecomast:</b> a male with the breasts (<i>masta</i>) of a woman (<i>gyne</i>).<br/> + +<b>Hepatic:</b> pertaining to the liver (<i>hepar</i>).<br/> + +<b>Holoblastic:</b> embryos in which the animal and vegetal cells divide +equally (<i>holon</i> = whole).<br/> + +<b>Hypermastism:</b> the possession of more than the normal breasts (<i>masta</i>).<br/> + +<b>Hypobranchial:</b> underneath (<i>hypo</i>) the gills.<br/> + +<b>Hypophysis:</b> sensitive-offshoot from the brain in the vertebrate.<br/> + +<b>Hyposoma:</b> see <b>Soma.</b><br/> + +<b>Lecith-:</b> pertaining to the yelk (<i>lecithus</i>); hence:—<br/> + Centrolecithal: eggs with the yelk in the centre.<br/> + Lecithoma: the yelk-sac.<br/> + Telolecithal: eggs with the yelk at one end.<br/> + +<b>Meroblastic:</b> cleaving in part (<i>meron</i>) only.<br/> + +<b>Meta-:</b> (in compounds) the “after” or secondary stage; hence:—<br/> + Metagaster: the secondary or permanent gut (<i>gaster</i>).<br/> + Metaplasm: secondary or differentiated plasm.<br/> + Metastoma: the secondary or permanent mouth (<i>stoma</i>).<br/> + Metazoa: the higher or later animals, made up of many cells.<br/> + Metovum: the mature or advanced ovum.<br/> + +<b>Metamera:</b> the segments into which the embryo breaks up.<br/> + +<b>Metamerism:</b> the segmentation of the embryo.<br/> + +<b>Monera:</b> the most primitive of the unicellular organisms.<br/> + +<b>Monism:</b> belief in the fundamental unity of all things.<br/> + +<b>Morphology:</b> the science of organic forms (generally equivalent to +anatomy).<br/> + +<b>Myotomes:</b> segments into which the muscles break up.<br/> + +<b>Nephra:</b> the kidneys; hence:—<br/> + Nephridia: the rudimentary kidney-organs.<br/> + Nephrotomes: the segments of the developing kidneys.<br/> + +<b>Ontogeny:</b> the science of the development of the individual (generally +equivalent to embryology).<br/> + +<b>Perigenesis:</b> the genesis of the movements in the vital particles.<br/> + +<b>Phagocytes:</b> cells that absorb food (<i>phagein</i> = to eat).<br/> + +<b>Phylogeny:</b> the science of the evolution of species (<i>phyla</i>).<br/> + +<b>Planocytes:</b> cells that move about (<i>planein</i>).<br/> + +<b>Plasm:</b> the colloid or jelly-like matter of which organisms are +composed; hence:—<br/> + Caryoplasm: the matter of the nucleus (<i>caryon</i>).<br/> + Cytoplasm: the matter of the body of the cell.<br/> + Deutoplasm: secondary or differentiated plasm.<br/> + Metaplasm: secondary or differentiated plasm.<br/> + Protoplasm: primitive or undifferentiated plasm.<br/> + +<b>Plasson:</b> the simplest form of plasm.<br/> + +<b>Plastidules:</b> small particles of plasm.<br/> + +<b>Polyspermism:</b> the penetration of more than one sperm-cell into the ovum.<br/> + +<b>Pro-</b> or <b>Prot:</b> (in compounds) the earlier form (opposed to <b>Meta</b>); hence:—<br/> + Prochorion: the first form of the chorion.<br/> + Progaster: the first or primitive stomach.<br/> + Pronephridia: the earlier form of the kidneys.<br/> + Prorenal: the earlier form of the kidneys.<br/> + Prostoma: the first or primitive mouth.<br/> + Protists: the earliest or unicellular organisms.<br/> + Provertebræ: the earliest phase of the vertebræ.<br/> + Protophyta: the primitive or unicellular plants.<br/> + Protoplasm: undifferentiated plasm.<br/> + Protozoa: the primitive or unicellular animals.<br/> + +<b>Renal:</b> pertaining to the kidneys (<i>renes</i>).<br/> + +<b>Scatulation:</b> packing or boxing-up (<i>scatula</i> = a box).<br/> + +<b>Sclerotomes:</b> segments into which the primitive skeleton falls.<br/> + +<b>Soma:</b> the body; hence:—<br/> + Cytosoma: the body of the cell (<i>cytos</i>).<br/> + Episoma: the upper or back-half of the embryonic body.<br/> + Somites: segments of the embryonic body.<br/> + Hyposoma: the under or belly-half of the embryonic body.<br/> + +<b>Teleology:</b> the belief in design and purpose (<i>telos</i>) in nature.<br/> + +<b>Telolecithal:</b> see <b>Lecith-.</b><br/> + +<b>Umbilical:</b> pertaining to the navel (<i>umbilicus</i>).<br/> + +<b>Vitelline:</b> pertaining to the yelk (<i>vitellus</i>). +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="pref03"></a>PREFACE</h2> + +<p class="center"> +[BY JOSEPH MCCABE] +</p> + +<p> +The work which we now place within the reach of every reader of the English +tongue is one of the finest productions of its distinguished author. The first +edition appeared in 1874. At that time the conviction of man’s natural +evolution was even less advanced in Germany than in England, and the work +raised a storm of controversy. Theologians—forgetting the commonest facts +of our individual development—spoke with the most profound disdain of the +theory that a Luther or a Goethe could be the outcome of development from a +tiny speck of protoplasm. The work, one of the most distinguished of them said, +was “a fleck of shame on the escutcheon of Germany.” To-day its +conclusion is accepted by influential clerics, such as the Dean of Westminster, +and by almost every biologist and anthropologist of distinction in Europe. +Evolution is not a laboriously reached conclusion, but a guiding truth, in +biological literature to-day. +</p> + +<p> There was ample evidence to substantiate the conclusion even in the first +edition of the book. But fresh facts have come to light in each decade, always +enforcing the general truth of man’s evolution, and at times making +clearer the line of development. Professor Haeckel embodied these in successive +editions of his work. In the fifth edition, of which this is a translation, +reference will be found to the very latest facts bearing on the evolution of +man, such as the discovery of the remarkable effect of mixing human blood with +that of the anthropoid ape. Moreover, the ample series of illustrations has +been considerably improved and enlarged; there is no scientific work published, +at a price remotely approaching that of the present edition, with so abundant +and excellent a supply of illustrations. When it was issued in Germany, a few +years ago, a distinguished biologist wrote in the <i>Frankfurter Zeitung</i> +that it would secure immortality for its author, the most notable critic of the +idea of immortality. And the <i>Daily Telegraph</i> reviewer described the +English version as a “handsome edition of Haeckel’s monumental +work,” and “an issue worthy of the subject and the author.” +</p> + +<p> +The influence of such a work, one of the most constructive that Haeckel has +ever written, should extend to more than the few hundred readers who are able +to purchase the expensive volumes of the original issue. Few pages in the story +of science are more arresting and generally instructive than this great picture +of “mankind in the making.” The horizon of the mind is healthily +expanded as we follow the search-light of science down the vast avenues of past +time, and gaze on the uncouth forms that enter into, or illustrate, the +line of our ancestry. And if the imagination recoils from the strange and +remote figures that are lit up by our search-light, and hesitates to accept +them as ancestral forms, science draws aside another veil and reveals another +picture to us. It shows us that each of us passes, in our embryonic +development, through a series of forms hardly less uncouth and unfamiliar. Nay, +it traces a parallel between the two series of forms. It shows us man beginning +his existence, in the ovary of the female infant, as a minute and simple speck +of jelly-like plasm. It shows us (from analogy) the fertilised ovum breaking +into a cluster of cohering cells, and folding and curving, until the limb-less, +head-less, long-tailed fœtus looks like a worm-shaped body. It then +points out how gill-slits and corresponding blood-vessels appear, as in a lowly +fish, and the fin-like extremities bud out and grow into limbs, and so on; +until, after a very clear ape-stage, the definite human form emerges from the +series of transformations. +</p> + +<p> +It is with this embryological evidence for our evolution that the present +volume is concerned. There are illustrations in the work that will make the +point clear at a glance. Possibly <i>too</i> clear; for the simplicity of the +idea and the eagerness to apply it at every point have carried many, who borrow +hastily from Haeckel, out of their scientific depth. Haeckel has never shared +their errors, nor encouraged their superficiality. He insists from the outset +that a complete parallel could not possibly be expected. Embryonic life itself +is subject to evolution. Though there is a general and substantial law—as +most of our English and American authorities admit—that the embryonic +series of forms recalls the ancestral series of forms, the parallel is blurred +throughout and often distorted. It is not the obvious resemblance of the +embryos of different animals, and their general similarity to our extinct +ancestors in this or that organ, on which we must rest our case. A careful +study must be made of the various stages through which all embryos pass, and an +effort made to prove their real identity and therefore genealogical relation. +</p> + +<p> +This is a task of great subtlety and delicacy. Many scientists have worked at +it together with Professor Haeckel—I need only name our own Professor +Balfour and Professor Ray Lankester—and the scheme is fairly complete. +But the general reader must not expect that even so clear a writer as Haeckel +can describe these intricate processes without demanding his very careful +attention. Most of the chapters in the present volume (and the second volume +will be less difficult) are easily intelligible to all; but there are points at +which the line of argument is necessarily subtle and complex. In the hope that +most readers will be induced to master even these more difficult chapters, I +will give an outline of the characteristic argument of the work. +Haeckel’s distinctive services in regard to man’s evolution have +been: (1) The construction of a complete ancestral tree, though, of course, +some of the stages in it are purely conjectural, and not final; (2) The tracing +of the remarkable reproduction of ancestral forms in the embryonic +development of the individual. Naturally, he has not worked alone in either +department. The second volume of this work will embody the first of these two +achievements; the present one is mainly concerned with the latter. It will be +useful for the reader to have a synopsis of the argument and an explanation of +some of the chief terms invented or employed by the author. +</p> + +<p> +The main theme of the work is that, in the course of their embryonic +development, all animals, including man, pass roughly and rapidly through a +series of forms which represents the succession of their ancestors in the past. +After a severe and extensive study of embryonic phenomena, Haeckel has drawn up +a “law” (in the ordinary scientific sense) to this effect, and has +called it “the biogenetic law,” or the chief law relating to the +evolution (<i>genesis</i>) of life (<i>bios</i>). This law is widely and +increasingly accepted by embryologists and zoologists. It is enough to quote a +recent declaration of the great American zoologist, President D. Starr Jordan: +“It is, of course, true that the life-history of the individual is an +epitome of the life-history of the race”; while a distinguished German +zoologist (Sarasin) has described it as being of the same use to the biologist +as spectrum analysis is to the astronomer. +</p> + +<p> +But the reproduction of ancestral forms in the course of the embryonic +development is by no means always clear, or even always present. Many of the +embryonic phases do not recall ancestral stages at all. They may have done so +originally, but we must remember that the embryonic life itself has been +subject to adaptive changes for millions of years. All this is clearly +explained by Professor Haeckel. For the moment, I would impress on the reader +the vital importance of fixing the distinction from the start. He must +thoroughly familiarise himself with the meaning of five terms. <i>Biogeny</i> +is the development of life in general (both in the individual and the species), +or the sciences describing it. <i>Ontogeny</i> is the development (embryonic +and post-embryonic) of the individual (<i>on</i>), or the science describing +it. <i>Phylogeny</i> is the development of the race or stem (<i>phulon</i>), or +the science describing it. Roughly, <i>ontogeny</i> may be taken to mean +embryology, and <i>phylogeny</i> what we generally call evolution. Further, the +embryonic phenomena sometimes reproduce ancestral forms, and they are then +called <i>palingenetic</i> (from <i>palin</i> = again): sometimes they do not +recall ancestral forms, but are later modifications due to adaptation, and they +are then called <i>cenogenetic</i> (from <i>kenos</i> = new or foreign). These +terms are now widely used, but the reader of Haeckel must understand them +thoroughly. +</p> + +<p> +The first five chapters are an easy account of the history of embryology and +evolution. The sixth and seventh give an equally clear account of the sexual +elements and the process of conception. But some of the succeeding chapters +must deal with embryonic processes so unfamiliar, and pursue them through so +wide a range of animals in a brief space, that, in spite of the 200 +illustrations, they will offer difficulty to many a reader. As our aim is to +secure, not a superficial acquiescence in conclusions, but a fair comprehension +of the truths of science, we have retained these chapters. However, I will give +a brief and clear outline of the argument, so that the reader with little +leisure may realise their value. +</p> + +<p> +When the animal ovum (egg-cell) has been fertilised, it divides and subdivides +until we have a cluster of cohering cells, externally not unlike a raspberry or +mulberry. This is the <i>morula</i> (= mulberry) stage. The cluster becomes +hollow, or filled with fluid in the centre, all the cells rising to the +surface. This is the <i>blastula</i> (hollow ball) stage. One half of the +cluster then bends or folds in upon the other, as one might do with a thin +indiarubber ball, and we get a vase-shaped body with hollow interior (the first +stomach, or “primitive gut”), an open mouth (the first or +“primitive mouth”), and a wall composed of two layers of cells (two +“germinal layers”). This is the <i>gastrula</i> (stomach) stage, +and the process of its formation is called <i>gastrulation</i>. A glance at the +illustration (Fig. 29) will make this perfectly clear. +</p> + +<p> +So much for the embryonic process in itself. The application to evolution has +been a long and laborious task. Briefly, it was necessary to show that +<i>all</i> the multicellular animals passed through these three stages, so that +our biogenetic law would enable us to recognise them as reminiscences of +ancestral forms. This is the work of Chapters VIII and IX. The difficulty can +be realised in this way: As we reach the higher animals the ovum has to take up +a large quantity of yelk, on which it may feed in developing. Think of the +bird’s “egg.” The effect of this was to flatten the germ (the +<i>morula</i> and <i>blastula</i>) from the first, and so give, at first sight, +a totally different complexion to what it has in the lowest animals. When we +pass the reptile and bird stage, the large yelk almost disappears (the germ now +being supplied with blood by the mother), but the germ has been permanently +altered in shape, and there are now a number of new embryonic processes +(membranes, blood-vessel connections, etc.). Thus it was no light task to trace +the identity of this process of <i>gastrulation</i> in all the animals. It has +been done, however; and with this introduction the reader will be able to +follow the proof. The conclusion is important. If all animals pass through the +curious gastrula stage, it must be because they all had a common ancestor of +that nature. To this conjectural ancestor (it lived before the period of +fossilisation begins) Haeckel gives the name of the <i>Gastræa,</i> and +in the second volume we shall see a number of living animals of this type +(“gastræads”). +</p> + +<p> +The line of argument is the same in the next chapter. After laborious and +careful research (though this stage is not generally admitted in the same sense +as the previous one), a fourth common stage was discovered, and given the name +of the <i>Cœlomula.</i> The blastula had one layer of cells, the +<i>blastoderm</i> (<i>derma</i> = skin): the gastrula two layers, the +<i>ectoderm</i> (“outer skin”) and <i>entoderm</i> (“inner +skin”). Now a third layer (<i>mesoderm</i> = middle skin) is formed, +by the growth inwards of two pouches or folds of the skin. The pouches blend +together, and form a single cavity (the body cavity, or cœlom), and its +two walls are two fresh “germinal layers.” Again, the identity of +the process has to be proved in all the higher classes of animals, and when +this is done we have another ancestral stage, the <i>Cœlomæa.</i> +</p> + +<p> +The remaining task is to build up the complex frame of the higher +animals—always showing the identity of the process (on which the +evolutionary argument depends) in enormously different conditions of embryonic +life—out of the four “germinal layers.” Chapter IX prepares +us for the work by giving us a very clear account of the essential structure of +the back-boned (vertebrate) animal, and the probable common ancestor of all the +vertebrates (a small fish of the lancelet type). Chapters XI–XIV then +carry out the construction step by step. The work is now simpler, in the sense +that we leave all the invertebrate animals out of account; but there are so +many organs to be fashioned out of the four simple layers that the reader must +proceed carefully. In the second volume each of these organs will be dealt with +separately, and the parallel will be worked out between its embryonic and its +phylogenetic (evolutionary) development. The general reader may wait for this +for a full understanding. But in the meantime the wonderful story of the +construction of all our organs in the course of a few weeks (the human frame is +perfectly formed, though less than two inches in length, by the twelfth week) +from so simple a material is full of interest. It would be useless to attempt +to summarise the process. The four chapters are themselves but a summary of it, +and the eighty fine illustrations of the process will make it sufficiently +clear. The last chapter carries the story on to the point where man at last +parts company with the anthropoid ape, and gives a full account of the +membranes or wrappers that enfold him in the womb, and the connection with the +mother. +</p> + +<p> +In conclusion, I would urge the reader to consult, at his free library perhaps, +the complete edition of this work, when he has read the present abbreviated +edition. Much of the text has had to be condensed in order to bring out the +work at our popular price, and the beautiful plates of the complete edition +have had to be omitted. The reader will find it an immense assistance if he can +consult the library edition. +</p> + +<p class="right"> +JOSEPH MCCABE +</p> + +<p> +<i>Cricklewood, March, 1906.</i> +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="pref04"></a>HAECKEL’S CLASSIFICATION OF THE ANIMAL WORLD</h2> + +<table border="1" cellspacing="0" cellpadding="4" summary="Classification of Unicellular animals (Protozoa)"> +<tr><td align="center" colspan="3"><b>Unicellular animals (Protozoa)</b></td></tr> + +<tr><td>1. Unnucleated</td><td>Bacteria<br/>Protamæbæ</td><td>Monera</td></tr> + +<tr><td rowspan="2">2. Nucleated</td><td><i>a.</i> Rhizopoda</td><td>Amœbina<br/>Radiolaria</td></tr> + +<tr><td><i>b.</i> Infusoria</td><td>Flagellata<br/>Ciliata</td></tr> + +<tr><td>3. Cell-colonies</td><td>Catallacta<br/>Blastæada</td><td> </td></tr> +</table> + +<p> +<br/><br/> +</p> + +<table border="1" cellspacing="0" cellpadding="4" summary="Classification of Multicellular animals (Metazoa)"> +<tr><td align="center" colspan="4"><b>Unicellular animals (Protozoa)</b></td></tr> + +<tr><td align="center" valign="middle" rowspan="4"><b>I<br/>Cœlenterata,</b><br/> +or <b>Zoophytes.</b><br/> +Animals without<br/> +body-cavity,<br/> +blood, or anus.</td> +<td><i>a.</i> Gastræads</td><td>Gastremaria<br/>Cyemaria</td><td> </td></tr> + +<tr><td><i>b.</i> Sponges</td><td>Protospongiæ<br/>Metaspongiæ</td><td> </td></tr> + +<tr><td><i>c.</i> Cnidaria<br/> (stinging animals)</td><td> +Hydrozoa<br/>Polyps<br/>Medusæ</td><td> </td></tr> + +<tr><td><i>d.</i> Platodes<br/> (flat-worms)</td><td>Platodaria<br/>Turbullaria<br/>Trematoda<br/>Cestoda</td><td> </td></tr> + + +<tr><td align="center" valign="middle" rowspan="9"> +<b>II<br/> +Cœlomaria</b> or<br/> +<b>Bilaterals.</b><br/> +Animals with<br/> +body-cavity and<br/> +anus, and generally<br/>blood.</td> +<td><i>a.</i> Vermalia<br/> (worm-like)</td> +<td>Rotatoria<br/>Strongylaria<br/>Prosopygia<br/>Frontonia</td><td> </td></tr> + +<tr><td><i>b.</i> Molluscs</td><td>Cochlides<br/>Conchades<br/>Teuthodes</td><td> </td></tr> + +<tr><td><i>c.</i> Articulates</td><td>Annelida<br/>Crustacea<br/>Tracheata</td><td> </td></tr> + +<tr><td><i>d.</i> Echinoderms</td><td>Monorchonia<br/>Pentorchonia</td><td> </td></tr> + +<tr><td><i>e.</i> Tunicates</td><td>Copelata<br/>Ascidiæ<br/>Thalidiæ</td> +<td> </td></tr> + +<tr><td rowspan="4"><i>f.</i> Vertebrates</td> +<td>I. Acrania-Lancelet<br/> (without skull)<br/>II. Craniota<br/> (with skull)<br/><i>a.</i> Cyclostomes<br/> (“round-mouthed”)</td><td> </td></tr> + +<tr><td><i>b.</i> Fishes</td><td>Selachii<br/>Ganoids<br/>Teleosts<br/>Dipneusts</td></tr> + +<tr><td><i>c.</i> Amphibia<br/><i>d.</i> Reptiles<br/><i>e.</i> Birds</td><td> </td></tr> + +<tr><td><i>f.</i> Mammal</td><td>Monotremes<br/>Marsupials<br/>Placentals:<br/> + Rodents<br/> + Edentates<br/> + Ungulates<br/> + Cetacea<br/> + Sirenia<br/> + Insectivora<br/> + Cheiroptera<br/> + Carnassia<br/> + Primates</td></tr> +</table> + +<p class="footnote"> +(This classification is given for the purpose of explaining Haeckel’s +use of terms in this volume. The general reader should bear in mind +that it differs very considerably from more recent schemes of +classification. He should compare the scheme framed by Professor E. +Ray Lankester.)</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2>THE EVOLUTION OF MAN</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap01"> +</a><span class='pagenum'><a name="Page_1" id="Page_1"></a></span> +Chapter I.<br/> +THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION</h2> + +<p>The field of natural phenomena into which I would +introduce my readers in the following chapters has a quite peculiar +place in the broad realm of scientific inquiry. There is no object +of investigation that touches man more closely, and the knowledge +of which should be more acceptable to him, than his own frame. But +among all the various branches of the natural history of mankind, +or <i>anthropology,</i> the story of his development by natural +means must excite the most lively interest. It gives us the key of +the great world-riddles at which the human mind has been working +for thousands of years. The problem of the nature of man, or the +question of man’s place in nature, and the cognate inquiries +as to the past, the earliest history, the present situation, and +the future of humanity—all these most important questions are +directly and intimately connected with that branch of study which +we call the science of the evolution of man, or, in one word, +“Anthropogeny” (the genesis of man). Yet it is an +astonishing fact that the science of the evolution of man does not +even yet form part of the scheme of general education. In fact, +educated people even in our day are for the most part quite +ignorant of the important truths and remarkable phenomena which +anthropogeny teaches us.</p> + +<p>As an illustration of this curious state of things, it may be +pointed out that most of what are considered to be +“educated” people do not know that every human being is +developed from an egg, or ovum, and that this egg is one simple +cell, like any other plant or animal egg. They are equally ignorant +that in the course of the development of this tiny, round egg-cell +there is first formed a body that is totally different from the +human frame, and has not the remotest resemblance to it. Most of +them have never seen such a human embryo in the earlier period of +its development, and do not know that it is quite indistinguishable +from other animal embryos. At first the embryo is no more than a +round cluster of cells, then it becomes a simple hollow sphere, the +wall of which is composed of a layer of cells. Later it approaches +very closely, at one period, to the anatomic structure of the +lancelet, afterwards to that of a fish, and again to the typical +build of the amphibia and mammals. As it continues to develop, a +form appears which is like those we find at the lowest stage of +mammal-life (such as the duck-bills), then a form that resembles +the marsupials, and only at a late stage a form that has a +resemblance to the ape; until at last the definite human form +emerges and closes the series of transformations. These suggestive +facts are, as I said, still almost unknown to the general +public—so completely unknown that, if one casually mentions +them, they are called in question or denied outright as +fairy-tales. Everybody knows that the butterfly emerges from the +pupa, and the pupa from a quite different thing called a larva, and +the larva from the butterfly’s egg. But few besides medical +men are aware that <i>man</i>, in the course of his individual +formation, passes through a series of transformations which are not +less surprising and wonderful than the familiar metamorphoses of +the butterfly.</p> + +<p>The mere description of these remarkable changes through which +man passes during his embryonic life should arouse considerable +interest. But the mind will experience a far keener satisfaction +when <span class='pagenum'><a name="Page_2" id="Page_2"></a></span>we trace these curious facts to their causes, and +when we learn to behold in them natural phenomena which are of the +highest importance throughout the whole field of human knowledge. +They throw light first of all on the “natural history of +creation,” then on psychology, or “the science of the +soul,” and through this on the whole of philosophy. And as +the general results of every branch of inquiry are summed up in +philosophy, all the sciences come in turn to be touched and +influenced more or less by the study of the evolution of man.</p> + +<p>But when I say that I propose to present here the most important +features of these phenomena and trace them to their causes, I take +the term, and I interpret my task, in a very much wider sense than +is usual. The lectures which have been delivered on this subject in +the universities during the last half-century are almost +exclusively adapted to medical men. Certainly, the medical man has +the greatest interest in studying the origin of the human body, +with which he is daily occupied. But I must not give here this +special description of the embryonic processes such as it has +hitherto been given, as most of my readers have not studied +anatomy, and are not likely to be entrusted with the care of the +adult organism. I must content myself with giving some parts of the +subject only in general outline, and must not enter upon all the +marvellous, but very intricate and not easily described, details +that are found in the story of the development of the human frame. +To understand these fully a knowledge of anatomy is needed. I will +endeavour to be as plain as possible in dealing with this branch of +science. Indeed, a sufficient general idea of the course of the +embryonic development of man can be obtained without going too +closely into the anatomic details. I trust we may be able to arouse +the same interest in this delicate field of inquiry as has been +excited already in other branches of science; though we shall meet +more obstacles here than elsewhere.</p> + +<p>The story of the evolution of man, as it has hitherto been +expounded to medical students, has usually been confined to +embryology—more correctly, <i>ontogeny</i>—or the +science of the development of the individual human organism. But +this is really only the first part of our task, the first half of +the story of the evolution of man in that wider sense in which we +understand it here. We must add as the second half—as another +and not less important and interesting branch of the science of the +evolution of the human stem—<i>phylogeny</i>: this may be +described as the science of the evolution of the various animal +forms from which the human organism has been developed in the +course of countless ages. Everybody now knows of the great +scientific activity that was occasioned by the publication of +Darwin’s <i>Origin of Species</i> in 1859. The chief direct +consequence of this publication was to provoke a fresh inquiry into +the origin of the human race, and this has proved beyond question +our gradual evolution from the lower species. We give the name of +“Phylogeny” to the science which describes this ascent +of man from the lower ranks of the animal world. The chief source +that it draws upon for facts is “Ontogeny,” or +embryology, the science of the development of the individual +organism. Moreover, it derives a good deal of support from <i> +paleontology,</i> or the science of fossil remains, and even more +from comparative anatomy, or <i>morphology.</i></p> + +<p>These two branches of our science—on the one side ontogeny +or embryology, and on the other phylogeny, or the science of +race-evolution—are most vitally connected. The one cannot be +understood without the other. It is only when the two branches +fully co-operate and supplement each other that +“Biogeny” (or the science of the genesis of life in the +widest sense) attains to the rank of a philosophic science. The +connection between them is not external and superficial, but +profound, intrinsic, and causal. This is a discovery made by recent +research, and it is most clearly and correctly expressed in the +comprehensive law which I have called “the fundamental law of +organic evolution,” or “the fundamental law of +biogeny.” This general law, to which we shall find ourselves +constantly recurring, and on the recognition of which depends +one’s whole insight into the story of evolution, may be +briefly expressed in the phrase: “The history of the +fœtus is a recapitulation of the history of the race”; +or, in other words, “Ontogeny is a recapitulation of +phylogeny.” It may be more fully stated as follows: The +series of forms through which the individual organism passes during +its development from the ovum to the complete bodily structure is a +brief, condensed repetition <span class='pagenum'><a name="Page_3" id="Page_3"></a></span>of the long series of forms which the animal +ancestors of the said organism, or the ancestral forms of the +species, have passed through from the earliest period of organic +life down to the present day.</p> + +<p>The causal character of the relation which connects embryology +with stem-history is due to the action of heredity and adaptation. +When we have rightly understood these, and recognised their great +importance in the formation of organisms, we can go a step further +and say: Phylogenesis is the mechanical cause of +ontogenesis.<a href="#linknote-1" name="linknoteref-1" id="linknoteref-1"><sup>[1]</sup></a> In other words, the development of the +stem, or race, is, in accordance with the laws of heredity and +adaptation, the cause of all the changes which appear in a +condensed form in the evolution of the fœtus.</p> + +<p class="footnote"> +<a name="linknote-1" id="linknote-1"></a> <a href="#linknoteref-1">[1]</a> +The term “genesis,” which occurs +throughout, means, of course, “birth” or origin. From +this we get: Biogeny = the origin of life (<i>bios</i>); +Anthropogeny = the origin of man (<i>anthropos</i>); Ontogeny = the +origin of the individual (<i>on</i>); Phylogeny = the origin of the +species (<i>phulon</i>); and so on. In each case the term may refer +to the process itself, or to the science describing the +process.—Translator. +</p> + +<p>The chain of manifold animal forms which represent the ancestry +of each higher organism, or even of man, according to the theory of +descent, always form a connected whole. We may designate this +uninterrupted series of forms with the letters of the alphabet: A, +B, C, D, E, etc., to Z. In apparent contradiction to what I have +said, the story of the development of the individual, or the +ontogeny of most organisms, only offers to the observer a part of +these forms; so that the defective series of embryonic forms would +run: A, B, D, F, H, K, M, etc.; or, in other cases, B, D, H, L, M, +N, etc. Here, then, as a rule, several of the evolutionary forms of +the original series have fallen out. Moreover, we often +find—to continue with our illustration from the +alphabet—one or other of the original letters of the +ancestral series represented by corresponding letters from a +different alphabet. Thus, instead of the Roman B and D, we often +have the Greek Β and Δ. In this case the text of the +biogenetic law has been corrupted, just as it had been abbreviated +in the preceding case. But, in spite of all this, the series of +ancestral forms remains the same, and we are in a position to +discover its original complexion.</p> + +<p>In reality, there is always a certain parallel between the two +evolutionary series. But it is obscured from the fact that in the +embryonic succession much is wanting that certainly existed in the +earlier ancestral succession. If the parallel of the two series +were complete, and if this great fundamental law affirming the +causal connection between ontogeny and phylogeny in the proper +sense of the word were directly demonstrable, we should only have +to determine, by means of the microscope and the dissecting knife, +the series of forms through which the fertilised ovum passes in its +development; we should then have before us a complete picture of +the remarkable series of forms which our animal ancestors have +successively assumed from the dawn of organic life down to the +appearance of man. But such a repetition of the ancestral history +by the individual in its embryonic life is very rarely complete. We +do not often find our full alphabet. In most cases the +correspondence is very imperfect, being greatly distorted and +falsified by causes which we will consider later. We are thus, for +the most part, unable to determine in detail, from the study of its +embryology, all the different shapes which an organism’s +ancestors have assumed; we usually—and especially in the case +of the human fœtus—encounter many gaps. It is true that +we can fill up most of these gaps satisfactorily with the help of +comparative anatomy, but we cannot do so from direct embryological +observation. Hence it is important that we find a large number of +lower animal forms to be still represented in the course of +man’s embryonic development. In these cases we may draw our +conclusions with the utmost security as to the nature of the +ancestral form from the features of the form which the embryo +momentarily assumes.</p> + +<p>To give a few examples, we can infer from the fact that the +human ovum is a simple cell that the first ancestor of our species +was a tiny unicellular being, something like the amœba. In +the same way, we know, from the fact that the human fœtus +consists, at the first, of two simple cell-layers (the <i> +gastrula</i>), that the <i>gastræa</i>, a form with two such +layers, was certainly in the line of our ancestry. A later human +embryonic form (the <i>chordula</i>) points just as clearly to a +worm-like ancestor (the <i>prochordonia</i>), the nearest living +relation of which is found among the actual ascidiæ. To this +succeeds a most important embryonic stage (<i>acrania</i>), in +which our headless fœtus <span class='pagenum'><a name="Page_4" id="Page_4"></a></span>presents, in the main, the structure of the +lancelet. But we can only indirectly and approximately, with the +aid of comparative anatomy and ontogeny, conjecture what lower +forms enter into the chain of our ancestry between the +gastræa and the chordula, and between this and the lancelet. +In the course of the historical development many intermediate +structures have gradually fallen out, which must certainly have +been represented in our ancestry. But, in spite of these many, and +sometimes very appreciable, gaps, there is no contradiction between +the two successions. In fact, it is the chief purpose of this work +to prove the real harmony and the original parallelism of the two. +I hope to show, on a substantial basis of facts, that we can draw +most important conclusions as to our genealogical tree from the +actual and easily-demonstrable series of embryonic changes. We +shall then be in a position to form a general idea of the wealth of +animal forms which have figured in the direct line of our ancestry +in the lengthy history of organic life.</p> + +<p>In this evolutionary appreciation of the facts of embryology we +must, of course, take particular care to distinguish sharply and +clearly between the primitive, palingenetic (or ancestral) +evolutionary processes and those due to cenogenesis.<a href="#linknote-2" name="linknoteref-2" id="linknoteref-2"><sup>[2]</sup></a> By +<i>palingenetic</i> processes, or embryonic <i>recapitulations,</i> +we understand all those phenomena in the development of the +individual which are transmitted from one generation to another by +heredity, and which, on that account, allow us to draw direct +inferences as to corresponding structures in the development of the +species. On the other hand, we give the name of <i>cenogenetic</i> +processes, or embryonic <i>variations,</i> to all those phenomena +in the fœtal development that cannot be traced to inheritance +from earlier species, but are due to the adaptation of the +fœtus, or the infant-form, to certain conditions of its +embryonic development. These cenogenetic phenomena are foreign or +later additions; they allow us to draw no direct inference whatever +as to corresponding processes in our ancestral history, but rather +hinder us from doing so.</p> + +<p class="footnote"> +<a name="linknote-2" id="linknote-2"></a> <a href="#linknoteref-2">[2]</a> +Palingenesis = new birth, or re-incarnation (<i>palin</i> = again, +<i>genesis</i> or <i>genea</i> = development); hence its application to the +phenomena which are recapitulated by heredity from earlier ancestral forms. +Cenogenesis = foreign or negligible development (<i>kenos</i> and <i> +genea</i>); hence, those phenomena which come later in the story of life to +disturb the inherited structure, by a fresh adaptation to +environment.—Translator. +</p> + +<p>This careful discrimination between the primary or palingenetic +processes and the secondary or cenogenetic is of great importance +for the purposes of the scientific history of a species, which has +to draw conclusions from the available facts of embryology, +comparative anatomy, and paleontology, as to the processes in the +formation of the species in the remote past. It is of the same +importance to the student of evolution as the careful distinction +between genuine and spurious texts in the works of an ancient +writer, or the purging of the real text from interpolations and +alterations, is for the student of philology. It is true that this +distinction has not yet been fully appreciated by many scientists. +For my part, I regard it as the first condition for forming any +just idea of the evolutionary process, and I believe that we must, +in accordance with it, divide embryology into two +sections—palingenesis, or the science of recapitulated forms; +and cenogenesis, or the science of supervening structures.</p> + +<p>To give at once a few examples from the science of man’s +origin in illustration of this important distinction, I may +instance the following processes in the embryology of man, and of +all the higher vertebrates, as <i>palingenetic</i>: the formation +of the two primary germinal layers and of the primitive gut, the +undivided structure of the dorsal nerve-tube, the appearance of a +simple axial rod between the medullary tube and the gut, the +temporary formation of the gill-clefts and arches, the primitive +kidneys, and so on.<a href="#linknote-3" name="linknoteref-3" id="linknoteref-3"><sup>[3]</sup></a> All these, and many other important +structures, have clearly been transmitted by a steady heredity from +the early ancestors of the mammal, and are, therefore, direct +indications of the presence of similar structures in the history of +the stem. On the other hand, this is certainly not the case with +the following embryonic forms, which we must describe as +cenogenetic processes: the formation of the yelk-sac, the +allantois, the placenta, the amnion, the serolemma, and the +chorion—or, generally speaking, the various fœtal +membranes and the corresponding changes in the blood vessels. +Further instances are: the dual structure of the heart cavity, the +temporary division of the plates of the primitive vertebræ +and <span class='pagenum'><a name="Page_5" id="Page_5"></a></span>lateral plates, the secondary closing of the ventral +and intestinal walls, the formation of the navel, and so on. All +these and many other phenomena are certainly not traceable to +similar structures in any earlier and completely-developed +ancestral form, but have arisen simply by adaptation to the +peculiar conditions of embryonic life (within the fœtal +membranes). In view of these facts, we may now give the following +more precise expression to our chief law of biogeny: The evolution +of the fœtus (or <i>ontogenesis</i>) is a condensed and +abbreviated recapitulation of the evolution of the stem (or <i> +phylogenesis</i>); and this recapitulation is the more complete in +proportion as the original development (or <i>palingenesis</i>) is +preserved by a constant heredity; on the other hand, it becomes +less complete in proportion as a varying adaptation to new +conditions increases the disturbing factors in the development (or +cenogenesis).</p> + +<p class="footnote"> +<a name="linknote-3" id="linknote-3"></a> <a href="#linknoteref-3">[3]</a> +All these, and the following structures, will be fully described in later +chapters.—Translator. +</p> + +<p>The cenogenetic alterations or distortions of the original +palingenetic course of development take the form, as a rule, of a +gradual displacement of the phenomena, which is slowly effected by +adaptation to the changed conditions of embryonic existence during +the course of thousands of years. This displacement may take place +as regards either the position or the time of a phenomenon.</p> + +<p>The great importance and strict regularity of the +time-variations in embryology have been carefully studied recently +by Ernest Mehnert, in his <i>Biomechanik</i> (Jena, 1898). He +contends that our biogenetic law has not been impaired by the +attacks of its opponents, and goes on to say: “Scarcely any +piece of knowledge has contributed so much to the advance of +embryology as this; its formulation is one of the most signal +services to general biology. It was not until this law passed into +the flesh and blood of investigators, and they had accustomed +themselves to see a reminiscence of ancestral history in embryonic +structures, that we witnessed the great progress which +embryological research has made in the last two decades.” The +best proof of the correctness of this opinion is that now the most +fruitful work is done in all branches of embryology with the aid of +this biogenetic law, and that it enables students to attain every +year thousands of brilliant results that they would never have +reached without it.</p> + +<p>It is only when one appreciates the cenogenetic processes in +relation to the palingenetic, and when one takes careful account of +the changes which the latter may suffer from the former, that the +radical importance of the biogenetic law is recognised, and it is +felt to be the most illuminating principle in the science of +evolution. In this task of discrimination it is the silver thread +in relation to which we can arrange all the phenomena of this realm +of marvels—the “Ariadne thread,” which alone +enables us to find our way through this labyrinth of forms. Hence +the brothers Sarasin, the zoologists, could say with perfect +justice, in their study of the evolution of the <i>Ichthyophis,</i> +that “the great biogenetic law is just as important for the +zoologist in tracing long-extinct processes as spectrum analyses is +for the astronomer.”</p> + +<p>Even at an earlier period, when a correct acquaintance with the +evolution of the human and animal frame was only just being +obtained—and that is scarcely eighty years ago!—the +greatest astonishment was felt at the remarkable similarity +observed between the embryonic forms, or stages of fœtal +development, in very different animals; attention was called even +then to their close resemblance to certain fully-developed animal +forms belonging to some of the lower groups. The older scientists +(Oken, Treviranus, and others) knew perfectly well that these lower +forms in a sense illustrated and fixed, in the hierarchy of the +animal world, a temporary stage in the evolution of higher forms. +The famous anatomist Meckel spoke in 1821 of a “similarity +between the development of the embryo and the series of +animals.” Baer raised the question in 1828 how far, within +the vertebrate type, the embryonic forms of the higher animals +assume the permanent shapes of members of lower groups. But it was +impossible fully to understand and appreciate this remarkable +resemblance at that time. We owe our capacity to do this to the +theory of descent; it is this that puts in their true light the +action of <i>heredity</i> on the one hand and <i>adaptation</i> on +the other. It explains to us the vital importance of their constant +reciprocal action in the production of organic forms. Darwin was +the first to teach us the great part that was played in this by the +ceaseless struggle for existence between living things, and to show +how, under the influence of this (by natural selection), new +species were produced and maintained solely by the interaction of +heredity and <span class='pagenum'><a name="Page_6" id="Page_6"></a></span>adaptation. It was thus Darwinism that first opened +our eyes to a true comprehension of the supremely important +relations between the two parts of the science of organic +evolution—Ontogeny and Phylogeny.</p> + +<p>Heredity and adaptation are, in fact, the two constructive +physiological functions of living things; unless we understand +these properly we can make no headway in the study of evolution. +Hence, until the time of Darwin no one had a clear idea of the real +nature and causes of embryonic development. It was impossible to +explain the curious series of forms through which the human embryo +passed; it was quite unintelligible why this strange succession of +animal-like forms appeared in the series at all. It had previously +been generally assumed that the man was found complete in all his +parts in the ovum, and that the development consisted only in an +unfolding of the various parts, a simple process of growth. This is +by no means the case. On the contrary, the whole process of the +development of the individual presents to the observer a connected +succession of different animal-forms; and these forms display a +great variety of external and internal structure. But <i>why</i> +each individual human being should pass through this series of +forms in the course of his embryonic development it was quite +impossible to say until Lamarck and Darwin established the theory +of descent. Through this theory we have at last detected the real +causes, the <i>efficient causes,</i> of the individual development; +we have learned that these <i>mechanical</i> causes suffice of +themselves to effect the formation of the organism, and that there +is no need of the <i>final</i> causes which were formerly assumed. +It is true that in the academic philosophies of our time these +final causes still figure very prominently; in the new philosophy +of nature we can entirely replace them by efficient causes. We +shall see, in the course of our inquiry, how the most wonderful and +hitherto insoluble enigmas in the human and animal frame have +proved amenable to a mechanical explanation, by causes acting +without prevision, through Darwin’s reform of the science of +evolution. We have everywhere been able to substitute unconscious +causes, acting from necessity, for conscious, purposive +causes.<a href="#linknote-4" name="linknoteref-4" id="linknoteref-4"><sup>[4]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-4" id="linknote-4"></a> <a href="#linknoteref-4">[4]</a> +The monistic or mechanical philosophy of nature holds that only unconscious, +necessary, efficient causes are at work in the whole field of nature, in +organic life as well as in inorganic changes. On the other hand, the dualist or +vitalist philosophy of nature affirms that unconscious forces are only at work +in the inorganic world, and that we find conscious, purposive, or final causes +in organic nature. +</p> + +<p>If the new science of evolution had done no more than this, +every thoughtful man would have to admit that it had accomplished +an immense advance in knowledge. It means that in the whole of +philosophy that tendency which we call monistic, in opposition to +the dualistic, which has hitherto prevailed, must be +accepted.<a href="#linknote-5" name="linknoteref-5" id="linknoteref-5"><sup>[5]</sup></a> At this point the science of human evolution +has a direct and profound bearing on the foundations of philosophy. +Modern anthropology has, by its astounding discoveries during the +second half of the nineteenth century, compelled us to take a +completely monistic view of life. Our bodily structure and its +life, our embryonic development and our evolution as a species, +teach us that the same laws of nature rule in the life of man as in +the rest of the universe. For this reason, if for no others, it is +desirable, nay, indispensable, that every man who wishes to form a +serious and philosophic view of life, and, above all, the expert +philosopher, should acquaint himself with the chief facts of this +branch of science.</p> + +<p class="footnote"> +<a name="linknote-5" id="linknote-5"></a> <a href="#linknoteref-5">[5]</a> +Monism is neither purely materialistic nor purely spiritualistic, but a +reconciliation of these two principles, since it regards the whole of nature as +one, and sees only efficient causes at work in it. Dualism, on the contrary, +holds that nature and spirit, matter and force, the world and God, inorganic +and organic nature, are separate and independent existences. Cf. <i>The Riddle +of the Universe,</i> chap. xii. +</p> + +<p> +The facts of embryology have so great and obvious a significance in this +connection that even in recent years dualist and teleological philosophers have +tried to rid themselves of them by simply denying them. This was done, for +instance, as regards the fact that man is developed from an egg, and that this +egg or ovum is a simple cell, as in the case of other animals. When I had +explained this pregnant fact and its significance in my <i>History of +Creation,</i> it was described in many of the theological journals as a +dishonest invention of my own. The fact that the embryos of man and the dog +are, at a certain stage of their development, almost indistinguishable was also +denied. When we examine the human embryo in the third or fourth week of its +development, we find it to be quite different in shape and structure from the +full-grown human being, but almost identical with that of the ape, the dog, the +rabbit, and +<span class='pagenum'><a name="Page_7" id="Page_7"></a></span> +other mammals, at the same stage of ontogeny. We find a bean-shaped body of +very simple construction, with a tail below and a pair of fins at the sides, +something like those of a fish, but very different from the limbs of man and +the mammals. Nearly the whole front half of the body is taken up by a shapeless +head without face, at the sides of which we find gill-clefts and arches as in +the fish. At this stage of its development the human embryo does not differ in +any essential detail from that of the ape, dog, horse, ox, etc., at a +corresponding period. This important fact can easily be verified at any moment +by a comparison of the embryos of man, the dog, rabbit, etc. Nevertheless, the +theologians and dualist philosophers pronounced it to be a materialistic +invention; even scientists, to whom the facts should be known, have sought to +deny them. +</p> + +<p>There could not be a clearer proof of the profound importance of +these embryological facts in favour of the monistic philosophy than +is afforded by these efforts of its opponents to get rid of them by +silence or denial. The truth is that these facts are most +inconvenient for them, and are quite irreconcilable with their +views. We must be all the more pressing on our side to put them in +their proper light. I fully agree with Huxley when he says, in his +<i>Man’s Place in Nature</i>: “Though these facts are +ignored by several well-known popular leaders, they are easy to +prove, and are accepted by all scientific men; on the other hand, +their importance is so great that those who have once mastered them +will, in my opinion, find few other biological discoveries to +astonish them.”</p> + +<p>We shall make it our chief task to study the evolution of +man’s bodily frame and its various organs in their external +form and internal structures. But I may observe at once that this +is accompanied step by step with a study of the evolution of their +functions. These two branches of inquiry are inseparably united in +the whole of anthropology, just as in zoology (of which the former +is only a section) or general biology. Everywhere the peculiar form +of the organism and its structures, internal and external, is +directly related to the special physiological functions which the +organism or organ has to execute. This intimate connection of +structure and function, or of the instrument and the work done by +it, is seen in the science of evolution and all its parts. Hence +the story of the evolution of structures, which is our immediate +concern, is also the history of the development of functions; and +this holds good of the human organism as of any other.</p> + +<p>At the same time, I must admit that our knowledge of the +evolution of functions is very far from being as complete as our +acquaintance with the evolution of structures. One might say, in +fact, that the whole science of evolution has almost confined +itself to the study of structures; the evolution of <i> +functions</i> hardly exists even in name. That is the fault of the +physiologists, who have as yet concerned themselves very little +about evolution. It is only in recent times that physiologists like +W. Engelmann, W. Preyer, M. Verworn, and a few others, have +attacked the evolution of functions.</p> + +<p>It will be the task of some future physiologist to engage in the +study of the evolution of functions with the same zeal and success +as has been done for the evolution of structures in morphogeny (the +science of the genesis of forms). Let me illustrate the close +connection of the two by a couple of examples. The heart in the +human embryo has at first a very simple construction, such as we +find in permanent form among the ascidiæ and other low +organisms; with this is associated a very simple system of +circulation of the blood. Now, when we find that with the +full-grown heart there comes a totally different and much more +intricate circulation, our inquiry into the development of the +heart becomes at once, not only an anatomical, but also a +physiological, study. Thus it is clear that the ontogeny of the +heart can only be understood in the light of its phylogeny (or +development in the past), both as regards function and structure. +The same holds true of all the other organs and their functions. +For instance, the science of the evolution of the alimentary canal, +the lungs, or the sexual organs, gives us at the same time, through +the exact comparative investigation of structure-development, most +important information with regard to the evolution of the functions +of these organs.</p> + +<p>This significant connection is very clearly seen in the +evolution of the nervous system. This system is in the economy of +the human body the medium of sensation, will, and even thought, the +highest of the psychic functions; in a word, of <span class='pagenum'><a name="Page_8" id="Page_8"></a></span>all the various functions which constitute the +proper object of psychology. Modern anatomy and physiology have +proved that these psychic functions are immediately dependent on +the fine structure and the composition of the central nervous +system, or the internal texture of the brain and spinal cord. In +these we find the elaborate cell-machinery, of which the psychic or +soul-life is the physiological function. It is so intricate that +most men still look upon the mind as something supernatural that +cannot be explained on mechanical principles.</p> + +<p>But embryological research into the gradual appearance and the +formation of this important system of organs yields the most +astounding and significant results. The first sketch of a central +nervous system in the human embryo presents the same very simple +type as in the other vertebrates. A spinal tube is formed in the +external skin of the back, and from this first comes a simple +spinal cord without brain, such as we find to be the permanent +psychic organ in the lowest type of vertebrate, the amphioxus. Not +until a later stage is a brain formed at the anterior end of this +cord, and then it is a brain of the most rudimentary kind, such as +we find permanently among the lower fishes. This simple brain +develops step by step, successively assuming forms which correspond +to those of the amphibia, the reptiles, the duck-bills, and the +lemurs. Only in the last stage does it reach the highly organised +form which distinguishes the apes from the other vertebrates, and +which attains its full development in man.</p> + +<p>Comparative physiology discovers a precisely similar growth. The +function of the brain, the psychic activity, rises step by step +with the advancing development of its structure.</p> + +<p>Thus we are enabled, by this story of the evolution of the +nervous system, to understand at length <i>the natural development +of the human mind</i> and its gradual unfolding. It is only with +the aid of embryology that we can grasp how these highest and most +striking faculties of the animal organism have been historically +evolved. In other words, a knowledge of the evolution of the spinal +cord and brain in the human embryo leads us directly to a +comprehension of the historic development (or phylogeny) of the +human mind, that highest of all faculties, which we regard as +something so marvellous and supernatural in the adult man. This is +certainly one of the greatest and most pregnant results of +evolutionary science. Happily our embryological knowledge of +man’s central nervous system is now so adequate, and agrees +so thoroughly with the complementary results of comparative anatomy +and physiology, that we are thus enabled to obtain a clear insight +into one of the highest problems of philosophy, the phylogeny of +the soul, or the ancestral history of the mind of man. Our chief +support in this comes from the embryological study of it, or the +ontogeny of the soul. This important section of psychology owes its +origin especially to W. Preyer, in his interesting works, such as +<i>The Mind of the Child. The Biography of a Baby</i> (1900), of +Milicent Washburn Shinn, also deserves mention. [See also +Preyer’s <i>Mental Development in the Child</i> +(translation), and Sully’s <i>Studies of Childhood</i> and +<i>Children’s Ways.</i>]</p> + +<p>In this way we follow the only path along which we may hope to +reach the solution of this difficult problem.</p> + +<p>Thirty-six years have now elapsed since, in my <i>General +Morphology,</i> I established phylogeny as an independent science +and showed its intimate causal connection with ontogeny; thirty +years have passed since I gave in my gastræa-theory the proof +of the justice of this, and completed it with the theory of +germinal layers. When we look back on this period we may ask, What +has been accomplished during it by the fundamental law of biogeny? +If we are impartial, we must reply that it has proved its fertility +in hundreds of sound results, and that by its aid we have acquired +a vast fund of knowledge which we should never have obtained +without it.</p> + +<p>There has been no dearth of attacks—often violent +attacks—on my conception of an intimate causal connection +between ontogenesis and phylogenesis; but no other satisfactory +explanation of these important phenomena has yet been offered to +us. I say this especially with regard to Wilhelm His’s theory +of a “mechanical evolution,” which questions the truth +of phylogeny generally, and would explain the complicated embryonic +processes without going beyond by simple physical +changes—such as the bending and folding of leaves by +electricity, the origin of cavities through unequal strain of the +tissues, the formation of processes by uneven growth, and so on. +But the <span class='pagenum'><a name="Page_9" id="Page_9"></a></span>fact is that these embryological phenomena themselves +demand explanation in turn, and this can only be found, as a rule, +in the corresponding changes in the long ancestral series, or in +the physiological functions of heredity and adaptation.</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap02"></a>Chapter II.<br/> +THE OLDER EMBRYOLOGY</h2> + +<p> +It is in many ways useful, on entering upon the study of any science, to cast a +glance at its historical development. The saying that “everything is best +understood in its growth” has a distinct application to science. While we +follow its gradual development we get a clearer insight into its aims and +objects. Moreover, we shall see that the present condition of the science of +human evolution, with all its characteristics, can only be rightly understood +when we examine its historical growth. This task will, however, not detain us +long. The study of man’s evolution is one of the latest branches of +natural science, whether you consider the embryological or the phylogenetic +section of it. +</p> + +<p> +Apart from the few germs of our science which we find in classical antiquity, +and which we shall notice presently, we may say that it takes its definite +rise, as a science, in the year 1759, when one of the greatest German +scientists, Caspar Friedrich Wolff, published his <i>Theoria generationis.</i> +That was the foundation-stone of the science of animal embryology. It was not +until fifty years later, in 1809, that Jean Lamarck published his +<i>Philosophie Zoologique</i>—the first effort to provide a base for the +theory of evolution; and it was another half-century before Darwin’s work +appeared (in 1859), which we may regard as the first scientific attainment of +this aim. But before we go further into this solid establishment of evolution, +we must cast a brief glance at that famous philosopher and scientist of +antiquity, who stood alone in this, as in many other branches of science, for +more than 2000 years: the “father of Natural History,” Aristotle. +</p> + +<p> +The extant scientific works of Aristotle deal with many different sides of +biological research; the most comprehensive of them is his famous <i>History of +Animals.</i> But not less interesting is the smaller work, On the <i>Generation +of Animals (Peri zoon geneseos).</i> This work treats especially of embryonic +development, and it is of great interest as being the earliest of its kind and +the only one that has come down to us in any completeness from classical +antiquity. +</p> + +<p> +Aristotle studied embryological questions in various classes of animals, and +among the lower groups he learned many most remarkable facts which we only +rediscovered between 1830 and 1860. It is certain, for instance, that he was +acquainted with the very peculiar mode of propagation of the cuttlefishes, or +cephalopods, in which a yelk-sac hangs out of the mouth of the fœtus. He +knew, also, that embryos come from the eggs of the bee even when they have not +been fertilised. This “parthenogenesis” (or virgin-birth) of the +bees has only been established in our time by the distinguished zoologist of +Munich, Siebold. He discovered that male bees come from the unfertilised, and +female bees only from the fertilised, eggs. Aristotle further states that some +kinds of fishes (of the genus <i>serranus</i>) are hermaphrodites, each +individual having both male and female organs and being able to fertilise +itself; this, also, has been recently confirmed. He knew that the embryo of +many fishes of the shark family is attached to the mother’s body by a +sort of placenta, or nutritive organ very rich in blood; apart from these, such +an arrangement is only found among the higher mammals and <span class='pagenum'><a name="Page_10" id="Page_10"></a></span>man. This placenta of +the shark was looked upon as legendary for a long time, until Johannes +Müller proved it to be a fact in 1839. Thus a number of remarkable +discoveries were found in Aristotle’s embryological work, proving a very +good acquaintance of the great scientist—possibly helped by his +predecessors—with the facts of ontogeny, and a great advance upon +succeeding generations in this respect. +</p> + +<p> +In the case of most of these discoveries he did not merely describe the fact, +but added a number of observations on its significance. Some of these +theoretical remarks are of particular interest, because they show a correct +appreciation of the nature of the embryonic processes. He conceives the +development of the individual as a new formation, in the course of which the +various parts of the body take shape successively. When the human or animal +frame is developed in the mother’s body, or separately in an egg, the +heart—which he regards as the starting-point and centre of the +organism—must appear first. Once the heart is formed the other organs +arise, the internal ones before the external, the upper (those above the +diaphragm) before the lower (or those beneath the diaphragm). The brain is +formed at an early stage, and the eyes grow out of it. These observations are +quite correct. And, if we try to form some idea from these data of +Aristotle’s general conception of the embryonic process, we find a dim +prevision of the theory which Wolff showed 2000 years afterwards to be the +correct view. It is significant, for instance, that Aristotle denied the +eternity of the individual in any respect. He said that the species or genus, +the group of similar individuals, might be eternal, but the individual itself +is temporary. It comes into being in the act of procreation, and passes away at +death. +</p> + +<p> +During the 2000 years after Aristotle no progress whatever was made in general +zoology, or in embryology in particular. People were content to read, copy, +translate, and comment on Aristotle. Scarcely a single independent effort at +research was made in the whole of the period. During the Middle Ages the spread +of strong religious beliefs put formidable obstacles in the way of independent +scientific investigation. There was no question of resuming the advance of +biology. Even when human anatomy began to stir itself once more in the +sixteenth century, and independent research was resumed into the structure of +the developed body, anatomists did not dare to extend their inquiries to the +unformed body, the embryo, and its development. There were many reasons for the +prevailing horror of such studies. It is natural enough, when we remember that +a Bull of Boniface VIII excommunicated every man who ventured to dissect a +human corpse. If the dissection of a developed body were a crime to be thus +punished, how much more dreadful must it have seemed to deal with the embryonic +body still enclosed in the womb, which the Creator himself had decently veiled +from the curiosity of the scientist! The Christian Church, then putting many +thousands to death for unbelief, had a shrewd presentiment of the menace that +science contained against its authority. It was powerful enough to see that its +rival did not grow too quickly. +</p> + +<p> +It was not until the Reformation broke the power of the Church, and a +refreshing breath of the spirit dissolved the icy chains that bound science, +that anatomy and embryology, and all the other branches of research, could +begin to advance once more. However, embryology lagged far behind anatomy. The +first works on embryology appear at the beginning of the sixteenth century. The +Italian anatomist, Fabricius ab Aquapendente, a professor at Padua, opened the +advance. In his two books (<i>De formato fœtu,</i> 1600, and <i>De +formatione fœtus,</i> 1604) he published the older illustrations and +descriptions of the embryos of man and other mammals, and of the hen. Similar +imperfect illustrations were given by Spigelius (<i>De formato fœtu,</i> +1631), and by Needham (1667) and his more famous compatriot, Harvey (1652), who +discovered the circulation of the blood in the animal body and formulated the +important principle, <i>Omne vivum ex vivo</i> (all life comes from +pre-existing life). The Dutch scientist, Swammerdam, published in his <i>Bible +of Nature</i> the earliest observations on the embryology of the frog and the +division of its egg-yelk. But the most important embryological studies in the +sixteenth century were those of the famous Italian, Marcello Malpighi, of +Bologna, who led the way both in zoology and botany. His treatises, <i>De +formatione pulli</i> and <i>De ovo incubato</i> (1687), contain the first +consistent description of the development of the chick in the fertilised egg. +</p> + +<p> +<span class='pagenum'><a name="Page_11" id="Page_11"></a></span>Here I ought to say a word about the important part played by the chick in the +growth of our science. The development of the chick, like that of the young of +all other birds, agrees in all its main features with that of the other chief +vertebrates, and even of man. The three highest classes of +vertebrates—mammals, birds, and reptiles (lizards, serpents, tortoises, +etc.)—have from the beginning of their embryonic development so striking +a resemblance in all the chief points of structure, and especially in their +first forms, that for a long time it is impossible to distinguish between them. +We have known now for some time that we need only examine the embryo of a bird, +which is the easiest to get at, in order to learn the typical mode of +development of a mammal (and therefore of man). As soon as scientists began to +study the human embryo, or the mammal-embryo generally, in its earlier stages +about the middle and end of the seventeenth century, this important fact was +very quickly discovered. It is both theoretically and practically of great +value. As regards the <i>theory</i> of evolution, we can draw the most weighty +inferences from this similarity between the embryos of widely different classes +of animals. But for the practical purposes of embryological research the +discovery is invaluable, because we can fill up the gaps in our imperfect +knowledge of the embryology of the mammals from the more thoroughly studied +embryology of the bird. Hens’ eggs are easily to be had in any quantity, +and the development of the chick may be followed step by step in artificial +incubation. The development of the mammal is much more difficult to follow, +because here the embryo is not detached and enclosed in a large egg, but the +tiny ovum remains in the womb until the growth is completed. Hence, it is very +difficult to keep up sustained observation of the various stages in any great +extent, quite apart from such extrinsic considerations as the cost, the +technical difficulties, and many other obstacles which we encounter when we +would make an extensive study of the fertilised mammal. The chicken has, +therefore, always been the chief object of study in this connection. The +excellent incubators we now have enable us to observe it in any quantity and at +any stage of development, and so follow the whole course of its formation step +by step. +</p> + +<p> +By the end of the seventeenth century Malpighi had advanced as far as it was +possible to do with the imperfect microscope of his time in the embryological +study of the chick. Further progress was arrested until the instrument and the +technical methods should be improved. The vertebrate embryos are so small and +delicate in their earlier stages that you cannot go very far into the study of +them without a good microscope and other technical aid. But this substantial +improvement of the microscope and the other apparatus did not take place until +the beginning of the nineteenth century. +</p> + +<p> +Embryology made scarcely any advance in the first half of the eighteenth +century, when the systematic natural history of plants and animals received so +great an impulse through the publication of Linné’s famous +<i>Systema Naturæ.</i> Not until 1759 did the genius arise who was to +give it an entirely new character, Caspar Friedrich Wolff. Until then +embryology had been occupied almost exclusively in unfortunate and misleading +efforts to build up theories on the imperfect empirical material then +available. +</p> + +<p> +The theory which then prevailed, and remained in favour throughout nearly the +whole of the eighteenth century, was commonly called at that time “the +evolution theory”; it is better to describe it as “the preformation +theory.”<a href="#linknote-6" name="linknoteref-6" id="linknoteref-6"><sup>[6]</sup></a> Its chief point is this: There is no new formation +of structures in the embryonic development of any organism, animal or plant, or +even of man; there is only a growth, or unfolding, of parts which have been +constructed or <i>pre-formed</i> from all eternity, though on a very small +scale and closely packed together. Hence, every living germ contains all the +organs and parts of the body, in the form and arrangement they will present +later, already within it, and thus the whole embryological process is merely an +<i>evolution</i> in the literal sense of the word, or an <i>unfolding,</i> of +parts that were pre-formed and folded up in it. So, for instance, we find in +the hen’s egg not merely a simple cell, that divides and subdivides and +forms germinal layers, and at last, after all kinds of variation and cleavage +and reconstruction, brings forth <span class='pagenum'><a name="Page_12" id="Page_12"></a></span>the body of the chick; but there is in every +egg from the first a complete chicken, with all its parts made and neatly +packed. These parts are so small or so transparent that the microscope cannot +detect them. In the hatching, these parts merely grow larger, and spread out in +the normal way. +</p> + +<p class="footnote"> +<a name="linknote-6" id="linknote-6"></a> <a href="#linknoteref-6">[6]</a> +This theory is usually known as the “evolution theory” in Germany, +in contradistinction to the “epigenesis theory.” But as it is the +latter that is called the “evolution theory” in England, France, +and Italy, and “evolution” and “epigenesis” are taken +to be synonymous, it seems better to call the first the “pre-formation +theory.” +</p> + +<p> +When this theory is consistently developed it becomes a “scatulation +theory.”<a href="#linknote-7" name="linknoteref-7" id="linknoteref-7"><sup>[7]</sup></a> According to its teaching, there was made in the +beginning one couple or one individual of each species of animal or plant; but +this one individual contained the germs of all the other individuals of the +same species who should ever come to life. As the age of the earth was +generally believed at that time to be fixed by the Bible at 5000 or 6000 years, +it seemed possible to calculate how many individuals of each species had lived +in the period, and so had been packed inside the first being that was created. +The theory was consistently extended to man, and it was affirmed that our +common parent Eve had had stored in her ovary the germs of all the children of +men. +</p> + +<p class="footnote"> +<a name="linknote-7" id="linknote-7"></a> <a href="#linknoteref-7">[7]</a> +“Packing theory” would be the literal translation. Scatula is the +Latin for a case or box.—Translator. +</p> + +<p> +The theory at first took the form of a belief that it was the <i>females</i> +who were thus encased in the first being. One couple of each species was +created, but the female contained in her ovary all the future individuals of +the species, of either sex. However, this had to be altered when the Dutch +microscopist, Leeuwenhoek, discovered the male spermatozoa in 1690, and showed +that an immense number of these extremely fine and mobile thread-like beings +exist in the male sperm (this will be explained in Chapter VII). This +astonishing discovery was further advanced when it was proved that these living +bodies, swimming about in the seminal fluid, were real animalcules, and, in +fact, were the pre-formed germs of the future generation. When the male and +female procreative elements came together at conception, these thread-like +spermatozoa (“seed-animals”) were supposed to penetrate into the +fertile body of the ovum and begin to develop there, as the plant seed does in +the fruitful earth. Hence, every spermatozoon was regarded as a +<i>homunculus,</i> a tiny complete man; all the parts were believed to be +pre-formed in it, and merely grew larger when it reached its proper medium in +the female ovum. This theory, also, was consistently developed in the sense +that in each of these thread-like bodies the whole of its posterity was +supposed to be present in the minutest form. Adam’s sexual glands were +thought to have contained the germs of the whole of humanity. +</p> + +<p> +This “theory of male scatulation” found itself at once in keen +opposition to the prevailing “female” theory. The two rival +theories at once opened a very lively campaign, and the physiologists of the +eighteenth century were divided into two great camps—the Animalculists +and the Ovulists—which fought vigorously. The animalculists held that the +spermatozoa were the true germs, and appealed to the lively movements and the +structure of these bodies. The opposing party of the Ovulists, who clung to the +older “evolution theory,” affirmed that the ovum is the real germ, +and that the spermatozoa merely stimulate it at conception to begin its growth; +all the future generations are stored in the ovum. This view was held by the +great majority of the biologists of the eighteenth century, in spite of the +fact that Wolff proved it in 1759 to be without foundation. It owed its +prestige chiefly to the circumstance that the most weighty authorities in the +biology and philosophy of the day decided in favour of it, especially Haller, +Bonnet, and Leibnitz. +</p> + +<p> +Albrecht Haller, professor at Göttingen, who is often called the father of +physiology, was a man of wide and varied learning, but he does not occupy a +very high position in regard to insight into natural phenomena. He made a +vigorous defence of the “evolutionary theory” in his famous work, +<i>Elementa physiologiae,</i> affirming: “There is no such thing as +formation (<i>nulla est epigenesis</i>). No part of the animal frame is made +before another; all were made together.” He thus denied that there was +any evolution in the proper sense of the word, and even went so far as to say +that the beard existed in the new-born child and the antlers in the hornless +fawn; all the parts were there in advance, and were merely hidden from the eye +of man for the time being. Haller even calculated the number of human beings +that God must have created on the sixth day and stored away in Eve’s +ovary. He put the number at 200,000 millions, assuming the age of the world to +be 6000 years, the average age of a human being to be thirty years, and the +population of the world at <span class='pagenum'><a name="Page_13" id="Page_13"></a></span>that time to be 1000 millions. And the famous Haller +maintained all this nonsense, in spite of its ridiculous consequences, even +after Wolff had discovered the real course of embryonic development and +established it by direct observation! +</p> + +<p> +Among the philosophers of the time the distinguished Leibnitz was the chief +defender of the “preformation theory,” and by his authority and +literary prestige won many adherents to it. Supported by his system of monads, +according to which body and soul are united in inseparable association and by +their union form the individual, or the “monad,” Leibnitz +consistently extended the “scatulation theory” to the soul, and +held that this was no more evolved than the body. He says, for instance, in his +<i>Théodicée</i>: “I mean that these souls, which one day +are to be the souls of men, are present in the seed, like those of other +species; in such wise that they existed in our ancestors as far back as Adam, +or from the beginning of the world, in the forms of organised bodies.” +</p> + +<p> +The theory seemed to receive considerable support from the observations of one +of its most zealous supporters, Bonnet. In 1745 he discovered, in the +plant-louse, a case of parthenogenesis, or virgin-birth, an interesting form of +reproduction that has lately been found by Siebold and others among various +classes of the articulata, especially crustacea and insects. Among these and +other animals of certain lower species the female may reproduce for several +generations without having been fertilised by the male. These ova that do not +need fertilisation are called “false ova,” pseudova or spores. +Bonnet saw that a female plant-louse, which he had kept in cloistral isolation, +and rigidly removed from contact with males, had on the eleventh day (after +forming a new skin for the fourth time) a living daughter, and during the next +twenty days ninety-four other daughters; and that all of them went on to +reproduce in the same way without any contact with males. It seemed as if this +furnished an irrefutable proof of the truth of the scatulation theory, as it +was held by the Ovulists; it is not surprising to find that the theory then +secured general acceptance. +</p> + +<p> +This was the condition of things when suddenly, in 1759, Caspar Friedrich Wolff +appeared, and dealt a fatal blow at the whole preformation theory with his new +theory of epigenesis. Wolff, the son of a Berlin tailor, was born in 1733, and +went through his scientific and medical studies, first at Berlin under the +famous anatomist Meckel, and afterwards at Halle. Here he secured his doctorate +in his twenty-sixth year, and in his academic dissertation (November 28th, +1759), the <i>Theoria generationis,</i> expounded the new theory of a real +development on a basis of epigenesis. This treatise is, in spite of its +smallness and its obscure phraseology, one of the most valuable in the whole +range of biological literature. It is equally distinguished for the mass of new +and careful observations it contains, and the far-reaching and pregnant ideas +which the author everywhere extracts from his observations and builds into a +luminous and accurate theory of generation. Nevertheless, it met with no +success at the time. Although scientific studies were then assiduously +cultivated owing to the impulse given by Linné—although botanists +and zoologists were no longer counted by dozens, but by hundreds, hardly any +notice was taken of Wolff’s theory. Even when he established the truth of +epigenesis by the most rigorous observations, and demolished the airy structure +of the preformation theory, the “exact” scientist Haller proved one +of the most strenuous supporters of the old theory, and rejected Wolff’s +correct view with a dictatorial “There is no such thing as +evolution.” He even went on to say that religion was menaced by the new +theory! It is not surprising that the whole of the physiologists of the second +half of the eighteenth century submitted to the ruling of this physiological +pontiff, and attacked the theory of epigenesis as a dangerous innovation. It +was not until more than fifty years afterwards that Wolff’s work was +appreciated. Only when Meckel translated into German in 1812 another valuable +work of Wolff’s on <i>The Formation of the Alimentary Canal</i> (written +in 1768), and called attention to its great importance, did people begin to +think of him once more; yet this obscure writer had evinced a profounder +insight into the nature of the living organism than any other scientist of the +eighteenth century. +</p> + +<p> +Wolff’s idea led to an appreciable advance over the whole field of +biology. There is such a vast number of new and important observations and +pregnant thoughts in his writings that we have only gradually learned to +appreciate them rightly in the course of the nineteenth <span class='pagenum'><a name="Page_14" id="Page_14"></a></span>century. He opened up +the true path for research in many directions. In the first place, his theory +of epigenesis gave us our first real insight into the nature of embryonic +development. He showed convincingly that the development of every organism +consists of a series of <i>new formations,</i> and that there is no trace +whatever of the complete form either in the ovum or the spermatozoon. On the +contrary, these are quite simple bodies, with a very different purport. The +embryo which is developed from them is also quite different, in its internal +arrangement and outer configuration, from the complete organism. There is no +trace whatever of preformation or in-folding of organs. To-day we can scarcely +call epigenesis a <i>theory,</i> because we are convinced it is a fact, and can +demonstrate it at any moment with the aid of the microscope. +</p> + +<p> +Wolff furnished the conclusive empirical proof of his theory in his classic +dissertation on <i>The Formation of the Alimentary Canal</i> (1768). In its +complete state the alimentary canal of the hen is a long and complex tube, with +which the lungs, liver, salivary glands, and many other small glands, are +connected. Wolff showed that in the early stages of the embryonic chick there +is no trace whatever of this complicated tube with all its dependencies, but +instead of it only a flat, leaf-shaped body; that, in fact, the whole embryo +has at first the appearance of a flat, oval-shaped leaf. When we remember how +difficult the exact observation of so fine and delicate a structure as the +early leaf-shaped body of the chick must have been with the poor microscopes +then in use, we must admire the rare faculty for observation which enabled +Wolff to make the most important discoveries in this most difficult part of +embryology. By this laborious research he reached the correct opinion that the +embryonic body of all the higher animals, such as the birds, is for some time +merely a flat, thin, leaf-shaped disk—consisting at first of one layer, +but afterwards of several. The lowest of these layers is the alimentary canal, +and Wolff followed its development from its commencement to its completion. He +showed how this leaf-shaped structure first turns into a groove, then the +margins of this groove fold together and form a closed canal, and at length the +two external openings of the tube (the mouth and anus) appear. +</p> + +<p> +Moreover, the important fact that the other systems of organs are developed in +the same way, from tubes formed out of simple layers, did not escape Wolff. The +nerveless system, muscular system, and vascular (blood-vessel) system, with all +the organs appertaining thereto, are, like the alimentary system, developed out +of simple leaf-shaped structures. Hence, Wolff came to the view by 1768 which +Pander developed in the <i>Theory of Germinal Layers</i> fifty years +afterwards. His principles are not literally correct; but he comes as near to +the truth in them as was possible at that time, and could be expected of him. +</p> + +<p> +Our admiration of this gifted genius increases when we find that he was also +the precursor of Goethe in regard to the metamorphosis of plants and of the +famous cellular theory. Wolff had, as Huxley showed, a clear presentiment of +this cardinal theory, since he recognised small microscopic globules as the +elementary parts out of which the germinal layers arose. +</p> + +<p> +Finally, I must invite special attention to the <i>mechanical</i> character of +the profound philosophic reflections which Wolff always added to his remarkable +observations. He was a great monistic philosopher, in the best meaning of the +word. It is unfortunate that his philosophic discoveries were ignored as +completely as his observations for more than half a century. We must be all the +more careful to emphasise the fact of their clear monistic tendency. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap03"> +<span class='pagenum'><a name="Page_15" id="Page_15"></a></span> +</a>Chapter III.<br/> +MODERN EMBRYOLOGY</h2> + +<p> +We may distinguish three chief periods in the growth of our science of human +embryology. The first has been considered in the preceding chapter; it embraces +the whole of the preparatory period of research, and extends from Aristotle to +Caspar Friedrich Wolff, or to the year 1759, in which the epoch-making +<i>Theoria generationis</i> was published. The second period, with which we +have now to deal, lasts about a century—that is to say, until the +appearance of Darwin’s <i>Origin of Species,</i> which brought about a +change in the very foundations of biology, and, in particular, of embryology. +The third period begins with Darwin. When we say that the second period lasted +a full century, we must remember that Wolff’s work had remained almost +unnoticed during half the time—namely, until the year 1812. During the +whole of these fifty-three years not a single book that appeared followed up +the path that Wolff had opened, or extended his theory of embryonic +development. We merely find his views—perfectly correct views, based on +extensive observations of fact—mentioned here and there as erroneous; +their opponents, who adhered to the dominant theory of preformation, did not +even deign to reply to them. This unjust treatment was chiefly due to the +extraordinary authority of Albrecht von Haller; it is one of the most +astonishing instances of a great authority, as such, preventing for a long time +the recognition of established facts. +</p> + +<p> +The general ignorance of Wolff’s work was so great that at the beginning +of the nineteenth century two scientists of Jena, Oken (1806) and Kieser +(1810), began independent research into the development of the alimentary canal +of the chick, and hit upon the right clue to the embryonic puzzle, without +knowing a word about Wolff’s important treatise on the same subject. They +were treading in his very footsteps without suspecting it. This can be easily +proved from the fact that they did not travel as far as Wolff. It was not until +Meckel translated into German Wolff’s book on the alimentary system, and +pointed out its great importance, that the eyes of anatomists and physiologists +were suddenly opened. At once a number of biologists instituted fresh +embryological inquiries, and began to confirm Wolff’s theory of +epigenesis. +</p> + +<p> +This resuscitation of embryology and development of the epigenesis-theory was +chiefly connected with the university of Würtzburg. One of the professors there +at that time was Döllinger, an eminent biologist, and father of the famous +Catholic historian who later distinguished himself by his opposition to the new +dogma of papal infallibility. Döllinger was both a profound thinker and an +accurate observer. He took the keenest interest in embryology, and worked at it +a good deal. However, he is not himself responsible for any important result in +this field. In 1816 a young medical doctor, whom we may at once designate as +Wolff’s chief successor, Karl Ernst von Baer, came to Würtzburg. +Baer’s conversations with Döllinger on embryology led to a fresh series +of most extensive investigations. Döllinger had expressed a wish that some +young scientist should begin again under his guidance an independent inquiry +into the development of the chick during the hatching of the egg. As neither he +nor Baer had money enough to pay for an incubator and the proper control of the +experiments, and for a competent artist to illustrate the various stages +observed, the lead of the enterprise was given to Christian Pander, a wealthy +friend of Baer’s who had been induced by Baer to come to Würtzburg. An +able engraver, Dalton, was engaged to do the copper-plates. In a short time the +embryology of the chick, in which Baer was taking the greatest indirect +interest, was so far advanced that Pander was able to sketch the main features +of it on the ground of Wolff’s theory in the dissertation he published in +1817. He clearly enunciated the theory of germinal layers which Wolff +<span class='pagenum'><a name="Page_16" id="Page_16"></a></span> +had anticipated, and established the truth of Wolff’s idea of a +development of the complicated systems of organs out of simple leaf-shaped +primitive structures. According to Pander, the leaf-shaped object in the +hen’s egg divides, before the incubation has proceeded twelve hours, into +two different layers, an external <i>serous</i> layer and an internal +<i>mucous</i> layer; between the two there develops later a third layer, the +<i>vascular</i> (blood-vessel) layer.<a href="#linknote-8" name="linknoteref-8" id="linknoteref-8"><sup>[8]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-8" id="linknote-8"></a> <a href="#linknoteref-8">[8]</a> +The technical terms which are bound to creep into this chapter will be fully +understood later on.—Translator. +</p> + +<p> +Karl Ernst von Baer, who had set afoot Pander’s investigation, and had +shown the liveliest interest in it after Pander’s departure from +Würtzburg, began his own much more comprehensive research in 1819. He published +the mature result nine years afterwards in his famous work, <i>Animal +Embryology: Observation and Reflection</i> (not translated). This classic work +still remains a model of careful observation united to profound philosophic +speculation. The first part appeared in 1828, the second in 1837. The book +proved to be the foundation on which the whole science of embryology has built +down to our own day. It so far surpassed its predecessors, and Pander in +particular, that it has become, after Wolff’s work, the chief base of +modern embryology. +</p> + +<p> +Baer was one of the greatest scientists of the nineteenth century, and +exercised considerable influence on other branches of biology as well. He built +up the theory of germinal layers, as a whole and in detail, so clearly and +solidly that it has been the starting-point of embryological research ever +since. He taught that in all the vertebrates first two and then four of these +germinal layers are formed; and that the earliest rudimentary organs of the +body arise by the conversion of these layers into tubes. He described the first +appearance of the vertebrate embryo, as it may be seen in the globular yelk of +the fertilised egg, as an oval disk which first divides into two layers. From +the upper or <i>animal</i> layer are developed all the organs which accomplish +the phenomena of animal life—the functions of sensation and motion, and +the covering of the body. From the lower or <i>vegetative</i> layer come the +organs which effect the vegetative life of the organism—nutrition, +digestion, blood-formation, respiration, secretion, reproduction, etc. +</p> + +<p> +Each of these original layers divides, according to Baer, into two thinner and +superimposed layers or plates. He calls the two plates of the animal layer, the +skin-stratum and muscle-stratum. From the upper of these plates, the +<i>skin-stratum,</i> the external skin, or outer covering of the body, the +central nervous system, and the sense-organs, are formed. From the lower, or +<i>muscle-stratum,</i> the muscles, or fleshy parts and the bony +skeleton—in a word, the motor organs—are evolved. In the same way, +Baer said, the lower or vegetative layer splits into two plates, which he calls +the vascular-stratum and the mucous-stratum. From the outer of the two (the +<i>vascular</i>) the heart, blood-vessels, spleen, and the other vascular +glands, the kidneys, and sexual glands, are formed. From the fourth or +<i>mucous</i> layer, in fine, we get the internal and digestive lining of the +alimentary canal and all its dependencies, the liver, lungs, salivary glands, +etc. Baer had, in the main, correctly judged the significance of these four +secondary embryonic layers, and he followed the conversion of them into the +tube-shaped primitive organs with great perspicacity. He first solved the +difficult problem of the transformation of this four-fold, flat, leaf-shaped, +embryonic disk into the complete vertebrate body, through the conversion of the +layers or plates into tubes. The flat leaves bend themselves in obedience to +certain laws of growth; the borders of the curling plates approach nearer and +nearer; until at last they come into actual contact. Thus out of the flat +gut-plate is formed a hollow gut-tube, out of the flat spinal plate a hollow +nerve-tube, from the skin-plate a skin-tube, and so on. +</p> + +<p> +Among the many great services which Baer rendered to embryology, especially +vertebrate embryology, we must not forget his discovery of the human ovum. +Earlier scientists had, as a rule, of course, assumed that man developed out of +an egg, like the other animals. In fact, the preformation theory held that the +germs of the whole of humanity were stored already in Eve’s ova. But the +real ovum escaped detection until the year 1827. This ovum is extremely small, +being a tiny round vesicle about the 1/120 of an inch in diameter; it can be +seen under very favourable circumstances with the naked eye as a tiny particle, +but is otherwise quite invisible. This particle is formed in the ovary inside a +much larger +<span class='pagenum'><a name="Page_17" id="Page_17"></a></span> +globule, which takes the name of the Graafian follicle, from its discoverer, +Graaf, and had previously been regarded as the true ovum. However, in 1827 Baer +proved that it was not the real ovum, which is much smaller, and is contained +within the follicle. (Compare the end of Chapter XXIX.) +</p> + +<p> +Baer was also the first to observe what is known as the <i>segmentation +sphere</i> of the vertebrate; that is to say, the round vesicle which first +develops out of the impregnated ovum, and the thin wall of which is made up of +a single layer of regular, polygonal (many-cornered) cells (see the +illustration in Chapter XII). Another discovery of his that was of great +importance in constructing the vertebrate stem and the characteristic +organisation of this extensive group (to which man belongs) was the detection +of the axial rod, or the <i>chorda dorsalis.</i> There is a long, round, +cylindrical rod of cartilage which runs down the longer axis of the vertebrate +embryo; it appears at an early stage, and is the first sketch of the spinal +column, the solid skeletal axis of the vertebrate. In the lowest of the +vertebrates, the amphioxus, the internal skeleton consists only of this cord +throughout life. But even in the case of man and all the higher vertebrates it +is round this cord that the spinal column and the brain are afterwards formed. +</p> + +<p> +However, important as these and many other discoveries of Baer’s were in +vertebrate embryology, his researches were even more influential, from the +circumstance that he was the first to employ the <i>comparative</i> method in +studying the development of the animal frame. Baer occupied himself chiefly +with the embryology of vertebrates (especially the birds and fishes). But he by +no means confined his attention to these, gradually taking the various groups +of the invertebrates into his sphere of study. As the general result of his +comparative embryological research, Baer distinguished four different modes of +development and four corresponding groups in the animal world. These chief +groups or types are: 1, the vertebrata; 2, the articulata; 3, the mollusca; and +4, all the lower groups which were then wrongly comprehended under the general +name of the radiata. Georges Cuvier had been the first to formulate this +distinction, in 1812. He showed that these groups present specific differences +in their whole internal structure, and the connection and disposal of their +systems of organs; and that, on the other hand, all the animals of the same +type—say, the vertebrates—essentially agreed in their inner +structure, in spite of the greatest superficial differences. But Baer proved +that these four groups are also quite differently developed from the ovum; and +that the series of embryonic forms is the same throughout for animals of the +same type, but different in the case of other animals. Up to that time the +chief aim in the classification of the animal kingdom was to arrange all the +animals from lowest to highest, from the infusorium to man, in one long and +continuous series. The erroneous idea prevailed nearly everywhere that there +was one uninterrupted chain of evolution from the lowest animal to the highest. +Cuvier and Baer proved that this view was false, and that we must distinguish +four totally different types of animals, on the ground of anatomic structure +and embryonic development. +</p> + +<p> +Baer’s epoch-making works aroused an extraordinary and widespread +interest in embryological research. Immediately afterwards we find a great +number of observers at work in the newly opened field, enlarging it in a very +short time with great energy by their various discoveries in detail. Next to +Baer’s comes the admirable work of Heinrich Rathke, of Königsberg (died +1860); he made an extensive study of the embryology, not only of the +invertebrates (crustaceans, insects, molluscs), but also, and particularly, of +the vertebrates (fishes, tortoises, serpents, crocodiles, etc.). We owe the +first comprehensive studies of mammal embryology to the careful research of +Wilhelm Bischoff, of Munich; his embryology of the rabbit (1840), the dog +(1842), the guinea-pig (1852), and the doe (1854), still form classical +studies. About the same time a great impetus was given to the embryology of the +invertebrates. The way was opened through this obscure province by the studies +of the famous Berlin zoologist, Johannes Müller, on the echinoderms. He was +followed by Albert Kölliker, of Würtzburg, writing on the cuttlefish (or the +cephalopods), Siebold and Huxley on worms and zoophytes, Fritz Muller +(Desterro) on the crustacea, Weismann on insects, and so on. The number of +workers in this field has greatly increased of late, and a quantity of new and +astonishing discoveries have been made. One notices, in several of these recent +works on +<span class='pagenum'><a name="Page_18" id="Page_18"></a></span> +embryology, that their authors are too little acquainted with comparative +anatomy and classification. Paleontology is, unfortunately, altogether +neglected by many of these new workers, although this interesting science +furnishes most important facts for phylogeny, and thus often proves of very +great service in ontogeny. +</p> + +<p> +A very important advance was made in our science in 1839, when the cellular +theory was established, and a new field of inquiry bearing on embryology was +suddenly opened. When the famous botanist, M. Schleiden, of Jena, showed in +1838, with the aid of the microscope, that every plant was made up of +innumerable elementary parts, which we call <i>cells,</i> a pupil of Johannes +Müller at Berlin, Theodor Schwann, applied the discovery at once to the animal +organism. He showed that in the animal body as well, when we examine its +tissues in the microscope, we find these cells everywhere to be the elementary +units. All the different tissues of the organism, especially the very +dissimilar tissues of the nerves, muscles, bones, external skin, mucous lining, +etc., are originally formed out of cells; and this is also true of all the +tissues of the plant. These cells are separate living beings; they are the +citizens of the State which the entire multicellular organism seems to be. This +important discovery was bound to be of service to embryology, as it raised a +number of new questions. What is the relation of the cells to the germinal +layers? Are the germinal layers composed of cells, and what is their relation +to the cells of the tissues that form later? How does the ovum stand in the +cellular theory? Is the ovum itself a cell, or is it composed of cells? These +important questions were now imposed on the embryologist by the cellular +theory. +</p> + +<p> +The most notable effort to answer these questions—which were attacked on +all sides by different students—is contained in the famous work, +<i>Inquiries into the Development of the Vertebrates</i> (not translated) of +Robert Remak, of Berlin (1851). This gifted scientist succeeded in mastering, +by a complete reform of the science, the great difficulties which the cellular +theory had at first put in the way of embryology. A Berlin anatomist, Carl +Boguslaus Reichert, had already attempted to explain the origin of the tissues. +But this attempt was bound to miscarry, since its not very clear-headed author +lacked a sound acquaintance with embryology and the cell theory, and even with +the structure and development of the tissue in particular. Remak at length +brought order into the dreadful confusion that Reichert had caused; he gave a +perfectly simple explanation of the origin of the tissues. In his opinion the +animal ovum is always <i>a simple cell</i> : the germinal layers which develop +out of it are always composed of cells; and these cells that constitute the +germinal layers arise simply from the continuous and repeated cleaving +(segmentation) of the original solitary cell. It first divides into two and +then into four cells; out of these four cells are born eight, then sixteen, +thirty-two, and so on. Thus, in the embryonic development of every animal and +plant there is formed first of all out of the simple egg cell, by a repeated +subdivision, a cluster of cells, as Kölliker had already stated in connection +with the cephalopods in 1844. The cells of this group spread themselves out +flat and form leaves or plates; each of these leaves is formed exclusively out +of cells. The cells of different layers assume different shapes, increase, and +differentiate; and in the end there is a further cleavage (differentiation) and +division of work of the cells within the layers, and from these all the +different tissues of the body proceed. +</p> + +<p> +These are the simple foundations of <i>histogeny,</i> or the science that +treats of the development of the tissues ( <i>hista</i>), as it was established +by Remak and Kölliker. Remak, in determining more closely the part which the +different germinal layers play in the formation of the various tissues and +organs, and in applying the theory of evolution to the cells and the tissues +they compose, raised the theory of germinal layers, at least as far as it +regards the vertebrates, to a high degree of perfection. +</p> + +<p> +Remak showed that three layers are formed out of the two germinal layers which +compose the first simple leaf-shaped structure of the vertebrate body (or the +“germinal disk”), as the lower layer splits into two plates. These +three layers have a very definite relation to the various tissues. First of +all, the cells which form the outer skin of the body (the epidermis), with its +various dependencies (hairs, nails, etc.)—that is to say, the entire +outer envelope of the body—are developed out of the outer or upper layer; +but there are also developed in a curious way out of the same layer the cells +which form the central nervous system, the +<span class='pagenum'><a name="Page_19" id="Page_19"></a></span>brain and the spinal cord. In the second place, the inner or lower germinal +layer gives rise only to the cells which form the epithelium (the whole inner +lining) of the alimentary canal and all that depends on it (the lungs, liver, +pancreas, etc.), or the tissues that receive and prepare the nourishment of the +body. Finally, the middle layer gives rise to all the other tissues of the +body, the muscles, blood, bones, cartilage, etc. Remak further proved that this +middle layer, which he calls “the motor-germinative layer,” +proceeds to subdivide into two secondary layers. Thus we find once more the +four layers which Baer had indicated. Remak calls the outer secondary leaf of +the middle layer (Baer’s “muscular layer”) the “skin +layer” (it would be better to say, skin-fibre layer); it forms the outer +wall of the body (the true skin, the muscles, etc.). To the inner secondary +leaf (Baer’s “vascular layer”) he gave the name of the +“alimentary-fibre layer”; this forms the outer envelope of the +alimentary canal, with the mesentery, the heart, the blood-vessels, etc. +</p> + +<p> +On this firm foundation provided by Remak for <i>histogeny,</i> or the science +of the formation of the tissues, our knowledge has been gradually built up and +enlarged in detail. There have been several attempts to restrict and even +destroy Remak’s principles. The two anatomists, Reichert (of Berlin) and +Wilhelm His (of Leipzic), especially, have endeavoured in their works to +introduce a new conception of the embryonic development of the vertebrate, +according to which the two primary germinal layers would not be the sole +sources of formation. But these efforts were so seriously marred by ignorance +of comparative anatomy, an imperfect acquaintance with ontogenesis, and a +complete neglect of phylogenesis, that they could not have more than a passing +success. We can only explain how these curious attacks of Reichert and His came +to be regarded for a time as advances by the general lack of discrimination and +of grasp of the true object of embryology. +</p> + +<p> +Wilhelm His published, in 1868, his extensive Researches into the <i>Earliest +Form of the Vertebrate Body,</i><a href="#linknote-9" name="linknoteref-9" id="linknoteref-9"><sup>[9]</sup></a> one of the curiosities of +embryological literature. The author imagines that he can build a +“mechanical theory of embryonic development” by merely giving an +exact description of the embryology of the chick, without any regard to +comparative anatomy and phylogeny, and thus falls into an error that is almost +without parallel in the history of biological literature. As the final result +of his laborious investigations, His tells us “that a comparatively +simple law of growth is the one essential thing in the first development. Every +formation, whether it consist in cleavage of layers, or folding, or complete +division, is a consequence of this fundamental law.” Unfortunately, he +does not explain what this “law of growth” is; just as other +opponents of the theory of selection, who would put in its place a great +“law of evolution,” omit to tell us anything about the nature of +this. Nevertheless, it is quite clear from His’s works that he imagines +constructive Nature to be a sort of skilful tailor. The ingenious operator +succeeds in bringing into existence, by “evolution,” all the +various forms of living things by cutting up in different ways the germinal +layers, bending and folding, tugging and splitting, and so on. +</p> + +<p class="footnote"> +<a name="linknote-9" id="linknote-9"></a> <a href="#linknoteref-9">[9]</a> +None of His’s works have been translated into English. +</p> + +<p> +His’s embryological theories excited a good deal of interest at the time +of publication, and have evoked a fair amount of literature in the last few +decades. He professed to explain the most complicated parts of organic +construction (such as the development of the brain) in the simplest way on +mechanical principles, and to derive them immediately from simple physical +processes (such as unequal distribution of strain in an elastic plate). It is +quite true that a mechanical or monistic explanation (or a reduction of natural +processes) is the ideal of modern science, and this ideal would be realised if +we could succeed in expressing these formative processes in mathematical +formulæ. His has, therefore, inserted plenty of numbers and measurements in his +embryological works, and given them an air of “exact” scholarship +by putting in a quantity of mathematical tables. Unfortunately, they are of no +value, and do not help us in the least in forming an “exact” +acquaintance with the embryonic phenomena. Indeed, they wander from the true +path altogether by neglecting the phylogenetic method; this, he thinks, is +“a mere by-path,” and is “not necessary at all for the +explanation of the facts of +<span class='pagenum'><a name="Page_20" id="Page_20"></a></span>embryology,” which are the direct consequence of physiological +principles. What His takes to be a simple physical process—for instance, +the folding of the germinal layers (in the formation of the medullary tube, +alimentary tube, etc.)—is, as a matter of fact, the direct result of the +growth of the various cells which form those organic structures; but these +growth-motions have themselves been transmitted by heredity from parents and +ancestors, and are only the hereditary repetition of countless phylogenetic +changes which have taken place for thousands of years in the race-history of +the said ancestors. Each of these historical changes was, of course, originally +due to adaptation; it was, in other words, physiological, and reducible to +mechanical causes. But we have, naturally, no means of observing them now. It +is only by the hypotheses of the science of evolution that we can form an +approximate idea of the organic links in this historic chain. +</p> + +<p> +All the best recent research in animal embryology has led to the confirmation +and development of Baer and Remak’s theory of the germinal layers. One of +the most important advances in this direction of late was the discovery that +the two primary layers out of which is built the body of all vertebrates +(including man) are also present in all the invertebrates, with the sole +exception of the lowest group, the unicellular protozoa. Huxley had detected +them in the medusa in 1849. He showed that the two layers of cells from which +the body of this zoophyte is developed correspond, both morphologically and +physiologically, to the two original germinal layers of the vertebrate. The +outer layer, from which come the external skin and the muscles, was then called +by Allman (1853) the “ectoderm” (outer layer, or skin); the inner +layer, which forms the alimentary and reproductory organs, was called the +“entoderm” (= inner layer). In 1867 and the following years the +discovery of the germinal layers was extended to other groups of the +invertebrates. In particular, the indefatigable Russian zoologist, Kowalevsky, +found them in all the most diverse sections of the invertebrates—the +worms, tunicates, echinoderms, molluscs, articulates, etc. +</p> + +<p> +In my monograph on the sponges (1872) I proved that these two primary germinal +layers are also found in that group, and that they may be traced from it right +up to man, through all the various classes, in identical form. This +“homology of the two primary germinal layers” extends through the +whole of the metazoa, or tissue-forming animals; that is to say, through the +whole animal kingdom, with the one exception of its lowest section, the +unicellular beings, or protozoa. These lowly organised animals do not form +germinal layers, and therefore do not succeed in forming true tissue. Their +whole body consists of a single cell (as is the case with the amœbæ and +infusoria), or of a loose aggregation of only slightly differentiated cells, +though it may not even reach the full structure of a single cell (as with the +monera). But in all other animals the ovum first grows into two primary layers, +the outer or <i>animal</i> layer (the ectoderm, epiblast, or ectoblast), and +the inner or <i>vegetal</i> layer (the entoderm, hypoblast, or endoblast); and +from these the tissues and organs are formed. The first and oldest organ of all +these metazoa is the primitive gut (or progaster) and its opening, the +primitive mouth (prostoma). The typical embryonic form of the metazoa, as it is +presented for a time by this simple structure of the two-layered body, is +called the <i>gastrula</i> ; it is to be conceived as the hereditary +reproduction of some primitive common ancestor of the metazoa, which we call +the <i>gastræa.</i> This applies to the sponges and other zoophyta, and to the +worms, the mollusca, echinoderma, articulata, and vertebrata. All these animals +may be comprised under the general heading of “gut animals,” or +metazoa, in contradistinction to the gutless protozoa. +</p> + +<p> +I have pointed out in my Study of the <i>Gastræa Theory</i> [not translated] +(1873) the important consequences of this conception in the morphology and +classification of the animal world. I also divided the realm of metazoa into +two great groups, the lower and higher metazoa. In the first are comprised the +<i>cœlenterata</i> (also called zoophytes, or plant-animals). In the lower +forms of this group the body consists throughout life merely of the primary +germinal layers, with the cells sometimes more and sometimes less +differentiated. But with the higher forms of the cœlentarata (the corals, +higher medusæ, ctenophoræ, and platodes) a middle layer, or <i>mesoderm,</i> +often of considerable size, is developed between the +<span class='pagenum'><a name="Page_21" id="Page_21"></a></span>other two layers; but blood and an internal cavity are still lacking. +</p> + +<p> +To the second great group of the metazoa I gave the name of the +<i>cœlomaria,</i> or <i>bilaterata</i> (or the bilateral higher forms). They +all have a cavity within the body (cœloma), and most of them have blood and +blood-vessels. In this are comprised the six higher stems of the animal +kingdom, the annulata and their descendants, the mollusca, echinoderma, +articulata, tunicata, and vertebrata. In all these bilateral organisms the +two-sided body is formed out of four secondary germinal layers, of which the +inner two construct the wall of the alimentary canal, and the outer two the +wall of the body. Between the two pairs of layers lies the cavity (cœloma). +</p> + +<p> +Although I laid special stress on the great morphological importance of this +cavity in my <i>Study of the Gastræa Theory,</i> and endeavoured to prove the +significance of the four secondary germinal layers in the organisation of the +cœlomaria, I was unable to deal satisfactorily with the difficult question of +the mode of their origin. This was done eight years afterwards by the brothers +Oscar and Richard Hertwig in their careful and extensive comparative studies. +In their masterly <i>Cœlum Theory: An Attempt to Explain the Middle Germinal +Layer</i> [not translated] (1881) they showed that in most of the metazoa, +especially in all the vertebrates, the body-cavity arises in the same way, by +the outgrowth of two sacs from the inner layer. These two cœlom-pouches proceed +from the rudimentary mouth of the gastrula, between the two primary layers. The +inner plate of the two-layered cœlom-pouch (the visceral layer) joins itself to +the entoderm; the outer plate (parietal layer) unites with the ectoderm. Thus +are formed the double-layered gut-wall within and the double-layered body-wall +without; and between the two is formed the cavity of the cœlom, by the blending +of the right and left cœlom-sacs. We shall see this more fully in Chapter X. +</p> + +<p> +The many new points of view and fresh ideas suggested by my gastræa theory and +Hertwig’s cœlom theory led to the publication of a number of writings on +the theory of germinal layers. Most of them set out to oppose it at first, but +in the end the majority supported it. Of late years both theories are accepted +in their essential features by nearly every competent man of science, and light +and order have been introduced into this once dark and contradictory field of +research. A further cause of congratulation for this solution of the great +embryological controversy is that it brought with it a recognition of the need +for phylogenetic study and explanation. +</p> + +<p> +Interest and practice in embryological research have been remarkably stimulated +during the past thirty years by this appreciation of phylogenetic methods. +Hundreds of assiduous and able observers are now engaged in the development of +comparative embryology and its establishment on a basis of evolution, whereas +they numbered only a few dozen not many decades ago. It would take too long to +enumerate even the most important of the countless valuable works which have +enriched embryological literature since that time. References to them will be +found in the latest manuals of embryology of Kölliker, Balfour, Hertwig, +Kollman, Korschelt, and Heider. +</p> + +<p> +Kölliker’s <i>Entwickelungsgeschichte des Menschen und der höherer +Thiere,</i> the first edition of which appeared forty-two years ago, had the +rare merit at that time of gathering into presentable form the scattered +attainments of the science, and expounding them in some sort of unity on the +basis of the cellular theory and the theory of germinal layers. Unfortunately, +the distinguished Würtzburg anatomist, to whom comparative anatomy, histology, +and ontogeny owe so much, is opposed to the theory of descent generally and to +Darwinism in particular. All the other manuals I have mentioned take a decided +stand on evolution. Francis Balfour has carefully collected and presented with +discrimination, in his <i>Manual of Comparative Embryology</i> (1880), the very +scattered and extensive literature of the subject; he has also widened the +basis of the gastræa theory by a comparative description of the rise of the +organs from the germinal layers in all the chief groups of the animal kingdom, +and has given a most thorough empirical support to the principles I have +formulated. A comparison of his work with the excellent <i>Text-book of the +Embryology of the Vertebrates</i> (1890) [translation 1895] of Korschelt and +Heider shows what astonishing progress has been made in the science in the +course of ten years. I would especially recommend the manuals of Julius +Kollmann and Oscar Hertwig to those readers who are stimulated to further study +by these chapters on human +<span class='pagenum'><a name="Page_22" id="Page_22"></a></span>embryology. Kollmann’s work is commendable for its clear treatment of the +subject and very fine original illustrations; its author adheres firmly to the +biogenetic law, and uses it throughout with considerable profit. That is not +the case in Oscar Hertwig’s recent <i>Text-book of the Embryology of Man +and the Mammals</i> [translations 1892 and 1899] (seventh edition 1902). This +able anatomist has of late often been quoted as an opponent of the biogenetic +law, although he himself had demonstrated its great value thirty years ago. His +recent vacillation is partly due to the timidity which our “exact” +scientists have with regard to hypotheses; though it is impossible to make any +headway in the explanation of facts without them. However, the purely +descriptive part of embryology in Hertwig’s <i>Text-book</i> is very +thorough and reliable. +</p> + +<p> +A new branch of embryological research has been studied very assiduously in the +last decade of the nineteenth century—namely, “experimental +embryology.” The great importance which has been attached to the +application of physical experiments to the living organism for the last hundred +years, and the valuable results that it has given to physiology in the study of +the vital phenomena, have led to its extension to embryology. I was the first +to make experiments of this kind during a stay of four months on the Canary +Island, Lanzerote, in 1866. I there made a thorough investigation of the almost +unknown embryology of the siphonophoræ. I cut a number of the embryos of these +animals (which develop freely in the water, and pass through a very curious +transformation), at an early stage, into several pieces, and found that a fresh +organism (more or less complete, according to the size of the piece) was +developed from each particle. More recently some of my pupils have made similar +experiments with the embryos of vertebrates (especially the frog) and some of +the invertebrates. Wilhelm Roux, in particular, has made extensive experiments, +and based on them a special “mechanical embryology,” which has +given rise to a good deal of discussion and controversy. Roux has published a +special journal for these subjects since 1895, the <i>Archiv für +Entwickelungsmechanik.</i> The contributions to it are very varied in value. +Many of them are valuable papers on the physiology and pathology of the embryo. +Pathological experiments—the placing of the embryo in abnormal +conditions—have yielded many interesting results; just as the physiology +of the normal body has for a long time derived assistance from the pathology of +the diseased organism. Other of these mechanical-embryological articles return +to the erroneous methods of His, and are only misleading. This must be said of +the many contributions of mechanical embryology which take up a position of +hostility to the theory of descent and its chief embryological +foundation—the biogenetic law. This law, however, when rightly +understood, is not opposed to, but is the best and most solid support of, a +sound mechanical embryology. Impartial reflection and a due attention to +paleontology and comparative anatomy should convince these one-sided +mechanicists that the facts they have discovered—and, indeed, the whole +embryological process—cannot be fully understood without the theory of +descent and the biogenetic law. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap04"></a>Chapter IV.<br/> +THE OLDER PHYLOGENY</h2> + +<p> +The embryology of man and the animals, the history of which we have reviewed in +the last two chapters, was mainly a descriptive science forty years ago. The +earlier investigations in this province were chiefly directed to the discovery, +by careful observation, of the wonderful facts of the embryonic development of +the animal body from the ovum. Forty years ago no one dared attack the question +of the <span class='pagenum'><a name="Page_23" id="Page_23"></a></span><i>causes</i> of these phenomena. For fully a century, from the year +1759, when Wolff’s solid <i>Theoria generationis</i> appeared, until +1859, when Darwin published his famous Origin of Species, the real causes of +the embryonic processes were quite unknown. No one thought of seeking the +agencies that effected this marvellous succession of structures. The task was +thought to be so difficult as almost to pass beyond the limits of human +thought. It was reserved for Charles Darwin to initiate us into the knowledge +of these causes. This compels us to recognise in this great genius, who wrought +a complete revolution in the whole field of biology, a founder at the same time +of a new period in embryology. It is true that Darwin occupied himself very +little with direct embryological research, and even in his chief work he only +touches incidentally on the embryonic phenomena; but by his reform of the +theory of descent and the founding of the theory of selection he has given us +the means of attaining to a real knowledge of the causes of embryonic +formation. That is, in my opinion, the chief feature in Darwin’s +incalculable influence on the whole science of evolution. +</p> + +<p> +When we turn our attention to this latest period of embryological research, we +pass into the second division of organic evolution—stem-evolution, or +phylogeny. I have already indicated in Chapter I the important and intimate +causal connection between these two sections of the science of +evolution—between the evolution of the individual and that of his +ancestors. We have formulated this connection in the biogenetic law; the +shorter evolution, that of the individual, or <i>ontogenesis,</i> is a rapid +and summary repetition, a condensed recapitulation, of the larger evolution, or +that of the species. In this principle we express all the essential points +relating to the causes of evolution; and we shall seek throughout this work to +confirm this principle and lend it the support of facts. When we look to its +<i>causal</i> significance, perhaps it would be better to formulate the +biogenetic law thus: “The evolution of the species and the stem ( +<i>phylon</i>) shows us, in the physiological functions of heredity and +adaptation, the conditioning causes on which the evolution of the individual +depends”; or, more briefly: “Phylogenesis is the mechanical cause +of ontogenesis.” +</p> + +<p> +But before we examine the great achievement by which Darwin revealed the causes +of evolution to us, we must glance at the efforts of earlier scientists to +attain this object. Our historical inquiry into these will be even shorter than +that into the work done in the field of ontogeny. We have very few names to +consider here. At the head of them we find the great French naturalist, Jean +Lamarck, who first established evolution as a scientific theory in 1809. Even +before his time, however, the chief philosopher, Kant, and the chief poet, +Goethe, of Germany had occupied themselves with the subject. But their efforts +passed almost without recognition in the eighteenth century. A +“philosophy of nature” did not arise until the beginning of the +nineteenth century. In the whole of the time before this no one had ventured to +raise seriously the question of the origin of species, which is the culminating +point of phylogeny. On all sides it was regarded as an insoluble enigma. +</p> + +<p> +The whole science of the evolution of man and the other animals is intimately +connected with the question of the nature of species, or with the problem of +the origin of the various animals which we group together under the name of +species. Thus the definition of the species becomes important. It is well known +that this definition was given by Linné, who, in his famous <i>Systema +Naturæ</i> (1735), was the first to classify and name the various groups of +animals and plants, and drew up an orderly scheme of the species then known. +Since that time “species” has been the most important and +indispensable idea in descriptive natural history, in zoological and botanical +classification; although there have been endless controversies as to its real +meaning. +</p> + +<p> +What, then, is this “organic species”? Linné himself appealed +directly to the Mosaic narrative; he believed that, as it is stated in +<i>Genesis,</i> one pair of each species of animals and plants was created in +the beginning, and that all the individuals of each species are the descendants +of these created couples. As for the hermaphrodites (organisms that have male +and female organs in one being), he thought it sufficed to assume the creation +of one sole individual, since this would be fully competent to propagate its +species. Further developing these mystic ideas, Linné went on to borrow from +<i>Genesis</i> the account of the deluge and of Noah’s ark as a ground +for a science of the geographical +<span class='pagenum'><a name="Page_24" id="Page_24"></a></span>and topographical distribution of organisms. He accepted the story that all the +plants, animals, and men on the earth were swept away in a universal deluge, +except the couples preserved with Noah in the ark, and ultimately landed on +Mount Ararat. This mountain seemed to Linné particularly suitable for the +landing, as it reaches a height of more than 16,000 feet, and thus provides in +its higher zones the several climates demanded by the various species of +animals and plants: the animals that were accustomed to a cold climate could +remain at the summit; those used to a warm climate could descend to the foot; +and those requiring a temperate climate could remain half-way down. From this +point the re-population of the earth with animals and plants could proceed. +</p> + +<p> +It was impossible to have any scientific notion of the method of evolution in +Linné’s time, as one of the chief sources of information, paleontology, +was still wholly unknown. This science of the fossil remains of extinct animals +and plants is very closely bound up with the whole question of evolution. It is +impossible to explain the origin of living organisms without appealing to it. +But this science did not rise until a much later date. The real founder of +scientific paleontology was Georges Cuvier, the most distinguished zoologist +who, after Linné, worked at the classification of the animal world, and +effected a complete revolution in systematic zoology at the beginning of the +nineteenth century. In regard to the nature of the species he associated +himself with Linné and the Mosaic story of creation, though this was more +difficult for him with his acquaintance with fossil remains. He clearly showed +that a number of quite different animal populations have lived on the earth; +and he claimed that we must distinguish a number of stages in the history of +our planet, each of which was characterised by a special population of animals +and plants. These successive populations were, he said, quite independent of +each other, and therefore the supernatural creative act, which was demanded as +the origin of the animals and plants by the dominant creed, must have been +repeated several times. In this way a whole series of different creative +periods must have succeeded each other; and in connection with these he had to +assume that stupendous revolutions or cataclysms—something like the +legendary deluge—must have taken place repeatedly. Cuvier was all the +more interested in these catastrophes or cataclysms as geology was just +beginning to assert itself, and great progress was being made in our knowledge +of the structure and formation of the earth’s crust. The various strata +of the crust were being carefully examined, especially by the famous geologist +Werner and his school, and the fossils found in them were being classified; and +these researches also seemed to point to a variety of creative periods. In each +period the earth’s crust, composed of the various strata, seemed to be +differently constituted, just like the population of animals and plants that +then lived on it. Cuvier combined this notion with the results of his own +paleontological and zoological research; and in his effort to get a consistent +view of the whole process of the earth’s history he came to form the +theory which is known as “the catastrophic theory,” or the theory +of terrestrial revolutions. According to this theory, there have been a series +of mighty cataclysms on the earth, and these have suddenly destroyed the whole +animal and plant population then living on it; after each cataclysm there was a +fresh creation of living things throughout the earth. As this creation could +not be explained by natural laws, it was necessary to appeal to an intervention +on the part of the Creator. This catastrophic theory, which Cuvier described in +a special work, was soon generally accepted, and retained its position in +biology for half a century. +</p> + +<p> +However, Cuvier’s theory was completely overthrown sixty years ago by the +geologists, led by Charles Lyell, the most distinguished worker in this field +of science. Lyell proved in his famous <i>Principles of Geology</i> (1830) that +the theory was false, in so far as it concerned the crust of the earth; that it +was totally unnecessary to bring in supernatural agencies or general +catastrophes in order to explain the structure and formation of the mountains; +and that we can explain them by the familiar agencies which are at work to-day +in altering and reconstructing the surface of the earth. These causes +are—the action of the atmosphere and water in its various forms (snow, +ice, fog, rain, the wear of the river, and the stormy ocean), and the volcanic +action which is exerted by the molten central +<span class='pagenum'><a name="Page_25" id="Page_25"></a></span> +mass. Lyell convincingly proved that these natural causes are quite adequate to +explain every feature in the build and formation of the crust. Hence +Cuvier’s theory of cataclysms was very soon driven out of the province of +geology, though it remained for another thirty years in undisputed authority in +biology. All the zoologists and botanists who gave any thought to the question +of the origin of organisms adhered to Cuvier’s erroneous idea of +revolutions and new creations. +</p> + +<p> +In order to illustrate the complete stagnancy of biology from 1830 to 1859 on +the question of the origin of the various species of animals and plants, I may +say, from my own experience, that during the whole of my university studies I +never heard a single word said about this most important problem of the +science. I was fortunate enough at that time (1852–1857) to have the most +distinguished masters for every branch of biological science. Not one of them +ever mentioned this question of the origin of species. Not a word was ever said +about the earlier efforts to understand the formation of living things, nor +about Lamarck’s <i>Philosophie Zoologique</i> which had made a fresh +attack on the problem in 1809. Hence it is easy to understand the enormous +opposition that Darwin encountered when he took up the question for the first +time. His views seemed to float in the air, without a single previous effort to +support them. The whole question of the formation of living things was +considered by biologists, until 1859, as pertaining to the province of religion +and transcendentalism; even in speculative philosophy, in which the question +had been approached from various sides, no one had ventured to give it serious +treatment. This was due to the dualistic system of Immanuel Kant, who taught a +natural system of evolution as far as the inorganic world was concerned; but, +on the whole, adopted a supernaturalist system as regards the origin of living +things. He even went so far as to say: “It is quite certain that we +cannot even satisfactorily understand, much less explain, the nature of an +organism and its internal forces on purely mechanical principles; it is so +certain, indeed, that we may confidently say: ‘It is absurd for a man to +imagine even that some day a Newton will arise who will explain the origin of a +single blade of grass by natural laws not controlled by +design’—such a hope is entirely forbidden us.” In these words +Kant definitely adopts the dualistic and teleological point of view for +biological science. +</p> + +<p> +Nevertheless, Kant deserted this point of view at times, particularly in +several remarkable passages which I have dealt with at length in my <i>Natural +History of Creation</i> (chap. v), where he expresses himself in the opposite, +or monistic, sense. In fact, these passages would justify one, as I showed, in +claiming his support for the theory of evolution. However, these monistic +passages are only stray gleams of light; as a rule, Kant adheres in biology to +the obscure dualistic ideas, according to which the forces at work in inorganic +nature are quite different from those of the organic world. This dualistic +system prevails in academic philosophy to-day—most of our philosophers +still regarding these two provinces as totally distinct. They put, on the one +side, the inorganic or “lifeless” world, in which there are at work +only mechanical laws, acting necessarily and without design; and, on the other, +the province of organic nature, in which none of the phenomena can be properly +understood, either as regards their inner nature or their origin, except in the +light of preconceived design, carried out by final or purposive causes. +</p> + +<p> +The prevalence of this unfortunate dualistic prejudice prevented the problem of +the origin of species, and the connected question of the origin of man, from +being regarded by the bulk of people as a scientific question at all until +1859. Nevertheless, a few distinguished students, free from the current +prejudice, began, at the commencement of the nineteenth century, to make a +serious attack on the problem. The merit of this attaches particularly to what +is known as “the older school of natural philosophy,” which has +been so much misrepresented, and which included Jean Lamarck, Buffon, Geoffroy +St. Hilaire, and Blainville in France; Wolfgang Goethe, Reinhold Treviranus, +Schelling, and Lorentz Oken in Germany [and Erasmus Darwin in England]. +</p> + +<p> +The gifted natural philosopher who treated this difficult question with the +greatest sagacity and comprehensiveness was Jean Lamarck. He was born at +Bazentin, in Picardy, on August 1st, 1744; he was the son of a clergyman, and +was destined for the Church. But he turned to seek glory in the army, and +eventually devoted himself to science. +</p> + +<p> +His <i>Philosophie Zoologique</i> was the +<span class='pagenum'><a name="Page_26" id="Page_26"></a></span> +first scientific attempt to sketch the real course of the origin of species, +the first “natural history of creation” of plants, animals, and +men. But, as in the case of Wolff’s book, this remarkably able work had +no influence whatever; neither one nor the other could obtain any recognition +from their prejudiced contemporaries. No man of science was stimulated to take +an interest in the work, and to develop the germs it contained of the most +important biological truths. The most distinguished botanists and zoologists +entirely rejected it, and did not even deign to reply to it. Cuvier, who lived +and worked in the same city, has not thought fit to devote a single syllable to +this great achievement in his memoir on progress in the sciences, in which the +pettiest observations found a place. In short, Lamarck’s <i>Philosophie +Zoologique</i> shared the fate of Wolff’s theory of development, and was +for half a century ignored and neglected. The German scientists, especially +Oken and Goethe, who were occupied with similar speculations at the same time, +seem to have known nothing about Lamarck’s work. If they had known it, +they would have been greatly helped by it, and might have carried the theory of +evolution much farther than they found it possible to do. +</p> + +<p> +To give an idea of the great importance of the <i>Philosophie Zoologique,</i> I +will briefly explain Lamarck’s leading thought. He held that there was no +essential difference between living and lifeless beings. Nature is one united +and connected system of phenomena; and the forces which fashion the lifeless +bodies are the only ones at work in the kingdom of living things. We have, +therefore, to use the same method of investigation and explanation in both +provinces. Life is only a physical phenomenon. All the plants and animals, with +man at their head, are to be explained, in structure and life, by mechanical or +efficient causes, without any appeal to final causes, just as in the case of +minerals and other inorganic bodies. This applies equally to the origin of the +various species. We must not assume any original creation, or repeated +creations (as in Cuvier’s theory), to explain this, but a natural, +continuous, and necessary evolution. The whole evolutionary process has been +uninterrupted. All the different kinds of animals and plants which we see +to-day, or that have ever lived, have descended in a natural way from earlier +and different species; all come from one common stock, or from a few common +ancestors. These remote ancestors must have been quite simple organisms of the +lowest type, arising by spontaneous generation from inorganic matter. The +succeeding species have been constantly modified by adaptation to their varying +environment (especially by use and habit), and have transmitted their +modifications to their successors by heredity. +</p> + +<p> +Lamarck was the first to formulate as a scientific theory the natural origin of +living things, including man, and to push the theory to its extreme +conclusions—the rise of the earliest organisms by spontaneous generation +(or abiogenesis) and the descent of man from the nearest related mammal, the +ape. He sought to explain this last point, which is of especial interest to us +here, by the same agencies which he found at work in the natural origin of the +plant and animal species. He considered use and habit (adaptation) on the one +hand, and heredity on the other, to be the chief of these agencies. The most +important modifications of the organs of plants and animals are due, in his +opinion, to the function of these very organs, or to the use or disuse of them. +To give a few examples, the woodpecker and the humming-bird have got their +peculiarly long tongues from the habit of extracting their food with their +tongues from deep and narrow folds or canals; the frog has developed the web +between his toes by his own swimming; the giraffe has lengthened his neck by +stretching up to the higher branches of trees, and so on. It is quite certain +that this use or disuse of organs is a most important factor in organic +development, but it is not sufficient to explain the origin of species. +</p> + +<p> +To adaptation we must add heredity as the second and not less important agency, +as Lamarck perfectly recognised. He said that the modification of the organs in +any one individual by use or disuse was slight, but that it was increased by +accumulation in passing by heredity from generation to generation. But he +missed altogether the principle which Darwin afterwards found to be the chief +factor in the theory of transformation—namely, the principle of natural +selection in the struggle for existence. It was partly owing to his failure to +detect this supremely important element, and partly to the poor condition of +all biological science at the time, that Lamarck did not +<span class='pagenum'><a name="Page_27" id="Page_27"></a></span> +succeed in establishing more firmly his theory of the common descent of man and +the other animals. +</p> + +<p> +Independently of Lamarck, the older German school of natural philosophy, +especially Reinhold Treviranus, in his <i>Biologie</i> (1802), and Lorentz +Oken, in his <i>Naturphilosophie</i> (1809), turned its attention to the +problem of evolution about the end of the eighteenth and beginning of the +nineteenth century. I have described its work in my <i>History of Creation</i> +(chap. iv). Here I can only deal with the brilliant genius whose evolutionary +ideas are of special interest—the greatest of German poets, Wolfgang +Goethe. With his keen eye for the beauties of nature, and his profound insight +into its life, Goethe was early attracted to the study of various natural +sciences. It was the favourite occupation of his leisure hours throughout life. +He gave particular and protracted attention to the theory of colours. But the +most valuable of his scientific studies are those which relate to that +“living, glorious, precious thing,” the organism. He made profound +research into the science of structures or morphology (morphæ = forms). Here, +with the aid of comparative anatomy, he obtained the most brilliant results, +and went far in advance of his time. I may mention, in particular, his +vertebral theory of the skull, his discovery of the pineal gland in man, his +system of the metamorphosis of plants, etc. These morphological studies led +Goethe on to research into the formation and modification of organic structures +which we must count as the first germ of the science of evolution. He +approaches so near to the theory of descent that we must regard him, after +Lamarck, as one of its earliest founders. It is true that he never formulated a +complete scientific theory of evolution, but we find a number of remarkable +suggestions of it in his splendid miscellaneous essays on morphology. Some of +them are really among the very basic ideas of the science of evolution. He +says, for instance (1807): “When we compare plants and animals in their +most rudimentary forms, it is almost impossible to distinguish between them. +But we may say that the plants and animals, beginning with an almost +inseparable closeness, gradually advance along two divergent lines, until the +plant at last grows in the solid, enduring tree and the animal attains in man +to the highest degree of mobility and freedom.” That Goethe was not +merely speaking in a poetical, but in a literal genealogical, sense of this +close affinity of organic forms is clear from other remarkable passages in +which he treats of their variety in outward form and unity in internal +structure. He believes that every living thing has arisen by the interaction of +two opposing formative forces or impulses. The internal or +“centripetal” force, the type or “impulse to +specification,” seeks to maintain the constancy of the specific forms in +the succession of generations: this is <i>heredity.</i> The external or +“centrifugal” force, the element of variation or “impulse to +metamorphosis,” is continually modifying the species by changing their +environment: this is <i>adaptation.</i> In these significant conceptions Goethe +approaches very close to a recognition of the two great mechanical factors +which we now assign as the chief causes of the formation of species. +</p> + +<p> +However, in order to appreciate Goethe’s views on morphology, one must +associate his decidedly monistic conception of nature with his pantheistic +philosophy. The warm and keen interest with which he followed, in his last +years, the controversies of contemporary French scientists, and especially the +struggle between Cuvier and Geoffroy St. Hilaire (see chap. iv of <i>The +History of Creation</i>), is very characteristic. It is also necessary to be +familiar with his style and general tenour of thought in order to appreciate +rightly the many allusions to evolution found in his writings. Otherwise, one +is apt to make serious errors. +</p> + +<p> +He approached so close, at the end of the eighteenth century, to the principles +of the science of evolution that he may well be described as the first +forerunner of Darwin, although he did not go so far as to formulate evolution +as a scientific system, as Lamarck did. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap05"></a> +<span class='pagenum'><a name="Page_28" id="Page_28"></a></span> +Chapter V.<br/> +THE MODERN SCIENCE OF EVOLUTION</h2> + +<p> +We owe so much of the progress of scientific knowledge to Darwin’s +<i>Origin of Species</i> that its influence is almost without parallel in the +history of science. The literature of Darwinism grows from day to day, not only +on the side of academic zoology and botany, the sciences which were chiefly +affected by Darwin’s theory, but in a far wider circle, so that we find +Darwinism discussed in popular literature with a vigour and zest that are given +to no other scientific conception. This remarkable success is due chiefly to +two circumstances. In the first place, all the sciences, and especially +biology, have made astounding progress in the last half-century, and have +furnished a very vast quantity of proofs of the theory of evolution. In +striking contrast to the failure of Lamarck and the older scientists to attract +attention to their effort to explain the origin of living things and of man, we +have this second and successful effort of Darwin, which was able to gather to +its support a large number of established facts. Availing himself of the +progress already made, he had very different scientific proofs to allege than +Lamarck, or St. Hilaire, or Goethe, or Treviranus had had. But, in the second +place, we must acknowledge that Darwin had the special distinction of +approaching the subject from an entirely new side, and of basing the theory of +descent on a consistent system, which now goes by the name of Darwinism. +</p> + +<p> +Lamarck had unsuccessfully attempted to explain the modification of organisms +that descend from a common form chiefly by the action of habit and the use of +organs, though with the aid of heredity. But Darwin’s success was +complete when he independently sought to give a mechanical explanation, on a +quite new ground, of this modification of plant and animal structures by +adaptation and heredity. He was impelled to his theory of selection on the +following grounds. He compared the origin of the various kinds of animals and +plants which we modify artificially—by the action of artificial selection +in horticulture and among domestic animals—with the origin of the species +of animals and plants in their natural state. He then found that the agencies +which we employ in the modification of forms by artificial selection are also +at work in Nature. The chief of these agencies he held to be “the +struggle for life.” The gist of this peculiarly Darwinian idea is given +in this formula: The struggle for existence produces new species without +premeditated design in the life of Nature, in the same way that the will of man +consciously selects new races in artificial conditions. The gardener or the +farmer selects new forms as he wills for his own profit, by ingeniously using +the agency of heredity and adaptation for the modification of structures; so, +in the natural state, the struggle for life is always unconsciously modifying +the various species of living things. This struggle for life, or competition of +organisms in securing the means of subsistence, acts without any conscious +design, but it is none the less effective in modifying structures. As heredity +and adaptation enter into the closest reciprocal action under its influence, +new structures, or alterations of structure, are produced; and these are +purposive in the sense that they serve the organism when formed, but they were +produced without any pre-conceived aim. +</p> + +<p> +This simple idea is the central thought of Darwinism, or the theory of +selection. Darwin conceived this idea at an early date, and then, for more than +twenty years, worked at the collection of empirical evidence in support of it +before he published his theory. His grandfather, Erasmus Darwin, was an able +scientist of the older school of natural philosophy, who published a number of +natural-philosophic works about the end of the eighteenth century. The most +important of them is his <i>Zoonomia,</i> published in 1794, in which he +expounds views similar to those of Goethe and Lamarck, without really knowing +anything of the work of these +<span class='pagenum'><a name="Page_29" id="Page_29"></a></span> +contemporaries. However, in the writings of the grandfather the plastic +imagination rather outran the judgment, while in Charles Darwin the two were +better balanced. +</p> + +<p> +Darwin did not publish any account of his theory until 1858, when Alfred Russel +Wallace, who had independently reached the same theory of selection, published +his own work. In the following year appeared the <i>Origin of Species,</i> in +which he develops it at length and supports it with a mass of proof. Wallace +had reached the same conclusion, but he had not so clear a perception as Darwin +of the effectiveness of natural selection in forming species, and did not +develop the theory so fully. Nevertheless, Wallace’s writings, especially +those on mimicry, etc., and an admirable work on <i>The Geographical +Distribution of Animals,</i> contain many fine original contributions to the +theory of selection. Unfortunately, this gifted scientist has since devoted +himself to spiritism.<a href="#linknote-10" name="linknoteref-10" id="linknoteref-10"><sup>[10]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-10" id="linknote-10"></a> <a href="#linknoteref-10">[10]</a> +Darwin and Wallace arrived at the theory quite independently. <i>Vide</i> +Wallace’s <i>Contributions to the Theory of Natural Selection</i> (1870) +and <i>Darwinism</i> (1891). +</p> + +<p> +Darwin’s <i>Origin of Species</i> had an extraordinary influence, though +not at first on the experts of the science. It took zoologists and botanists +several years to recover from the astonishment into which they had been thrown +through the revolutionary idea of the work. But its influence on the special +sciences with which we zoologists and botanists are concerned has increased +from year to year; it has introduced a most healthy fermentation in every +branch of biology, especially in comparative anatomy and ontogeny, and in +zoological and botanical classification. In this way it has brought about +almost a revolution in the prevailing views. +</p> + +<p> +However, the point which chiefly concerns us here—the extension of the +theory to man—was not touched at all in Darwin’s first work in +1859. It was believed for several years that he had no thought of applying his +principles to man, but that he shared the current idea of man holding a special +position in the universe. Not only ignorant laymen (especially several +theologians), but also a number of men of science, said very naively that +Darwinism in itself was not to be opposed; that it was quite right to use it to +explain the origin of the various species of plants and animals, but that it +was totally inapplicable to man. +</p> + +<p> +In the meantime, however, it seemed to a good many thoughtful people, laymen as +well as scientists, that this was wrong; that the descent of man from some +other animal species, and immediately from some ape-like mammal, followed +logically and necessarily from Darwin’s reformed theory of evolution. +Many of the acuter opponents of the theory saw at once the justice of this +position, and, as this consequence was intolerable, they wanted to get rid of +the whole theory. +</p> + +<p> +The first scientific application of the Darwinian theory to man was made by +Huxley, the greatest zoologist in England. This able and learned scientist, to +whom zoology owes much of its progress, published in 1863 a small work entitled +<i>Evidence as to Man’s Place in Nature.</i> In the extremely important +and interesting lectures which made up this work he proved clearly that the +descent of man from the ape followed necessarily from the theory of descent. If +that theory is true, we are bound to conceive the animals which most closely +resemble man as those from which humanity has been gradually evolved. About the +same time Carl Vogt published a larger work on the same subject. We must also +mention Gustav Jaeger and Friedrich Rolle among the zoologists who accepted and +taught the theory of evolution immediately after the publication of +Darwin’s book, and maintained that the descent of man from the lower +animals logically followed from it. The latter published, in 1866, a work on +the origin and position of man. +</p> + +<p> +About the same time I attempted, in the second volume of my <i>General +Morphology</i> (1866), to apply the theory of evolution to the whole organic +kingdom, including man.<a href="#linknote-11" name="linknoteref-11" id="linknoteref-11"><sup>[11]</sup></a> I endeavoured to sketch the probable +ancestral trees of the various classes of the animal world, the protists, and +the plants, as it seemed necessary to do on Darwinian principles, and as we can +actually do now with a high degree of confidence. If the theory of descent, +which Lamarck first clearly formulated and Darwin thoroughly established, is +true, we should be able to draw up a natural classification of plants and +animals in the light of their genealogy, and to conceive the large and small +divisions of +<span class='pagenum'><a name="Page_30" id="Page_30"></a></span> +the system as the branches and twigs of an ancestral tree. The eight +genealogical tables which I inserted in the second volume of the <i>General +Morphology</i> are the first sketches of their kind. In Chapter 27, +particularly, I trace the chief stages in man’s ancestry, as far as it is +possible to follow it through the vertebrate stem. I tried especially to +determine, as well as one could at that time, the position of man in the +classification of the mammals and its genealogical significance. I have greatly +improved this attempt, and treated it in a more popular form, in chaps. +xxvi–xxviii of my <i>History of Creation</i> (1868).<a href="#linknote-12" name="linknoteref-12" id="linknoteref-12"><sup>[12]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-11" id="linknote-11"></a> <a href="#linknoteref-11">[11]</a> +Huxley spoke of this “as one of the greatest scientific works ever +published.”—Translator. +</p> + +<p class="footnote"> +<a name="linknote-12" id="linknote-12"></a> <a href="#linknoteref-12">[12]</a> +Of which Darwin said that the <i>Descent of Man</i> would probably never have +been written if he had seen it earlier.—Translator. +</p> + +<p> +It was not until 1871, twelve years after the appearance of <i>The Origin of +Species,</i> that Darwin published the famous work which made the +much-contested application of his theory to man, and crowned the splendid +structure of his system. This important work was <i>The Descent of Man, and +Selection in Relation to Sex.</i> In this Darwin expressly drew the conclusion, +with rigorous logic, that man also must have been developed out of lower +species, and described the important part played by sexual selection in the +elevation of man and the other higher animals. He showed that the careful +selection which the sexes exercise on each other in regard to sexual relations +and procreation, and the æsthetic feeling which the higher animals develop +through this, are of the utmost importance in the progressive development of +forms and the differentiation of the sexes. The males choosing the handsomest +females in one class of animals, and the females choosing only the +finest-looking males in another, the special features and the sexual +characteristics are increasingly accentuated. In fact, some of the higher +animals develop in this connection a finer taste and judgment than man himself. +But, even as regards man, it is to this sexual selection that we owe the +family-life, which is the chief foundation of civilisation. The rise of the +human race is due for the most part to the advanced sexual selection which our +ancestors exercised in choosing their mates. +</p> + +<p> +Darwin accepted in the main the general outlines of man’s ancestral tree, +as I gave it in the <i>General Morphology</i> and the <i>History of +Creation,</i> and admitted that his studies led him to the same conclusion. +That he did not at once apply the theory to man in his first work was a +commendable piece of discretion; such a sequel was bound to excite the +strongest opposition to the whole theory. The first thing to do was to +establish it as regards the animal and plant worlds. The subsequent extension +to man was bound to be made sooner or later. +</p> + +<p> +It is important to understand this very clearly. If all living things come from +a common root, man must be included in the general scheme of evolution. On the +other hand, if the various species were separately created, man, too, must have +been created, and not evolved. We have to choose between these two +alternatives. This cannot be too frequently or too strongly emphasised. +<i>Either</i> all the species of animals and plants are of supernatural +origin—created, not evolved—and in that case man also is the +outcome of a creative act, as religion teaches, <i>or</i> the different species +have been evolved from a few common, simple ancestral forms, and in that case +man is the highest fruit of the tree of evolution. +</p> + +<p> +We may state this briefly in the following principle—<i>The descent of +man from the lower animals is a special deduction which inevitably follows from +the general inductive law of the whole theory of evolution.</i> In this +principle we have a clear and plain statement of the matter. Evolution is in +reality nothing but a great induction, which we are compelled to make by the +comparative study of the most important facts of morphology and physiology. But +we must draw our conclusion according to the laws of induction, and not attempt +to determine scientific truths by direct measurement and mathematical +calculation. In the study of living things we can scarcely ever directly and +fully, and with mathematical accuracy, determine the nature of phenomena, as is +done in the simpler study of the inorganic world—in chemistry, physics, +mineralogy, and astronomy. In the latter, especially, we can always use the +simplest and absolutely safest method—that of mathematical determination. +But in biology this is quite impossible for various reasons; one very obvious +reason being that most of the facts of the science are very complicated and +much too intricate to allow a direct mathematical analysis. The greater part of +the phenomena that biology deals with are +<span class='pagenum'><a name="Page_31" id="Page_31"></a></span> +complicated <i>historical processes,</i> which are related to a far-reaching +past, and as a rule can only be approximately estimated. Hence we have to +proceed by <i>induction</i>—that is to say, to draw general conclusions, +stage by stage, and with proportionate confidence, from the accumulation of +detailed observations. These inductive conclusions cannot command absolute +confidence, like mathematical axioms; but they approach the truth, and gain +increasing probability, in proportion as we extend the basis of observed facts +on which we build. The importance of these inductive laws is not diminished +from the circumstance that they are looked upon merely as temporary +acquisitions of science, and may be improved to any extent in the progress of +scientific knowledge. The same may be said of the attainments of many other +sciences, such as geology or archeology. However much they may be altered and +improved in detail in the course of time, these inductive truths may retain +their substance unchanged. +</p> + +<p> +Now, when we say that the theory of evolution in the sense of Lamarck and +Darwin is an inductive law—in fact, the greatest of all biological +inductions—we rely, in the first place, on the facts of paleontology. +This science gives us some direct acquaintance with the historical phenomena of +the changes of species. From the situations in which we find the fossils in the +various strata of the earth we gather confidently, in the first place, that the +living population of the earth has been gradually developed, as clearly as the +earth’s crust itself; and that, in the second place, several different +populations have succeeded each other in the various geological periods. Modern +geology teaches that the formation of the earth has been gradual, and unbroken +by any violent revolutions. And when we compare together the various kinds of +animals and plants which succeed each other in the history of our planet, we +find, in the first place, a constant and gradual increase in the number of +species from the earliest times until the present day; and, in the second +place, we notice that the forms in each great group of animals and plants also +constantly improve as the ages advance. Thus, of the vertebrates there are at +first only the lower fishes; then come the higher fishes, and later the +amphibia. Still later appear the three higher classes of vertebrates—the +reptiles, birds, and mammals, for the first time; only the lowest and least +perfect forms of the mammals are found at first; and it is only at a very late +period that placental mammals appear, and man belongs to the latest and +youngest branch of these. Thus perfection of form increases as well as variety +from the earliest to the latest stage. That is a fact of the greatest +importance. It can only be explained by the theory of evolution, with which it +is in perfect harmony. If the different groups of plants and animals do really +descend from each other, we must expect to find this increase in their number +and perfection under the influence of natural selection, just as the succession +of fossils actually discloses it to us. +</p> + +<p> +Comparative anatomy furnishes a second series of facts which are of great +importance for the forming of our inductive law. This branch of morphology +compares the adult structures of living things, and seeks in the great variety +of organic forms the stable and simple law of organisation, or the common type +or structure. Since Cuvier founded this science at the beginning of the +nineteenth century it has been a favourite study of the most distinguished +scientists. Even before Cuvier’s time Goethe had been greatly stimulated +by it, and induced to take up the study of morphology. Comparative osteology, +or the philosophic study and comparison of the bony skeleton of the +vertebrates—one of its most interesting sections—especially +fascinated him, and led him to form the theory of the skull which I mentioned +before. Comparative anatomy shows that the internal structure of the animals of +each stem and the plants of each class is the same in its essential features, +however much they differ in external appearance. Thus man has so great a +resemblance in the chief features of his internal organisation to the other +mammals that no comparative anatomist has ever doubted that he belongs to this +class. The whole internal structure of the human body, the arrangement of its +various systems of organs, the distribution of the bones, muscles, +blood-vessels, etc., and the whole structure of these organs in the larger and +the finer scale, agree so closely with those of the other mammals (such as the +apes, rodents, ungulates, cetacea, marsupials, etc.) that their external +differences are of no account whatever. We learn further from comparative +anatomy that the chief features of animal structure +<span class='pagenum'><a name="Page_32" id="Page_32"></a></span> +are so similar in the various classes (fifty to sixty in number altogether) +that they may all be comprised in from eight to twelve great groups. But even +in these groups, the stem-forms or animal types, certain organs (especially the +alimentary canal) can be proved to have been originally the same for all. We +can only explain by the theory of evolution this essential unity in internal +structure of all these animal forms that differ so much in outward appearance. +This wonderful fact can only be really understood and explained when we regard +the internal resemblance as an inheritance from common-stem forms, and the +external differences as the effect of adaptation to different environments. +</p> + +<p> +In recognising this, comparative anatomy has itself advanced to a higher stage. +Gegenbaur, the most distinguished of recent students of this science, says that +with the theory of evolution a new period began in comparative anatomy, and +that the theory in turn found a touch stone in the science. “Up to now +there is no fact in comparative anatomy that is inconsistent with the theory of +evolution; indeed, they all lead to it. In this way the theory receives back +from the science all the service it rendered to its method.” Until then +students had marvelled at the wonderful resemblance of living things in their +inner structure without being able to explain it. We are now in a position to +explain the causes of this, by showing that this remarkable agreement is the +necessary consequence of the inheriting of common stem-forms; while the +striking difference in outward appearance is a result of adaptation to changes +of environment. Heredity and adaptation alone furnish the true explanation. +</p> + +<p> +But one special part of comparative anatomy is of supreme interest and of the +utmost philosophic importance in this connection. This is the science of +rudimentary or useless organs; I have given it the name of +“dysteleology” in view of its philosophic consequences. Nearly +every organism (apart from the very lowest), and especially every +highly-developed animal or plant, including man, has one or more organs which +are of no use to the body itself, and have no share in its functions or vital +aims. Thus we all have, in various parts of our frame, muscles which we never +use, as, for instance, in the shell of the ear and adjoining parts. In most of +the mammals, especially those with pointed ears, these internal and external +ear-muscles are of great service in altering the shell of the ear, so as to +catch the waves of sound as much as possible. But in the case of man and other +short-eared mammals these muscles are useless, though they are still present. +Our ancestors having long abandoned the use of them, we cannot work them at all +to-day. In the inner corner of the eye we have a small crescent-shaped fold of +skin; this is the last relic of a third inner eye-lid, called the nictitating +(winking) membrane. This membrane is highly developed and of great service in +some of our distant relations, such as fishes of the shark type and several +other vertebrates; in us it is shrunken and useless. In the intestines we have +a process that is not only quite useless, but may be very harmful—the +vermiform appendage. This small intestinal appendage is often the cause of a +fatal illness. If a cherry-stone or other hard body is unfortunately squeezed +through its narrow aperture during digestion, a violent inflammation is set up, +and often proves fatal. This appendix has no use whatever now in our frame; it +is a dangerous relic of an organ that was much larger and was of great service +in our vegetarian ancestors. It is still large and important in many vegetarian +animals, such as apes and rodents. +</p> + +<p> +There are similar rudimentary organs in all parts of our body, and in all the +higher animals. They are among the most interesting phenomena to which +comparative anatomy introduces us; partly because they furnish one of the +clearest proofs of evolution, and partly because they most strikingly refute +the teleology of certain philosophers. The theory of evolution enables us to +give a very simple explanation of these phenomena. +</p> + +<p> +We have to look on them as organs which have fallen into disuse in the course +of many generations. With the decrease in the use of its function, the organ +itself shrivels up gradually, and finally disappears. There is no other way of +explaining rudimentary organs. Hence they are also of great interest in +philosophy; they show clearly that the <i>monistic</i> or mechanical view of +the organism is the only correct one, and that the <i>dualistic</i> or +teleological conception is wrong. The ancient legend of the direct creation of +man according to a pre-conceived plan and the empty phrases about +<span class='pagenum'><a name="Page_33" id="Page_33"></a></span> +“design” in the organism are completely shattered by them. It would +be difficult to conceive a more thorough refutation of teleology than is +furnished by the fact that all the higher animals have these rudimentary +organs. +</p> + +<p> +The theory of evolution finds its broadest inductive foundation in the natural +classification of living things, which arranges all the various forms in larger +and smaller groups, according to their degree of affinity. These groupings or +categories of classification—the varieties, species, genera, families, +orders, classes, etc.—show such constant features of coordination and +subordination that we are bound to look on them as <i>genealogical,</i> and +represent the whole system in the form of a branching tree. This is the +genealogical tree of the variously related groups; their likeness in form is +the expression of a real affinity. As it is impossible to explain in any other +way the natural tree-like form of the system of organisms, we must regard it at +once as a weighty proof of the truth of evolution. The careful construction of +these genealogical trees is, therefore, not an amusement, but the chief task of +modern classification. +</p> + +<p> +Among the chief phenomena that bear witness to the inductive law of evolution +we have the geographical distribution of the various species of animals and +plants over the surface of the earth, and their topographical distribution on +the summits of mountains and in the depths of the ocean. The scientific study +of these features—the “science of distribution,” or chorology +(<i>chora</i> = a place)—has been pursued with lively interest since the +discoveries made by Alexander von Humboldt. Until Darwin’s time the work +was confined to the determination of the facts of the science, and chiefly +aimed at settling the spheres of distribution of the existing large and small +groups of living things. It was impossible at that time to explain the causes +of this remarkable distribution, or the reasons why one group is found only in +one locality and another in a different place, and why there is this manifold +distribution at all. Here, again, the theory of evolution has given us the +solution of the problem. It furnishes the only possible explanation when it +teaches that the various species and groups of species descend from common +stem-forms, whose ever-branching offspring have gradually spread themselves by +migration over the earth. For each group of species we must admit a +“centre of production,” or common home; this is the original +habitat in which the ancestral form was developed, and from which its +descendants spread out in every direction. Several of these descendants became +in their turn the stem-forms for new groups of species, and these also +scattered themselves by active and passive migration, and so on. As each +migrating organism found a different environment in its new home, and adapted +itself to it, it was modified, and gave rise to new forms. +</p> + +<p> +This very important branch of science that deals with active and passive +migration was founded by Darwin, with the aid of the theory of evolution; and +at the same time he advanced the true explanation of the remarkable relation or +similarity of the living population in any locality to the fossil forms found +in it. Moritz Wagner very ably developed his idea under the title of “the +theory of migration.” In my opinion, this famous traveller has rather +over-estimated the value of his theory of migration when he takes it to be an +indispensable condition of the formation of new species and opposes the theory +of selection. The two theories are not opposed in their main features. +Migration (by which the stem-form of a new species is isolated) is really only +a special case of selection. The striking and interesting facts of chorology +can be explained only by the theory of evolution, and therefore we must count +them among the most important of its inductive bases. +</p> + +<p> +The same must be said of all the remarkable phenomena which we perceive in the +economy of the living organism. The many and various relations of plants and +animals to each other and to their environment, which are treated in +<i>bionomy</i> (from <i>nomos,</i> law or norm, and <i>bios,</i> life), the +interesting facts of parasitism, domesticity, care of the young, social habits, +etc., can only be explained by the action of heredity and adaptation. Formerly +people saw only the guidance of a beneficent Providence in these phenomena; +to-day we discover in them admirable proofs of the theory of evolution. It is +impossible to understand them except in the light of this theory and the +struggle for life. +</p> + +<p> +Finally, we must, in my opinion, count among the chief inductive bases of the +<span class='pagenum'><a name="Page_34" id="Page_34"></a></span> +theory of evolution the fœtal development of the individual organism, the whole +science of embryology or ontogeny. But as the later chapters will deal with +this in detail, I need say nothing further here. I shall endeavour in the +following pages to show, step by step, how the whole of the embryonic phenomena +form a massive chain of proof for the theory of evolution; for they can be +explained in no other way. In thus appealing to the close causal connection +between ontogenesis and phylogenesis, and taking our stand throughout on the +biogenetic law, we shall be able to prove, stage by stage, from the facts of +embryology, the evolution of man from the lower animals. +</p> + +<p> +The general adoption of the theory of evolution has definitely closed the +controversy as to the nature or definition of the species. The word has no +<i>absolute</i> meaning whatever, but is only a group-name, or category of +classification, with a purely relative value. In 1857, it is true, a famous and +gifted, but inaccurate and dogmatic, scientist, Louis Agassiz, attempted to +give an absolute value to these “categories of classification.” He +did this in his <i>Essay on Classification,</i> in which he turns upside down +the phenomena of organic nature, and, instead of tracing them to their natural +causes, examines them through a theological prism. The true species (<i>bona +species</i>) was, he said, an “incarnate idea of the Creator.” +Unfortunately, this pretty phrase has no more scientific value than all the +other attempts to save the absolute or intrinsic value of the species. +</p> + +<p> +The dogma of the fixity and creation of species lost its last great champion +when Agassiz died in 1873. The opposite theory, that all the different species +descend from common stem-forms, encounters no serious difficulty to-day. All +the endless research into the nature of the species, and the possibility of +several species descending from a common ancestor, has been closed to-day by +the removal of the sharp limits that had been set up between species and +varieties on the one hand, and species and genera on the other. I gave an +analytic proof of this in my monograph on the sponges (1872), having made a +very close study of variability in this small but highly instructive group, and +shown the impossibility of making any dogmatic distinction of species. +According as the classifier takes his ideas of genus, species, and variety in a +broader or in a narrower sense, he will find in the small group of the sponges +either one genus with three species, or three genera with 238 species, or 113 +genera with 591 species. Moreover, all these forms are so connected by +intermediate forms that we can convincingly prove the descent of all the +sponges from a common stem-form, the olynthus. +</p> + +<p> +Here, I think, I have given an analytic solution of the problem of the origin +of species, and so met the demand of certain opponents of evolution for an +actual instance of descent from a stem-form. Those who are not satisfied with +the synthetic proofs of the theory of evolution which are provided by +comparative anatomy, embryology, paleontology, dysteleology, chorology, and +classification, may try to refute the analytic proof given in my treatise on +the sponge, the outcome of five years of assiduous study. I repeat: It is now +impossible to oppose evolution on the ground that we have no convincing example +of the descent of all the species of a group from a common ancestor. The +monograph on the sponges furnishes such a proof, and, in my opinion, an +indisputable proof. Any man of science who will follow the protracted steps of +my inquiry and test my assertions will find that in the case of the sponges we +can follow the actual evolution of species in a concrete case. And if this is +so, if we can show the origin of all the species from a common form in one +single class, we have the solution of the problem of man’s origin, +because we are in a position to prove clearly his descent from the lower +animals. +</p> + +<p> +At the same time, we can now reply to the often-repeated assertion, even heard +from scientists of our own day, that the descent of man from the lower animals, +and proximately from the apes, still needs to be “proved with +certainty.” These “certain proofs” have been available for a +long time; one has only to open one’s eyes to see them. It is a mistake +to seek them in the discovery of intermediate forms between man and the ape, or +the conversion of an ape into a human being by skilful education. The proofs +lie in the great mass of empirical material we have already collected. They are +furnished in the strongest form by the data of comparative anatomy and +embryology, completed by paleontology. It is not a question now of detecting +new proofs of the evolution of man, but of examining +<span class='pagenum'><a name="Page_35" id="Page_35"></a></span> +and understanding the proofs we already have. +</p> + +<p> +I was almost alone thirty-six years ago when I made the first attempt, in my +<i>General Morphology,</i> to put organic science on a mechanical foundation +through Darwin’s theory of descent. The association of ontogeny and +phylogeny and the proof of the intimate causal connection between these two +sections of the science of evolution, which I expounded in my work, met with +the most spirited opposition on nearly all sides. The next ten years were a +terrible “struggle for life” for the new theory. But for the last +twenty-five years the tables have been turned. The phylogenetic method has met +with so general a reception, and found so prolific a use in every branch of +biology, that it seems superfluous to treat any further here of its validity +and results. The proof of it lies in the whole morphological literature of the +last three decades. But no other science has been so profoundly modified in its +leading thoughts by this adoption, and been forced to yield such far-reaching +consequences, as that science which I am now seeking to +establish—monistic anthropogeny. +</p> + +<p> +This statement may seem to be rather audacious, since the very next branch of +biology, anthropology in the stricter sense, makes very little use of these +results of anthropogeny, and sometimes expressly opposes them.<a href="#linknote-13" name="linknoteref-13" id="linknoteref-13"><sup>[13]</sup></a> This +applies especially to the attitude which has characterised the German +Anthropological Society (the <i>Deutsche Gesellschaft fur Anthropologie</i>) +for some thirty years. Its powerful president, the famous pathologist, Rudolph +Virchow, is chiefly responsible for this. Until his death (September 5th, 1902) +he never ceased to reject the theory of descent as unproven, and to ridicule +its chief consequence—the descent of man from a series of mammal +ancestors—as a fantastic dream. I need only recall his well-known +expression at the Anthropological Congress at Vienna in 1894, that “it +would be just as well to say man came from the sheep or the elephant as from +the ape.” +</p> + +<p class="footnote"> +<a name="linknote-13" id="linknote-13"></a> <a href="#linknoteref-13">[13]</a> +This does not apply to English anthropologists, who are almost all +evolutionists. +</p> + +<p> +Virchow’s assistant, the secretary of the German Anthropological Society, +Professor Johannes Ranke of Munich, has also indefatigably opposed +transformism: he has succeeded in writing a work in two volumes (<i>Der +Mensch</i>), in which all the facts relating to his organisation are explained +in a sense hostile to evolution. This work has had a wide circulation, owing to +its admirable illustrations and its able treatment of the most interesting +facts of anatomy and physiology—exclusive of the sexual organs! But, as +it has done a great deal to spread erroneous views among the general public, I +have included a criticism of it in my <i>History of Creation,</i> as well as +met Virchow’s attacks on anthropogeny. +</p> + +<p> +Neither Virchow, nor Ranke, nor any other “exact” anthropologist, +has attempted to give any other natural explanation of the origin of man. They +have either set completely aside this “question of questions” as a +transcendental problem, or they have appealed to religion for its solution. We +have to show that this rejection of the rational explanation is totally without +justification. The fund of knowledge which has accumulated in the progress of +biology in the nineteenth century is quite adequate to furnish a rational +explanation, and to establish the theory of the evolution of man on the solid +facts of his embryology. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap06"></a> +<span class='pagenum'><a name="Page_36" id="Page_36"></a></span> +Chapter VI.<br/> +THE OVUM AND THE AMŒBA</h2> + +<p> +In order to understand clearly the course of human embryology, we must select +the more important of its wonderful and manifold processes for fuller +explanation, and then proceed from these to the innumerable features of less +importance. The most important feature in this sense, and the best +starting-point for ontogenetic study, is the fact that man is developed from an +ovum, and that this ovum is a simple cell. The human ovum does not materially +differ in form and composition from that of the other mammals, whereas there is +a distinct difference between the fertilised ovum of the mammal and that of any +other animal. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus01"></a> +<img src="images/fig1.gif" width="209" height="86" alt="Fig.1 The human ovum" /> +<p class="caption">Fig. 1—<b>The human ovum.</b> The globular mass of +yelk (<i>b</i>) is enclosed by a transparent membrane (the ovolemma or zona +pellucida [<i>a</i>]), and contains a noncentral nucleus (the germinal vesicle, +<i>c</i>). Cf. Fig. 14.</p> +</div> + +<p>This fact is so important that few should be unaware of its +extreme significance; yet it was quite unknown in the first quarter of the +nineteenth century. As we have seen, the human and mammal ovum was not +discovered until 1827, when Carl Ernst von Baer detected it. Up to that time +the larger vesicles, in which the real and much smaller ovum is contained, had +been wrongly regarded as ova. The important circumstance that this mammal ovum +is a simple cell, like the ovum of other animals, could not, of course, be +recognised until the cell theory was established. This was not done, by +Schleiden for the plant and Schwann for the animal, until 1838. As we have +seen, this cell theory is of the greatest service in explaining the human frame +and its embryonic development. Hence we must say a few words about the actual +condition of the theory and the significance of the views it has suggested. +</p> + +<p> +In order properly to appreciate the cellular theory, the most important element +in our science, it is necessary to understand in the first place that the cell +is a <i>unified organism,</i> a self-contained living being. When we +anatomically dissect the fully-formed animal or plant into its various organs, +and then examine the finer structure of these organs with the microscope, we +are surprised to find that all these different parts are ultimately made up of +the same structural element or unit. This common unit of structure is the cell. +It does not matter whether we thus dissect a leaf, flower, or fruit, or a bone, +muscle, gland, or bit of skin, etc.; we find in every case the same ultimate +constituent, which has been called the cell since Schleiden’s discovery. +There are many opinions as to its real nature, but the essential point in our +view of the cell is to look upon it as a self-contained or independent living +unit. It is, in the words of Brucke, “an elementary organism.” We +may define it most precisely as the ultimate organic unit, and, as the cells +are the sole active principles in every vital function, we may call them the +“plastids,” or “formative elements.” This unity is +found in both the anatomic structure and the physiological function. In the +case of the protists, the entire organism usually consists of a single +independent cell throughout life. But in the tissue-forming animals and plants, +which are the great majority, the organism begins its career as a simple cell, +and then grows into a cell-community, or, more correctly, an organised +cell-state. Our own body is not really the simple unity that it is generally +supposed to be. On the contrary, it is a very elaborate social system of +countless microscopic organisms, a colony or commonwealth, made up of +innumerable independent units, or very different tissue-cells. +</p> + +<p> +<span class='pagenum'><a name="Page_37" id="Page_37"></a></span>In reality, the term “cell,” which existed long before the cell +theory was formulated, is not happily chosen. Schleiden, who first brought it +into scientific use in the sense of the cell theory, gave this name to the +elementary organisms because, when you find them in the dissected plant, they +generally have the appearance of chambers, like the cells in a bee-hive, with +firm walls and a fluid or pulpy content. But some cells, especially young ones, +are entirely without the enveloping membrane, or stiff wall. Hence we now +generally describe the cell as a living, viscous particle of protoplasm, +enclosing a firmer nucleus in its albuminoid body. There may be an enclosing +membrane, as there actually is in the case of most of the plants; but it may be +wholly lacking, as is the case with most of the animals. There is no membrane +at all in the first stage. The young cells are usually round, but they vary +much in shape later on. Illustrations of this will be found in the cells of the +various parts of the body shown in Figs. 3–7. +</p> + +<p> +Hence the essential point in the modern idea of the cell is that it is made up +of two different active constituents—an inner and an outer part. The +smaller and inner part is the nucleus (or <i>caryon</i> or <i>cytoblastus,</i> +Fig. 1<i>c</i> and Fig. 2<i>k</i>). The outer and larger part, which encloses +the other, is the body of the cell (<i>celleus, cytos,</i> or <i>cytosoma</i>). +The soft living substance of which the two are composed has a peculiar chemical +composition, and belongs to the group of the albuminoid plasma-substances +(“formative matter”), or protoplasm. The essential and +indispensable element of the nucleus is called nuclein (or caryoplasm); that of +the cell body is called plastin (or cytoplasm). In the most rudimentary cases +both substances seem to be quite simple and homogeneous, without any visible +structure. But, as a rule, when we examine them under a high power of the +microscope, we find a certain structure in the protoplasm. The chief and most +common form of this is the fibrous or net-like “thready structure” +(Frommann) and the frothy “honeycomb structure” (Bütschli). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus02"></a> +<img src="images/fig2.gif" width="155" height="138" alt="Fig.2 Stem-cell of one of the echinoderms" /> +<p class="caption">Fig. 2—<b>Stem-cell of one of the echinoderms</b> (cytula, or +“first segmentation-cell” = fertilised ovum), after <i>Hertwig. +k</i> is the nucleus or caryon.</p> +</div> + +<p> +The shape or outer form of the cell is infinitely varied, in accordance with +its endless power of adapting itself to the most diverse activities or +environments. In its simplest form the cell is globular (Fig. 2). This normal +round form is especially found in cells of the simplest construction, and those +that are developed in a free fluid without any external pressure. In such cases +the nucleus also is not infrequently round, and located in the centre of the +cell-body (Fig. 2<i>k</i>). In other cases, the cells have no definite shape; +they are constantly changing their form owing to their automatic movements. +This is the case with the amœbæ (Fig. 15 and 16) and the amœboid travelling +cells (Fig. 11), and also with very young ova (Fig. 13).However, as a rule, the +cell assumes a definite form in the course of its career. In the tissues of the +multicellular organism, in which a number of similar cells are bound together +in virtue of certain laws of heredity, the shape is determined partly by the +form of their connection and partly by their special functions. Thus, for +instance, we find in the mucous lining of our tongue very thin and delicate +flat cells of roundish shape (Fig. 3). In the outer skin we find similar, but +harder, covering cells, joined together by saw-like edges (Fig. 4). In the +liver and other glands there are thicker and softer cells, linked together in +rows (Fig. 5). +</p> + +<p> +The last-named tissues (Figs. 3–5) belong to the simplest and most +primitive type, the group of the “covering-tissues,” or epithelia. +In these “primary tissues” (to which the germinal layers belong) +simple cells of the same kind are arranged in layers. The arrangement and shape +are more complicated in the “secondary tissues,” which are +gradually developed out of the primary, as in the tissues of the muscles, +nerves, bones, etc. In the bones, for instance, which belong to the group of +supporting or connecting organs, +<span class='pagenum'><a name="Page_38" id="Page_38"></a></span> +the cells (Fig. 6) are star-shaped, and are joined together by numbers of +net-like interlacing processes; so, also, in the tissues of the teeth (Fig. 7), +and in other forms of supporting-tissue, in which a soft or hard substance +(intercellular matter, or base) is inserted between the cells. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus03"></a><a name="illus04"></a><a name="illus05"></a> +<img src="images/fig3.gif" width="431" height="167" alt="Fig.3 Three epithelial +cells. Fig. 4 Five spiny or grooved cells. Fig. 5 Ten liver-cells." /> +<p class="caption">Fig. 3—<b>Three epithelial cells</b> from the mucous +lining of the tongue.<br/> +Fig. 4—<b>Five spiny or grooved cells,</b> with edges joined, from the +outer skin (epidermis): one of them (<i>b</i>) is isolated.<br/> +Fig. 5—<b>Ten liver-cells:</b> one of them (<i>b</i>) has two nuclei.</p> +</div> + +<p> +The cells also differ very much in size. The great majority of them are +invisible to the naked eye, and can be seen only through the microscope (being +as a rule between 1/2500 and 1/250 inch in diameter). There are many of the +smaller plastids—such as the famous bacteria—which only come into +view with a very high magnifying power. On the other hand, many cells attain a +considerable size, and run occasionally to several inches in diameter, as do +certain kinds of rhizopods among the unicellular protists (such as the +radiolaria and thalamophora). Among the tissue-cells of the animal body many of +the muscular fibres and nerve fibres are more than four inches, and sometimes +more than a yard, in length. Among the largest cells are the yelk-filled ova; +as, for instance, the yellow “yolk” in the hen’s egg, which +we shall describe later (Fig. 15). +</p> + +<p> +Cells also vary considerably in structure. In this connection we must first +distinguish between the active and passive components of the cell. It is only +the former, or <i>active</i> parts of the cell, that really live, and effect +that marvellous world of phenomena to which we give the name of “organic +life.” The first of these is the inner nucleus (<i>caryoplasm</i>), and +the second the body of the cell (<i>cytoplasm</i>). The <i> passive</i> +portions come third; these are subsequently formed from the others, and I have +given them the name of “plasma-products.” They are partly external +(cell-membranes and intercellular matter) and partly internal (cell-sap and +cell-contents). +</p> + +<p> +The nucleus (or caryon), which is usually of a simple roundish form, is quite +structureless at first (especially in very young cells), and composed of +homogeneous nuclear matter or caryoplasm (Fig. 2<i>k</i>). But, as a rule, it +forms a sort of vesicle later on, in which we can distinguish a more solid +<i>nuclear base (caryobasis)</i> and a softer or fluid <i>nuclear sap +(caryolymph).</i> In a mesh of the nuclear network (or it may be on the inner +side of the nuclear envelope) there is, as a rule, a dark, very opaque, solid +body, called the <i>nucleolus.</i> Many of the nuclei contain several of these +nucleoli (as, for instance, the germinal vesicle of the ova of fishes and +amphibia). Recently a very small, but particularly important, part of the +nucleus has been distinguished as the <i>central body</i> (centrosoma)—a +tiny particle that is originally found in the nucleus itself, but is usually +outside it, in the cytoplasm; as a rule, fine threads stream out from it in the +cytoplasm. From the position of the central body with regard to the other parts +it seems probable that it has a high physiological importance as a centre of +movement; but it is lacking in many cells. +</p> + +<p> +The cell-body also consists originally, and in its simplest form, of a +homogeneous viscid plasmic matter. But, as a rule, +<span class='pagenum'><a name="Page_39" id="Page_39"></a></span> +only the smaller part of it is formed of the living active cell-substance +(protoplasm); the greater part consists of dead, passive plasma-products +(metaplasm). It is useful to distinguish between the inner and outer of these. +External plasma-products (which are thrust out from the protoplasm as solid +“structural matter”) are the cell-membranes and the intercellular +matter. The <i>internal</i> plasma-products are either the fluid cell-sap or +hard structures. As a rule, in mature and differentiated cells these various +parts are so arranged that the protoplasm (like the caryoplasm in the round +nucleus) forms a sort of skeleton or framework. The spaces of this network are +filled partly with the fluid cell-sap and partly by hard structural products. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus06"></a> +<img src="images/fig6.gif" width="287" height="251" alt="Fig.6 Nine +star-shaped bone cells." /> +<p class="caption">Fig. 6—<b>Nine star-shaped bone-cells,</b> with +interlaced branches.</p> +</div> + +<p> +The simple round ovum, which we take as the starting-point of our study (Figs. +1 and 2), has in many cases the vague, indifferent features of the typical +primitive cell. As a contrast to it, and as an instance of a very highly +differentiated plastid, we may consider for a moment a large nerve-cell, or +ganglionic cell, from the brain. The ovum stands potentially for the entire +organism—in other words, it has the faculty of building up out of itself +the whole multicellular body. It is the common parent of all the countless +generations of cells which form the different tissues of the body; it unites +all their powers in itself, though only potentially or in germ. In complete +contrast to this, the neural cell in the brain (Fig. 9) develops along one +rigid line. It cannot, like the ovum, beget endless generations of cells, of +which some will become skin-cells, others muscle-cells, and others again +bone-cells. But, on the other hand, the nerve-cell has become fitted to +discharge the highest functions of life; it has the powers of sensation, will, +and thought. It is a real soul-cell, or an elementary organ of the psychic +activity. It has, therefore, a most elaborate and delicate structure. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus07"></a> +<img src="images/fig7.gif" width="203" height="154" alt="Fig.7 Eleven star-shaped cells." /> +<p class="caption">Fig. 7—<b>Eleven star-shaped cells</b> from the enamel +of a tooth, joined together by their branchlets.</p> +</div> + +<p> Numbers of extremely fine threads, like the electric wires at a +large telegraphic centre, cross and recross in the delicate protoplasm of the +nerve cell, and pass out in the branching processes which proceed from it and +put it in communication with other nerve-cells or nerve-fibres (<i>a, b</i>). +We can only partly follow their intricate paths in the fine matter of the body +of the cell. +</p> + +<p> +Here we have a most elaborate apparatus, the delicate structure of which we are +just beginning to appreciate through our most powerful microscopes, but whose +significance is rather a matter of +<span class='pagenum'><a name="Page_40" id="Page_40"></a></span> +conjecture than knowledge. Its intricate structure corresponds to the very +complicated functions of the mind. Nevertheless, this elementary organ of +psychic activity—of which there are thousands in our brain—is +nothing but a single cell. Our whole mental life is only the joint result of +the combined activity of all these nerve-cells, or soul-cells. In the centre of +each cell there is a large transparent nucleus, containing a small and dark +nuclear body. Here, as elsewhere, it is the nucleus that determines the +individuality of the cell; it proves that the whole structure, in spite of its +intricate composition, amounts to only a single cell. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus08"></a> +<img src="images/fig8.gif" width="216" height="145" alt="Fig.8 Unfertilised +ovum of an echinoderm." /> +<p class="caption">Fig. 8—<b>Unfertilised ovum of an echinoderm</b> (from +<i>Hertwig</i>). The vesicular nucleus (or “germinal vesicle”) is +globular, half the size of the round ovum, and encloses a nuclear framework, in +the central knot of which there is a dark nucleolus (the “germinal +spot”).</p> +</div> + +<p> +In contrast with this very elaborate and very strictly differentiated psychic +cell (Fig. 9), we have our ovum (Figs. 1 and 2), which has hardly any structure +at all. But even in the case of the ovum we must infer from its properties that +its protoplasmic body has a very complicated chemical composition and a fine +molecular structure which escapes our observation. This presumed molecular +structure of the plasm is now generally admitted; but it has never been seen, +and, indeed, lies far beyond the range of microscopic vision. It must not be +confused—as is often done—with the structure of the plasm (the +fibrous network, groups of granules, honey-comb, etc.) which does come within +the range of the microscope. +</p> + +<p> +But when we speak of the cells as the elementary organisms, or structural +units, or “ultimate individualities,” we must bear in mind a +certain restriction of the phrases. I mean, that the cells are not, as is often +supposed, the very lowest stage of organic individuality. There are yet more +elementary organisms to which I must refer occasionally. These are what we call +the “cytodes” (<i>cytos</i> = cell), certain living, independent +beings, consisting only of a particle of <i> plasson</i>—an albuminoid +substance, which is not yet differentiated into caryoplasm and cytoplasm, but +combines the properties of both. Those remarkable beings called the <i> +monera</i>—especially the chromacea and bacteria—are specimens of +these simple cytodes. (Compare Chapter XIX.) To be quite accurate, then, we +must say: the elementary organism, or the ultimate individual, is found in two +different stages. The first and lower stage is the cytode, which consists +merely of a particle of plasson, or quite simple plasm. The second and higher +stage is the cell, which is already divided or differentiated into nuclear +matter and cellular matter. We comprise both kinds—the cytodes and the +cells—under the name of <i>plastids</i> (“formative +particles”), because they are the real builders of the organism. However, +these cytodes are not found, as a rule, in the higher animals and plants; here +we have only real cells with a nucleus. Hence, in these tissue-forming +organisms (both plant and animal) the organic unit always consists of two +chemically and anatomically different parts—the outer cell-body and the +inner nucleus. +</p> + +<p> +In order to convince oneself that this cell is really an independent organism, +we have only to observe the development and vital phenomena of one of them. We +see then that it performs all the essential functions of life—both +vegetal and animal—which we find in the entire organism. Each of these +tiny beings grows and nourishes itself independently. It takes its food from +the surrounding fluid; sometimes, even, the naked cells take in solid particles +at certain points of their surface—in other words, “eat” +them—without needing any special mouth and stomach for the purpose (cf. +Fig. 19). +</p> + +<p> +Further, each cell is able to reproduce itself. This multiplication, in most +cases, takes the form of a simple cleavage, sometimes direct, sometimes +indirect; the simple direct (or “amitotic”) division is less +common, and is found, for instance, in the blood cells (Fig. 10). In these the +nucleus first divides into two equal parts by constriction. The indirect (or +“mitotic”) +<span class='pagenum'><a name="Page_41" id="Page_41"></a></span> +<span class='pagenum'><a name="Page_42" id="Page_42"></a></span> +cleavage is much more frequent; in this the caryoplasm of the nucleus and the +cytoplasm of the cell-body act upon each other in a peculiar way, with a +partial dissolution (<i>caryolysis</i>), the formation of knots and loops +(<i>mitosis</i>), and a movement of the halved plasma-particles towards two +mutually repulsive poles of attraction (<i>caryokinesis,</i> Fig. 11.) +</p> + +<div class="fig" style="width:100%;"> +<a name="illus09"></a> +<img src="images/fig9.gif" width="328" height="556" alt="Fig.9 A large branching nerve-cell" /> +<p class="caption">Fig. 9—<b>A large branching nerve-cell, or +“soul-cell”,</b> from the brain of an electric fish +(<i>Torpedo</i>). In the middle of the cell is the large transparent round +<i>nucleus,</i> one <i>nucleolus,</i> and, within the latter again, a +<i>nucleolinus.</i> The protoplasm of the cell is split into innumerable fine +threads (or fibrils), which are embedded in intercellular matter, and are +prolonged into the branching processes of the cell (<i>b</i>). One branch +(<i>a</i>) passes into a nerve-fibre. (From <i>Max Schultze.</i>)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus10"></a> +<img src="images/fig10.gif" width="120" height="145" alt="Fig.10 Blood-cells, multiplying by direct division" /> +<p class="caption">Fig. 10—<b>Blood-cells, multiplying by direct +division,</b> from the blood of the embryo of a stag. Originally, each +blood-cell has a nucleus and is round (<i>a</i>). When it is going to multiply, +the nucleus divides into two (<i>b, c, d</i>). Then the protoplasmic body is +constricted between the two nuclei, and these move away from each other +(<i>e</i>). Finally, the constriction is complete, and the cell splits into two +daughter-cells (<i>f</i>). (From <i>Frey.</i>)</p> +</div> + +<p> +The intricate physiological processes which accompany this +“mitosis” have been very closely studied of late years. The inquiry +has led to the detection of certain laws of evolution which are of extreme +importance in connection with heredity. As a rule, two very different parts of +the nucleus play an important part in these changes. They are: the +<i>chromatin,</i> or coloured nuclear substance, which has a peculiar property +of tingeing itself deeply with certain colouring matters (carmine, hæmatoxylin, +etc.), and the <i>achromin</i> (or <i>linin,</i> or <i> achromatin</i>), a +colourless nuclear substance that lacks this property. The latter generally +forms in the dividing cell a sort of spindle, at the poles of which there is a +very small particle, also colourless, called the “central body” +(<i>centrosoma</i>). This acts as the centre or focus in a “sphere of +attraction” for the granules of protoplasm in the surrounding cell-body, +and assumes a star-like appearance (the cell-star, or <i>monaster</i>). The two +central bodies, standing opposed to each other at the poles of the nuclear +spindle, form “the double-star” (or <i>amphiaster,</i> Fig. 11, B +C). The chromatin often forms a long, irregularly-wound thread—“the +coil” (<i>spirema,</i> Fig. A). At the commencement of the cleavage it +gathers at the equator of the cell, between the stellar poles, and forms a +crown of U-shaped loops (generally four or eight, or some other definite +number). The loops split lengthwise into two halves (B), and these back away +from each other towards the poles of the spindle (C). Here each group forms a +crown once more, and this, with the corresponding half of the divided spindle, +forms a fresh nucleus (D). Then the protoplasm of the cell-body begins to +contract in the middle, and gather about the new daughter-nuclei, and at last +the two daughter-cells become independent beings. +</p> + +<p> +Between this common mitosis, or <i>indirect</i> cell-division—which is +the normal cleavage-process in most cells of the higher animals and +plants—and the simple <i> direct</i> division (Fig. 10) we find every +grade of segmentation; in some circumstances even one kind of division may be +converted into another. +</p> + +<p> +The plastid is also endowed with the functions of movement and sensation. The +single cell can move and creep about, when it has space for free movement and +is not prevented by a hard envelope; it then thrusts out at its surface +processes like fingers, and quickly withdraws them again, and thus changes its +shape (Fig. 12). Finally, the young cell is sensitive, or more or less +responsive to stimuli; it makes certain movements on the application of +chemical and mechanical irritation. Hence we can ascribe to the individual cell +all the chief functions which we comprehend under the general heading of +“life”—sensation, movement, nutrition, and reproduction. All +these properties of the multicellular and highly developed animal are also +found in the single animal-cell, at least in its younger stages. There is no +longer any doubt about this, and so we may regard it as a solid and important +base of our physiological conception of the elementary organism. +</p> + +<p> +Without going any further here into these very interesting phenomena of the +life of the cell, we will pass on to consider the application of the cell +theory to the ovum. Here comparative research yields the important result that +<i>every ovum is at first a simple cell.</i> I say this is very important, +because our whole science of embryology now resolves itself into the problem: +“How does the multicellular +<span class='pagenum'><a name="Page_43" id="Page_43"></a></span> +organism arise from the unicellular?” Every organic individual is at +first a simple cell, and as such an elementary organism, or a unit of +individuality. This cell produces a cluster of cells by segmentation, and from +these develops the multicellular organism, or individual of higher rank. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus11"></a> +<img src="images/fig11.gif" width="130" height="460" alt="Fig. 11 Indirect or +mitotic cell-division." /> +<p class="caption"><b>A. Mother-cell</b><br/> (Knot, spirema)<br/> 1. <b>Nuclear +threads</b> (chromosomata) (coloured nuclear matter, chromatin)<br/> 2. Nuclear +membrane<br/> 3. Nuclear sap<br/> 4. Cytosoma<br/> 5. Protoplasm of the +cell-body<br/> <br/> <br/> <b>B. Mother-star,</b> the loops beginning to split +lengthways (nuclear membrane gone)<br/> 1. Star-like appearance in +cytoplasm<br/> 2. Centrosoma (sphere of attraction)<br/> 3. Nuclear spindle +(achromin, colourless matter)<br/> 4. Nuclear loops (chromatin, coloured +matter)<br/> <br/> <br/> <br/> <b>C. The two daughter-stars,</b><br/> produced +by the breaking of the loops of the mother-star (moving away)<br/> 1. Upper +daughter-crown<br/> 2. Connecting threads of the two crowns (achromin)<br/> 3. +Lower daughter-crown<br/> 4. Double-star (amphiaster)<br/> <br/> <br/> <br/> +<b>D. The two daughter-cells,</b><br/> produced by the complete division of the +two nuclear halves (cytosomata still connected at the equator) (Double-knot, +Dispirema)<br/> 1. Upper daughter-nucleus<br/> 2. Equatorial constriction of +the cell-body<br/> 3. Lower daughter-nucleus.</p> +<p class="caption">Fig. 11—<b>Indirect or mitotic +cell-division</b> (with caryolysis and caryokinesis) from the skin of the larva +of a salamander. (From <i>Rabl.</i>).</p> +</div> + +<p> +When we examine a little closer the original features of the ovum, we notice +the extremely significant fact that in its first stage the ovum is just the +same simple and indefinite structure in the case of man and all the animals +(Fig. 13). We are unable to detect any material difference between them, either +in outer shape or internal constitution. Later, though the ova remain +unicellular, they differ in size and shape, enclose various kinds of +yelk-particles, have different envelopes, and so on. But when we examine them +at their birth, in the ovary of the female animal, we find them to be always of +the same form in the first stages of their life. In the beginning each ovum is +a very simple, roundish, naked, mobile cell, without a membrane; it consists +merely of a particle of cytoplasm enclosing a nucleus (Fig. 13). Special names +have been given to these parts of the ovum; the cell-body is called the +<i>yelk</i> (<i>vitellus</i>), and the cell-nucleus the <i>germinal +vesicle.</i> As a rule, the +<span class='pagenum'><a name="Page_44" id="Page_44"></a></span> +nucleus of the ovum is soft, and looks like a small pimple or vesicle. Inside +it, as in many other cells, there is a nuclear skeleton or frame and a third, +hard nuclear body (the <i> nucleolus</i>). In the ovum this is called the +<i>germinal spot.</i> Finally, we find in many ova (but not in all) a still +further point within the germinal spot, a “nucleolin,” which goes +by the name of the germinal point. The latter parts (germinal spot and germinal +point) have, apparently, a minor importance, in comparison with the other two +(the yelk and germinal vesicle). In the yelk we must distinguish the active +<i>formative yelk</i> (or protoplasm = first plasm) from the passive <i> +nutritive yelk</i> (or deutoplasm = second plasm). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus12"></a> +<img src="images/fig12.gif" width="180" height="181" alt="Fig.12 Mobile +cells from the inflamed eye of a frog." /> +<p class="caption">Fig. 12—<b>Mobile cells from the +inflamed eye of a frog</b> (from the watery fluid of the eye, the <i>humor +aqueus</i>). The naked cells creep freely about, by (like the amœba or +rhizopods) protruding fine processes from the uncovered protoplasmic body. +These bodies vary continually in number, shape, and size. The nucleus of these +amœboid lymph-cells (“travelling cells,” or planocytes) is +invisible, because concealed by the numbers of fine granules which are +scattered in the protoplasm. (From <i>Frey.</i>)</p> +</div> + +<p> +In many of the lower animals (such as sponges, polyps, and medusæ) the naked +ova retain their original simple appearance until impregnation. But in most +animals they at once begin to change; the change consists partly in the +formation of connections with the yelk, which serve to nourish the ovum, and +partly of external membranes for their protection (the ovolemma, or +prochorion). A membrane of this sort is formed in all the mammals in the course +of the embryonic process. The little globule is surrounded by a thick capsule +of glass-like transparency, the <i> zona pellucida,</i> or <i>ovolemma +pellucidum</i> (Fig. 14). When we examine it closely under the microscope, we +see very fine radial streaks in it, piercing the <i> zona,</i> which are really +very narrow canals. The human ovum, whether fertilised or not, cannot be +distinguished from that of most of the other mammals. It is nearly the same +everywhere in form, size, and composition. When it is fully formed, it has a +diameter of (on an average) about 1/120 of an inch. When the mammal ovum has +been carefully isolated, and held against the light on a glass-plate, it may be +seen as a fine point even with the naked eye. The ova of most of the higher +mammals are about the same size. The diameter of the ovum is almost always +between 1/250 to 1/125 inch. It has always the same globular shape; the same +characteristic membrane; the same transparent germinal vesicle with its dark +germinal spot. Even when we use the most powerful microscope with its highest +power, we can detect no material difference between the ova of man, the ape, +the dog, and so on. I do not mean to say that there are no differences between +the ova of these different mammals. On the contrary, we are bound to assume +that there are such, at least as regards chemical composition. Even the ova of +different men must differ from each other; otherwise we should not have a +different individual from each ovum. It is true that our crude and imperfect +apparatus cannot detect these subtle individual differences, which are probably +in the molecular structure. However, such a striking resemblance of their ova +in form, so great as to seem to be a complete similarity, is a strong proof of +the common parentage of man and the other mammals. From the common germ-form we +infer a common stem-form. On the other hand, there are striking peculiarities +by which we can easily distinguish the fertilised ovum of the mammal from the +fertilised ovum of the birds, amphibia, fishes, and other vertebrates (see the +close of Chap. XXIX). +</p> + +<p> +The fertilised bird-ovum (Fig. 15) is notably different. It is true that in its +earliest stage (Fig. 13 E) this ovum also is very like that of the mammal (Fig. +13 F). But afterwards, while still within the oviduct, it takes up a quantity +of nourishment and works this into the familiar large yellow yelk. When we +examine a very young ovum in the hen’s oviduct, we +<span class='pagenum'><a name="Page_45" id="Page_45"></a></span> +find it to be a simple, small, naked, amœboid cell, just like the young ova of +other animals (Fig. 13). But it then grows to the size we are familiar with in +the round yelk of the egg. The nucleus of the ovum, or the germinal vesicle, is +thus pressed right to the surface of the globular ovum, and is embedded there +in a small quantity of transparent matter, the so-called white yelk. This forms +a round white spot, which is known as the “tread” +(<i>cicatricula</i>) (Fig. 15 <i>b</i>). From the tread a thin column of the +white yelk penetrates through the yellow yelk to the centre of the globular +cell, where it swells into a small, central globule (wrongly called the +yelk-cavity, or <i>latebra,</i> Fig. 15 <i>d′</i>). The yellow +yelk-matter which surrounds this white yelk has the appearance in the egg (when +boiled hard) of concentric layers (<i>c</i>). The yellow yelk is also enclosed +in a delicate structureless membrane (the <i>membrana vitellina, a</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus13"></a> +<img src="images/fig13.gif" width="274" height="363" alt="Fig.13 Ova of +various animals, executing amœboid movements." /> +<p class="caption">Fig. 13—<b>Ova of various animals, executing +amœboid movements,</b> magnified. All the ova are naked cells of varying shape. +In the dark fine-grained protoplasm (yelk) is a large vesicular nucleus (the +germinal vesicle), and in this is seen a nuclear body (the germinal spot), in +which again we often see a germinal point. Figs. <i>A1–A4</i> represent +the ovum of a sponge (<i>Leuculmis echinus</i>) in four successive movements. +<i>B1–B8</i> are the ovum of a parasitic crab (<i>Chondracanthus +cornutus</i>), in eight successive movements. (From <i>Edward von Beneden.</i>) +<i>C1–C5</i> show the ovum of the cat in various stages of movement (from +<i> Pflüger</i>); Fig. <i>D</i> the ovum of a trout; <i>E</i> the ovum of a +chicken; <i>F</i> a human ovum.</p> +</div> + +<p> +As the large yellow ovum of the bird +<span class='pagenum'><a name="Page_46" id="Page_46"></a></span> +attains a diameter of several inches in the bigger birds, and encloses round +yelk-particles, there was formerly a reluctance to consider it as a simple +cell. This was a mistake. Every animal that has only one cell-nucleus, every +amœba, every gregarina, every infusorium, is unicellular, and remains +unicellular whatever variety of matter it feeds on. So the ovum remains a +simple cell, however much yellow yelk it afterwards accumulates within its +protoplasm. It is, of course, different, with the bird’s egg when it has +been fertilised. The ovum then consists of as many cells as there are nuclei in +the tread. Hence, in the fertilised egg which we eat daily, the yellow yelk is +already a multicellular body. Its tread is composed of several cells, and is +now commonly called the <i>germinal disc.</i> We shall return to this +<i>discogastrula</i> in Chap. IX. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus14"></a> +<img src="images/fig14.gif" width="231" height="230" alt="Fig.14 The +human ovum." /> +<p class="caption">Fig. 14—<b>The human ovum,</b> taken from the female +ovary, magnified. The whole ovum is a simple round cell. The chief part of the +globular mass is formed by the nuclear yelk (<i>deutoplasm</i>), which is +evenly distributed in the active protoplasm, and consists of numbers of fine +yelk-granules. In the upper part of the yelk is the transparent round germinal +vesicle, which corresponds to the <i> nucleus.</i> This encloses a darker +granule, the germinal spot, which shows a <i>nucleolus.</i> The globular yelk +is surrounded by the thick transparent germinal membrane (<i>ovolemma,</i> or +<i> zona pellucida</i>). This is traversed by numbers of lines as fine as +hairs, which are directed radially towards the centre of the ovum. These are +called the pore-canals; it is through these that the moving spermatozoa +penetrate into the yelk at impregnation.</p> +</div> + +<p> +When the mature bird-ovum has left the ovary and been fertilised in the +oviduct, it covers itself with various membranes which are secreted from the +wall of the oviduct. First, the large clear albuminous layer is deposited +around the yellow yelk; afterwards, the hard external shell, with a fine inner +skin. All these gradually forming envelopes and processes are of no importance +in the formation of the embryo; they serve merely for the protection of the +original simple ovum. We sometimes find extraordinarily large eggs with strong +envelopes in the case of other animals, such as fishes of the shark type. Here, +also, the ovum is originally of the same character as it is in the mammal; it +is a perfectly simple and naked cell. But, as in the case of the bird, a +considerable quantity of nutritive yelk is accumulated inside the original yelk +as food for the developing embryo; and various coverings are formed round the +egg. The ovum of many other animals has the same internal and external +features. They have, however, only a physiological, not a morphological, +importance; they have no direct influence on the formation of the fœtus. They +are partly consumed as food by the embryo, and partly serve as protective +envelopes. Hence we may leave them out of consideration altogether here, and +restrict ourselves to material points—<i>to the substantial identity of +the original ovum in man and the rest of the animals</i> (Fig. 13). +</p> + +<p> +Now, let us for the first time make use of our biogenetic law; and directly +apply this fundamental law of evolution to the human ovum. We reach a very +simple, but very important, conclusion. <i> From</i> +<span class='pagenum'><a name="Page_47" id="Page_47"></a></span> +<i>the fact that the human ovum and that of all other animals consists of a +single cell, it follows immediately, according to the biogenetic law, that all +the animals, including man, descend from a unicellular organism.</i> If our +biogenetic law is true, if the embryonic development is a summary or condensed +recapitulation of the stem-history—and there can be no doubt about +it—we are bound to conclude, from the fact that all the ova are at first +simple cells, that all the multicellular organisms originally sprang from a +unicellular being. And as the original ovum in man and all the other animals +has the same simple and indefinite appearance, we may assume with some +probability that this unicellular stem-form was the common ancestor of the +whole animal world, including man. However, this last hypothesis does not seem +to me as inevitable and as absolutely certain as our first conclusion. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus15"></a> +<img src="images/fig15.gif" width="236" height="134" alt="Fig.15 A fertilised ovum from the oviduct of a hen" /> +<p class="caption">Fig. 15—<b>A fertilised ovum from the oviduct of a +hen.</b> The yellow yelk (<i>c</i>) consists of several concentric layers +(<i>d</i>), and is enclosed in a thin yelk-membrane (<i>a</i>). The nucleus or +germinal vesicle is seen above in the cicatrix or “tread” +(<i>b</i>). From that point the white yelk penetrates to the central +yelk-cavity (<i>d′</i>). The two kinds of yelk do not differ very much.</p> +</div> + +<p>This inference from the unicellular embryonic form to the +unicellular ancestor is so simple, but so important, that we cannot +sufficiently emphasise it. We must, therefore, turn next to the question +whether there are to-day any unicellular organisms, from the features of which +we may draw some approximate conclusion as to the unicellular ancestors of the +multicellular organisms. The answer is: Most certainly there are. There are +assuredly still unicellular organisms which are, in their whole nature, really +nothing more than permanent ova. There are independent unicellular organisms of +the simplest character which develop no further, but reproduce themselves as +such, without any further growth. We know to-day of a great number of these +little beings, such as the gregarinæ, flagellata, acineta, infusoria, etc. +However, there is one of them that has an especial interest for us, because it +at once suggests itself when we raise our question, and it must be regarded as +the unicellular being that approaches nearest to the real ancestral form. This +organism is the <i>Amœba.</i> +</p> + +<div class="fig" style="width:100%;"> +<a name="illus16"></a> +<img src="images/fig16.gif" width="221" height="155" alt="Fig.16 A creeping amœba." /> +<p class="caption">Fig. 16—<b>A creeping amœba</b> (highly magnified). +The whole organism is a simple naked cell, and moves about by means of the +changing arms which it thrusts out of and withdraws into its protoplasmic body. +Inside it is the roundish nucleus with its nucleolus.</p> +</div> + +<p>For a long time now we have comprised under the general name of +amœbæ a number of microscopic unicellular organisms, which are very widely +distributed, especially in fresh-water, but also in the ocean; in fact, they +have lately been discovered in damp soil. There are also parasitic amœbæ which +live inside other animals. When we place one of these amœbæ in a drop of water +under the microscope and examine it with a high power, it generally appears as +a roundish particle of a very irregular and varying shape (Figs. 16 and 17). In +its soft, slimy, semi-fluid substance, which consists of protoplasm, we see +only the solid globular particle it contains, the nucleus. This unicellular +body moves about continually, creeping in every direction on the glass on which +we are examining it. The movement is effected by the shapeless body thrusting +out finger-like processes at various parts of its surface; and these are slowly +but continually changing, and drawing the rest of the body after them. After a +time, perhaps, the action changes. The amœba suddenly stands still, withdraws +its projections, and assumes a globular shape. In a little while, however, the +round body begins to expand again, thrusts out arms in another +<span class='pagenum'><a name="Page_48" id="Page_48"></a></span> +direction, and moves on once more. These changeable processes are called +“false feet,” or pseudopodia, because they act physiologically as +feet, yet are not special organs in the anatomic sense. They disappear as +quickly as they come, and are nothing more than temporary projections of the +semi-fluid and structureless body. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus17"></a> +<img src="images/fig17.gif" width="276" height="303" alt="Fig.17 Division +of a unicellular amœba." /> +<p class="caption">Fig. 17—<b>Division of a +unicellular amœba</b> (<i>Amœba polypodia</i>) in six stages. (From <i>F. E. +Schultze.</i>) the dark spot is the nucleus, the lighter spot a contractile +vacuole in the protoplasm. The latter reforms in one of the daughter-cells.)</p> +</div> + +<p> +If you touch one of these creeping amœbæ with a needle, or put a drop of acid +in the water, the whole body at once contracts in consequence of this +mechanical or physical stimulus. As a rule, the body then resumes its globular +shape. In certain circumstances—for instance, if the impurity of the +water lasts some time—the amœba begins to develop a covering. It exudes a +membrane or capsule, which immediately hardens, and assumes the appearance of a +round cell with a protective membrane. The amœba either takes its food directly +by imbibition of matter floating in the water, or by pressing into its +protoplasmic body solid particles with which it comes in contact. The latter +process may be observed at any moment by forcing it to eat. If finely ground +colouring matter, such as carmine or indigo, is put into the water, you can see +the body of the amœba pressing these coloured particles into itself, the +substance of the cell closing round them. The amœba can take in food in this +way at any point on its surface, without having any special organs for +intussusception and digestion, or a real mouth or gut. +</p> + +<p> +The amœba grows by thus taking in food and dissolving the particles eaten in +its protoplasm. When it reaches a certain size by this continual feeding, it +begins to reproduce. This is done by the simple process of cleavage (Fig. 17). +First, the nucleus divides into two parts. Then the protoplasm is separated +between the two new nuclei, and the whole cell splits into two daughter-cells, +the protoplasm gathering about each of the nuclei. The thin bridge of +protoplasm which at first connects the daughter-cells soon breaks. Here we have +the simple form of direct cleavage of the nuclei. Without mitosis, or formation +of threads, the homogeneous nucleus divides into two halves. These move away +from each other, and become centres of attraction for the enveloping matter, +the protoplasm. The same direct cleavage of the nuclei is also witnessed in the +reproduction of many other protists, while other unicellular organisms show the +indirect division of the cell. +</p> + +<p> +Hence, although the amœba is nothing but a simple cell, it is evidently able to +accomplish all the functions of the multicellular organism. It moves, feels, +nourishes itself, and reproduces. Some kinds of these amœbæ can be seen with +the naked eye, but most of them are microscopically small. It is for the +following reasons that we regard the amœbæ as the unicellular organisms which +have +<span class='pagenum'><a name="Page_49" id="Page_49"></a></span> +special phylogenetic (or evolutionary) relations to the ovum. In many of the +lower animals the ovum retains its original naked form until fertilisation, +develops no membranes, and is then often indistinguishable from the ordinary +amœba. Like the amœbæ, these naked ova may thrust out processes, and move about +as travelling cells. In the sponges these mobile ova move about freely in the +maternal body like independent amœbæ (Fig. 17). They had been observed by +earlier scientists, but described as foreign bodies—namely, parasitic +amœbæ, living parasitically on the body of the sponge. Later, however, it was +discovered that they were not parasites, but the ova of the sponge. We also +find this remarkable phenomenon among other animals, such as the graceful, +bell-shaped zoophytes, which we call polyps and medusæ. Their ova remain naked +cells, which thrust out amœboid projections, nourish themselves, and move +about. When they have been fertilised, the multicellular organism is formed +from them by repeated segmentation. +</p> + +<p> +It is, therefore, no audacious hypothesis, but a perfectly sound conclusion, to +regard the amœba as the particular unicellular organism which offers us an +approximate illustration of the ancient common unicellular ancestor of all the +metazoa, or multicellular animals. The simple naked amœba has a less definite +and more original character than any other cell. Moreover, there is the fact +that recent research has discovered such amœba-like cells everywhere in the +mature body of the multicellular animals. They are found, for instance, in the +human blood, side by side with the red corpuscles, as colourless blood-cells; +and it is the same with all the vertebrates. They are also found in many of the +invertebrates—for instance, in the blood of the snail. I showed, in 1859, +that these colourless blood-cells can, like the independent amœbæ, take up +solid particles, or “eat” (whence they are called <i>phagocytes</i> += “eating-cells,” Fig. 19). Lately, it has been discovered that +many different cells may, if they have room enough, execute the same movements, +creeping about and eating. They behave just like amœbæ (Fig. 12). It has also +been shown that these “travelling-cells,” or <i>planocytes,</i> +play an important part in man’s physiology and pathology (as means of +transport for food, infectious matter, bacteria, etc.). +</p> + +<p> +The power of the naked cell to execute these characteristic amœba-like +movements comes from the contractility (or automatic mobility) of its +protoplasm. This seems to be a universal property of young cells. When they are +not enclosed by a firm membrane, or confined in a “cellular +prison,” they can always accomplish these amœboid movements. This is true +of the naked ova as well as of any other naked cells, of the +“travelling-cells,” of various kinds in connective tissue, +lymph-cells, mucus-cells, etc. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus18"></a> +<img src="images/fig18.gif" width="203" height="129" alt="Fig.18. Ovum of a +sponge." /> +<p class="caption">Fig. 18—<b>Ovum of a sponge</b> (<i>Olynthus</i>). +The ovum creeps about in a body of the sponge by thrusting out ever-changing +processes. It is indistinguishable from the common amœba.)</p> +</div> + +<p>We have now, by our study of the ovum and the comparison of it +with the amœba, provided a perfectly sound and most valuable foundation for +both the embryology and the evolution of man. We have learned that the human +ovum is a simple cell, that this ovum is not materially different from that of +other mammals, and that we may infer from it the existence of a primitive +unicellular ancestral form, with a substantial resemblance to the amœba. +</p> + +<p> +The statement that the earliest progenitors of the human race were simple cells +of this kind, and led an independent unicellular life like the amœba, has not +only been ridiculed as the dream of a natural philosopher, but also been +violently censured in theological journals as “shameful and +immoral.” But, as I observed in my essay <i>On the Origin and Ancestral +Tree of the Human Race</i> in 1870, this offended piety must equally protest +against the “shameful and immoral” fact that each human individual +is developed from a simple ovum, and that this human ovum is indistinguishable +from those of the other mammals, and in its earliest stage is like a naked +amœba. +<span class='pagenum'><a name="Page_50" id="Page_50"></a></span> +We can show this to be a fact any day with the microscope, and it is little use +to close one’s eyes to “immoral” facts of this kind. It is as +indisputable as the momentous conclusions we draw from it and as the vertebrate +character of man (see Chap. XI). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus19"></a> +<img src="images/fig19.gif" width="355" height="116" alt="Fig.19 Blood-cells +that eat, or phagocytes, from a naked sea-snail." /> +<p class="caption">Fig. 19—<b>Blood-cells that eat, or phagocytes, +from a naked sea-snail</b> (<i>Thetis</i>), greatly magnified. I was the first +to observe in the blood-cells of this snail the important fact that “the +blood-cells of the invertebrates are unprotected pieces of plasm, and take in +food, by means of their peculiar movements, like the amœbæ.” I had (in +Naples, on May 10th, 1859) injected into the blood-vessels of one of these +snails an infusion of water and ground indigo, and was greatly astonished to +find the blood-cells themselves more or less filled with the particles of +indigo after a few hours. After repeated injections I succeeded in +“observing the very entrance of the coloured particles in the +blood-cells, which took place just in the same way as with the amœba.” I +have given further particulars about this in my <i>Monograph on the +Radiolaria.</i></p> +</div> + +<p> +We now see very clearly how extremely important the cell theory has been for +our whole conception of organic nature. “Man’s place in +nature” is settled beyond question by it. Apart from the cell theory, man +is an insoluble enigma to us. Hence philosophers, and especially physiologists, +should be thoroughly conversant with it. The soul of man can only be really +understood in the light of the cell-soul, and we have the simplest form of this +in the amœba. Only those who are acquainted with the simple psychic functions +of the unicellular organisms and their gradual evolution in the series of lower +animals can understand how the elaborate mind of the higher vertebrates, and +especially of man, was gradually evolved from them. The academic psychologists +who lack this zoological equipment are unable to do so. +</p> + +<p> +This naturalistic and realistic conception is a stumbling-block to our modern +idealistic metaphysicians and their theological colleagues. Fenced about with +their transcendental and dualistic prejudices, they attack not only the +monistic system we establish on our scientific knowledge, but even the plainest +facts which go to form its foundation. An instructive instance of this was seen +a few years ago, in the academic discourse delivered by a distinguished +theologian, Willibald Beyschlag, at Halle, January 12th, 1900, on the occasion +of the centenary festival. The theologian protested violently against the +“materialistic dustmen of the scientific world who offer our people the +diploma of a descent from the ape, and would prove to them that the genius of a +Shakespeare or a Goethe is merely a distillation from a drop of primitive +mucus.” Another well-known theologian protested against “the +horrible idea that the greatest of men, Luther and Christ, were descended from +a mere globule of protoplasm.” Nevertheless, not a single informed and +impartial scientist doubts the fact that these greatest men were, like all +other men—and all other vertebrates—developed from an impregnated +ovum, and that this simple nucleated globule of protoplasm has the same +chemical constitution in all the mammals. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap07"></a><span class='pagenum'><a name="Page_51" +id="Page_51"></a></span>Chapter VII.<br/> +CONCEPTION</h2> + +<p> +The recognition of the fact that every man begins his individual existence as a +simple cell is the solid foundation of all research into the genesis of man. +From this fact we are forced, in virtue of our biogenetic law, to draw the +weighty phylogenetic conclusion that the earliest ancestors of the human race +were also unicellular organisms; and among these protozoa we may single out the +vague form of the amœba as particularly important (cf. Chapter VI). That these +unicellular ancestral forms did once exist follows directly from the phenomena +which we perceive every day in the fertilised ovum. The development of the +multicellular organism from the ovum, and the formation of the germinal layers +and the tissues, follow the same laws in man and all the higher animals. It +will, therefore, be our next task to consider more closely the impregnated ovum +and the process of conception which produces it. +</p> + +<p> +The process of impregnation or sexual conception is one of those phenomena that +people love to conceal behind the mystic veil of supernatural power. We shall +soon see, however, that it is a purely mechanical process, and can be reduced +to familiar physiological functions. Moreover, this process of conception is of +the same type, and is effected by the same organs, in man as in all the other +mammals. The pairing of the male and female has in both cases for its main +purpose the introduction of the ripe matter of the male seed or sperm into the +female body, in the sexual canals of which it encounters the ovum. Conception +then ensues by the blending of the two. +</p> + +<p> +We must observe, first, that this important process is by no means so widely +distributed in the animal and plant world as is commonly supposed. There is a +very large number of lower organisms which propagate unsexually, or by +monogamy; these are especially the sexless monera (chromacea, bacteria, etc.) +but also many other protists, such as the amœbæ, foraminifera, radiolaria, +myxomycetæ, etc. In these the multiplication of individuals takes place by +unsexual reproduction, which takes the form of cleavage, budding, or +spore-formation. The copulation of two coalescing cells, which in these cases +often precedes the reproduction, cannot be regarded as a sexual act unless the +two copulating plastids differ in size or structure. On the other hand, sexual +reproduction is the general rule with all the higher organisms, both animal and +plant; very rarely do we find asexual reproduction among them. There are, in +particular, no cases of parthenogenesis (virginal conception) among the +vertebrates. +</p> + +<p> +Sexual reproduction offers an infinite variety of interesting forms in the +different classes of animals and plants, especially as regards the mode of +conception, and the conveyance of the spermatozoon to the ovum. These features +are of great importance not only as regards conception itself, but for the +development of the organic form, and especially for the differentiation of the +sexes. There is a particularly curious correlation of plants and animals in +this respect. The splendid studies of Charles Darwin and Hermann Müller on the +fertilisation of flowers by insects have given us very interesting particulars +of this.<a href="#linknote-14" name="linknoteref-14" id="linknoteref-14"><sup>[14]</sup></a> This reciprocal service has given rise to a most intricate +sexual apparatus. Equally elaborate structures have been developed in man and +the higher animals, serving partly for the isolation of the sexual products on +each side, partly for bringing them together in conception. But, however +interesting these phenomena are in themselves, we cannot go into them here, as +they have only a minor importance—if any at all—in the real process +of conception. We must, however, try to get a very clear idea of this process +and the meaning of sexual reproduction. +</p> + +<p class="footnote"> +<a name="linknote-14" id="linknote-14"></a> <a href="#linknoteref-14">[14]</a> +See Darwin’s work, <i>On the Various Contrivances by which Orchids are +Fertilised</i> (1862). +</p> + +<p> +<span class='pagenum'><a name="Page_52" id="Page_52"></a></span>In every act of conception we have, as I said, to consider two different kinds +of cells—a female and a male cell. The female cell of the animal organism +is always called the ovum (or <i>ovulum,</i> egg, or egg-cell); the male cells +are known as the sperm or seed-cells, or the spermatozoa (also spermium and +zoospermium). The ripe ovum is, on the whole, one of the largest cells we know. +It attains colossal dimensions when it absorbs great quantities of nutritive +yelk, as is the case with birds and reptiles and many of the fishes. In the +great majority of the animals the ripe ovum is rich in yelk and much larger +than the other cells. On the other hand, the next cell which we have to +consider in the process of conception, the male sperm-cell or spermatozoon, is +one of the smallest cells in the animal body. Conception usually consists in +the bringing into contact with the ovum of a slimy fluid secreted by the male, +and this may take place either inside or out of the female body. This fluid is +called sperm, or the male seed. Sperm, like saliva or blood, is not a simple +fluid, but a thick agglomeration of innumerable cells, swimming about in a +comparatively small quantity of fluid. It is not the fluid, but the independent +male cells that swim in it, that cause conception. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus20"></a> +<img src="images/fig20.gif" width="356" height="212" alt="Fig.20 Spermia or +spermatozoa of various mammals." /> +<p class="caption">Fig. 20—<b>Spermia or spermatozoa of various +mammals.</b> The pear-shaped flattened nucleus is seen from the front in +<i>I</i> and sideways in <i>II. k</i> is the nucleus, <i>m</i> its middle part +(protoplasm), <i> s</i> the mobile, serpent-like tail (or whip); <i>M</i> four +human spermatozoa, <i>A</i> four spermatozoa from the ape; <i>K</i> from the +rabbit; <i>H</i> from the mouse; <i>C</i> from the dog; <i> S</i> from the +pig.</p> +</div> + +<p> +The spermatozoa of the great majority of animals have two characteristic +features. Firstly, they are extraordinarily small, being usually the smallest +cells in the body; and, secondly, they have, as a rule, a peculiarly lively +motion, which is known as spermatozoic motion. The shape of the cell has a good +deal to do with this motion. In most of the animals, and also in many of the +lower plants (but not the higher) each of these spermatozoa has a very small, +naked cell-body, enclosing an elongated nucleus, and a long thread hanging from +it (Fig. 20). It was long before we could recognise that these structures are +simple cells. They were formerly held to be special organisms, and were called +“seed animals” (spermato-zoa, or spermato-zoidia); they are now +scientifically known as <i>spermia</i> or <i>spermidia,</i> or as +<i>spermatosomata</i> (seed-bodies) or <i>spermatofila</i> (seed threads). It +took a good deal of comparative research to convince us that each of these +spermatozoa is really a simple cell. They have the same shape as in many other +vertebrates and most of the invertebrates. However, in many of the lower +animals they have quite a different shape. Thus, for instance, in the craw fish +they are large round cells, without any movement, equipped with stiff +outgrowths like bristles (Fig. 21 <i> f</i> ). They have also a peculiar +form in some of the worms, such as the thread-worms (<i>filaria</i>); in this +case they are sometimes +<span class='pagenum'><a name="Page_53" id="Page_53"></a></span> +amœboid and like very small ova (Fig. 21 <i> c</i> to <i>e</i>). But in most of +the lower animals (such as the sponges and polyps) they have the same pine-cone +shape as in man and the other animals (Fig. 21 <i>a, h</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus21"></a> +<img src="images/fig21.gif" width="236" height="154" alt="Fig.21 Spermatozoa or +spermidia of various animals." /> +<p class="caption">Fig. 21—<b>Spermatozoa or spermidia of various +animals.</b> (From <i>Lang</i>). <i>a</i> of a fish, <i>b</i> of a turbellaria +worm (with two side-lashes), <i>c</i> to <i>e</i> of a nematode worm (amœboid +spermatozoa), <i>f</i> from a craw fish (star-shaped), <i>g</i> from the +salamander (with undulating membrane), <i>h</i> of an annelid (<i>a</i> and +<i>h</i> are the usual shape).</p> +</div> + +<p> +When the Dutch naturalist Leeuwenhoek discovered these thread-like lively +particles in 1677 in the male sperm, it was generally believed that they were +special, independent, tiny animalcules, like the infusoria, and that the whole +mature organism existed already, with all its parts, but very small and packed +together, in each spermatozoon (see p.12). We now know that the mobile +spermatozoa are nothing but simple and real cells, of the kind that we call +“ciliated” (equipped with lashes, or <i>cilia</i>). In the previous +illustrations we have distinguished in the spermatozoon a head, trunk, and +tail. The “head” (Fig. 20 <i>k</i>) is merely the oval nucleus of +the cell; the body or middle-part (<i>m</i>) is an accumulation of cell-matter; +and the tail (<i>s</i>) is a thread-like prolongation of the same. +</p> + +<p> +Moreover, we now know that these spermatozoa are not at all a peculiar form of +cell; precisely similar cells are found in various other parts of the body. If +they have many short threads projecting, they are called <i>ciliated</i>; if +only one long, whip-shaped process (or, more rarely, two or four), +<i>caudate</i> (tailed) cells. +</p> + +<p> +Very careful recent examination of the spermia, under a very high microscopic +power (Fig. 22 a, b), has detected some further details in the finer structure +of the ciliated cell, and these are common to man and the anthropoid ape. The +head (<i>k</i>) encloses the elliptic nucleus in a thin envelope of cytoplasm; +it is a little flattened on one side, and thus looks rather pear-shaped from +the front (<i>b</i>). In the central piece (<i>m</i>) we can distinguish a +short neck and a longer connective piece (with central body). The tail consists +of a long main section (<i>h</i>) and a short, very fine tail (<i>e</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus22"></a> +<img src="images/fig22.gif" width="184" height="300" alt="Fig.22 A single human +spermatozoon." /> +<p class="caption">Fig. 22—<b>A single human spermatozoon</b> +magnified; a shows it from the broader and b from the narrower side. <i>k</i> +head (with nucleus), <i>m</i> middle-stem, <i>h</i> long-stem, and <i>e</i> +tail. (From <i> Retzius.</i>)</p> +</div> + +<p>The process of fertilisation by sexual conception consists, therefore, +essentially in the coalescence and fusing together of two different cells. The +lively spermatozoon travels towards the ovum by its serpentine movements, and +bores its way into the female cell (Fig. 23). The nuclei of both sexual cells, +attracted by a certain “affinity,” approach each other and melt +into one. +</p> + +<p> +The fertilised cell is quite another thing from the unfertilised cell. For if +we must regard the spermia as real cells no less than the ova, and the process +of conception as a coalescence of the two, we must consider the resultant cell +as a quite new and independent organism. It bears in the cell and nuclear +matter of the penetrating spermatozoon a part of the father’s body, and +in the protoplasm and caryoplasm of the ovum a part of the mother’s body. +This is clear from the fact that the child inherits many features from both +parents. It inherits from the father by means of the spermatozoon, and from the +mother by means of the ovum. The +<span class='pagenum'><a name="Page_54" id="Page_54"></a></span> +actual blending of the two cells produces a third cell, which is the germ of +the child, or the new organism conceived. One may also say of this sexual +coalescence that the <i> stem-cell is a simple hermaphrodite</i>; it unites +both sexual substances in itself. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus23"></a> +<img src="images/fig23.gif" width="216" height="164" alt="Fig.23 The +fertilisation of the ovum by the spermatozoon." /> +<p class="caption">Fig. 23—<b>The fertilisation of the ovum by the +spermatozoon</b> (of a mammal). One of the many thread-like, lively spermidia +pierces through a fine pore-canal into the nuclear yelk. The nucleus of the +ovum is invisible.</p> +</div> + +<p>I think it necessary to emphasise the fundamental importance of +this simple, but often unappreciated, feature in order to have a correct and +clear idea of conception. With that end, I have given a special name to the new +cell from which the child develops, and which is generally loosely called +“the fertilised ovum,” or “the first segmentation +sphere.” I call it “the stem-cell” (<i>cytula</i>). The name +“stem-cell” seems to me the simplest and most suitable, because all +the other cells of the body are derived from it, and because it is, in the +strictest sense, the stem-father and stem-mother of all the countless +generations of cells of which the multicellular organism is to be composed. +That complicated molecular movement of the protoplasm which we call +“life” is, naturally, something quite different in this stem-cell +from what we find in the two parent-cells, from the coalescence of which it has +issued. <i>The life of the stem-cell or cytula is the product or resultant of +the paternal life-movement that is conveyed in the spermatozoon and the +maternal life-movement that is contributed by the ovum.</i> +</p> + +<p> +The admirable work done by recent observers has shown that the individual +development, in man and the other animals, commences with the formation of a +simple “stem-cell” of this character, and that this then passes, by +repeated segmentation (or cleavage), into a cluster of cells, known as +“the segmentation sphere” or “segmentation cells.” The +process is most clearly observed in the ova of the echinoderms (star-fishes, +sea-urchins, etc.). The investigations of Oscar and Richard Hertwig were +chiefly directed to these. The main results may be summed up as follows:— +</p> + +<p> +Conception is preceded by certain preliminary changes, which are very +necessary—in fact, usually indispensable—for its occurrence. They +are comprised under the general heading of “Changes prior to +impregnation.” In these the original nucleus of the ovum, the germinal +vesicle, is lost. Part of it is extruded, and part dissolved in the cell +contents; only a very small part of it is left to form the basis of a fresh +nucleus, the <i>pronucleus femininus.</i> It is the latter alone that combines +in conception with the invading nucleus of the fertilising spermatozoon (the +<i>pronucleus masculinus</i>). +</p> + +<p> +The impregnation of the ovum commences with a decay of the germinal vesicle, or +the original nucleus of the ovum (Fig. 8). We have seen that this is in most +unripe ova a large, transparent, round vesicle. This germinal vesicle contains +a viscous fluid (the <i>caryolymph</i>). The firm nuclear frame +(<i>caryobasis</i>) is formed of the enveloping membrane and a mesh-work of +nuclear threads running across the interior, which is filled with the nuclear +sap. In a knot of the network is contained the dark, stiff, opaque nuclear +corpuscle or nucleolus. When the impregnation of the ovum sets in, the greater +part of the germinal vesicle is dissolved in the cell; the nuclear membrane and +mesh-work disappear; the nuclear sap is distributed in the protoplasm; a small +portion of the nuclear base is extruded; another small portion is left, and is +converted into the secondary nucleus, or the female pro-nucleus (Fig. 24 <i>e +k</i>). +</p> + +<p> +The small portion of the nuclear base which is extruded from the impregnated +ovum is known as the “directive bodies” or “polar +cells”; there are many disputes as to their origin and significance, but +we are as yet imperfectly acquainted with them. As a rule, they are two small +round granules, of the same size and appearance as the remaining pro-nucleus. +They are detached cell-buds; their separation from the large mother-cell takes +<span class='pagenum'><a name="Page_55" id="Page_55"></a></span> +place in the same way as in ordinary “indirect cell-division.” +Hence, the polar cells are probably to be conceived as “abortive +ova,” or “rudimentary ova,” which proceed from a simple +original ovum by cleavage in the same way that several sperm-cells arise from +one “sperm-mother-cell,” in reproduction from sperm. The male +sperm-cells in the testicles must undergo similar changes in view of the coming +impregnation as the ova in the female ovary. In this maturing of the sperm each +of the original seed-cells divides by double segmentation into four +daughter-cells, each furnished with a fourth of the original nuclear matter +(the hereditary chromatin); and each of these four descendant cells becomes a +<i> spermatozoon,</i> ready for impregnation. Thus is prevented the doubling of +the chromatin in the coalescence of the two nuclei at conception. As the two +polar cells are extruded and lost, and have no further part in the +fertilisation of the ovum, we need not discuss them any further. But we must +give more attention to the female pro-nucleus which alone remains after the +extrusion of the polar cells and the dissolving of the germinal vesicle (Fig. +23 <i>e k</i>). This tiny round corpuscle of chromatin now acts as a centre of +attraction for the invading spermatozoon in the large ripe ovum, and coalesces +with its “head,” the male pro-nucleus. The product of this +blending, which is the most important part of the act of impregnation, is the +stem-nucleus, or the first segmentation nucleus (<i>archicaryon</i>)—that +is to say, the nucleus of the new-born embryonic stem-cell or “first +segmentation cell.” This stem-cell is the starting point of the +subsequent embryonic processes. +</p> + +<p> +Hertwig has shown that the tiny transparent ova of the echinoderms are the most +convenient for following the details of this important process of impregnation. +We can, in this case, easily and successfully accomplish artificial +impregnation, and follow the formation of the stem-cell step by step within the +space of ten minutes. If we put ripe ova of the star-fish or sea-urchin in a +watch glass with sea-water and add a drop of ripe sperm-fluid, we find each +ovum impregnated within five minutes. Thousands of the fine, mobile ciliated +cells, which we have described as “sperm-threads” (Fig. 20), make +their way to the ova, owing to a sort of chemical sensitive action which may be +called “smell.” But only one of these innumerable spermatozoa is +chosen—namely, the one that first reaches the ovum by the serpentine +motions of its tail, and touches the ovum with its head. At the spot where the +point of its head touches the surface of the ovum the protoplasm of the latter +is raised in the form of a small wart, the “impregnation rise” +(Fig. 25 <i>A</i>). The spermatozoon then bores its way into this with its +head, the tail outside wriggling about all the time (Fig. 25 <i>B, C</i>). +Presently the tail also disappears within the ovum. At the same time the ovum +secretes a thin external yelk-membrane (Fig. 25 <i> C</i>), starting from the +point of impregnation; and this prevents any more spermatozoa from entering. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus24"></a> +<img src="images/fig24.gif" width="159" height="142" alt="Fig.24 An impregnated +echinoderm ovum." /> +<p class="caption">Fig. 24—<b>An impregnated echinoderm ovum,</b> with +small homogeneous nucleus (<i>e k</i>).<br/>(From <i>Hertwig.</i>)</p> +</div> + +<p> +Inside the impregnated ovum we now see a rapid series of most important +changes. The pear-shaped head of the sperm-cell, or the “head of the +spermatozoon,” grows larger and rounder, and is converted into the male +pro-nucleus (Fig. 26 <i>s k</i>). This has an attractive influence on the fine +granules or particles which are distributed in the protoplasm of the ovum; they +arrange themselves in lines in the figure of a star. But the attraction or the +“affinity” between the two nuclei is even stronger. They move +towards each other inside the yelk with increasing speed, the male (Fig. 27 +<i>s k</i>) going more quickly than the female nucleus (<i>e k</i>). The tiny +male nucleus takes with it the radiating mantle which spreads like a star about +it. At last the two sexual nuclei touch (usually in the centre of the globular +ovum), lie close together, are flattened at the points of contact, and coalesce +into a common mass. The small central particle of +<span class='pagenum'><a name="Page_56" id="Page_56"></a></span> +nuclein which is formed from this combination of the nuclei is the +stem-nucleus, or the first segmentation nucleus; the new-formed cell, the +product of the impregnation, is our stem-cell, or “first segmentation +sphere” (Fig. 2). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus25"></a> +<img src="images/fig25.gif" width="376" height="160" alt="Fig. +25 Impregnation of the ovum of a star-fish." /> +<p class="caption">Fig. 25—<b>Impregnation of the ovum of a +star-fish.</b> (From <i>Hertwig.</i>) Only a small part of the surface of the +ovum is shown. One of the numerous spermatozoa approaches the +“impregnation rise” (<i>A</i>), touches it (<i>B</i>), and then +penetrates into the protoplasm of the ovum (<i>C</i>).</p> +</div> + +<p> +Hence the one essential point in the process of sexual reproduction or +impregnation is the formation of a new cell, the stem-cell, by the combination +of two originally different cells, the female ovum and the male spermatozoon. +This process is of the highest importance, and merits our closest attention; +all that happens in the later development of this first cell and in the life of +the organism that comes of it is determined from the first by the chemical and +morphological composition of the stem-cell, its nucleus and its body. We must, +therefore, make a very careful study of the rise and structure of the +stem-cell. +</p> + +<p> +The first question that arises is as to the two different active elements, the +nucleus and the protoplasm, in the actual coalescence. It is obvious that the +nucleus plays the more important part in this. Hence Hertwig puts his theory of +conception in the principle: “Conception consists in the copulation of +two cell-nuclei, which come from a male and a female cell.” And as the +phenomenon of heredity is inseparably connected with the reproductive process, +we may further conclude that these two copulating nuclei “convey the +characteristics which are transmitted from parents to offspring.” In this +sense I had in 1866 (in the ninth chapter of the <i>General Morphology</i>) +ascribed to the reproductive nucleus the function of generation and +<i>heredity,</i> and to the nutritive protoplasm the duties of nutrition and +<i>adaptation.</i> As, moreover, there is a complete coalescence of the +mutually attracted nuclear substances in conception, and the new nucleus formed +(the stem-nucleus) is the real starting-point for the development of the fresh +organism, the further conclusion may be drawn that the male nucleus conveys to +the child the qualities of the father, and the female nucleus the features of +the mother. We must not forget, however, that the protoplasmic bodies of the +copulating cells also fuse together in the act of impregnation; the cell-body +of the invading spermatozoon (the trunk and tail of the male ciliated cell) is +dissolved in the yelk of the female ovum. This coalescence is not so important +as that of the nuclei, but it must not be overlooked; and, though this process +is not so well known to us, we see clearly at least the formation of the +star-like figure (the radial arrangement of the particles in the plasma) in it +(Figs. 26–27). +</p> + +<p> +The older theories of impregnation generally went astray in regarding the large +ovum as the sole base of the new organism, and only ascribed to the +spermatozoon the work of stimulating and originating its development. The +stimulus which it gave to the ovum was sometimes thought to be purely chemical, +at other times rather physical (on the principle of transferred movement), or +again a mystic and transcendental process. This error was partly due to the +imperfect knowledge at that time of the facts of impregnation, and partly to +the striking +<span class='pagenum'><a name="Page_57" id="Page_57"></a></span> +difference in the sizes of the two sexual cells. Most of the earlier observers +thought that the spermatozoon did not penetrate into the ovum. And even when +this had been demonstrated, the spermatozoon was believed to disappear in the +ovum without leaving a trace. However, the splendid research made in the last +three decades with the finer technical methods of our time has completely +exposed the error of this. It has been shown that the tiny sperm-cell is <i>not +subordinated to, but coordinated with,</i> the large ovum. The nuclei of the +two cells, as the vehicles of the hereditary features of the parents, are of +equal physiological importance. In some cases we have succeeded in proving that +the mass of the active nuclear substance which combines in the copulation of +the two sexual nuclei is originally the same for both. +</p> + +<p> +These morphological facts are in perfect harmony with the familiar +physiological truth that the child inherits from both parents, and that on the +average they are equally distributed. I say “on the average,” +because it is well known that a child may have a greater likeness to the father +or to the mother; that goes without saying, as far as the primary sexual +characters (the sexual glands) are concerned. But it is also possible that the +determination of the latter—the weighty determination whether the child +is to be a boy or a girl—depends on a slight qualitative or quantitative +difference in the nuclein or the coloured nuclear matter which comes from both +parents in the act of conception. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus26"></a> +<img src="images/fig26.gif" width="338" height="169" alt="Figs. +26 and 27 Impregnation of the ovum of the sea-urchin." /> +<p class="caption">Figs. 26 and 27.—<b>Impregnation of the ovum of the +sea-urchin.</b> (From <i>Hertwig.</i>) In Fig. 26 the little sperm-nucleus +(<i>sk</i>) moves towards the larger nucleus of the ovum (<i>ek</i>). In Fig. +27 they nearly touch, and are surrounded by the radiating mantle of +protoplasm.</p> +</div> + +<p> +The striking differences of the respective sexual cells in size and shape, +which occasioned the erroneous views of earlier scientists, are easily +explained on the principle of division of labour. The inert, motionless ovum +grows in size according to the quantity of provision it stores up in the form +of nutritive yelk for the development of the germ. The active swimming +sperm-cell is reduced in size in proportion to its need to seek the ovum and +bore its way into its yelk. These differences are very conspicuous in the +higher animals, but they are much less in the lower animals. In those protists +(unicellular plants and animals) which have the first rudiments of sexual +reproduction the two copulating cells are at first quite equal. In these cases +the act of impregnation is nothing more than a sudden <i>growth,</i> in which +the originally simple cell doubles its volume, and is thus prepared for +reproduction (cell-division). Afterwards slight differences are seen in the +size of the copulating cells; though the smaller ones still have the same shape +as the larger ones. It is only when the difference in size is very pronounced +that a notable difference in shape is found: the sprightly sperm-cell changes +more in shape and the ovum in size. +</p> + +<p> +Quite in harmony with this new conception of the <i>equivalence of the two +gonads,</i> or the equal physiological importance of the male and female +sex-cells and their equal share in the process of heredity, is the important +fact established by Hertwig (1875), that in normal impregnation only one single +spermatozoon +<span class='pagenum'><a name="Page_58" id="Page_58"></a></span> +copulates with one ovum; the membrane which is raised on the surface of the +yelk immediately after one sperm-cell has penetrated (Fig. 25 <i>C</i>) +prevents any others from entering. All the rivals of the fortunate penetrator +are excluded, and die without. But if the ovum passes into a morbid state, if +it is made stiff by a lowering of its temperature or stupefied with narcotics +(chloroform, morphia, nicotine, etc.), two or more spermatozoa may penetrate +into its yelk-body. We then witness <i>polyspermism.</i> The more Hertwig +chloroformed the ovum, the more spermatozoa were able to bore their way into +its unconscious body. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus28"></a> +<img src="images/fig28.gif" width="209" height="196" alt="Fig.28 Stem-cell of a +rabbit." /> +<p class="caption">Fig. 28—<b>Stem-cell of a rabbit,</b> magnified. In +the centre of the granular protoplasm of the fertilised ovum (<i>d</i>) is seen +the little, bright stem-nucleus, <i>z</i> is the ovolemma, with a mucous +membrane (<i>h</i>). <i>s</i> are dead spermatozoa.</p> +</div> + +<p>These remarkable facts of impregnation are also of the greatest +interest in psychology, especially as regards the theory of the cell-soul, +which I consider to be its chief foundation. The phenomena we have described +can only be understood and explained by ascribing a certain lower degree of +psychic activity to the sexual principles. They <i>feel</i> each other’s +proximity, and are drawn together by a <i>sensitive</i> impulse (probably +related to smell); they <i>move</i> towards each other, and do not rest until +they fuse together. Physiologists may say that it is only a question of a +peculiar physico-chemical phenomenon, and not a psychic action; but the two +cannot be separated. Even the psychic functions, in the strict sense of the +word, are only complex physical processes, or “psycho-physical” +phenomena, which are determined in all cases exclusively by the chemical +composition of their material substratum. +</p> + +<p> +The monistic view of the matter becomes clear enough when we remember the +radical importance of impregnation as regards heredity. It is well known that +not only the most delicate bodily structures, but also the subtlest traits of +mind, are transmitted from the parents to the children. In this the chromatic +matter of the male nucleus is just as important a vehicle as the large +caryoplasmic substance of the female nucleus; the one transmits the mental +features of the father, and the other those of the mother. The blending of the +two parental nuclei determines the individual psychic character of the child. +</p> + +<p> +But there is another important psychological question—the most important +of all—that has been definitely answered by the recent discoveries in +connection with conception. This is the question of the immortality of the +soul. No fact throws more light on it and refutes it more convincingly than the +elementary process of conception that we have described. For this copulation of +the two sexual nuclei (Figs. 26 and 27) indicates the precise moment at which +the individual begins to exist. All the bodily and mental features of the +new-born child are the sum-total of the hereditary qualities which it has +received in reproduction from parents and ancestors. All that man acquires +afterwards in life by the exercise of his organs, the influence of his +environment, and education—in a word, by adaptation—cannot +obliterate that general outline of his being which he inherited from his +parents. But this hereditary disposition, the essence of every human soul, is +not “eternal,” but “temporal”; it comes into being only +at the moment when the sperm-nucleus of the father and the nucleus of the +maternal ovum meet and fuse together. It is clearly irrational to assume an +“eternal life without end” for an individual phenomenon, the +commencement of which we can indicate to a moment by direct visual observation. +</p> + +<p> +The great importance of the process of impregnation in answering such questions +is quite clear. It is true that conception has never been studied +microscopically in all its details in the human case—notwithstanding its +occurrence at every moment—for reasons that are +<span class='pagenum'><a name="Page_59" id="Page_59"></a></span> +obvious enough. However, the two cells which need consideration, the female +ovum and the male spermatozoon, proceed in the case of man in just the same way +as in all the other mammals; the human fœtus or embryo which results from +copulation has the same form as with the other animals. Hence, no scientist who +is acquainted with the facts doubts that the processes of impregnation are just +the same in man as in the other animals. +</p> + +<p> +The stem-cell which is produced, and with which every man begins his career, +cannot be distinguished in appearance from those of other mammals, such as the +rabbit (Fig. 28). In the case of man, also, this stem-cell differs materially +from the original ovum, both in regard to form (morphologically), in regard to +material composition (chemically), and in regard to vital properties +(physiologically). It comes partly from the father and partly from the mother. +Hence it is not surprising that the child who is developed from it inherits +from both parents. The vital movements of each of these cells form a sum of +mechanical processes which in the last analysis are due to movements of the +smallest vital parts, or the molecules, of the living substance. If we agree to +call this active substance <i>plasson,</i> and its molecules <i> +plastidules,</i> we may say that the individual physiological character of each +of these cells is due to its molecular plastidule-movement. <i>Hence, the +plastidule-movement of the cytula is the resultant of the combined +plastidule-movements of the female ovum and the male +sperm-cell.</i><a href="#linknote-15" name="linknoteref-15" id="linknoteref-15"><sup>[15]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-15" id="linknote-15"></a> <a href="#linknoteref-15">[15]</a> +The plasson of the stem-cell or cytula may, from the anatomical point of view, +be regarded as homogeneous and structureless, like that of the monera. This is +not inconsistent with our hypothetical ascription to the plastidules (or +molecules of the plasson) of a complex molecular structure. The complexity of +this is the greater in proportion to the complexity of the organism that is +developed from it and the length of the chain of its ancestry, or to the +multitude of antecedent processes of heredity and adaptation. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap08"></a>Chapter VIII.<br/> +THE GASTRÆA THEORY</h2> + +<p> +There is a substantial agreement throughout the animal world in the first +changes which follow the impregnation of the ovum and the formation of the +stem-cell; they begin in all cases with the segmentation of the ovum and the +formation of the germinal layers. The only exception is found in the protozoa, +the very lowest and simplest forms of animal life; these remain unicellular +throughout life. To this group belong the amœbae, gregarinæ, rhizopods, +infusoria, etc. As their whole organism consists of a single cell, they can +never form germinal layers, or definite strata of cells. But all the other +animals—all the tissue-forming animals, or <i>metazoa,</i> as we call +them, in contradistinction to the protozoa—construct real germinal layers +by the repeated cleavage of the impregnated ovum. This we find in the lower +cnidaria and worms, as well as in the more highly-developed molluscs, +echinoderms, articulates, and vertebrates. +</p> + +<p> +In all these metazoa, or multicellular animals, the chief embryonic processes +are substantially alike, although they often seem to a superficial observer to +differ considerably. The stem-cell that proceeds from the impregnated ovum +always passes by repeated cleavage into a number of simple cells. These cells +are all direct descendants of the stem-cell, and are, for reasons we shall see +presently, called segmentation-cells. The repeated cleavage of the stem-cell, +which gives rise to these segmentation-spheres, has long been known as +“segmentation.” Sooner or later the segmentation-cells join +together to form a round (at first, globular) embryonic sphere +(<i>blastula</i>); they then form into two very different groups, and arrange +themselves +<span class='pagenum'><a name="Page_60" id="Page_60"></a></span> +in two separate strata—the two <i>primary germinal layers.</i> These +enclose a digestive cavity, the primitive gut, with an opening, the primitive +mouth. We give the name of the <i>gastrula</i> to the important embryonic form +that has these primitive organs, and the name of <i>gastrulation</i> to the +formation of it. This ontogenetic process has a very great significance, and is +the real starting-point of the construction of the multicellular animal body. +</p> + +<p> +The fundamental embryonic processes of the cleavage of the ovum and the +formation of the germinal layers have been very thoroughly studied in the last +thirty years, and their real significance has been appreciated. They present a +striking variety in the different groups, and it was no light task to prove +their essential identity in the whole animal world. But since I formulated the +gastræa theory in 1872, and afterwards (1875) reduced all the various forms of +segmentation and gastrulation to one fundamental type, their identity may be +said to have been established. We have thus mastered the law of unity which +governs the first embryonic processes in all the animals. +</p> + +<p> +Man is like all the other higher animals, especially the apes, in regard to +these earliest and most important processes. As the human embryo does not +essentially differ, even at a much later stage of development—when we +already perceive the cerebral vesicles, the eyes, ears, gill-arches, +etc.—from the similar forms of the other higher mammals, we may +confidently assume that they agree in the earliest embryonic processes, +segmentation and the formation of germinal layers. This has not yet, it is +true, been established by observation. We have never yet had occasion to +dissect a woman immediately after impregnation and examine the stem-cell or the +segmentation-cells in her oviduct. However, as the earliest human embryos we +have examined, and the later and more developed forms, agree with those of the +rabbit, dog, and other higher mammals, no reasonable man will doubt but that +the segmentation and formation of layers are the same in both cases. +</p> + +<p> +But the special form of segmentation and layer formation which we find in the +mammal is by no means the original, simple, palingenetic form. It has been much +modified and cenogenetically altered by a very complex adaptation to embryonic +conditions. We cannot, therefore, understand it altogether in itself. In order +to do this, we have to make a <i>comparative</i> study of segmentation and +layer-formation in the animal world; and we have especially to seek the +original, <i>palingenetic</i> form from which the modified <i>cenogenetic</i> +(see p. 4) form has gradually been developed. +</p> + +<p> +This original unaltered form of segmentation and layer-formation is found +to-day in only one case in the vertebrate-stem to which man belongs—the +lowest and oldest member of the stem, the wonderful lancelet or amphioxus (cf. +Chapters XVI and XVII). But we find a precisely similar palingenetic form of +embryonic development in the case of many of the invertebrate animals, as, for +instance, the remarkable ascidia, the pond-snail (<i>Limnæus</i>), and +arrow-worm (<i>Sagitta</i>), and many of the echinoderms and cnidaria, such as +the common star-fish and sea-urchin, many of the medusæ and corals, and the +simpler sponges (<i>Olynthus</i>). We may take as an illustration the +palingenetic segmentation and germinal layer-formation in an eight-fold insular +coral, which I discovered in the Red Sea, and described as <i>Monoxenia +Darwinii.</i> +</p> + +<p> +The impregnated ovum of this coral (Fig. 29 A, B) first splits into two equal +cells (C). First, the nucleus of the stem-cell and its central body divide into +two halves. These recede from and repel each other, and act as centres of +attraction on the surrounding protoplasm; in consequence of this, the +protoplasm is constricted by a circular furrow, and, in turn, divides into two +halves. Each of the two segmentation-cells thus produced splits in the same way +into two equal cells. The four segmentation-cells (grand-daughters of the +stem-cell) lie in one plane. Now, however, each of them subdivides into two +equal halves, the cleavage of the nucleus again preceding that of the +surrounding protoplasm. The eight cells which thus arise break into sixteen, +these into thirty-two, and then (each being constantly halved) into sixty-four, +128, and so on.<a href="#linknote-16" name="linknoteref-16" id="linknoteref-16"><sup>[16]</sup></a> The final result of this +<span class='pagenum'><a name="Page_62" id="Page_62"></a></span> +repeated cleavage is the formation of a globular cluster of similar +segmentation-cells, which we call the mulberry-formation or morula. The cells +are thickly pressed together like the parts of a mulberry or blackberry, and +this gives a lumpy appearance to the surface of the sphere (Fig. +E).<a href="#linknote-17" name="linknoteref-17" id="linknoteref-17"><sup>[17]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-16" id="linknote-16"></a> <a href="#linknoteref-16">[16]</a> +The number of segmentation-cells thus produced increases geometrically in the +original gastrulation, or the purest palingenetic form of cleavage. However, in +different animals the number reaches a different height, so that the morula, +and also the blastula, may consist sometimes of thirty-two, sometimes of +sixty-four, and sometimes of 128, or more, cells. +</p> + +<p class="footnote"> +<a name="linknote-17" id="linknote-17"></a> <a href="#linknoteref-17">[17]</a> +The segmentation-cells which make up the morula after the close of the +palingenetic cleavage seem usually to be quite similar, and to present no +differences as to size, form, and composition. That, however, does not prevent +them from differentiating into animal and vegetative cells, even during the +cleavage. +<span class='pagenum'><a name="Page_61" id="Page_61"></a></span> +</p> + +<div class="fig" style="width:100%;"> +<a name="illus29"></a> +<img src="images/fig29.gif" width="279" height="476" alt="Gastrulation of a coral." /> +<p class="caption">Fig. 29—<b>Gastrulation of a coral</b> (<i>Monoxenia +Darwinii</i>). A, B, stem-cell (cytula) or impregnated ovum. In Figure A +(immediately after impregnation) the nucleus is invisible. In Figure B (a +little later) it is quite clear. C two segmentation-cells. D four +segmentation-cells. E mulberry-formation (morula). F blastosphere (blastula). G +blastula (transverse section). H depula, or hollowed blastula (transverse +section). I gastrula (longitudinal section). K gastrula, or cup-sphere, +external appearance.)</p> +</div> + +<p class="p2"> +When the cleavage is thus ended, the mulberry-like mass changes into a hollow +globular sphere. Watery fluid or jelly gathers inside the globule; the +segmentation-cells are loosened, and all rise to the surface. There they are +flattened by mutual pressure, and assume the shape of truncated pyramids, and +arrange themselves side by side in one regular layer (Figs. F, G). This layer +of cells is called the germinal membrane (or blastoderm); the homogeneous cells +which compose its simple structure are called blastodermic cells; and the whole +hollow sphere, the walls of which are made of the preceding, is called the +<i>blastula</i> or <i> blastosphere.</i><a href="#linknote-18" name="linknoteref-18" id="linknoteref-18"><sup>[18]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-18" id="linknote-18"></a> <a href="#linknoteref-18">[18]</a> +The blastula of the lower animals must not be confused with the very different +blastula of the mammal, which is properly called the <i>gastrocystis</i> or +<i>blastocystis.</i> This <i>cenogenetic</i> gastrocystis and the +<i>palingenetic</i> blastula are sometimes very wrongly comprised under the +common name of blastula or vesicula blastodermica. +</p> + +<p> +In the case of our coral, and of many other lower forms of animal life, the +young embryo begins at once to move independently and swim about in the water. +A fine, long, thread-like process, a sort of whip or lash, grows out of each +blastodermic cell, and this independently executes vibratory movements, slow at +first, but quicker after a time (Fig. F). In this way each blastodermic cell +becomes a ciliated cell. The combined force of all these vibrating lashes +causes the whole blastula to move about in a rotatory fashion. In many other +animals, especially those in which the embryo develops within enclosed +membranes, the ciliated cells are only formed at a later stage, or even not +formed at all. The blastosphere may grow and expand by the blastodermic cells +(at the surface of the sphere) dividing and increasing, and more fluid is +secreted in the internal cavity. There are still to-day some organisms that +remain throughout life at the structural stage of the blastula—hollow +vesicles that swim about by a ciliary movement in the water, the wall of which +is composed of a single layer of cells, such as the volvox, the magosphæra, +synura, etc. We shall speak further of the great phylogenetic significance of +this fact in Chapter XIX. +</p> + +<p> +A very important and remarkable process now follows—namely, the curving +or invagination of the blastula (Fig. H). The vesicle with a single layer of +cells for wall is converted into a cup with a wall of two layers of cells (cf. +Figs. G, H, I). A certain spot at the surface of the sphere is flattened, and +then bent inward. This depression sinks deeper and deeper, growing at the cost +of the internal cavity. The latter decreases as the hollow deepens. At last the +internal cavity disappears altogether, the inner side of the blastoderm (that +which lines the depression) coming to lie close on the outer side. At the same +time, the cells of the two sections assume different sizes and shapes; the +inner cells are more round and the outer more oval (Fig. I). In this way the +embryo takes the form of a cup or jar-shaped body, with a wall made up of two +layers of cells, the inner cavity of which opens to the outside at one end (the +spot where the depression was originally formed). We call this very important +and interesting embryonic form the “cup-embryo” or +“cup-larva” (<i>gastrula,</i> Fig. 29, I longitudinal section, K +external view). I have in my <i>Natural History of Creation</i> given the name +of <i>depula</i> to the remarkable intermediate form which appears at the +passage of the blastula into the gastrula. In this intermediate stage there are +two cavities in the embryo—the original cavity (<i>blastocœl</i>) which +is disappearing, and the primitive gut-cavity (<i>progaster</i>) which is +forming. +</p> + +<p> +I regard the gastrula as the most important and significant embryonic form in +the animal world. In all real animals (that is, excluding the unicellular +protists) the segmentation of the ovum produces either a pure, primitive, +palingenetic gastrula (Fig. 29 I, K) or an equally instructive cenogenetic +form, which has been developed in time from the first, and can be directly +reduced to it. It is certainly a fact of the greatest interest and +instructiveness that animals of the most different stems—vertebrates and +tunicates, molluscs and articulates, echinoderms and annelids, cnidaria and +sponges—proceed from one and the same embryonic form. In illustration I +give a few +<span class='pagenum'><a name="Page_63" id="Page_63"></a></span> +pure gastrula forms from various groups of animals (Figs. 30–35, +explanation given below each). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus30"></a> +<a name="illus31"></a> +<a name="illus32"></a> +<a name="illus33"></a> +<a name="illus34"></a> +<a name="illus35"></a> +<img src="images/fig30.gif" width="368" height="292" alt="Fig.30 Gastrula of a very simple +primitive-gut animal or gastræad. Fig. 31 Gastrula of a worm. Fig. +32 Gastrula of an echinoderm. Fig. 33 Gastrula of an arthropod. Fig. +34 Gastrula of a mollusc. Fig. 35 Gastrula of a vertebrate." /> +<p class="caption">Fig. 30 (<i>A</i>)—<b>Gastrula of a very simple +primitive-gut animal</b> or <b>gastræad</b> (gastrophysema). +(<i>Haeckel.</i>)<br/> Fig. 31 (<i>B</i>)—<b>Gastrula of a worm</b> +(<i>Sagitta</i>). (From <i>Kowalevsky.</i>)<br/> Fig. 32 +(<i>C</i>)—<b>Gastrula of an echinoderm</b> (star-fish, <i>Uraster</i>), +not completely folded in (depula). (From <i>Alexander Agassiz.</i>)<br/> Fig. +33 (<i>D</i>)—<b>Gastrula of an arthropod</b> (primitive crab, +<i>Nauplius</i>) (as 32).<br/> Fig. 34 (<i>E</i>)—<b>Gastrula of a +mollusc</b> (pond-snail, <i>Linnæus</i>). (From <i>Karl Rabl.</i>)<br/> Fig. 35 +(<i>F</i>)—<b>Gastrula of a vertebrate</b> (lancelet, <i>Amphioxus</i>). +(From <i>Kowalevsky.</i>) (Front view.)<br/> In each figure <i>d</i> is the +primitive-gut cavity, <i>o</i> primitive mouth,<br/> <i>s</i> +segmentation-cavity, <i>i</i> entoderm (gut-layer), <i>e</i> ectoderm (skin +layer).</p> +</div> + +<p> +In view of this extraordinary significance of the gastrula, we must make a very +careful study of its original structure. As a rule, the typical gastrula is +very small, being invisible to the naked eye, or at the most only visible as a +fine point under very favourable conditions, and measuring generally 1/500 to +1/250 of an inch (less frequently 1/50 inch, or even more) in diameter. In +shape it is usually like a roundish drinking-cup. Sometimes it is rather oval, +at other times more ellipsoid or spindle-shaped; in some cases it is half +round, or even almost round, and in others lengthened out, or almost +cylindrical. +</p> + +<p> +I give the name of primitive gut (<i>progaster</i>) and primitive mouth +(<i>prostoma</i>) to the internal cavity of the gastrula-body and its opening; +because this cavity is the first rudiment of the digestive cavity of the +organism, and the opening originally served to take food into it. Naturally, +the primitive gut and mouth change very considerably afterwards in the various +classes of animals. In most of the cnidaria and many of the annelids (worm-like +animals) they remain unchanged throughout life. But in most of the +<span class='pagenum'><a name="Page_64" id="Page_64"></a></span> +higher animals, and so in the vertebrates, only the larger central part of the +later alimentary canal develops from the primitive gut; the later mouth is a +fresh development, the primitive mouth disappearing or changing into the anus. +We must therefore distinguish carefully between the primitive gut and mouth of +the gastrula and the later alimentary canal and mouth of the fully developed +vertebrate.<a href="#linknote-19" name="linknoteref-19" id="linknoteref-19"><sup>[19]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-19" id="linknote-19"></a> <a href="#linknoteref-19">[19]</a> +My distinction (1872) between the primitive gut and mouth and the later +permanent stomach (<i>metagaster</i>) and mouth (<i>metastoma</i>) has been +much criticised; but it is as much justified as the distinction between the +primitive kidneys and the permanent kidneys. Professor E. Ray-Lankester +suggested three years afterwards (1875) the name <i>archenteron</i> for the +primitive gut, and <i>blastoporus</i> for the primitive mouth. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus36"></a> +<img src="images/fig36.gif" width="324" height="192" alt="Fig.36 Gastrula of a lower sponge +(olynthus)." /> +<p class="caption">Fig. 36—<b>Gastrula of a lower sponge</b> (lynthus). <i>A</i> +external view, <i>B</i> longitudinal section through the axis, <i>g</i> +primitive-gut cavity, a primitive mouth-aperture, <i>i</i> inner cell-layer +(entoderm, endoblast, gut-layer), <i>e</i> external cell-layer (outer germinal +layer, ectoderm, ectoblast, or skin-layer).</p> +</div> + +<p> +The two layers of cells which line the gut-cavity and compose its wall are of +extreme importance. These two layers, which are the sole builders of the whole +organism, are no other than the two primary germinal layers, or the primitive +germ-layers. I have spoken in the introductory section (Chapter III) of their +radical importance. The outer stratum is the skin-layer, or <i>ectoderm</i> +(Figs. 30–35<i>e</i>); the inner stratum is the gut-layer, or +<i>entoderm</i> (<i>i</i>). The former is often also called the ectoblast, or +epiblast, and the latter the endoblast, or hypoblast. <i>From these two primary +germinal layers alone is developed the entire organism of all the metazoa or +multicellular animals.</i> The skin-layer forms the external skin, the +gut-layer forms the internal skin or lining of the body. Between these two +germinal layers are afterwards developed the middle germinal layer +(<i>mesoderma</i>) and the body-cavity (<i>cœloma</i>) filled with blood or +lymph. +</p> + +<p> +The two primary germinal layers were first distinguished by Pander in 1817 in +the incubated chick. Twenty years later (1849) Huxley pointed out that in many +of the lower zoophytes, especially the medusæ, the whole body consists +throughout life of these two primary germinal layers. Soon afterwards (1853) +Allman introduced the names which have come into general use; he called the +outer layer the <i>ectoderm</i> (“outer-skin”), and the inner the +<i>entoderm</i> (“inner-skin”). But in 1867 it was shown, +particularly by Kowalevsky, from comparative observation, that even in +invertebrates, also, of the most different classes—annelids, molluscs, +echinoderms, and articulates—the body is developed out of the same two +primary layers. Finally, I discovered them (1872) in the lowest tissue-forming +animals, the sponges, and proved in my gastræa theory that these two layers +must be regarded as identical throughout the animal world, from the sponges and +corals to the insects and vertebrates, including man. This fundamental +“homology +<span class='pagenum'><a name="Page_65" id="Page_65"></a></span> +[identity] of the primary germinal layers and the primitive gut” has been +confirmed during the last thirty years by the careful research of many able +observers, and is now pretty generally admitted for the whole of the metazoa. +</p> + +<p> +As a rule, the cells which compose the two primary germinal layers show +appreciable differences even in the gastrula stage. Generally (if not always) +the cells of the skin-layer or ectoderm (Figs. 36 <i>c</i> and 37 <i>e</i>) are +the smaller, more numerous, and clearer; while the cells of the gut-layer, or +entoderm (<i>i</i>), are larger, less numerous, and darker. The protoplasm of +the ectodermic (outer) cells is clearer and firmer than the thicker and softer +cell-matter of the entodermic (inner) cells; the latter are, as a rule, much +richer in yelk-granules (albumen and fatty particles) than the former. Also the +cells of the gut-layer have, as a rule, a stronger affinity for colouring +matter, and take on a tinge in a solution of carmine, aniline, etc., more +quickly and appreciably than the cells of the skin-layer. The nuclei of the +entoderm-cells are usually roundish, while those of the ectoderm-cells are +oval. +</p> + +<p> +When the doubling-process is complete, very striking histological differences +between the cells of the two layers are found (Fig. 37). The tiny, light +ectoderm-cells (<i>e</i>) are sharply distinguished from the larger and darker +entoderm-cells (<i>i</i>). Frequently this differentiation of the cell-forms +sets in at a very early stage, during the segmentation-process, and is already +very appreciable in the blastula. +</p> + +<p> +We have, up to the present, only considered that form of segmentation and +gastrulation which, for many and weighty reasons, we may regard as the +original, primordial, or palingenetic form. We might call it +“equal” or homogeneous segmentation, because the divided cells +retain a resemblance to each other at first (and often until the formation of +the blastoderm). We give the name of the “bell-gastrula,” or <i> +archigastrula,</i> to the gastrula that succeeds it. In just the same form as +in the coral we considered (<i>Monoxenia,</i> Fig. 29), we find it in the +lowest zoophyta (the gastrophysema, Fig. 30), and the simplest sponges +(olynthus, Fig. 36); also in many of the medusæ and hydrapolyps, lower types of +worms of various classes (brachiopod, arrow-worm, Fig. 31), tunicates +(ascidia), many of the echinoderms (Fig. 32), lower articulates (Fig. 33), and +molluscs (Fig. 34), and, finally, in a slightly modified form, in the lowest +vertebrate (the amphioxus, Fig. 35). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus37"></a> +<img src="images/fig37.gif" width="208" height="146" alt="Fig.37 Cells from the +two primary germinal layers." /> +<p class="caption">Fig. 37—<b>Cells from the two primary germinal +layers</b> of the mammal (from both layers of the blastoderm). <i>i</i> larger +and darker cells of the inner stratum, the vegetal layer or entoderm. <i>e</i> +smaller and clearer cells from the outer stratum, the animal layer or +ectoderm.</p> +</div> + +<p> +The gastrulation of the amphioxus is especially interesting because this lowest +and oldest of all the vertebrates is of the highest significance in connection +with the evolution of the vertebrate stem, and therefore with that of man +(compare Chapters XVI and XVII). Just as the comparative anatomist traces the +most elaborate features in the structures of the various classes of vertebrates +to divergent development from this simple primitive vertebrate, so comparative +embryology traces the various secondary forms of vertebrate gastrulation to the +simple, primary formation of the germinal layers in the amphioxus. Although +this formation, as distinguished from the cenogenetic modifications of the +vertebrate, may on the whole be regarded as palingenetic, it is nevertheless +different in some features from the quite primitive gastrulation such as we +have, for instance, in the <i>Monoxenia</i> (Fig. 29) and the <i>Sagitta.</i> +Hatschek rightly observes that the segmentation of the ovum in the amphioxus is +not strictly equal, but almost equal, and approaches the unequal. The +difference in size between the two groups of cells continues to be very +noticeable in the further course of the segmentation; the smaller animal cells +of the upper hemisphere divide more quickly than the larger vegetal cells of +the lower (Fig. 38 <i>A, B</i>). Hence the blastoderm, which forms the +single-layer wall of the globular blastula at the end of the cleavage-process, +does not consist of +<span class='pagenum'><a name="Page_66" id="Page_66"></a></span> +homogeneous cells of equal size, as in the Sagitta and the Monoxenia; the cells +of the upper half of the blastoderm (the mother-cells of the ectoderm) are more +numerous and smaller, and the cells of the lower half (the mother-cells of the +entoderm) less numerous and larger. Moreover, the segmentation-cavity of the +blastula (Fig. 38 <i>C, h</i>) is not quite globular, but forms a flattened +spheroid with unequal poles of its vertical axis. While the blastula is being +folded into a cup at the vegetal pole of its axis, the difference in the size +of the blastodermic cells increases (Fig. 38 <i>D, E</i>); it is most +conspicuous when the invagination is complete and the segmentation-cavity has +disappeared (Fig. 38 <i>F</i>). The larger vegetal cells of the entoderm are +richer in granules, and so darker than the smaller and lighter animal cells of +the ectoderm. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus38"></a> +<img src="images/fig38.gif" width="324" height="229" alt="Fig.38 Gastrulation of the +amphioxus." /> +<p class="caption">Fig. 38—<b>Gastrulation of the amphioxus,</b> from +<i> Hatschek</i> (vertical section through the axis of the ovum). <i>A, B, +C</i> three stages in the formation of the blastula; <i>D, E</i> curving of the +blastula; <i>F</i> complete gastrula. <i>h</i> segmentation-cavity. <i>g</i> +primitive gut-cavity.</p> +</div> + +<p> +But the unequal gastrulation of the amphioxus diverges from the typical equal +cleavage of the <i>Sagitta,</i> the <i>Monoxenia</i> (Fig. 29), and the +<i>Olynthus</i> (Fig. 36), in another important particular. The pure +archigastrula of the latter forms is uni-axial, and it is round in its whole +length in transverse section. The vegetal pole of the vertical axis is just in +the centre of the primitive mouth. This is not the case in the gastrula of the +amphioxus. During the folding of the blastula the ideal axis is already bent on +one side, the growth of the blastoderm (or the increase of its cells) being +brisker on one side than on the other; the side that grows more quickly, and so +is more curved (Fig. 39 <i> v</i>), will be the anterior or belly-side, the +opposite, flatter side will form the back (<i>d</i>). The primitive mouth, +which at first, in the typical archigastrula, lay at the vegetal pole of the +main axis, is forced away to the dorsal side; and whereas its two lips lay at +first in a plane at right angles to the chief axis, they are now so far thrust +aside that their plane cuts the axis at a sharp angle. The dorsal lip is +therefore the upper and more forward, the ventral lip the lower and hinder. In +the latter, at the ventral passage of the entoderm into the ectoderm, there lie +side by side a pair of very large cells, one to the right and one to the left +(Fig. 39 <i>p</i>): these are the important polar cells of the primitive mouth, +or “the primitive cells of the mesoderm.” In consequence of these +considerable variations arising in the course of the gastrulation, the +primitive uni-axial form of the archigastrula in the amphioxus has already +become tri-axial, and thus the two-sidedness, or bilateral symmetry, of the +vertebrate body has already been determined. This has been transmitted from the +amphioxus to all the other modified gastrula-forms of the vertebrate stem. +</p> + +<p> +Apart from this bilateral structure, the gastrula of the amphioxus resembles +the typical archigastrula of the lower animals (Figs. 30–36) in +developing the two primary germinal layers from a single layer of cells. This +is clearly the oldest and original form of the metazoic embryo. Although the +animals I have mentioned belong to the most diverse classes, they nevertheless +agree with each other, and many more animal forms, in having retained to the +present day, by a conservative heredity, this palingenetic form of gastrulation +which they have from their +<span class='pagenum'><a name="Page_67" id="Page_67"></a></span> +earliest common ancestors. But this is not the case with the great majority of +the animals. With these the original embryonic process has been gradually more +or less altered in the course of millions of years by adaptation to new +conditions of development. Both the segmentation of the ovum and the subsequent +gastrulation have in this way been considerably changed. In fact, these +variations have become so great in the course of time that the segmentation was +not rightly understood in most animals, and the gastrula was unrecognised. It +was not until I had made an extensive comparative study, lasting a considerable +time (in the years 1866–75), in animals of the most diverse classes, that +I succeeded in showing the same common typical process in these apparently very +different forms of gastrulation, and tracing them all to one original form. I +regard all those that diverge from the primary palingenetic gastrulation as +secondary, modified, and cenogenetic. The more or less divergent form of +gastrula that is produced may be called a secondary, modified gastrula, or a +<i> metagastrula.</i> The reader will find a scheme of these different kinds of +segmentation and gastrulation at the close of this chapter. +</p> + +<p> +By far the most important process that determines the various cenogenetic forms +of gastrulation is the change in the nutrition of the ovum and the accumulation +in it of nutritive yelk. By this we understand various chemical substances +(chiefly granules of albumin and fat-particles) which serve exclusively as +reserve-matter or food for the embryo. As the metazoic embryo in its earlier +stages of development is not yet able to obtain its food and so build up the +frame, the necessary material has to be stored up in the ovum. Hence we +distinguish in the ova two chief elements—the active formative yelk +(protoplasm) and the passive food-yelk (deutoplasm, wrongly spoken of as +“the yelk”). In the little palingenetic ova, the segmentation of +which we have already considered, the yelk-granules are so small and so +regularly distributed in the protoplasm of the ovum that the even and repeated +cleavage is not affected by them. But in the great majority of the animal ova +the food-yelk is more or less considerable, and is stored in a certain part of +the ovum, so that even in the unfertilised ovum the “granary” can +clearly be distinguished from the formative plasm. As a rule, the +formative-yelk (with the germinal vesicle) then usually gathers at one pole and +the food-yelk at the other. The first is the <i> animal,</i> and the second the +<i>vegetal,</i> pole of the vertical axis of the ovum. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus39"></a> +<img src="images/fig39.gif" width="208" height="155" alt="Fig.39 Gastrula of the +amphioxus, seen from left side." /> +<p class="caption">Fig. 39—<b>Gastrula of the amphioxus, seen from +left side</b> (diagrammatic median section). (From <i>Hatschek.</i>) <i>g</i> +primitive gut, <i>u</i> primitive mouth, <i>p</i> peristomal pole-cells, +<i>i</i> entoderm, <i>e</i> ectoderm, <i>d</i> dorsal side, <i>v</i> ventral +side.</p> +</div> + +<p>In these “telolecithal” ova, or ova with the yelk at +one end (for instance, in the cyclostoma and amphibia), the gastrulation then +usually takes place in such a way that in the cleavage of the impregnated ovum +the animal (usually the upper) half splits up more quickly than the vegetal +(lower). The contractions of the active protoplasm, which effect this continual +cleavage of the cells, meet a greater resistance in the lower vegetal half from +the passive deutoplasm than in the upper animal half. Hence we find in the +latter more but smaller, and in the former fewer but larger, cells. The animal +cells produce the external, and the vegetal cells the internal, germinal layer. +</p> + +<p> +Although this unequal segmentation of the cyclostoma, ganoids, and amphibia +seems at first sight to differ from the original equal segmentation (for +instance, in the monoxenia, Fig. 29), they both have this in common, that the +cleavage process throughout affects the <i>whole</i> cell; hence Remak called +it <i>total</i> segmentation, and the ova in question <i>holoblastic,</i> or +“whole-cleaving.” It is otherwise with the second chief group of +ova, which he distinguished from these as <i> meroblastic,</i> or +“partially-cleaving ”: to this class belong the familiar large eggs +of birds and reptiles, and of most fishes. The inert mass of the passive +food-yelk is so +<span class='pagenum'><a name="Page_68" id="Page_68"></a></span> +large in these cases that the protoplasmic contractions of the active yelk +cannot effect any further cleavage. In consequence, there is only a partial +segmentation. While the protoplasm in the animal section of the ovum continues +briskly to divide, multiplying the nuclei, the deutoplasm in the vegetal +section remains more or less undivided; it is merely consumed as food by the +forming cells. The larger the accumulation of food, the more restricted is the +process of segmentation. It may, however, continue for some time (even after +the gastrulation is more or less complete) in the sense that the vegetal +cell-nuclei distributed in the deutoplasm slowly increase by cleavage; as each +of them is surrounded by a small quantity of protoplasm, it may afterwards +appropriate a portion of the food-yelk, and thus form a real +“yelk-cell” (<i>merocyte</i>). When this vegetal cell-formation +continues for a long time, after the two primary germinal layers have been +formed, it takes the name of the “after-segmentation.” +</p> + +<p> +The meroblastic ova are only found in the larger and more highly developed +animals, and only in those whose embryo needs a longer time and richer +nourishment within the fœtal membranes. According as the yelk-food accumulates +at the centre or at the side of the ovum, we distinguish two groups of dividing +ova, periblastic and discoblastic. In the periblastic the food-yelk is in the +centre, enclosed inside the ovum (hence they are also called +“centrolecithal” ova): the formative yelk surrounds the food-yelk, +and so suffers itself a superficial cleavage. This is found among the +articulates (crabs, spiders, insects, etc.). In the discoblastic ova the +food-yelk gathers at one side, at the vegetal or lower pole of the vertical +axis, while the nucleus of the ovum and the great bulk of the formative yelk +lie at the upper or animal pole (hence these ova are also called +“telolecithal”). In these cases the cleavage of the ovum begins at +the upper pole, and leads to the formation of a dorsal discoid embryo. This is +the case with all meroblastic vertebrates, most fishes, the reptiles and birds, +and the oviparous mammals (the monotremes). +</p> + +<p> +The gastrulation of the discoblastic ova, which chiefly concerns us, offers +serious difficulties to microscopic investigation and philosophic +consideration. These, however, have been mastered by the comparative +embryological research which has been conducted by a number of distinguished +observers during the last few decades—especially the brothers Hertwig, +Rabl, Kupffer, Selenka, Rückert, Goette, Rauber, etc. These thorough and +careful studies, aided by the most perfect modern improvements in technical +method (in tinting and dissection), have given a very welcome support to the +views which I put forward in my work, <i>On the Gastrula and the Segmentation +of the Animal Ovum</i> [not translated], in 1875. As it is very important to +understand these views and their phylogenetic foundation clearly, not only as +regards evolution in general, but particularly in connection with the genesis +of man, I will give here a brief statement of them as far as they concern the +vertebrate-stem:— +</p> + +<p> +1. All the vertebrates, including man, are phylogenetically (or genealogically) +related—that is, are members of one single natural stem. +</p> + +<p> +2. Consequently, the embryonic features in their individual development must +also have a genetic connection. +</p> + +<p> +3. As the gastrulation of the amphioxus shows the original palingenetic form in +its simplest features, that of the other vertebrates must have been derived +from it. +</p> + +<p> +4. The cenogenetic modifications of the latter are more appreciable the more +food-yelk is stored up in the ovum. +</p> + +<p> +5. Although the mass of the food-yelk may be very large in the ova of the +discoblastic vertebrates, nevertheless in every case a blastula is developed +from the morula, as in the holoblastic ova. +</p> + +<p> +6. Also, in every case, the gastrula develops from the blastula by curving or +invagination. +</p> + +<p> +7. The cavity which is produced in the fœtus by this curving is, in each case, +the primitive gut (progaster), and its opening the primitive mouth (prostoma). +</p> + +<p> +8. The food-yelk, whether large or small, is always stored in the ventral wall +of the primitive gut; the cells (called “merocytes”) which may be +formed in it subsequently (by “after-segmentation”) also belong to +the inner germinal layer, like the cells which immediately enclose the +primitive gut-cavity. +</p> + +<p> +9. The primitive mouth, which at first lies below at the lower pole of the +vertical axis, is forced, by the growth of the yelk, backwards and then +upwards, +<span class='pagenum'><a name="Page_69" id="Page_69"></a></span> +towards the dorsal side of the embryo; the vertical axis of the primitive gut +is thus gradually converted into horizontal. +</p> + +<p> +10. The primitive mouth is closed sooner or later in all the vertebrates, and +does not evolve into the permanent mouth-aperture; it rather corresponds to the +“properistoma,” or region of the anus. From this important point +the formation of the middle germinal layer proceeds, between the two primary +layers. +</p> + +<p> +The wide comparative studies of the scientists I have named have further shown +that in the case of the discoblastic higher vertebrates (the three classes of +amniotes) the primitive mouth of the embryonic disc, which was long looked for +in vain, is found always, and is nothing else than the familiar +“primitive groove.” Of this we shall see more as we proceed. +Meantime we realise that gastrulation may be reduced to one and the same +process in all the vertebrates. Moreover, the various forms it takes in the +invertebrates can always be reduced to one of the four types of segmentation +described above. In relation to the distinction between total and partial +segmentation, the grouping of the various forms is as follows:— +</p> + +<table class="text" border="1" cellspacing="0" cellpadding="4" summary= +"Grouping of various forms showing distinction between total and partial +segmentation."> +<tr> +<td>I. Palingenetic<br/> + (primitive) segmentation.</td> <td>1. +Equal segmentation<br/> (bell-gastrula).</td> <td +align="center" valign="middle" rowspan="2">A. Total segmentation<br/> (without +independent<br/> food-yelk).</td> </tr> + +<tr> +<td rowspan="3">II. Cenogenetic segmentation<br/> + (modified by adaptation).</td> <td +align="left">2. Unequal segmentation<br/> (hooded +gastrula).</td> </tr> + +<tr> +<td>3. Discoid segmentation<br/> (discoid +gastrula).</td> <td align="center" valign="middle" rowspan="2">B. Partial +segmentation<br/> (with independent<br/> food-yelk).</td> </tr> + +<tr> +<td>4. Superficial segmentation<br/> + (spherical gastrula).</td> </tr> +</table> + +<p> +The lowest metazoa we know—namely, the lower zoophyta (sponges, simple +polyps, etc.)—remain throughout life at a stage of development which +differs little from the gastrula; their whole body consists of two layers of +cells. This is a fact of extreme importance. We see that man, and also other +vertebrates, pass quickly through a stage of development in which they consist +of two layers, just as these lower zoophyta do throughout life. If we apply our +biogenetic law to the matter, we at once reach this important conclusion. +“Man and all the other animals which pass through the two-layer stage, or +gastrula-form, in the course of their embryonic development, must descend from +a primitive simple stem-form, the whole body of which consisted throughout life +(as is the case with the lower zoophyta to-day) merely of two cell-strata or +germinal layers.” We will call this primitive stem-form, with which we +shall deal more fully later on, the <i> gastræa</i>—that is to say, +“primitive-gut animal.” +</p> + +<p> +According to this gastræa-theory there was originally in all the multicellular +animals <i>one organ</i> with the same structure and function. This was the +primitive gut; and the two primary germinal layers which form its wall must +also be regarded as identical in all. This important homology or identity of +the primary germinal layers is proved, on the one hand, from the fact that the +gastrula was originally formed in the same way in all cases—namely, by +the curving of the blastula; and, on the other hand, by the fact that in every +case the same fundamental organs arise from the germinal layers. The outer or +animal layer, or ectoderm, always forms the chief organs of animal +life—the skin, nervous system, sense-organs, etc.; the inner or vegetal +layer, or entoderm, gives rise to the chief organs of vegetative life—the +organs of nourishment, digestion, blood-formation, etc. +</p> + +<p> +In the lower zoophyta, whose body remains at the two-layer stage throughout +life, the gastræads, the simplest sponges (<i>Olynthus</i>), and polyps +(<i>Hydra</i>), these two groups of functions, animal and vegetative, are +strictly divided between the two simple primary layers. Throughout life the +outer or animal layer acts simply as a covering for the body, and accomplishes +its movement and sensation. The inner or vegetative layer of cells acts +throughout life as a gut-lining, or nutritive layer of enteric cells, and often +also yields the reproductive cells. +</p> + +<p> +The best known of these “gastræads,” or “gastrula-like +animals,” is the common fresh-water polyp (<i>Hydra</i>). This simplest +of all the cnidaria has, it is true, a crown of tentacles round its mouth. Also +its outer germinal layer has certain special modifications. But these are +secondary additions, and the inner germinal layer is a simple stratum of cells. +On the whole, the hydra has preserved to our day by heredity the simple +structure of our primitive ancestor, the <i> gastræa</i> (cf. Chapter XIX). +</p> + +<p> +<span class='pagenum'><a name="Page_70" id="Page_70"></a></span>In all other +animals, particularly the vertebrates, the gastrula is merely a brief +transitional stage. Here the two-layer stage of the embryonic development is +quickly succeeded by a three-layer, and then a four-layer, stage. With the +appearance of the four superimposed germinal layers we reach again a firm and +steady standing-ground, from which we may follow the further, and much more +difficult and complicated, course of embryonic development. +</p> + +<p class="center"> +SUMMARY OF THE CHIEF DIFFERENCES IN THE OVUM-SEGMENTATION AND GASTRULATION OF +ANIMALS.<br/> +The animal stems are indicated by the +letters <i> a–g</i>: <i>a</i> Zoophyta. <i>b</i> Annelida.<br/> <i>c</i> +Mollusca. <i>d</i> Echinoderma. <i>e</i> Articulata. <i> f</i> Tunicata. +<i>g</i> Vertebrata. +</p> + +<table class="text" border="1" cellspacing="0" cellpadding="4" summary= +"Summary of the chief differences in the ovum-segmentation and gastrulation of +animals."> +<tr> +<td align="center" valign="middle" rowspan="2"><b>I.<br/> Total<br/> +Segmentation.</b><br/> Holoblastic ova.<br/> <br/> <br/> <br/> <br/> <br/> +<br/> <b>Gastrula without<br/> separate<br/> food-yelk.</b><br/> +Hologastrula.</td> <td align="center" valign="middle"><b>I. Primitive<br/> +Segmentation.</b><br/> Archiblastic ova.<br/> <br/> <b>Bell-gastrula</b><br/> +(archigastrula.)</td> <td><i>a.</i> Many lower zoophyta +(sponges,<br/> hydrapolyps, medusæ, simpler +corals).<br/> <i>b.</i> Many lower annelids (sagitta, phoronis,<br/> + many nematoda, etc., terebratula, argiope,<br/> + pisidium).<br/> <i>c.</i> Some lower molluscs.<br/> +<i>d.</i> Many echinoderms.<br/> <i>e.</i> A few lower articulata (some +brachiopods,<br/> copepods: Tardigrades, +pteromalina).<br/> <i>f.</i> Many tunicata.<br/> <i>g.</i> The acrania +(amphioxus).</td> </tr> + +<tr> +<td align="center" valign="middle"><b>II. Unequal<br/> Segmentation.</b><br/> +Amphiblastic ova.<br/> <br/> <b>Hooded-gastrula</b><br/> (amphigastrula).</td> +<td><i>a.</i> Many zoophyta (sponges, medusæ,<br/> + corals, siphonophoræ, ctenophora).<br/> <i>b.</i> Most +worms.<br/> <i>c.</i> Most molluscs.<br/> <i>d.</i> Many echinoderms +(viviparous species and<br/> some others).<br/> +<i>e.</i> Some of the lower articulata (both crustacea<br/> + and tracheata).<br/> <i>f.</i> Many tunicata.<br/> +<i>g.</i> Cyclostoma, the oldest fishes, amphibia,<br/> + mammals (not including man).</td> </tr> + +<tr> +<td align="center" valign="middle" rowspan="2"><b>II.<br/> Partial +Segmentation.</b><br/> Meroblastic ova.<br/> <br/> <b>Gastrula with<br/> +separate<br/> food-yelk.</b><br/> Merogastrula.</td> <td align="center" +valign="middle"><b>III. Discoid<br/> Segmentation.</b><br/> Discoblastic +ova.<br/> <br/> <b>Discoid gastrula.</b></td> <td><i>c.</i> +Cephalopods or cuttlefish.<br/> <i>e.</i> Many articulata, wood-lice, +scorpions, etc.<br/> <i>g.</i> Primitive fishes, bony fishes, reptiles, +birds,<br/> monotremes.</td> </tr> + +<tr> +<td align="center" valign="middle"><b>IV. Superficial<br/> +Segmentation.</b><br/> Periblastic ova.<br/> <b>Spherical-gastrula.</b></td> +<td><i>e.</i> The great majority of the articulata<br/> + (crustaceans, myriapods, arachnids, insects).</td> +</tr> +</table> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap09"></a> +<span class='pagenum'><a name="Page_71" id="Page_71"></a></span>Chapter IX.<br/> +THE GASTRULATION OF THE VERTEBRATE<a href="#linknote-20" name="linknoteref-20" id="linknoteref-20"><sup>[20]</sup></a></h2> + +<p class="footnote"> +<a name="linknote-20" id="linknote-20"></a> <a href="#linknoteref-20">[20]</a> +Cf. Balfour’s <i>Manual of Comparative Embryology,</i> vol. ii; Theodore +Morgan’s <i>The Development of the Frog’s Egg.</i> +</p> + +<p> +The remarkable processes of gastrulation, ovum-segmentation, and formation of +germinal layers present a most conspicuous variety. There is to-day only the +lowest of the vertebrates, the amphioxus, that exhibits the original form of +those processes, or the palingenetic gastrulation which we have considered in +the preceding chapter, and which culminates in the formation of the +archigastrula (Fig. 38). In all other extant vertebrates these fundamental +processes have been more or less modified by adaptation to the conditions of +embryonic development (especially by changes in the food-yelk); they exhibit +various cenogenetic types of the formation of germinal layers. However, the +different classes vary considerably from each other. In order to grasp the +unity that underlies the manifold differences in these phenomena and their +historical connection, it is necessary to bear in mind always the unity of the +vertebrate-stem. This “phylogenetic unity,” which I developed in my +<i>General Morphology</i> in 1866, is now generally admitted. All impartial +zoologists agree to-day that all the vertebrates, from the amphioxus and the +fishes to the ape and man, descend from a common ancestor, “the primitive +vertebrate.” Hence the embryonic processes, by which each individual +vertebrate is developed, must also be capable of being reduced to one common +type of embryonic development; and this primitive type is most certainly +exhibited to-day by the amphioxus. +</p> + +<p> +It must, therefore, be our next task to make a comparative study of the various +forms of vertebrate gastrulation, and trace them backwards to that of the +lancelet. Broadly speaking, they fall first into two groups: the older +cyclostoma, the earliest fishes, most of the amphibia, and the viviparous +mammals, have <i> holoblastic</i> ova—that is to say, ova with total, +unequal segmentation; while the younger cyclostoma, most of the fishes, the +cephalopods, reptiles, birds, and monotremes, have <i> meroblastic</i> ova, or +ova with partial discoid segmentation. A closer study of them shows, however, +that these two groups do not present a natural unity, and that the historical +relations between their several divisions are very complicated. In order to +understand them properly, we must first consider the various modifications of +gastrulation in these classes. We may begin with that of the amphibia. +</p> + +<p> +The most suitable and most available objects of study in this class are the +eggs of our indigenous amphibia, the tailless frogs and toads, and the tailed +salamander. In spring they are to be found in clusters in every pond, and +careful examination of the ova with a lens is sufficient to show at least the +external features of the segmentation. In order to understand the whole process +rightly and follow the formation of the germinal layers and the gastrula, the +ova of the frog and salamander must be carefully hardened; then the thinnest +possible sections must be made of the hardened ova with the microtome, and the +tinted sections must be very closely compared under a powerful microscope. +</p> + +<p> +The ova of the frog or toad are globular in shape, about the twelfth of an inch +in diameter, and are clustered in jelly-like masses, which are lumped together +in the case of the frog, but form long strings in the case of the toad. When we +examine the opaque, grey, brown, or blackish ova closely, we find that the +upper half is darker than the lower. The middle of the upper half is in many +species black, while the middle of the lower half is white.<a href="#linknote-21" name="linknoteref-21" id="linknoteref-21"><sup>[21]</sup></a> In this +way we get a definite axis of the ovum with two poles. To give a clear +<span class='pagenum'><a name="Page_72" id="Page_72"></a></span> +idea of the segmentation of this ovum, it is best to compare it with a globe, +on the surface of which are marked the various parallels of longitude and +latitude. The superficial dividing lines between the different cells, which +come from the repeated segmentation of the ovum, look like deep furrows on the +surface, and hence the whole process has been given the name of furcation. In +reality, however, this “furcation,” which was formerly regarded as +a very mysterious process, is nothing but the familiar, repeated +cell-segmentation. Hence also the segmentation-cells which result from it are +real cells. +</p> + +<p class="footnote"> +<a name="linknote-21" id="linknote-21"></a> <a href="#linknoteref-21">[21]</a> +The colouring of the eggs of the amphibia is caused by the accumulation of +dark-colouring matter at the animal pole of the ovum. In consequence of this, +the animal cells of the ectoderm are darker than the vegetal cells of the +entoderm. We find the reverse of this in the case of most animals, the +protoplasm of the entoderm cells being usually darker and coarser-grained. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus40"></a> +<img src="images/fig40.gif" width="354" height="291" alt="Fig.40. The cleavage of the frog’s +ovum." /> +<p class="caption">Fig. 40—<b>The cleavage of the frog’s ovum</b> +(magnified). A stem-cell. <i> B</i> the first two segmentation-cells. <i>C</i> four cells. <i> +D</i> eight cells (4 animal and 4 vegetative). <i>E</i> twelve cells (8 animal +and 4 vegetative). <i>F</i> sixteen cells (8 animal and 8 vegetative). <i>G</i> +twenty-four cells (16 animal and 8 vegetative). <i>H</i> thirty-two cells. +<i>I</i> forty-eight cells. <i>K</i> sixty-four cells. <i>L</i> ninety-six +cells. <i>M</i> 160 cells (128 animal and 32 vegetative).</p> +</div> + +<p> +The unequal segmentation which we observe in the ovum of the amphibia has the +special feature of beginning at the upper and darker pole (the north pole of +the terrestrial globe in our illustration), and slowly advancing towards the +lower and brighter pole (the south pole). Also the upper and darker hemisphere +remains in this position throughout the course of the segmentation, and its +cells multiply much more briskly. Hence the cells of the lower hemisphere are +found to be larger and less numerous. The cleavage of the stem-cell (Fig. 40 +<i>A</i>) begins with the formation of a complete furrow, which starts from the +north pole and reaches to the south (<i>B</i>). An hour later a second furrow +arises in the same way, and this cuts the first at a right angle (Fig. 40 <i> +C</i>). The ovum is thus divided into four equal parts. Each of these four +“segmentation cells” has an upper and darker and a lower, brighter +half. A few hours later a third furrow appears, vertically to the first two +(Fig. 40 <i>D</i>). The globular germ now consists of eight cells, four smaller +ones above (northern) and four larger ones below (southern). Next, each of the +four upper ones divides into two halves by a cleavage beginning from the north +pole, so that we now have eight above and four below (Fig. 40 <i>E</i>). Later, +the +<span class='pagenum'><a name="Page_73" id="Page_73"></a></span> +four new longitudinal divisions extend gradually to the lower cells, and the +number rises from twelve to sixteen (<i>F</i>). Then a second circular furrow +appears, parallel to the first, and nearer to the north pole, so that we may +compare it to the north polar circle. In this way we get twenty-four +segmentation-cells—sixteen upper, smaller, and darker ones, and eight +smaller and brighter ones below (<i>G</i>). Soon, however, the latter also +sub-divide into sixteen, a third or “meridian of latitude” +appearing, this time in the southern hemisphere: this makes thirty-two cells +altogether (<i>H</i>). Then eight new longitudinal lines are formed at the +north pole, and these proceed to divide, first the darker cells above and +afterwards the lighter southern cells, and finally reach the south pole. In +this way we get in succession forty, forty-eight, fifty-six, and at last +sixty-four cells (<i>I, K</i>). In the meantime, the two hemispheres differ +more and more from each other. Whereas the sluggish lower hemisphere long +remains at thirty-two cells, the lively northern hemisphere briskly sub-divides +twice, producing first sixty-four and then 128 cells (<i>L, M</i>). Thus we +reach a stage in which we count on the surface of the ovum 128 small cells in +the upper half and thirty-two large ones in the lower half, or 160 altogether. +The dissimilarity of the two halves increases: while the northern breaks up +into a great number of small cells, the southern consists of a much smaller +number of larger cells. Finally, the dark cells of the upper half grow almost +over the surface of the ovum, leaving only a small circular spot +<span class='pagenum'><a name="Page_74" id="Page_74"></a></span> +</p> + +<p> +at the south pole, where the large and clear cells of the lower half are +visible. This white region at the south pole corresponds, as we shall see +afterwards, to the primitive mouth of the gastrula. The whole mass of the inner +and larger and clearer cells (including the white polar region) belongs to the +entoderm or ventral layer. The outer envelope of dark smaller cells forms the +ectoderm or skin-layer. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus41"></a> +<img src="images/fig41.gif" width="286" height="328" alt="Figs. 41-44. Four vertical sections +of the fertilised ovum of the toad, in four successive stages of development." /> +<p class="caption">Figs. 41–44—<b>Four vertical sections of the +fertilised ovum of the toad,</b> in four successive stages of development. The +letters have the same meaning throughout: <i>F</i> segmentation-cavity. +<i>D</i> covering of same (<i>D</i> dorsal half of the embryo, <i>P</i> ventral +half). <i>P</i> yelk-stopper (white round field at the lower pole). <i>Z</i> +yelk-cells of the entoderm (Remak’s “glandular embryo”). +<i>N</i> primitive gut cavity (progaster or Rusconian alimentary cavity). The +primitive mouth (prostoma) is closed by the yelk-stopper, <i>P. s</i> partition +between the primitive gut cavity (<i>N</i>) and the segmentation cavity +(<i>F</i>). <i>k k′,</i> section of the large circular lip-border of the +primitive mouth (the Rusconian anus). The line of dots between <i>k</i> and +<i>k′</i> indicates the earlier connection of the yelk-stopper +(<i>P</i>) with the central mass of the yelk-cells (<i>Z</i>). In Fig. 44 the +ovum has turned 90°, so that the back of the embryo is uppermost and the +ventral side down. (From <i>Stricker.</i>).</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus45"></a> +<img src="images/fig45.gif" width="193" height="181" alt="Blastula of the +water-salamander." /> +<p class="caption">Fig. 45—<b>Blastula of the water-salamander</b> +(<i>Triton</i>). <i>fh</i> segmentation-cavity, <i>dz</i> yelk-cells, <i>rz</i> +border-zone. (From <i>Hertwig.</i>)</p> +</div> + +<p> +In the meantime, a large cavity, full of fluid, has been formed within the +globular body—the segmentation-cavity or embryonic cavity +(<i>blastocœl,</i> Figs. 41–44 <i>F</i>). It extends considerably as the +cleavage proceeds, and afterwards assumes an almost semi-circular form (Fig. 41 +<i>F</i>). The frog-embryo now represents a modified embryonic vesicle or +<i>blastula,</i> with hollow animal half and solid vegetal half. +</p> + +<p> +Now a second, narrower but longer, cavity arises by a process of folding at the +lower pole, and by the falling away from each other of the white entoderm-cells +(Figs. 41–44 <i>N</i>). This is the primitive gut-cavity or the gastric +cavity of the gastrula, progaster or archenteron. It was first observed in the +ovum of the amphibia by Rusconi, and so called the Rusconian cavity. The reason +of its peculiar narrowness here is that it is, for the most part, full of +yelk-cells of the entoderm. These also stop up the whole of the wide opening of +the primitive mouth, and form what is known as the “yelk-stopper,” +which is seen freely at the white round spot at the south pole (<i>P</i>). +Around it the ectoderm is much thicker, and forms the border of the primitive +mouth, the most important part of the embryo (Fig. 44 <i>k, k′</i>). +Soon the primitive gut-cavity stretches further and further at the expense of +the segmentation-cavity (<i>F</i>), until at last the latter disappears +altogether. The two cavities are only separated by a thin partition (Fig. 43 +<i>s</i>). With the formation of the primitive gut our frog-embryo has reached +the gastrula stage, though it is clear that this cenogenetic amphibian gastrula +is very different from the real palingenetic gastrula we have considered (Figs. +30–36). +</p> + +<p> +In the growth of this hooded gastrula we cannot sharply mark off the various +stages which we distinguish successively in the bell-gastrula as morula and +gastrula. Nevertheless, it is not difficult to reduce the whole cenogenetic or +disturbed development of this amphigastrula to the true palingenetic formation +of the archigastrula of the amphioxus. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus46"></a> +<img src="images/fig46.gif" width="187" height="109" alt="Fig.46. Embryonic vesicle of triton." /> +<p class="caption">Fig. 46—<b>Embryonic vesicle of triton</b> +(<i>blastula</i>), outer view, with the transverse fold of the primitive mouth +(<i>u</i>). (From <i>Hertwig.</i>)</p> +</div> + +<p>This reduction becomes easier if, after considering the +gastrulation of the tailless amphibia (frogs and toads), we glance for a moment +at that of the tailed amphibia, the salamanders. In some of the latter, that +have only recently been carefully studied, and that are phylogenetically older, +the process is much simpler and clearer than is the case with the former and +longer known. Our common water-salamander (<i>Triton taeniatus</i>) is a +particularly good subject for observation. Its nutritive yelk is much smaller +and its formative yelk less obscured with black pigment-cells than in the case +of the frog; and its gastrulation has better retained the original palingenetic +character. It was first described by Scott and Osborn (1879), and Oscar Hertwig +especially made a careful study of it (1881), and rightly pointed out its great +importance in helping us to understand the vertebrate development. Its globular +blastula (Fig. 45) consists of loosely-aggregated, +<span class='pagenum'><a name="Page_75" id="Page_75"></a></span> +yelk-filled entodermic cells or yelk-cells (<i>dz</i>) in the lower vegetal +half; the upper, animal half encloses the hemispherical segmentation-cavity +(<i>fh</i>), the curved roof of which is formed of two or three strata of small +ectodermic cells. At the point where the latter pass into the former (at the +equator of the globular vesicle) we have the border zone (<i>rz</i>). The +folding which leads to the formation of the gastrula takes place at a spot in +this border zone, the primitive mouth (Fig. 46 <i>u</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus47"></a> +<img src="images/fig47.gif" width="211" height="186" alt="Fig. 47 Sagittal section of a +hooded-embryo (depula) of triton." /> +<p class="caption">Fig. 47—<b>Sagittal section of a hooded-embryo</b> +(<i>depula</i>) <b>of triton</b> (blastula at the commencement of +gastrulation). <i>ak</i> outer germinal layer, <i>ik</i> inner germinal layer, +<i>fh</i> segmentation-cavity, ud primitive gut, <i>u</i> primitive mouth, +<i>dl</i> and <i>vl</i> dorsal and ventral lips of the mouth, <i>dz</i> +yelk-cells. (From <i> Hertwig.</i>)</p> +</div> + +<p> +Unequal segmentation takes place in some of the cyclostoma and in the oldest +fishes in just the same way as in most of the amphibia. Among the cyclostoma +(“round-mouthed”) the familiar lampreys are particularly +interesting. In respect of organisation and development they are half-way +between the acrania (lancelet) and the lowest real fishes (<i>Selachii</i>); +hence I divided the group of the cyclostoma in 1886 from the real fishes with +which they were formerly associated, and formed of them a special class of +vertebrates. The ovum-segmentation in our common river-lamprey (<i>Petromyzon +fluviatilis</i>) was described by Max Schultze in 1856, and afterwards by Scott +(1882) and Goette (1890). +</p> + +<p> +Unequal total segmentation follows the same lines in the oldest fishes, the +selachii and ganoids, which are directly descended from the cyclostoma. The +primitive fishes (<i>Selachii</i>), which we must regard as the ancestral group +of the true fishes, were generally considered, until a short time ago, to be +discoblastic. It was not until the beginning of the twentieth century that +Bashford Dean made the important discovery in Japan that one of the oldest +living fishes of the shark type (<i>Cestracion japonicus</i>) has the same +total unequal segmentation as the amphiblastic plated fishes +(<i>ganoides</i>).<a href="#linknote-22" name="linknoteref-22" id="linknoteref-22"><sup>[22]</sup></a> This is particularly interesting in connection +with our subject, because the few remaining survivors of this division, which +was so numerous in paleozoic times, exhibit three different types of +gastrulation. +</p> + +<p class="footnote"> +<a name="linknote-22" id="linknote-22"></a> <a href="#linknoteref-22">[22]</a> +Bashford Dean, <i>Holoblastic Cleavage in the Egg of a Shark, Cestracion +japonicus Macleay. Annotationes zoologicae japonenses,</i> vol. iv, Tokio, +1901. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus48"></a> +<img src="images/fig48.gif" width="215" height="179" alt="Fig. 48 Sagittal section of the +gastrula of the water-salamander." /> +<p class="caption">Fig. 48—<b>Sagittal section of the gastrula of the +water-salamander</b> (<i>Triton</i>). (From <i>Hertwig.</i>) Letters as in Fig. +47; except—<i>p</i> yelk-stopper, <i> mk</i> beginning of the middle +germinal layer.)</p> +</div> + +<p> +The oldest and most conservative forms of the modern ganoids are the scaly +sturgeons (Sturiones), plated fishes of great evolutionary importance, the eggs +of which are eaten as caviar; their cleavage is not essentially different from +that of the lampreys and the amphibia. On the other hand, the most modern of +the plated fishes, the beautifully scaled bony pike of the North American +rivers (Lepidosteus), approaches the osseous fishes, and is discoblastic like +them. A third genus (Amia) is midway between the sturgeons and the latter. +</p> + +<p> +The group of the lung-fishes (<i>Dipneusta</i> or <i>Dipnoi</i>) is closely +connected with the older ganoids. In respect of their whole organisation they +are midway between the gill-breathing fishes and the lung-breathing amphibia; +they share with the former the shape of the body and limbs, and with the latter +the form of the heart +<span class='pagenum'><a name="Page_76" id="Page_76"></a></span> +and lungs. Of the older dipnoi (<i>Paladipneusta</i>) we have now only one +specimen, the remarkable Ceratodus of East Australia; its amphiblastic +gastrulation has been recently explained by Richard Semon (cf. Chapter XXI). +That of the two modern dipneusta, of which <i> Protopterus</i> is found in +Africa and <i>Lepidosiren</i> in America, is not materially different. (Cf. +Fig. 51.) +</p> + +<div class="fig" style="width:100%;"> +<a name="illus49"></a> +<img src="images/fig49.gif" width="306" height="100" alt="Fig. 49. Ovum-segmentation in the lamprey." /> +<p class="caption">Fig. 49—<b>Ovum-segmentation of the lamprey</b> (<i>Petromyzon +fluviatalis</i>), in four successive stages. The small cells of the upper +(animal) hemisphere divide much more quickly than the cells of the lower +(vegetal) hemisphere.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus50"></a> +<img src="images/fig50.gif" width="468" height="152" alt="Fig.50. Gastrulation of the lamprey." /> +<p class="caption">Fig. 50—<b>Gastrulation of the lamprey</b> (<i>Petromyzon +fluviatilis</i>). A blastula, with wide embryonic cavity (blastocoel, +<i>bl</i>), <i>g</i> incipient invagination. <i>B</i> depula, with advanced +invagination, from the primitive mouth (<i>g</i>). <i>C</i> gastrula, with +complete primitive gut: the embryonic cavity has almost disappeared in +consequence of invagination.</p> +</div> + +<p> +All these amphiblastic vertebrates, <i>Petromyzon</i> and <i> Cestracion, +Accipenser</i> and <i>Ceratodus,</i> and also the salamanders and batrachia, +belong to the old, conservative groups of our stem. Their unequal +ovum-segmentation and gastrulation have many peculiarities in detail, but can +always be reduced with comparative ease to the original cleavage and +gastrulation of the lowest vertebrate, the amphioxus; and this is little +removed, as we have seen, from the very simple archigastrula of the +<i>Sagitta</i> and <i>Monoxenia</i> (see Fig. 29–36). All these and many +other classes of animals generally agree in the circumstance that in +segmentation their +<span class='pagenum'><a name="Page_77" id="Page_77"></a></span> +ovum divides into a large number of cells by repeated cleavage. All such ova +have been called, after Remak, “whole-cleaving” +(<i>holoblasta</i>), because their division into cells is complete or total. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus51"></a> +<img src="images/fig51.gif" width="314" height="332" alt="Fig.51. Gastrulation of ceratodus." /> +<p class="caption">Fig. +51—<b>Gastrulation of ceratodus</b> (from <i>Semon</i>). <i>A</i> and +<i>C</i> stage with four cells, <i>B</i> and <i>D</i> with sixteen cells. +<i>A</i> and <i>B</i> are seen from above, <i> C</i> and <i>D</i> sideways. +<i>E</i> stage with thirty-two cells; <i>F</i> blastula; <i>G</i> gastrula in +longitudinal section. <i> fh</i> segmentation-cavity. <i>gh</i> primitive gut +or gastric cavity.</p> +</div> + +<p> +In a great many other classes of animals this is not the case, as we find (in +the vertebrate stem) among the birds, reptiles, and most of the fishes; among +the insects and most of the spiders and crabs (of the articulates); and the +cephalopods (of the molluscs). In all these animals the mature ovum, and the +stem-cell that arises from it in fertilisation, consist of two different and +separate parts, which we have called formative yelk and nutritive yelk. The +formative yelk alone consists of living protoplasm, and is the active, +evolutionary, and nucleated part of the ovum; this alone divides in +segmentation, and produces the numerous cells which make up the embryo. On the +other hand, the nutritive yelk is merely a passive part of the contents of the +ovum, a subordinate element which contains nutritive material (albumin, fat, +etc.), and so represents in a sense the provision-store of the developing +embryo. The latter takes a quantity of food out of this store, and finally +consumes it all. Hence the nutritive yelk is of great indirect importance in +embryonic development, though it has no direct share in it. It either does not +divide at all, or only later on, and does not generally consist of cells. It is +sometimes large and sometimes small, but generally many times larger than the +formative yelk; and hence it is +<span class='pagenum'><a name="Page_78" id="Page_78"></a></span> +that it was formerly thought the more important of the two. As the respective +significance of these two parts of the ovum is often wrongly described, it must +be borne in mind that the nutritive yelk is only a secondary addition to the +primary cell, it is an inner enclosure, not an external appendage. All ova that +have this independent nutritive yelk are called, after Remak, +“partially-cleaving” (<i>meroblasta</i>). Their segmentation is +incomplete or partial. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus52"></a> +<img src="images/fig52.gif" width="202" height="123" alt="Fig.52. Ovum of a +deep-sea bony fish." /> +<p class="caption">Fig. 52—<b>Ovum of a deep-sea bony fish.</b> +<i>b</i> protoplasm of the stem-cell, <i>k</i> nucleus of same, <i>d</i> clear +globule of albumin, the nutritive yelk, <i>f</i> fat-globule of same, <i>c</i> +outer membrane of the ovum, or ovolemma.)</p> +</div> + +<p> +There are many difficulties in the way of understanding this partial +segmentation and the gastrula that arises from it. We have only recently +succeeded, by means of comparative research, in overcoming these difficulties, +and reducing this cenogenetic form of gastrulation to the original palingenetic +type. This is comparatively easy in the small meroblastic ova which contain +little nutritive yelk—for instance, in the marine ova of a bony fish, the +development of which I observed in 1875 at Ajaccio in Corsica. I found them +joined together in lumps of jelly, floating on the surface of the sea; and, as +the little ovula were completely transparent, I could easily follow the +development of the germ step by step. These ovula are glossy and colourless +globules of little more than the 50th of an inch. Inside a structureless, thin, +but firm membrane (<i>ovolemma,</i> Fig. 52 <i>c</i>) we find a large, quite +clear, and transparent globule of albumin (<i>d</i>). At both poles of its axis +this globule has a pit-like depression. In the pit at the upper, animal pole +(which is turned downwards in the floating ovum) there is a bi-convex lens +composed of protoplasm, and this encloses the nucleus (<i>k</i>); this is the +formative yelk of the stem-cell, or the germinal disk (<i>b</i>). The small +fat-globule (<i>f</i>) and the large albumin-globule (<i>d</i>) together form +the nutritive yelk. Only the formative yelk undergoes cleavage, the nutritive +yelk not dividing at all at first. +</p> + +<p> +The segmentation of the lens-shaped formative yelk (<i>b</i>) proceeds quite +independently of the nutritive yelk, and in perfect geometrical order. +</p> + +<p> +When the mulberry-like cluster of cells has been formed, the border-cells of +the lens separate from the rest and travel into the yelk and the border-layer. +From this the blastula is developed; the regular bi-convex lens being converted +into a disk, like a watch-glass, with thick borders. This lies on the upper and +less curved polar surface of the nutritive yelk like the watch glass on the +yelk. Fluid gathers between the outer layer and the border, and the +segmentation-cavity is formed. The gastrula is then formed by invagination, or +a kind of turning-up of the edge of the blastoderm. In this process the +segmentation-cavity disappears. +</p> + +<p> +The space underneath the entoderm corresponds to the primitive gut-cavity, and +is filled with the decreasing food-yelk (<i>n</i>). Thus the formation of the +gastrula of our fish is complete. In contrast to the two chief forms of +gastrula we considered previously, we give the name of discoid gastrula +(<i>discogastrula,</i> Fig. 54) to this third principal type. +</p> + +<p> +Very similar to the discoid gastrulation of the bony fishes is that of the hags +or myxinoida, the remarkable cyclostomes that live parasitically in the +body-cavity of fishes, and are distinguished by several notable peculiarities +from their nearest relatives, the lampreys. While the amphiblastic ova of the +latter are small and develop like those of the amphibia, the cucumber-shaped +ova of the hag are about an inch long, and form a discoid gastrula. Up to the +present it has only been observed in one species (<i>Bdellostoma Stouti</i>), +by Dean and Doflein (1898). +</p> + +<p> +It is clear that the important features which distinguish the discoid gastrula +from the other chief forms we have considered are determined by the large +food-yelk. This takes no direct part in the building of the germinal layers, +and completely fills the primitive gut-cavity of the gastrula, even protruding +at the mouth-opening. If we imagine the original bell-gastrula (Figs. +30–36) trying to swallow a +<span class='pagenum'><a name="Page_79" id="Page_79"></a></span> +ball of food which is much bigger than itself, it would spread out round it in +discoid shape in the attempt, just as we find to be the case here (Fig. 54). +Hence we may derive the discoid gastrula from the original bell-gastrula, +through the intermediate stage of the hooded gastrula. It has arisen through +the accumulation of a store of food-stuff at the vegetal pole, a +“nutritive yelk” being thus formed in contrast to the +“formative yelk.” Nevertheless, the gastrula is formed here, as in +the previous cases, by the folding or invagination of the blastula. We can, +therefore, reduce this cenogenetic form of the discoid segmentation to the +palingenetic form of the primitive cleavage. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus53"></a> +<img src="images/fig53.gif" width="420" height="153" alt="Fig.53. Ovum-segmentation of a bony fish." /> +<p class="caption">Fig. 53—<b>Ovum-segmentation of a bony fish.</b> <i>A</i> first +cleavage of the stem-cell (<i>cytula</i>), <i>B</i> division of same into four +segmentation-cells (only two visible), <i>C</i> the germinal disk divides into +the blastoderm (<i>b</i>) and the periblast (<i>p</i>). <i>d</i> nutritive +yelk, <i>f</i> fat-globule, <i>c</i> ovolemma, <i>z</i> space between the +ovolemma and the ovum, filled with a clear fluid.)</p> +</div> + +<p> +This reduction is tolerably easy and confident in the case of the small ovum of +our deep-sea bony fish, but it becomes difficult and uncertain in the case of +the large ova that we find in the majority of the other fishes and in all the +reptiles and birds. In these cases the food-yelk is, in the first place, +comparatively colossal, the formative yelk being almost invisible beside it; +and, in the second place, the food-yelk contains a quantity of different +elements, which are known as “yelk-granules, yelk-globules, yelk-plates, +yelk-flakes, yelk-vesicles,” and so on. Frequently these definite +elements in the yelk have been described as real cells, and it has been wrongly +stated that a portion of the embryonic body is built up from these cells. This +is by no means the case. In every case, however large it is—and even when +cell-nuclei travel into it during the cleavage of the border—the +nutritive yelk remains a dead accumulation of food, which is taken into the gut +during embryonic development and consumed by the embryo. The latter develops +solely from the living formative yelk of the stem-cell. This is equally true of +the ova of our small bony fishes and of the colossal ova of the primitive +fishes, reptiles, and birds. +</p> + +<p> +The gastrulation of the primitive fishes or selachii (sharks and rays) has been +carefully studied of late years by Ruckert, Rabl, and H.E. Ziegler in +particular, and is very important in the sense that this group is the oldest +among living fishes, and their gastrulation can be derived directly from that +of the cyclostoma by the accumulation of a large quantity of food-yelk. The +oldest sharks (<i>Cestracion</i>) still have the unequal segmentation inherited +from the cyclostoma. But while in this case, as in the case of the amphibia, +the small ovum completely divides into cells in segmentation, this is no longer +so in the great majority of the selachii (or <i>Elasmobranchii</i>). In these +the contractility of the active protoplasm no longer suffices to break up the +huge mass of the passive deutoplasm completely into cells; this is only +possible in the upper or dorsal part, but not in the lower or ventral section. +Hence we find in the primitive fishes a blastula with a small eccentric +segmentation-cavity (Fig. 55 <i>b</i>), the wall of which varies greatly in +composition. The circular border of the germinal disk which connects the roof +and floor of the segmentation-cavity corresponds to the border-zone at the +equator of the amphibian ovum. In the middle of its hinder border we have the +beginning of the invagination of the primitive gut +<span class='pagenum'><a name="Page_80" id="Page_80"></a></span> +(Fig. 56 <i>ud</i>); it extends gradually from this spot (which corresponds to +the Rusconian anus of the amphibia) forward and around, so that the primitive +mouth becomes first crescent-shaped and then circular, and, as it opens wider, +surrounds the ball of the larger food-yelk. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus54"></a> +<img src="images/fig54.gif" width="201" height="130" alt="Fig.54. Discoid +gastrula (discogastrula) of a bony fish." /> +<p class="caption">Fig. 54—<b>Discoid gastrula</b> +(<i>discogastrula</i>) <b>of a bony fish.</b> <i>e</i> ectoderm, <i>i</i> +entoderm, <i>w</i> border-swelling or primitive mouth, <i>n</i> albuminous +globule of the nutritive yelk, <i>f</i> fat-globule of same, <i>c</i> external +membrane (ovolemma), <i>d</i> partition between entoderm and ectoderm (earlier +the segmentation-cavity.)</p> +</div> + +<p> +Essentially different from the wide-mouthed discoid gastrula of most of the +selachii is the narrow-mouthed discoid gastrula (or <i> epigastrula</i>) of the +amniotes, the reptiles, birds, and monotremes; between the two—as an +intermediate stage—we have the <i>amphigastrula</i> of the amphibia. The +latter has developed from the amphigastrula of the ganoids and dipneusts, +whereas the discoid amniote gastrula has been evolved from the amphibian +gastrula by the addition of food-yelk. This change of gastrulation is still +found in the remarkable ophidia (<i>Gymnophiona, Cœcilia,</i> or +<i>Peromela</i>), serpent-like amphibia that live in moist soil in the tropics, +and in many respects represent the transition from the gill-breathing amphibia +to the lung-breathing reptiles. Their embryonic development has been explained +by the fine studies of the brothers Sarasin of <i>Ichthyophis glutinosa</i> at +Ceylon (1887), and those of August Brauer of the <i>Hypogeophis rostrata</i> in +the Seychelles (1897). It is only by the historical and comparative study of +these that we can understand the difficult and obscure gastrulation of the +amniotes. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus55"></a> +<img src="images/fig55.gif" width="210" height="121" alt="Fig. 55 Longitudinal +section through the blastula of a shark." /> +<p class="caption">Fig. 55—<b>Longitudinal section through the +blastula of a shark</b> (<i>Pristiuris</i>). (From <i>Ruckert.</i>) (Looked at +from the left; to the right is the hinder end, <i>H,</i> to the left the fore +end, <i>V.</i>) <i>B</i> segmentation-cavity, <i> kz</i> cells of the germinal +membrane, <i>dk</i> yelk-nuclei.</p> +</div> + +<p> +The bird’s egg is particularly important for our purpose, because most of +the chief studies of the development of the vertebrates are based on +observations of the hen’s egg during hatching. The mammal ovum is much +more difficult to obtain and study, and for this practical and obvious reason +very rarely thoroughly investigated. But we can get hens’ eggs in any +quantity at any time, and, by means of artificial incubation, follow the +development of the embryo step by step. The bird’s egg differs +considerably from the tiny mammal ovum in size, a large quantity of food-yelk +accumulating within the original yelk or the protoplasm of the ovum. This is +the yellow ball which we commonly call the yolk of the egg. In order to +understand the bird’s egg aright—for it is very often quite wrongly +explained—we must examine it in its original condition, and follow it +from the very beginning of its development in the bird’s ovary. We then +see that the original ovum is a quite small, naked, and simple cell with a +nucleus, not differing in either size or shape from the original ovum of the +mammals and other animals (cf. Fig. 13 <i> E</i>). As in the case of all the +craniota (animals with a skull), the original or primitive ovum +(<i>protovum</i>) is covered with a continuous layer of small cells. This +membrane is the follicle, from which the ovum afterwards issues. Immediately +underneath it the structureless yelk-membrane is secreted from the yelk. +</p> + +<p> +The small primitive ovum of the bird begins very early to take up into itself a +quantity of food-stuff through the yelk-membrane, and work it up into the +“yellow yelk.” In this way the ovum +<span class='pagenum'><a name="Page_81" id="Page_81"></a></span> +enters on its second stage (the metovum), which is many times larger than the +first, but still only a single enlarged cell. Through the accumulation of the +store of yellow yelk within the ball of protoplasm the nucleus it contains (the +germinal vesicle) is forced to the surface of the ball. Here it is surrounded +by a small quantity of protoplasm, and with this forms the lens-shaped +formative yelk (Fig. 15 <i>b</i>). This is seen on the yellow yelk-ball, at a +certain point of the surface, as a small round white spot—the +“tread” (<i>cicatricula</i>). From this point a thread-like column +of white nutritive yelk (<i>d</i>), which contains no yellow yelk-granules, and +is softer than the yellow food-yelk, proceeds to the middle of the yellow +yelk-ball, and forms there a small central globule of white yelk (Fig. 15 <i> +d</i>). The whole of this white yelk is not sharply separated from the yellow +yelk, which shows a slight trace of concentric layers in the hard-boiled egg +(Fig. 15 <i>c</i>). We also find in the hen’s egg, when we break the +shell and take out the yelk, a round small white disk at its surface which +corresponds to the tread. But this small white “germinal disk” is +now further developed, and is really the gastrula of the chick. The body of the +chick is formed from it alone. The whole white and yellow yelk-mass is without +any significance for the formation of the embryo, it being merely used as food +by the developing chick. The clear, glarous mass of albumin that surrounds the +yellow yelk of the bird’s egg, and also the hard chalky shell, are only +formed within the oviduct round the impregnated ovum. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus56"></a> +<img src="images/fig56.gif" width="329" height="121" alt="Fig.56. Longitudinal section of the blastula of +a shark (Pristiurus) at the beginning of gastrulation." /> +<p class="caption">Fig. +56—<b>Longitudinal section of the blastula of a shark</b> +(<i>Pristiurus</i>) at the beginning of gastrulation. (From <i> Ruckert.</i>) +(Seen from the left.) <i>V</i> fore end, <i>H</i> hind end, <i>B</i> +segmentation-cavity, <i>ud</i> first trace of the primitive gut, <i>dk</i> +yelk-nuclei, <i>fd</i> fine-grained yelk, <i>gd</i> coarse-grained yelk.</p> +</div> + +<p> +When the fertilisation of the bird’s ovum has taken place within the +mother’s body, we find in the lens-shaped stem-cell the progress of flat, +discoid segmentation (Fig. 57). First two equal segmentation-cells (<i>A</i>) +are formed from the ovum. These divide into four (<i>B</i>), then into eight, +sixteen (<i>C</i>), thirty-two, sixty-four, and so on. The cleavage of the +cells is always preceded by a division of their nuclei. The cleavage surfaces +between the segmentation-cells appear at the free surface of the tread as +clefts. The first two divisions are vertical to each other, in the form of a +cross (<i>B</i>). Then there are two more divisions, which cut the former at an +angle of forty-five degrees. The tread, which thus becomes the germinal disk, +now has the appearance of an eight-rayed star. A circular cleavage next taking +place round the middle, the eight triangular cells divide into sixteen, of +which eight are in the middle and eight distributed around (<i>C</i>). +Afterwards circular clefts and radial clefts, directed towards the centre, +alternate more or less irregularly (<i>D, E</i>). In most of the amniotes the +formation of concentric and radial clefts is irregular from the very first; and +so also in the hen’s egg. But the final outcome of the cleavage-process +is once more the formation of a large number of small cells of a similar +nature. As in the case of the fish-ovum, these segmentation-cells form a round, +lens-shaped disk, which corresponds to the morula, and is embedded in a small +depression of the white yelk. Between the lens-shaped disk of the morula-cells +and the underlying white yelk a small cavity is now formed by the accumulation +of fluid, as in the fishes. Thus we get the peculiar and not easily +recognisable blastula of the bird (Fig. 58). The small segmentation-cavity +(<i>fh</i>) is very flat and much compressed. The upper or dorsal wall +(<i>dw</i>) is formed of a single layer of clear, distinctly separated cells; +this +<span class='pagenum'><a name="Page_82" id="Page_82"></a></span> +corresponds to the upper or animal hemisphere of the triton-blastula (Fig. 45). +The lower or ventral wall of the flat dividing space (<i>vw</i>) is made up of +larger and darker segmentation-cells; it corresponds to the lower or vegetal +hemisphere of the blastula of the water-salamander (Fig. 45 <i>dz</i>). The +nuclei of the yelk-cells, which are in this case especially numerous at the +edge of the lens-shaped blastula, travel into the white yelk, increase by +cleavage, and contribute even to the further growth of the germinal disk by +furnishing it with food-stuff. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus57"></a> +<img src="images/fig57.gif" width="314" height="206" alt="Fig. 57 Diagram of discoid segmentation in +the bird’s ovum." /> +<p class="caption">Fig. 57—<b>Diagram of discoid segmentation in the bird’s +ovum</b> (magnified). Only the formative yelk (the tread) is shown in these six +figures (<i>A</i> to <i>F</i>), because cleavage only takes place in this. The +much larger food-yelk, which does not share in the cleavage, is left out and +merely indicated by the dark ring without.</p> +</div> + +<p> +The invagination or the folding inwards of the bird-blastula takes place in +this case also at the hinder pole of the subsequent chief axis, in the middle +of the hind border of the round germinal disk (Fig. 59 <i>s</i>). At this spot +we have the most brisk cleavage of the cells; hence the cells are more numerous +and smaller here than in the fore-half of the germinal disk. The +border-swelling or thick edge of the disk is less clear but whiter behind, and +is more sharply separated from contiguous parts. In the middle of its hind +border there is a white, crescent-shaped groove—Koller’s +sickle-groove (Fig. 59 <i>s</i>); a small projecting process in the centre of +it is called the sickle-knob (<i>sk</i>). This important cleft is the primitive +mouth, which was described for a long time as the “primitive +groove.” If we make a vertical section through this part, we see that a +flat and broad cleft stretches under the germinal disk forwards from the +primitive mouth; this is the primitive gut (Fig. 60 <i>ud</i>). Its roof or +dorsal wall is formed by the folded upper part of the blastula, and its floor +or ventral wall by the white yelk (<i>wd</i>), in which a number of yelk-nuclei +(<i>dk</i>) are distributed. There is a brisk multiplication of these at the +edge of the germinal disk, especially in the neighbourhood of the sickle-shaped +primitive mouth. +</p> + +<p> +We learn from sections through later stages of this discoid bird-gastrula that +the primitive gut-cavity, extending forward from the primitive mouth as a flat +pouch, undermines the whole region of the round flat lens-shaped blastula (Fig. +61 <i> ud</i>). At the same time, the segmentation-cavity gradually disappears +altogether, the folded inner germinal layer (<i>ik</i>) placing itself from +underneath on the overlying outer germinal layer (<i>ak</i>). The typical +process of invagination, though greatly disguised, can thus be clearly seen in +this case, as Goette and Rauber, and more recently Duval (Fig. 61), have shown. +</p> + +<p> +The older embryologists (Pander, Baer, Remak), and, in recent times especially, +<span class='pagenum'><a name="Page_83" id="Page_83"></a></span> +His, Kölliker, and others, said that the two primary germinal layers of the +hen’s ovum—the oldest and most frequent subject of +observation!—arose by horizontal cleavage of a simple germinal disk. In +opposition to this accepted view, I affirmed in my <i>Gastræa Theory</i> (1873) +that the discoid bird-gastrula, like that of all other vertebrates, is formed +by folding (or invagination), and that this typical process is merely altered +in a peculiar way and disguised by the immense accumulation of food-yelk and +the flat spreading of the discoid blastula at one part of its surface. I +endeavoured to establish this view by the derivation of the vertebrates from +one source, and especially by proving that the birds descend from the reptiles, +and these from the amphibia. If this is correct, the discoid gastrula of the +amniotes must have been formed by the folding-in of a hollow blastula, as has +been shown by Remak and Rusconi of the discoid gastrula of the amphibia, their +direct ancestors. The accurate and extremely careful observations of the +authors I have mentioned (Goette, Rauber, and Duval) have decisively proved +this +<span class='pagenum'><a name="Page_84" id="Page_84"></a></span> +recently for the birds; and the same has been done for the reptiles by the fine +studies of Kupffer, Beneke, Wenkebach, and others. In the shield-shaped +germinal disk of the lizard (Fig. 62), the crocodile, the tortoise, and other +reptiles, we find in the middle of the hind border (at the same spot as the +sickle groove in the bird) a transverse furrow (<i>u</i>), which leads into a +flat, pouch-like, blind sac, the primitive gut. The fore (dorsal) and hind +(ventral) lips of the transverse furrow correspond exactly to the lips of the +primitive mouth (or sickle-groove) in the birds. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus58"></a> +<img src="images/fig58.gif" width="385" height="285" alt="Fig.58. Vertical section of the +bastula of a hen. Fig. 59. The germinal disk of the hen’s ovum at the beginning +of gastrulation. Fig. 60. Longitudinal section of the germinal disk of a +siskin." /> +<p class="caption">Fig. +58—<b>Vertical section of the blastula of a hen</b> +(<i>discoblastula</i>). <i> fh</i> segmentation-cavity, <i>dw</i> dorsal wall +of same, <i> vw</i> ventral wall, passing directly into the white yelk +(<i>wd</i>). (From <i>Duval.</i>)<br/> Fig. 59—<b>The germinal disk of +the hen’s ovum at the beginning of gastrulation;</b> <i>A</i> before +incubation, <i>B</i> in the first hour of incubation. (From <i>Koller.</i>) +<i>ks</i> germinal-disk, <i>V</i> its fore and <i>H</i> its hind border; <i> +es</i> embryonic shield, <i>s</i> sickle-groove, <i>sk</i> sickle knob, +<i>d</i> yelk.<br/> Fig. 60—<b>Longitudinal section of the germinal disk +of a siskin</b> (<i>discogastrula</i>). (From <i>Duval.</i>) <i>ud</i> +primitive gut, <i>vl, hl</i> fore and hind lips of the primitive mouth (or +sickle-edge); <i>ak</i> outer germinal layer, <i>ik</i> inner germinal layer, +<i>dk</i> yelk-nuclei, <i>wd</i> white yelk.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus61"></a> +<img src="images/fig61.gif" width="443" height="87" alt="Fig.61. Longitudinal section +of the discoid gastrula of the nightingale." /> +<p class="caption">Fig. 61—<b>Longitudinal section of the discoid +gastrula of the nightingale.</b> (From <i>Duval.</i>) <i>ud</i> primitive gut, +<i> vl, hl</i> fore and hind lips of the primitive mouth; <i>ak, ik</i> outer +and inner germinal layers; <i>vr</i> fore-border of the discogastrula.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus62"></a> +<img src="images/fig62.gif" width="246" height="221" alt="Fig.62. Germinal disk +of the lizard." /> +<p class="caption">Fig. 62—<b>Germinal disk of the lizard</b> +(<i>Lacerta agilis</i>). (From <i>Kupffer.</i>) <i>u</i> primitive mouth, <i> +s</i> sickle, <i>es</i> embryonic shield, <i>hf</i> and <i>df</i> light and +dark germinative area.</p> +</div> + +<p> +The gastrulation of the mammals must be derived from this special embryonic +development of the reptiles and birds. This latest and most advanced class of +the vertebrates has, as we shall see afterwards, evolved at a comparatively +recent date from an older group of reptiles; and all these amniotes must have +come originally from a common stem-form. Hence the distinctive embryonic +process of the mammal must have arisen by cenogenetic modifications from the +older form of gastrulation of the reptiles and birds. Until we admit this +thesis we cannot understand the formation of the germinal layers in the mammal, +and therefore in man. +</p> + +<p> +I first advanced this fundamental principle in my essay <i>On the Gastrulation +of Mammals</i> (1877), and sought to show in this way that I assumed a gradual +degeneration of the food-yelk and the yelk-sac on the way from the proreptiles +to the mammals. “The cenogenetic process of adaptation,” I said, +“which has occasioned the atrophy of the rudimentary yelk-sac of the +mammal, is perfectly clear. It is due to the fact that the young of the mammal, +whose ancestors were certainly oviparous, now remain a long time in the womb. +As the great store of food-yelk, which the oviparous ancestors gave to the egg, +became superfluous in their descendants owing to the long carrying in the womb, +and the maternal blood in the wall of the uterus made itself the chief source +of nourishment, the now useless yelk-sac was bound to atrophy by embryonic +adaptation.” +</p> + +<p> +My opinion met with little approval at the time; it was vehemently attacked by +Kölliker, Hensen, and His in particular. However, it has been gradually +accepted, and has recently been firmly established by a large number of +excellent studies of mammal gastrulation, especially by Edward Van +Beneden’s studies of the rabbit and bat, Selenka’s on the +marsupials and rodents, Heape’s and Lieberkühn’s on the mole, +Kupffer and Keibel’s on the rodents, Bonnet’s on the ruminants, +etc. From the general comparative point of view, Carl Rabl in his theory of the +mesoderm, Oscar Hertwig in the latest edition of his Manual (1902), and +Hubrecht in his <i>Studies in Mammalian Embryology</i> (1891), have supported +the opinion, and sought to derive the peculiarly modified gastrulation of the +mammal from that of the reptile. +</p> + +<p> +In the meantime (1884) the studies of Wilhelm Haacke and Caldwell provided a +proof of the long-suspected and very interesting fact, that the lowest mammals, +the monotremes, <i>lay eggs,</i> like the birds and reptiles, and are not +viviparous like the other mammals. Although the gastrulation of the monotremes +was not really known until studied by Richard +<span class='pagenum'><a name="Page_85" id="Page_85"></a></span> +Semon in 1894, there could be little doubt, in view of the great size of their +food-yelk, that their ovum-segmentation was discoid, and led to the formation +of a sickle-mouthed discogastrula, as in the case of the reptiles and birds. +Hence I had, in 1875 (in my essay on <i>The Gastrula and Ovum-segmentation of +Animals</i>), counted the monotremes among the discoblastic vertebrates. This +hypothesis was established as a fact nineteen years afterwards by the careful +observations of Semon; he gave in the second volume of his great work, +<i>Zoological Journeys in Australia</i> (1894), the first description and +correct explanation of the discoid gastrulation of the monotremes. The +fertilised ova of the two living monotremes (<i>Echidna</i> and <i> +Ornithorhynchus</i>) are balls of one-fifth of an inch in diameter, enclosed in +a stiff shell; but they grow considerably during development, so that when laid +the egg is three times as large. The structure of the plentiful yelk, and +especially the relation of the yellow and the white yelk, are just the same as +in the reptiles and birds. As with these, partial cleavage takes place at a +spot on the surface at which the small formative yelk and the nucleus it +encloses are found. First is formed a lens-shaped circular germinal disk. This +is made up of several strata of cells, but it spreads over the yelk-ball, and +thus becomes a one-layered blastula. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus63"></a> +<img src="images/fig63.gif" width="202" height="205" alt="Fig.63. Ovum of the +opossum (Didelphys) divided into four." /> +<p class="caption">Fig. 63—<b>Ovum of the opossum</b> +(<i>Didelphys</i>) <b> divided into four.</b> (From <i>Selenka.</i>) <i>b</i> +the four segmentation-cells, <i>r</i> directive body, <i>c</i> unnucleated +coagulated matter, <i>p,</i> albumin-membrane.</p> +</div> + +<p> +If we then imagine the yelk it contains to be dissolved and replaced by a clear +liquid, we have the characteristic blastula of the higher mammals. In these the +gastrulation proceeds in two phases, as Semon rightly observes: firstly, +formation of the entoderm by cleavage at the centre and further growth at the +edge; secondly, invagination. In the monotremes more primitive conditions have +been retained better than in the reptiles and birds. In the latter, before the +commencement of the gastrula-folding, we have, at least at the periphery, a +two-layered embryo forming from the cleavage. But in the monotremes the +formation of the cenogenetic entoderm does not precede the invagination; hence +in this case the construction of the germinal layers is less modified than in +the other amniota. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus64"></a> +<img src="images/fig64.gif" width="198" height="169" alt="Fig.64. Blastula of the +opossum (Didelphys)." /> +<p class="caption">Fig. 64—<b>Blastula of the opossum</b> +(<i>Didelphys</i>). (From <i>Selenka.</i>) <i>a</i> animal pole of the +blastula, <i> v</i> vegetal pole, <i>en</i> mother-cell of the entoderm, <i> +ex</i> ectodermic cells, <i>s</i> spermia, <i>ib</i> unnucleated yelk-balls +(remainder of the food-yelk), <i>p</i> albumin membrane.</p> +</div> + +<p> +The marsupials, a second sub-class, come next to the oviparous monotremes, the +oldest of the mammals. But as in their case the food-yelk is already atrophied, +and the little ovum develops within the mother’s body, the partial +cleavage has been reconverted into total. One section of the marsupials still +show points of agreement with the monotremes, while another section of them, +according to the splendid investigations of Selenka, form a connecting-link +between these and the placentals. +</p> + +<p> +The fertilised ovum of the opossum (<i>Didelphys</i>) divides, according to +Selenka, first into two, then four, then eight equal cells; hence the +segmentation is at first equal or homogeneous. But in the course of the +cleavage a larger cell, distinguished by its less clear plasm and its +containing more yelk-granules (the mother cell of the entoderm, Fig. 64 +<i>en</i>), +<span class='pagenum'><a name="Page_86" id="Page_86"></a></span> +separates from the others; the latter multiply more rapidly than the former. +As, further, a quantity of fluid gathers in the morula, we get a round +blastula, the wall of which is of varying thickness, like that of the amphioxus +(Fig. 38 <i>E</i>) and the amphibia (Fig. 45). The upper or animal hemisphere +is formed of a large number of small cells; the lower or vegetal hemisphere of +a small number of large cells. One of the latter, distinguished by its size +(Fig. 64 <i>en</i>), lies at the vegetal pole of the blastula-axis, at the +point where the primitive mouth afterwards appears. This is the mother-cell of +the entoderm; it now begins to multiply by cleavage, and the daughter-cells +(Fig. 65 <i> i</i>) spread out from this spot over the inner surface of the +blastula, though at first only over the vegetal hemisphere. The less clear +entodermic cells (<i>i</i>) are distinguished at first by their rounder shape +and darker nuclei from the higher, clearer, and longer entodermic cells +(<i>e</i>), afterwards both are greatly flattened, the inner blastodermic cells +more than the outer. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus65"></a> +<img src="images/fig65.gif" width="361" height="235" alt="Fig.65. Blastula of the opossum +(Didelphys) at the beginning of gastrulation. Fig. 66. Oval gastrula of the +opossum (Didelphys), about eight hours old." /> +<p class="caption">Fig. 65—<b>Blastula of the opossum</b> +(<i>Didelphys</i>) at the beginning of gastrulation. (From <i>Selenka.</i>) +<i>e</i> ectoderm, <i>i</i> entoderm; <i>a</i> animal pole, <i>u</i> primitive +mouth at the vegetal pole, <i>f</i> segmentation-cavity, <i>d</i> unnucleated +yelk-balls (relics of the reduced food-yelk), c nucleated curd (without +yelk-granules)<br/> Fig. 66—<b>Oval gastrula of the opossum</b> +(<i>Didelphys</i>), about eight hours old. (From <i>Selenka</i>) (external +view).)</p> +</div> + +<p> +The unnucleated yelk-balls and curd (Fig. 65 <i>d</i>) that we find in the +fluid of the blastula in these marsupials are very remarkable; they are the +relics of the atrophied food-yelk, which was developed in their ancestors, the +monotremes, and in the reptiles. +</p> + +<p> +In the further course of the gastrulation of the opossum the oval shape of the +gastrula (Fig. 66) gradually changes into globular, a larger quantity of fluid +accumulating in the vesicle. At the same time, the entoderm spreads further and +further over the inner surface of the ectoderm (<i>e</i>). A globular vesicle +is formed, the wall of which consists of two thin simple strata of cells; the +cells of the outer germinal layer are rounder, and those of the inner layer +flatter. In the region of the primitive mouth (<i>p</i>) the cells are less +flattened, and multiply briskly. From this point—from the hind (ventral) +lip of the primitive mouth, which extends in a central cleft, the primitive +groove—the construction of the mesoderm proceeds. +</p> + +<p> +Gastrulation is still more modified and curtailed cenogenetically in the +placentals than in the marsupials. It was first accurately known to us by the +distinguished investigations of Edward Van Beneden in 1875, the first object of +study being the ovum of the rabbit. But as man also belongs to this sub-class, +and as his as yet unstudied gastrulation cannot be materially different from +that of the other placentals, it merits the closest attention. We have, in the +first place, the peculiar feature that the two first segmentation-cells that +proceed from the cleavage of the fertilised ovum (Fig. 68) are of different +sizes and natures; the difference is sometimes greater, sometimes less (Fig. +69). One of these first daughter-cells of the ovum is a little +<span class='pagenum'><a name="Page_87" id="Page_87"></a></span> +larger, clearer, and more transparent than the other. Further, the smaller cell +takes a colour in carmine, osmium, etc., more strongly than the larger. By +repeated cleavage of it a morula is formed, and from this a blastula, which +changes in a very characteristic way into the greatly modified gastrula. When +the number of the segmentation-cells in the mammal embryo has reached +ninety-six (in the rabbit, about seventy hours after impregnation) the fœtus +assumes a form very like the archigastrula (Fig. 72). The spherical embryo +consists of a central mass of thirty-two soft, round cells with dark nuclei, +which are flattened into polygonal shape by mutual pressure, and colour +dark-brown with osmic acid (Fig. 72 <i>i</i>). This dark central group of cells +is surrounded by a lighter spherical membrane, consisting of sixty-four +cube-shaped, small, and fine-grained cells which lie close together in a single +stratum, and only colour slightly in osmic acid (Fig. 72 <i>e</i>). The authors +who regard this embryonic form as the primary gastrula of the placental +conceive the outer layer as the ectoderm and the inner as the entoderm. The +entodermic membrane is only interrupted at one spot, one, two, or three of the +ectodermic cells being loose there. These form the yelk-stopper, and fill up +the mouth of the gastrula (<i>a</i>). The central primitive gut-cavity +(<i>d</i>) is full of entodermic cells. The uni-axial type of the mammal +gastrula is accentuated in this way. However, opinions still differ +considerably as to the real nature of this “provisional gastrula” +of the placental and its relation to the blastula into which it is converted. +</p> + +<p> +As the gastrulation proceeds a large spherical blastula is formed from this +peculiar solid amphigastrula of the placental, as we saw in the case of the +marsupial. The accumulation of fluid in the solid gastrula (Fig. 73 A) leads to +the formation of an eccentric cavity, the group of the darker entodermic cells +(<i>hy</i>) remaining directly attached at one spot with the round enveloping +stratum of the lighter ectodermic cells (<i>ep</i>). This spot corresponds to +the original primitive mouth (prostoma or blastoporus). From this important +spot the inner germinal layer spreads all round on the inner surface of the +outer layer, the cell-stratum of which forms the wall of the hollow sphere; the +extension proceeds from the vegetal towards the animal pole. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus67"></a> +<img src="images/fig67.gif" width="198" height="209" alt="Fig.67. Longitudinal +section through the oval gastrula of the opossum." /> +<p class="caption">Fig. 67—<b>Longitudinal section through the oval +gastrula of the opossum</b> (Fig. 69). (From <i>Selenka.</i>) <i>p</i> +primitive mouth, <i>e</i> ectoderm, <i>i</i> entoderm, <i>d</i> yelk remains in +the primitive gut-cavity (<i>u</i>).</p> +</div> + +<p> +The cenogenetic gastrulation of the placental has been greatly modified by +secondary adaptation in the various groups of this most advanced and youngest +sub-class of the mammals. Thus, for instance, we find in many of the rodents +(guinea-pigs, mice, etc.) <i> apparently</i> a temporary inversion of the two +germinal layers. This is due to a folding of the blastodermic wall by what is +called the “girder,” a plug-shaped growth of Rauber’s +“roof-layer.” It is a thin layer of flat epithelial cells, that is +freed from the surface of the blastoderm in some of the rodents; it has no more +significance in connection with the general course of placental gastrulation +than the conspicuous departure from the usual globular shape in the blastula of +some of the ungulates. In some pigs and ruminants it grows into a thread-like, +long and thin tube. +</p> + +<p> +Thus the gastrulation of the placentals, which diverges most from that of the +amphioxus, the primitive form, is reduced to the original type, the +invagination of a modified blastula. Its chief peculiarity is that the folded +part of the blastoderm does not form a completely closed (only open at the +primitive mouth) blind sac, as is usual; but this blind sac has a wide opening +at the ventral curve (opposite to the dorsal mouth); and through this opening +the primitive gut communicates from the first with the embryonic cavity of the +blastula. The folded crest-shaped +<span class='pagenum'><a name="Page_88" id="Page_88"></a></span> +entoderm grows with a free circular border on the inner surface of the entoderm +towards the vegetal pole; when it has reached this, and the inner surface of +the blastula is completely grown over, the primitive gut is closed. This +remarkable direct transition of the primitive gut-cavity into the +segmentation-cavity is explained simply by the assumption that in most of the +mammals the yelk-mass, which is still possessed by the oldest forms of the +class (the monotremes) and their ancestors (the reptiles), is atrophied. This +proves the essential unity of gastrulation in all the vertebrates, in spite of +the striking differences in the various classes. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus68"></a> +<img src="images/fig68.gif" width="170" height="170" alt="Fig.68. Stem-cell of the mammal ovum (from the +rabbit). Fig. 69. Incipient cleavage of the mammal ovum (from the rabbit). Fig. +70. The first four segmentation-cells of the mammal ovum (from the rabbit). +Fig. 71. Mammal ovum with eight segmentation-cells (from the rabbit)." /> +<p class="caption">Fig. 68—<b>Stem-cell of the mammal ovum</b> (from the +rabbit).<br/> <i>k</i> stem-nucleus, <i>n</i> nuclear corpuscle, <i>p</i> +protoplasm of the stem-cell, <i>z</i> modified zona pellucida, <i>h</i> +outer albuminous membrane, <i>s</i> dead sperm-cells.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus69"></a> +<img src="images/fig69.gif" width="170" height="170" alt="Fig. 69 Incipient cleavage of the mammal ovum (from the +rabbit)." /> +<p class="caption">Fig. 69—<b>Incipient cleavage of the mammal ovum</b> (from +the rabbit). The stem-cell has divided into two unequal cells, one lighter +(<i>e</i>) and one darker (<i>i</i>). <i>z</i> zona pellucida, <i>h</i> outer +albuminous membrane, <i>s</i> dead sperm-cell.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus70"></a> +<img src="images/fig70.gif" width="170" height="170" alt="Fig. 70 The first four segmentation-cells of the mammal +ovum (from the rabbit)." /> +<p class="caption">Fig. 70—<b>The first four +segmentation-cells of the mammal ovum</b> (from the rabbit).<br/> <i>e</i> the +two larger (and lighter) cells, <i>i</i> the two smaller (and darker) +cells, <i>z</i> zona pellucida, <i>h</i> outer albuminous membrane.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus71"></a> +<img src="images/fig71.gif" width="170" height="170" alt="Fig. 71 Mammal ovum with eight segmentation-cells (from the rabbit)." /> +<p class="caption">Fig. 71—<b>Mammal ovum with eight segmentation-cells</b> +(from the rabbit). <i>e</i> four larger and lighter cells, <i>i</i> four smaller +and darker cells, <i>z</i> zona pellucida, <i>h</i> outer albuminous membrane.</p> +</div> + +<p> +In order to complete our consideration of the important processes of +segmentation and gastrulation, we will, in conclusion, cast a brief glance at +the fourth chief type—superficial segmentation. In the vertebrates this +form is not found at all. But it plays the chief part in the large stem of the +articulates—the insects, +<span class='pagenum'><a name="Page_89" id="Page_89"></a></span> +spiders, myriapods, and crabs. The distinctive form of gastrula that comes of +it is the “vesicular gastrula” (<i>Perigastrula</i>). +</p> + +<p> +In the ova which undergo this superficial cleavage the formative yelk is +sharply divided from the nutritive yelk, as in the preceding cases of the ova +of birds, reptiles, fishes, etc.; the formative yelk alone undergoes cleavage. +But while in the ova with discoid gastrulation the formative yelk is not in the +centre, but at one pole of the uni-axial ovum, and the food-yelk gathered at +the other pole, in the ova with superficial cleavage we find the formative yelk +spread over the whole surface of the ovum; it encloses spherically the +food-yelk, which is accumulated in the middle of the ova. As the segmentation +only affects the former and not the latter, it is bound to be entirely +“superficial”; the store of food in the middle is quite untouched +by it. As a rule, it proceeds in regular geometrical progression. In the end +the whole of the formative yelk divides into a number of small and homogeneous +cells, which lie close together in a single stratum on the entire surface of +the ovum, and form a superficial blastoderm. This blastoderm is a simple, +completely closed vesicle, the internal cavity of which is entirely full of +food-yelk. This real blastula only differs from that of the primitive ova in +its chemical composition. In the latter the content is water or a watery jelly; +in the former it is a thick mixture, rich in food-yelk, of albuminous and fatty +substances. As this quantity of food-yelk fills the centre of the ovum before +cleavage begins, there is no difference in this respect between the morula and +the blastula. The two stages rather agree in this. +</p> + +<p> +When the blastula is fully formed, we have again in this case the important +folding or invagination that determines gastrulation. The space between the +skin-layer and the gut-layer (the remainder of the segmentation-cavity) remains +full of food-yelk, which is gradually used up. This is the only material +difference between our vesicular gastrula (<i>perigastrula</i>) and the +original form of the bell-gastrula (<i>archigastrula</i>). Clearly the one has +been developed from the other in the course of time, owing to the accumulation +of food-yelk in the centre of the ovum.<a href="#linknote-23" name="linknoteref-23" id="linknoteref-23"><sup>[23]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-23" id="linknote-23"></a> <a href="#linknoteref-23">[23]</a> +On the reduction of all forms of gastrulation to the original palingenetic form +see especially the lucid treatment of the subject in Arnold Lang’s +<i>Manual of Comparative Anatomy</i> (1888), Part I. +</p> + +<p> +We must count it an important advance that we are thus in a position to reduce +all the various embryonic phenomena in the different groups of animals to these +four principal forms of segmentation and gastrulation. Of these four forms we +must regard one only as the original palingenetic, and the other three as +cenogenetic and derivative. The unequal, the discoid, and the superficial +segmentation have all clearly arisen by secondary adaptation from the primary +segmentation; and the chief cause of their development has been the gradual +formation of the food-yelk, and the increasing antithesis between animal and +vegetal halves of the ovum, or between ectoderm (skin-layer) and entoderm +(gut-layer). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus72"></a> +<img src="images/fig72.gif" width="206" height="182" alt="Fig.72. Gastrula of the +placental mammal (epigastrula from the rabbit), longitudinal section through +the axis." /> +<p class="caption">Fig. 72—<b>Gastrula of the placental mammal</b> +(epigastrula from the rabbit), longitudinal section through the axis. <i>e</i> +ectodermic cells (sixty-four, lighter and smaller), <i>i</i> entodermic cells +(thirty-two, darker and larger), <i> d</i> central entodermic cell, filling the +primitive gut-cavity, <i>o</i> peripheral entodermic cell, stopping up the +opening of the primitive mouth (yelk-stopper in the Rusconian anus).</p> +</div> + +<p> +The numbers of careful studies of animal gastrulation that have been made in +the last few decades have completely established the views I have expounded, +and which I first advanced in the years 1872–76. For a time they were +greatly disputed by many embryologists. Some said that the original embryonic +form of the metazoa was not the gastrula, but the “planula”—a +double-walled vesicle with closed cavity and without mouth-aperture; the latter +was supposed to pierce through gradually. It was afterwards shown that this +planula (found in several sponges, etc.) was a later evolution from the +gastrula. +</p><br/> + +<div class="fig" style="width:100%;"> +<a name="illus73"></a> +<img src="images/fig73.gif" width="307" height="172" alt="Fig.73. Gastrula of the rabbit." /> +<p class="caption">Fig. +73—<b>Gastrula of the rabbit.</b> A as a solid, spherical cluster of +cells, B changing into the embryonic vesicle, <i>bp</i> primitive mouth, <i> +ep</i> ectoderm, <i>hy</i> entoderm.</p> +</div> + +<p> +<span class='pagenum'><a name="Page_90" id="Page_90"></a></span>It was also shown that what is called delamination—the rise of the two +primary germinal layers by the folding of the surface of the blastoderm (for +instance, in the <i>Geryonidæ</i> and other medusæ)—was a secondary +formation, due to cenogenetic variations from the original invagination of the +blastula. The same may be said of what is called “immigration,” in +which certain cells or groups of cells are detached from the simple layer of +the blastoderm, and travel into the interior of the blastula; they attach +themselves to the inner wall of the blastula, and form a second internal +epithelial layer—that is to say, the entoderm. In these and many other +controversies of modern embryology the first requisite for clear and natural +explanation is a careful and discriminative distinction between palingenetic +(hereditary) and cenogenetic (adaptive) processes. If this is properly attended +to, we find evidence everywhere of the biogenetic law. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap10"></a>Chapter X.<br/> +THE CŒLOM THEORY</h2> + +<p> +The two “primary germinal layers” which the gastræa theory has +shown to be the first foundation in the construction of the body are found in +this simplest form throughout life only in animals of the lowest grade—in +the gastræads, olynthus (the stem-form of the sponges), hydra, and similar very +simple animals. In all the other animals new strata of cells are formed +subsequently between these two primary body-layers, and these are generally +comprehended under the title of the middle layer, or <i>mesoderm.</i> As a +rule, the various products of this middle layer afterwards constitute the great +bulk of the animal frame, while the original entoderm, or internal germinal +layer, is restricted to the clothing of the alimentary canal and its glandular +appendages; and, on the other hand, the ectoderm, or external germinal layer, +furnishes the outer clothing of the body, the skin and nervous system. +</p> + +<p> +In some large groups of the lower animals, such as the sponges, corals, and +flat-worms, the middle germinal layer +<span class='pagenum'><a name="Page_91" id="Page_91"></a></span> +remains a single connected mass, and most of the body is developed from it; +these have been called the three-layered metazoa, in opposition to the +two-layered animals described. Like the two-layered animals, they have no +body-cavity—that is to say, no cavity distinct from the alimentary +system. On the other hand, all the higher animals have this real body-cavity +(<i>cœloma</i>), and so are called <i>cœlomaria.</i> In all these we can +distinguish <i>four</i> secondary germinal layers, which develop from the two +primary layers. To the same class belong all true vermalia (excepting the +platodes), and also the higher typical animal stems that have been evolved from +them—molluscs, echinoderms, articulates, tunicates, and vertebrates. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus74"></a> +<img src="images/fig74.gif" width="363" height="198" alt="Figs. 74 and 75. Diagram of the four secondary +terminal layers." /> +<p class="caption">Figs. 74 and 75—<b>Diagram of the four +secondary germinal layers,</b> transverse section through the metazoic embryo: +Fig. 74 of an annelid, Fig. 75 of a vermalian. <i>a</i> primitive gut, <i> +dd</i> ventral glandular layer, <i>df</i> ventral fibre-layer, <i> hm</i> +skin-fibre-layer, <i>hs</i> skin-sense-layer, <i>u</i> beginning of the +rudimentary kidneys, <i>n</i> beginning of the nerve-plates.</p> +</div> + +<p> +The body-cavity (<i>cœloma</i>) is therefore a new acquisition of the animal +body, much younger than the alimentary system, and of great importance. I first +pointed out this fundamental significance of the cœlom in my <i>Monograph on +the Sponges</i> (1872), in the section which draws a distinction between the +body-cavity and the gut-cavity, and which follows immediately on the germ-layer +theory and the ancestral tree of the animal kingdom (the first sketch of the +gastræa theory). Up to that time these two principal cavities of the animal +body had been confused, or very imperfectly distinguished; chiefly because +Leuckart, the founder of the cœlenterata group (1848), has attributed a +body-cavity, but not a gut-cavity, to these lowest metazoa. In reality, the +truth is just the other way about. +</p> + +<p> +The ventral cavity, the original organ of nutrition in the multicellular +animal-body, is the oldest and most important organ of all the metazoa, and, +together with the primitive mouth, is formed in every case in the gastrula as +the primitive gut; it is only at a much later stage that the body-cavity, which +is entirely wanting in the cœlenterata, is developed in some of the metazoa +between the ventral and the body wall. The two cavities are entirely different +in content and purport. The alimentary cavity (<i>enteron</i>) serves the +purpose of digestion; it contains water and food taken from without, as well as +the pulp (chymus) formed from this by digestion. On the other hand, the +body-cavity, quite distinct from the gut and closed externally, has nothing to +do with digestion; it encloses the gut itself and its glandular appendages, and +also contains the sexual products and a certain amount of blood or lymph, a +fluid that is transuded through the ventral wall. +</p> + +<p> +As soon as the body-cavity appears, the ventral wall is found to be separated +from the enclosing body-wall, but the two continue to be directly connected at +various points. We can also then always distinguish a number of different +layers of tissue in both walls—at least two in each. These tissue-layers +are formed originally from four different simple cell-layers, which are the +much-discussed four secondary germinal layers. The outermost of these, the +skin-sense-layer (Figs. 74, 75 <i>hs</i>), and the innermost, the +gut-gland-layer (<i>dd</i>), remain at first simple epithelia or +covering-layers. The one covers the outer surface of the body, the other the +inner +<span class='pagenum'><a name="Page_92" id="Page_92"></a></span> +surface of the ventral wall; hence they are called confining or limiting +layers. Between them are the two middle-layers, or mesoblasts, which enclose +the body-cavity. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus76"></a> +<img src="images/fig76.gif" width="170" height="138" alt="Fig.76. Coelomula of +sagitta." /> +<p class="caption">Fig. 76—<b>Cœlomula of sagitta</b> (gastrula with a +couple of cœlom-pouches. (From <i>Kowalevsky.</i>) <i> bl.p</i> primitive +mouth, <i>al</i> primitive gut, <i>pv</i> cœlom-folds, <i>m</i> permanent +mouth.</p> +</div> + +<p> +The four secondary germinal layers are so distributed in the structure of the +body in all the cœlomaria (or all metazoa that have a body-cavity) that the +outer two, joined fast together, constitute the body-wall, and the inner two +the ventral wall; the two walls are separated by the cavity of the cœlom. Each +of the walls is made up of a limiting layer and a middle layer. The two +limiting layers chiefly give rise to epithelia, or covering-tissues, and glands +and nerves, while the middle layers form the great bulk of the fibrous tissue, +muscles, and connective matter. Hence the latter have also been called fibrous +or muscular layers. The outer middle layer, which lies on the inner side of the +skin-sense-layer, is the skin fibre-layer; the inner middle layer, which +attaches from without to the ventral glandular layer, is the ventral fibre +layer. The former is usually called briefly the parietal, and the latter the +visceral layer or mesoderm. Of the many different names that have been given to +the four secondary germinal layers, the following are those most in use +to-day:— +</p> + +<table class="text" border="1" cellspacing="0" cellpadding="4" summary= "Names +that have been given to the four secondary germinal layers."> +<tr> +<td><b>1. Skin-sense-layer</b><br/> (outer +limiting layer).</td> <td><b>I. Neural layer</b><br/> + (<i>neuroblast</i>).</td> <td rowspan="2" +valign="middle">The two secondary germinal<br/> layers of the body-wall:<br/> +I. Epithelial.<br/> II. Fibrous.</td> </tr> + +<tr> +<td><b>2. Skin-fibre-layer</b><br/> (outer +middle layer).</td> <td><b>II. Parietal layer</b><br/> + (<i>myoblast</i>).</td> </tr> + +<tr> +<td><b>3. Gut-fibre-layer</b><br/> (inner +middle layer).</td> <td><b>III. Visceral layer</b><br/> + (<i>genoblast</i>).</td> <td +rowspan="2">The two secondary germinal<br/> layers +of the gut-wall:<br/> III. Fibrous.<br/> IV. Epithelial.</td> </tr> + +<tr> +<td><b>4. Gut-gland-layer</b><br/> (inner +limiting layer).</td> <td><b>IV. Enteral layer</b><br/> + (<i>enteroblast</i>)</td> </tr> +</table> + +<p> +The first scientist to recognise and clearly distinguish the four secondary +germinal layers was Baer. It is true that he was not quite clear as to their +origin and further significance, and made several mistakes in detail in +explaining them. But, on the whole, their great importance did not escape him. +However, in later years his view had to be given up in consequence of more +accurate observations. Remak then propounded a three-layer theory, which was +generally accepted. These theories of cleavage, however, began to give way +thirty years ago, when Kowalevsky (1871) showed that in the case of +<i>Sagitta</i> (a very clear and typical subject of gastrulation) the two +middle germinal layers and the two limiting layers arise not by cleavage, but +by folding—by a secondary invagination of the primary inner germ-layer. +This invagination or folding proceeds from the primitive mouth, at the two +sides of which (right and left) a couple of pouches are formed. As these +cœlom-pouches or cœlom-sacs detach themselves from the primitive gut, a double +body-cavity is formed (Figs. 74–76). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus77"></a> +<img src="images/fig77.gif" width="196" height="173" alt="Fig.77. Coelomula of +sagitta, in section." /> +<p class="caption">Fig. 77—<b>Cœlomula of sagitta,</b> in section. +(From <i>Hertwig.</i>) <i>D</i> dorsal side, <i>V</i> ventral side, <i> ik</i> +inner germinal layer, <i>mv</i> visceral mesoblast, <i> lh</i> body-cavity, +<i>mp</i> parietal mesoblast, <i>ak</i> outer germinal layer.</p> +</div> + +<p> +The same kind of cœlom-formation as in sagitta was afterwards found by +Kowalevsky in brachiopods and other invertebrates, and in the lowest +vertebrate—the amphioxus. Further instances were discovered by two +English embryologists, to whom we owe very considerable advance in +ontogeny—E. Ray-Lankester and F. Balfour. On the strength of these and +other studies, as well as most extensive research of their own, the brothers +Oscar and Richard Hertwig constructed in 1881 +<span class='pagenum'><a name="Page_93" id="Page_93"></a></span> +the Cœlom Theory. In order to appreciate fully the great merit of this +illuminating and helpful theory, one must remember what a chaos of +contradictory views was then represented by the “problem of the +mesoderm,” or the much-disputed “question of the origin of the +middle germinal layer.” The cœlom theory brought some light and order +into this infinite confusion by establishing the following points: 1. The +body-cavity originates in the great majority of animals (especially in all the +vertebrates) in the same way as in sagitta: a couple of pouches or sacs are +formed by folding inwards at the primitive mouth, between the two primary +germinal layers; as these pouches detach from the primitive gut, a pair of +cœlom-sacs (right and left) are formed; the coalescence of these produces a +simple body-cavity. 2. When these cœlom-embryos develop, not as a pair of +hollow pouches, but as solid layers of cells (in the shape of a pair of +mesodermal streaks)—as happens in the higher vertebrates—we have a +secondary (cenogenetic) modification of the primary (palingenetic) structure; +the two walls of the pouches, inner and outer, have been pressed together by +the expansion of the large food-yelk. 3. Hence the mesoderm consists from the +first of <i>two</i> genetically distinct layers, which do not originate by the +cleavage of a primary simple middle layer (as Remak supposed). 4. These two +middle layers have, in all vertebrates, and the great majority of the +invertebrates, the same radical significance for the construction of the animal +body; the inner middle layer, or the visceral mesoderm, (gut-fibre layer), +attaches itself to the original entoderm, and forms the fibrous, muscular, and +connective part of the visceral wall; the outer middle layer, or the parietal +mesoderm (skin-fibre-layer), attaches itself to the original ectoderm and forms +the fibrous, muscular, and connective part of the body-wall. 5. It is only at +the point of origination, the primitive mouth and its vicinity, that the four +secondary germinal layers are directly connected; from this point the two +middle layers advance forward separately between the two primary germinal +layers, to which they severally attach themselves. 6. The further separation or +differentiation of the four secondary germinal layers and their division into +the various tissues and organs take place especially in the later fore-part or +head of the embryo, and extend backwards from there towards the primitive +mouth. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus78"></a> +<img src="images/fig78.gif" width="180" height="122" alt="Fig.78. Section of a +young sagitta." /> +<p class="caption">Fig. 78—<b>Section of a young sagitta.</b> (From +<i> Hertwig.</i>) <i>dh</i> visceral cavity, <i>ik</i> and <i>ak</i> inner and +outer limiting layers, <i>mv</i> and <i>mp</i> inner and outer middle layers, +<i>lk</i> body-cavity, <i>dm</i> and <i>vm</i> dorsal and visceral +mesentery.</p> +</div> + +<p> +All animals in which the body-cavity demonstrably arises in this way from the +primitive gut (vertebrates, tunicates, echinoderms, articulates, and a part of +the vermalia) were comprised by the Hertwigs under the title of enterocœla, and +were contrasted with the other groups of the pseudocœla (with false +body-cavity) and the cœlenterata (with no body-cavity). However, this radical +distinction and the views as to classification which it occasioned have been +shown to be untenable. Further, the absolute differences in tissue-formation +which the Hertwigs set up between the enterocœla and pseudocœla cannot be +sustained in this connection. For these and other reasons their cœlom-theory +has been much criticised and partly abandoned. Nevertheless, it has rendered a +great and lasting service in the solution of the difficult problem of the +mesoderm, and a material part of it will certainly be retained. I consider it +an especial merit of the theory that it has established the identity of the +development of the two middle layers in all the vertebrates, and has traced +them as cenogenetic modifications back to the original palingenetic form of +development that we still find in the amphioxus. Carl Rabl comes to the same +conclusion in his able Theory of the Mesoderm, and so do Ray-Lankester, Rauber, +Kupffer, Ruckert, Selenka, Hatschek, and others. There is a general agreement +in these and many other recent writers that all the different forms of +cœlom-construction, like those of gastrulation, follow one and the same strict +hereditary law in the vast vertebrate stem; in spite of their apparent +differences, they +<span class='pagenum'><a name="Page_94" id="Page_94"></a></span> +are all only cenogenetic modifications of one palingenetic type, and this +original type has been preserved for us down to the present day by the +invaluable amphioxus. +</p> + +<p> +But before we go into the regular cœlomation of the amphioxus, we will glance +at that of the arrow-worm (<i>Sagitta</i>), a remarkable deep-sea worm that is +interesting in many ways for comparative anatomy and ontogeny. On the one hand, +the transparency of the body and the embryo, and, on the other hand, the +typical simplicity of its embryonic development, make the sagitta a most +instructive object in connection with various problems. The class of the +<i>chætogatha,</i> which is only represented by the cognate genera of +<i>Sagitta</i> and <i> Spadella,</i> is in another respect also a most +remarkable branch of the extensive vermalia stem. It was therefore very +gratifying that Oscar Hertwig (1880) fully explained the anatomy, +classification, and evolution of the chætognatha in his careful monograph. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus79"></a> +<img src="images/fig79.gif" width="415" height="185" alt="Figs. 79 and 80. Transverse section of +amphioxus-larvae." /> +<p class="caption">Figs. 79 and 80.—<b>Transverse section of +amphioxus-larvæ.</b> (From <i>Hatschek.</i>) Fig. 79 at the commencement of +cœlom formation (still without segments), Fig. 80 at the stage with four +primitive segments. <i>ak, ik, mk</i> outer, inner, and middle germinal layer, +<i>hp</i> horn plate, <i>mp</i> medullary plate, <i>ch</i> chorda, * and * +disposition of the cœlom-pouches, <i>lh</i> body-cavity.)</p> +</div> + +<p> +The spherical blastula that arises from the impregnated ovum of the sagitta is +converted by a folding at one pole into a typical archigastrula, entirely +similar to that of the <i>Monoxenia</i> which I described (Chapter VIII, Fig. +29). This oval, uni-axial cup-larva (circular in section) becomes bilateral (or +tri-axial) by the growth of a couple of cœlom-pouches from the primitive gut +(Figs. 76, 77). To the right and left a sac-shaped fold appears towards the top +pole (where the permanent mouth, <i>m,</i> afterwards arises). The two sacs are +at first separated by a couple of folds of the entoderm (Fig. 76 <i> pv</i>), +and are still connected with the primitive gut by wide apertures; they also +communicate for a short time with the dorsal side (Fig. 77 <i>d</i>). Soon, +however, the cœlom-pouches completely separate from each other and from the +primitive gut; at the same time they enlarge so much that they close round the +primitive gut (Fig. 78). But in the middle line of the dorsal and ventral sides +the pouches remain separated, their approaching walls joining here to form a +thin vertical partition, the mesentery (<i>dm</i> and <i>vm</i>). Thus <i> +Sagitta</i> has throughout life a double body-cavity (Fig. 78 <i> lk</i>), and +the gut is fastened to the body-wall both above and below by a +mesentery—below by the ventral mesentery (<i>vm</i>), and above by the +dorsal mesentery (<i>dm</i>). The inner layer of the two cœlom-pouches +(<i>mv</i>) attaches itself to the entoderm (<i>ik</i>), and forms with it the +visceral wall. The outer layer (<i>mp</i>) attaches itself to the ectoderm +(<i>ak</i>), and forms with it the outer body-wall. Thus we have in +<i>Sagitta</i> a perfectly clear and simple illustration of the original +cœlomation of the enterocœla. This palingenetic fact is the more important, as +the greater part of the two body-cavities in <i>Sagitta</i> changes afterwards +into sexual glands—the fore or female part into a pair of ovaries, and +the hind or male part into a pair of testicles. +</p> + +<p> +Cœlomation takes place with equal clearness and transparency in the case of +<span class='pagenum'><a name="Page_95" id="Page_95"></a></span> +the amphioxus, the lowest vertebrate, and its nearest relatives, the +invertebrate tunicates, the sea-squirts. However, in these two stems, which we +class together as <i> Chordonia,</i> this important process is more complex, as +two other processes are associated with it—the development of the chorda +from the entoderm and the separation of the medullary plate or nervous centre +from the ectoderm. Here again the skulless amphioxus has preserved to our own +time by tenacious heredity the chief phenomena in their original form, while it +has been more or less modified by embryonic adaptation in all the other +vertebrates (with skulls). Hence we must once more thoroughly understand the +palingenetic embryonic features of the lancelet before we go on to consider the +cenogenetic forms of the craniota. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus81"></a> +<img src="images/fig81.gif" width="347" height="165" alt="Figs. 81 and 82. Transverse section of amphioxus +embryo." /> +<p class="caption">Figs. 81 and 82.—<b>Transverse section of amphioxus +embryo.</b> Fig. 81 at the stage with five somites, Fig. 82 at the stage with +eleven somites. (From <i>Hatschek.</i>) <i>ak</i> outer germinal layer, +<i>mp</i> medullary plate, <i>n</i> nerve-tube, <i>ik</i> inner germinal layer, +<i>dh</i> visceral cavity, <i>lh</i> body-cavity, <i>mk</i> middle germinal +layer (<i>mk</i><sub>1</sub> parietal, <i>mk</i><sub>2</sub> visceral), +<i>us</i> primitive segment, <i> ch</i> chorda.</p> +</div> + +<p> +The cœlomation of the amphioxus, which was first observed by Kowalevsky in +1867, has been very carefully studied since by Hatschek (1881). According to +him, there are first formed on the bilateral gastrula we have already +considered (Figs. 36, 37) three parallel longitudinal folds—one single +ectodermal fold in the central line of the dorsal surface, and a pair of +entodermic folds at the two sides of the former. The broad ectodermal fold that +first appears in the middle line of the flattened dorsal surface, and forms a +shallow longitudinal groove, is the beginning of the central nervous system, +the medullary tube. Thus the primary outer germinal layer divides into two +parts, the middle medullary plate (Fig. 81 <i>mp</i>) and the horny-plate +(<i>ak</i>), the beginning of the outer skin or epidermis. As the parallel +borders of the concave medullary plate fold towards each other and grow +underneath the horny-plate, a cylindrical tube is formed, the medullary tube +(Fig. 82 <i>n</i>); this quickly detaches itself altogether from the +horny-plate. At each side of the medullary tube, between it and the alimentary +tube (Figs. 79–82 <i>dh</i>), the two parallel longitudinal folds grow +out of the dorsal wall of the alimentary tube, and these form the two +cœlom-pouches (Figs. 80, 81 <i> lh</i>). This part of the entoderm, which thus +represents the first structure of the middle germinal layer, is shown darker +than the rest of the inner germinal layer in Figs. 79–82. The edges of +the folds meet, and thus form closed tubes (Fig. 81 in section). +</p> + +<p> +During this interesting process the outline of a third very important organ, +the chorda or axial rod, is being formed between the two cœlom-pouches. This +first foundation of the skeleton, a solid cylindrical cartilaginous rod, is +formed in the middle line of the dorsal primitive gut-wall, from the entodermal +cell-streak that remains here between the two cœlom-pouches (Figs. 79–82 +<i>ch</i>). The chorda appears at first in the shape of a flat longitudinal +fold or a shallow groove (Figs. 80, 81); it does not become a solid cylindrical +cord until after separation from the primitive gut (Fig. 82). Hence we might +say that the dorsal wall of the primitive gut forms three parallel longitudinal +folds at this important period—one single fold and a pair of folds. The +single middle fold becomes the chorda, and lies immediately below the groove of +the ectoderm, which becomes the medullary +<span class='pagenum'><a name="Page_96" id="Page_96"></a></span> +tube; the pair of folds to the right and left lie at the sides between the +former and the latter, and form the cœlom-pouches. The part of the primitive +gut that remains after the cutting off of these three dorsal primitive organs +is the permanent gut; its entoderm is the gut-gland-layer or enteric layer. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus83"></a> +<img src="images/fig83.gif" width="401" height="190" alt="Figs. 83 and 84. Chordula of the amphioxus." /> +<p class="caption">Figs. +83 and 84—<b>Chordula of the amphioxus.</b> Fig. 83 median +longitudinal section (seen from the left). Fig. 84 transverse section. (From +<i>Hatschek.</i>) In Fig. 83 the cœlom-pouches are omitted, in order to show +the chordula more clearly. Fig. 84 is rather diagrammatic. <i>h</i> +horny-plate, <i>m</i> medullary tube, <i>n</i> wall of same (<i>n′</i> dorsal, +<i>n″</i> ventral), <i> ch</i> chorda, <i>np</i> neuroporus, <i>ne</i> canalis +neurentericus, <i>d</i> gut-cavity, <i>r</i> gut dorsal wall, <i> b</i> gut +ventral wall, <i>z</i> yelk-cells in the latter, <i>u</i> primitive mouth, +<i>o</i> mouth-pit, <i>p</i> promesoblasts (primitive or polar cells of the +mesoderm), <i>w</i> parietal layer, <i>v</i> visceral layer of the mesoderm, +<i>c</i> cœlom, <i>f</i> rest of the segmentation-cavity.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus85"></a> +<img src="images/fig85.gif" width="401" height="224" alt="Figs. 85 and 86. Chordula of the amphibia (the +ringed adder)." /> +<p class="caption">Figs. 85 and 86—<b>Chordula of the amphibia</b> (the +ringed adder). (From <i>Goette.</i>) Fig. 85 median longitudinal section (seen +from the left), Fig. 86 transverse section (slightly diagrammatic). Lettering +as in Figs. 83 and 84.</p> +</div> + +<p> +I give the name of <i>chordula</i> or <i>chorda-larva</i> to the embryonic +stage of the vertebrate organism which is represented by the amphioxus larva at +this period (Figs. 83, 84, in the third period of development according to +Hatschek). (Strabo and Plinius give the name of <i>cordula</i> or +<i>cordyla</i> to young fish larvæ.) I ascribe the utmost phylogenetic +significance to it, as it is found in all the chorda-animals (tunicates as well +as vertebrates) in essentially the same form. Although the accumulation of +food-yelk greatly modifies the form of the chordula in the higher vertebrates, +it remains the same in its main features throughout. In all +<span class='pagenum'><a name="Page_97" id="Page_97"></a></span> +cases the nerve-tube (<i>m</i>) lies on the dorsal side of the bilateral, +worm-like body, the gut-tube (<i>d</i>) on the ventral side, the chorda +(<i>ch</i>) between the two, on the long axis, and the cœlom pouches (<i>c</i>) +at each side. In every case these primitive organs develop in the same way from +the germinal layers, and the same organs always arise from them in the mature +chorda-animal. Hence we may conclude, according to the laws of the theory of +descent, that all these chordonia or chordata (tunicates and vertebrates) +descend from an ancient common ancestral form, which we may call +<i>Chordæa.</i> We should regard this long-extinct <i>Chordæa,</i> if it were +still in existence, as a special class of unarticulated worm +(<i>chordaria</i>). It is especially noteworthy that neither the dorsal +nerve-tube nor the ventral gut-tube, nor even the chorda that lies between +them, shows any trace of articulation or segmentation; even the two cœlom-sacs +are not segmented at first (though in the amphioxus they quickly divide into a +series of parts by transverse +<span class='pagenum'><a name="Page_98" id="Page_98"></a></span> +folding). These ontogenetic facts are of the greatest importance for the +purpose of learning those ancestral forms of the vertebrates which we have to +seek in the group of the unarticulated vermalia. The cœlom-pouches were +originally sexual glands in these ancient chordonia. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus87"></a> +<img src="images/fig87.gif" width="430" height="203" alt="Figs. 87 and 88. Diagrammatic vertical section of +coelomula-embryos of vertebrates." /> +<p class="caption">Figs. 87 and +88—<b>Diagrammatic vertical section of cœlomula-embryos of +vertebrates.</b> (From <i>Hertwig.</i>) Fig. 87, vertical section +<i>through</i> the primitive mouth, Fig. 88, vertical section <i>before</i> the +primitive mouth. <i>u</i> primitive mouth, <i>ud</i> primitive gut. <i>d</i> +yelk, <i>dk</i> yelk-nuclei, <i>dh</i> gut-cavity, <i>lh</i> body-cavity, +<i>mp</i> medullary plate, <i>ch</i> chorda plate, <i>ak</i> and <i>ik</i> +outer and inner germinal layers, <i>pb</i> parietal and <i>vb</i> visceral +mesoblast.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus89"></a> +<img src="images/fig89.gif" width="430" height="212" alt="Figs. 89 and 90. Transverse section of coelomula +embryos of triton." /> +<p class="caption">Figs. 89 and 90—<b>Transverse section of cœlomula +embryos of triton.</b> (From <i>Hertwig.</i>) Fig. 89, section <i>through</i> +the primitive mouth. Fig. 90, section in front of the primitive mouth, <i>u</i> +primitive mouth. <i>dh</i> gut-cavity, <i>dz</i> yelk-cells, <i>dp</i> +yelk-stopper, <i>ak</i> outer and <i>ik</i> inner germinal layer, <i>pb</i> +parietal and <i>vb</i> visceral middle layer, <i>m</i> medullary plate, +<i>ch</i> chorda.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus91"></a> +<img src="images/fig91.gif" width="294" height="464" alt="Fig.91 A, B, C. Vertical section +of the dorsal part of three triton-embryos." /> +<p class="caption">Fig. 91. <i>A, B, +C.</i>—<b>Vertical section of the dorsal part of three +triton-embryos.</b> (From <i>Hertwig.</i>) In Fig. <i>A</i> the medullary +swellings (the parallel borders of the medullary plate) begin to rise; in Fig. +<i>B</i> they grow towards each other; in Fig. <i>C</i> they join and form the +medullary tube. <i>mp</i> medullary plate, <i>mf</i> medullary folds, <i>n</i> +nerve-tube, <i>ch</i> chorda, <i>lh</i> body-cavity, <i>mk</i><sub>1</sub> and +<i>mk</i><sub>2</sub> parietal and visceral mesoblasts, <i>uv</i> +primitive-segment cavities, <i>ak</i> ectoderm, <i>ik</i> entoderm, <i>dz</i> +yelk-cells, <i>dh</i> gut-cavity.</p> +</div> + +<p> +From the evolutionary point of view the cœlom-pouches are, in any case, older +than the chorda; since they also develop in the same way as in the chordonia in +a number of invertebrates which have no chorda (for instance, <i>Sagitta,</i> +Figs. 76–78). Moreover, in the amphioxus the first outline of the chorda +appears later than that of the cœlom-sacs. Hence we must, according to the +biogenetic law, postulate a special intermediate form between the gastrula and +the chordula, which we will call <i> cœlomula,</i> an unarticulated, worm-like +body with primitive gut, primitive mouth, and a double body-cavity, but no +chorda. This embryonic form, the bilateral <i>cœlomula</i> (Fig. 81), may in +turn be regarded as the ontogenetic reproduction (maintained by heredity) of an +ancient ancestral form of the cœlomaria, the <i> Cœlomæa</i> (cf. Chapter XX). +</p> + +<p> +In <i>Sagitta</i> and other worm-like animals the two cœlom-pouches (presumably +gonads or sex-glands) are separated by a complete median partition, the dorsal +and ventral mesentery (Fig. 78 <i>dm, vm</i>); but in the vertebrates only the +upper part of this vertical partition is maintained, and forms the dorsal +mesentery. This mesentery afterwards takes the form of a thin membrane, which +fastens the visceral tube to the chorda (or the vertebral column). At the under +side of the visceral tube the cœlom-sacs blend together, their inner or median +walls breaking down and disappearing. The body-cavity then forms a single +simple hollow, in which the gut is quite free, or only attached to the dorsal +wall by means of the mesentery. +</p> + +<p> +The development of the body-cavity and the formation of the <i> chordula</i> in +the higher vertebrates is, like that of the <i> gastrula,</i> chiefly modified +by the pressure of the food-yelk on the embryonic structures, which forces its +hinder part into +<span class='pagenum'><a name="Page_99" id="Page_99"></a></span> +a discoid expansion. These cenogenetic modifications seem to be so great that +until twenty years ago these important processes were totally misunderstood. It +was generally believed that the body-cavity in man and the higher vertebrates +was due to the division of a simple middle layer, and that the latter arose by +cleavage from one or both of the primary germinal layers. The truth was brought +to light at last by the comparative embryological research of the Hertwigs. +They showed in their <i>Cœlom Theory</i> (1881) that all vertebrates are true +enterocœla, and that in every case a pair of cœlom-pouches are developed from +the primitive gut by folding. The cenogenetic chordula-forms of the craniotes +must therefore be derived from the palingenetic embryology of the amphioxus in +the same way as I had previously proved for their gastrula-forms. +</p> + +<p> +The chief difference between the cœlomation of the acrania (<i>amphioxus</i>) +and the other vertebrates (with skulls—craniotes) is that the two +cœlom-folds of the primitive gut in the former are from the first hollow +vesicles, filled with fluid, but in the latter are empty pouches, the layers of +which (inner and outer) close with each other. In common parlance we still call +a pouch or pocket by that name, whether it is full or empty. It is different in +ontogeny; in some of our embryological literature ordinary logic does not count +for very much. In many of the manuals and large treatises on this science it is +proved that vesicles, pouches, or sacs deserve that name only when they are +inflated and filled with a clear fluid. When they are not so filled (for +instance, when the primitive gut of the gastrula is filled with yelk, or when +the walls of the empty cœlom-pouches are pressed together), these vesicles must +not be cavities any longer, but “solid structures.” +</p> + +<p> +The accumulation of food-yelk in the ventral wall of the primitive gut (Figs. +85, 86) is the simple cause that converts the sac-shaped cœlom-pouches of the +acrania into the leaf-shaped cœlom-streaks of the craniotes. To convince +ourselves of this we need only compare, with Hertwig, the palingenetic cœlomula +of the amphioxus (Figs. 80, 81) with the corresponding cenogenetic form of the +amphibia (Figs. 89–90), and construct the simple diagram that connects +the two (Figs. 87, 88). If we imagine the ventral half of the primitive +gut-wall in the amphioxus embryo (Figs. 79–84) distended with food-yelk, +the vesicular cœlom-pouches (<i>lh</i>) must be pressed together by this, and +forced to extend in the shape of a thin double plate between the gut-wall and +body-wall (Figs. 86, 87). This expansion follows a downward and forward +direction. They are not directly connected with these two walls. The real +unbroken connection between the two middle layers and the primary germ-layers +is found right at the back, in the region of the primitive mouth (Fig. 87 +<i>u</i>). At this important spot we have the source of embryonic development +(<i>blastocrene</i>), or “zone of growth,” from which the +cœlomation (and also the gastrulation) originally proceeds. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus92"></a> +<img src="images/fig92.gif" width="250" height="81" alt="Fig.92. Transverse +section of the chordula-embryo of a bird (from a hen’s egg at the close of the +first day of incubation)." /> +<p class="caption">Fig. 92—<b>Transverse section of the +chordula-embryo of a bird</b> (from a hen’s egg at the close of the first +day of incubation). (From <i>Kölliker.</i>) <i>h</i> horn-plate (ectoderm), +<i>m</i> medullary plate, <i>Rf</i> dorsal folds of same, <i>Pv</i> medullary +furrow, <i>ch</i> chorda, <i>uwp</i> median (inner) part of the middle layer +(median wall of the cœlom-pouches), <i>sp</i> lateral (outer) part of same, or +lateral plates, <i>uwh</i> structure of the body-cavity, <i>dd</i> +gut-gland-layer.</p> +</div> + +<p> +Hertwig even succeeded in showing, in the cœlomula-embryo of the water +salamander (<i>Triton</i>), between the first structures of the two middle +layers, the relic of the body-cavity, which is represented in the diagrammatic +transitional form (Figs. 87, 88). In sections both through the primitive mouth +itself (Fig. 89) and in front of it (Fig. 90) the two middle layers (<i>pb</i> +and <i>vb</i>) diverge from each other, and disclose the two body-cavities as +narrow clefts. At the primitive-mouth itself (Fig. 90 <i>u</i>) we can +penetrate into them from without. It is only here at the border of the +primitive mouth that we can show the direct transition of the two middle layers +into the two limiting layers or primary germinal layers. +</p> + +<p> +The structure of the chorda also shows the same features in these +cœlomula-embryos of the amphibia (Fig. 91) as in the amphioxus (Figs. +79–82). It arises from the entodermic cell-streak, which forms the middle +dorsal-line of the primitive gut, and occupies the space between the flat +cœlom-pouches (Fig. 91 <i>A</i>). +<span class='pagenum'><a name="Page_100" id="Page_100"></a></span> +While the nervous centre is formed here in the middle line of the back and +separated from the ectoderm as “medullary tube,” there takes place +at the same time, directly underneath, the severance of the chorda from the +entoderm (Fig. 91 <i>A, B, C</i>). Under the chorda is formed (out of the +ventral entodermic half of the gastrula) the permanent gut or visceral cavity +(<i>enteron</i>) (Fig. 91 <i>B, dh</i>). This is done by the coalescence, under +the chorda in the median line, of the two dorsal side-borders of the +gut-gland-layer (<i>ik</i>), which were previously separated by the +chorda-plate (Fig. 91 <i>A, ch</i>); these now alone form the clothing of the +visceral cavity (<i>dh</i>) (enteroderm, Fig. 91 <i>C</i>). All these important +modifications take place at first in the fore or head-part of the embryo, and +spread backwards from there; here at the hinder end, the region of the +primitive mouth, the important border of the mouth (or <i>properistoma</i>) +remains for a long time the source of development or the zone of fresh +construction, in the further building-up of the organism. One has only to +compare carefully the illustrations given (Figs. 85–91) to see that, as a +fact, the cenogenetic cœlomation of the amphibia can be deduced directly from +the palingenetic form of the acrania (Figs. 79–84). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus93"></a> +<img src="images/fig93.gif" width="327" height="95" alt="Fig.93. Transverse section of the +vertebrate-embryo of a bird (from a hen’s egg on the second day of +incubation)." /> +<p class="caption">Fig. +93—<b>Transverse section of the vertebrate-embryo of a bird</b> (from +a hen’s egg on the second day of incubation). (From <i>Kölliker.</i>) +<i>h</i> horn-plate, <i>mr</i> medullary tube, <i>ch</i> chorda, <i>uw</i> +primitive segments, <i> uwh</i> primitive-segment cavity (median relic of the +cœlom), <i>sp</i> lateral cœlom-cleft, <i>hpl</i> skin-fibre-layer, <i>df</i> +gut-fibre-layer, <i>ung</i> primitive-kidney passage, <i> ao</i> primitive +aorta, <i>dd</i> gut-gland-layer.</p> +</div> + +<p> +The same principle holds good for the amniotes, the reptiles, birds, and +mammals, although in this case the processes of cœlomation are more modified +and more difficult to identify on account of the colossal accumulation of +food-yelk and the corresponding notable flattening of the germinal disk. +However, as the whole group of the amniotes has been developed at a +comparatively late date from the class of the amphibia, their cœlomation must +also be directly traceable to that of the latter. This is really possible as a +matter of fact; even the older illustrations showed an essential identity of +features. Thus forty years ago Kölliker gave, in the first edition of his +<i>Human Embryology</i> (1861), some sections of the chicken-embryo, the +features of which could at once be reduced to those already described and +explained in the sense of Hertwig’s cœlom-theory. A section through the +embryo in the hatched hen’s egg towards the close of the first day of +incubation shows in the middle of the dorsal surface a broad ectodermic +medullary groove (Fig. 92 <i>Rf</i>), and underneath the middle of the chorda +(<i>ch</i>) and at each side of it a couple of broad mesodermic layers +(<i>sp</i>). These enclose a narrow space or cleft (<i>uwh</i>), which is +nothing else than the structure of the body-cavity. The two layers that enclose +it—the upper parietal layer (<i>hpl</i>) and the lower visceral layer +(<i>df</i>)—are pressed together from without, but clearly +distinguishable. This is even clearer a little later, when the medullary furrow +is closed into the nerve-tube (Fig. 93 <i>mr</i>). +</p> + +<p> +Special importance attaches to the fact that here again the four secondary +germinal layers are already sharply distinct, and easily separated from each +other. There is only one very restricted area in which they are connected, and +actually pass into each other; this is the region of the primitive mouth, which +is contracted in the amniotes into a dorsal longitudinal cleft, the primitive +groove. Its two lateral lip-borders form the <i>primitive streak,</i> which has +long been recognised as the most important embryonic source and starting-point +of further processes. Sections through this primitive streak (Figs. 94 and 95) +show that the two primary germinal layers grow at an early stage (in the +discoid gastrula of the chick, a few hours after incubation) into the primitive +<span class='pagenum'><a name="Page_101" id="Page_101"></a></span> +streak (<i>x</i>), and that the two middle layers extend outward from this +thickened axial plate (<i>y</i>) to the right and left between the former. The +plates of the cœlom-layers, the parietal skin-fibre-layer (<i>m</i>) and the +visceral gut-fibre-layer (<i>f</i>), are seen to be still pressed close +together, and only diverge later to form the body-cavity. Between the inner +borders of the two flat cœlom-pouches lies the chorda (Fig. 95 <i>x</i>), which +here again develops from the middle line of the dorsal wall of the primitive +gut. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus94"></a> +<img src="images/fig94.gif" width="319" height="214" alt="Transverse section of the primitive +streak (primitive mouth) of the chick." /> +<p class="caption">Figs. 94 and 95—<b>Transverse section of +the primitive-streak (primitive mouth) of the chick.</b> Fig. 94 a few hours +after the commencement of incubation, Fig. 95 a little later. (From <i> +Waldeyer.</i>) <i>h</i> horn-plate, <i>n</i> nerve-plate, <i>m</i> +skin-fibre-layer, <i>f</i> gut-fibre-layer, <i>d</i> gut-gland-layer, <i>y</i> +primitive streak or axial plate, in which all four germinal layers meet, +<i>x</i> structure of the chorda, <i>u</i> region of the later primitive +kidneys.</p> +</div> + +<p> +Cœlomation takes place in the vertebrates in just the same way as in the birds +and reptiles. This was to be expected, as the characteristic gastrulation of +the mammal has descended from that of the reptiles. In both cases a discoid +gastrula with primitive streak arises from the segmented ovum, a two-layered +germinal disk with long and small hinder primitive mouth. Here again the two +primary germinal layers are only directly connected (Fig. 96 <i> pr</i>) along +the primitive streak (at the folding-point of the blastula), and from this spot +(the border of the primitive mouth) the middle germinal layers (<i>mk</i>) grow +out to right and left between the preceding. In the fine illustration of the +cœlomula of the rabbit which Van Beneden has given us (Fig. 96) one can clearly +see that each of the four secondary germinal layers consists of a single +stratum of cells. +</p> + +<p> +Finally, we must point out, as a fact of the utmost importance for our +anthropogeny and of great general interest, that the four-layered cœlomula of +man has just the same construction as that of the rabbit (Fig. 96). A vertical +section that Count Spee made through the primitive mouth or streak of a very +young human germinal disk (Fig. 97) clearly shows that here again the four +secondary germ-layers are inseparably connected only at the primitive streak, +and that here also the two flattened cœlom-pouches (<i>mk</i>) extend outwards +to right and left from the primitive mouth between the outer and inner germinal +layers. In this case, too, the middle germinal layer consists from the first of +two separate strata of cells, the parietal (<i>mp</i>) and visceral (<i>mv</i>) +mesoblasts. +</p> + +<p> +These concordant results of the best recent investigations (which have been +confirmed by the observations of a number of scientists I have not enumerated) +prove the unity of the vertebrate-stem in point of cœlomation, no less than of +gastrulation. In both respects the invaluable amphioxus—the sole survivor +of the acrania—is found to be the original model that has preserved for +us in palingenetic form by a tenacious heredity these +<span class='pagenum'><a name="Page_102" id="Page_102"></a></span> +most important embryonic processes. From this primary model of construction we +can cenogenetically deduce all the embryonic forms of the other vertebrates, +the craniota, by secondary modifications. My thesis of the universal formation +of the gastrula by folding of the blastula has now been clearly proved for all +the vertebrates; so also has been Hertwig’s thesis of the origin of the +middle germinal layers by the folding of a couple of cœlom-pouches which appear +at the border of the primitive mouth. Just as the gastræa-theory explains the +origin and identity of the two primary layers, so the cœlom-theory explains +those of the four secondary layers. The point of origin is always the +properistoma, the border of the original primitive mouth of the gastrula, at +which the two primary layers pass directly into each other. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus96"></a> +<img src="images/fig96.gif" width="304" height="165" alt="Fig.96. Transverse section of the +primitive groove (or primitive mouth) of a rabbit." /> +<p class="caption">Fig. 96—<b>Transverse section of the +primitive groove (or primitive mouth) of a rabbit.</b> (From <i>Van +Beneden.</i>) <i> pr</i> primitive mouth, <i>ul</i> lips of same (primitive +lips), <i>ak</i> and <i>ik</i> outer and inner germinal layers, <i>mk</i> +middle germinal layer, <i>mp</i> parietal layer, <i>mv</i> visceral layer of +the mesoderm.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus97"></a> +<img src="images/fig97.gif" width="274" height="154" alt="Fig.97. Transverse section of the +primitive mouth (or groove) of a human embryo (at the coelomula stage)." /> +<p class="caption">Fig. +97—<b>Transverse section of the primitive mouth (or groove) of a +human embryo</b> (at the cœlomula stage). (From <i>Count Spee.</i>) <i>pr</i> +primitive mouth, <i>ul</i> lips of same (primitive folds), <i>ak</i> and +<i>ik</i> outer and inner germinal layers, <i>mk</i> middle layer, <i>mp</i> +parietal layer, <i>mv</i> visceral layer of the mesoblasts.</p> +</div> + +<p> +Moreover, the cœlomula is important as the immediate source of the chordula, +the embryonic reproduction of the ancient, typical, unarticulated, worm-like +form, which has an axial chorda between the dorsal nerve-tube and the ventral +gut-tube. This instructive chordula (Figs. 83–86) provides a valuable +support of our phylogeny; it indicates the important moment in our stem-history +at which the stem of the chordonia (tunicates and vertebrates) parted for ever +from the divergent stems of the other metazoa (articulates, echinoderms, and +molluscs). +</p> + +<p> +I may express here my opinion, in the form of a chordæa-theory, that the +characteristic chordula-larva of the chordonia has in reality this great +significance—it is the typical reproduction (preserved by heredity) of +the ancient common stem-form of all the vertebrates and tunicates, the +long-extinct <i>Chordæa.</i> We will return in Chapter XX to these worm-like +ancestors, which stand out as luminous points in the obscure stem-history of +the invertebrate ancestors of our race. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap11"></a><span class='pagenum'><a name="Page_103" +id="Page_103"></a></span>Chapter XI.<br/> +THE VERTEBRATE CHARACTER OF MAN</h2> + +<p> +We have now secured a number of firm standing-places in the labyrinthian course +of our individual development by our study of the important embryonic forms +which we have called the cytula, morula, blastula, gastrula, cœlomula, and +chordula. But we have still in front of us the difficult task of deriving the +complicated frame of the human body, with all its different parts, organs, +members, etc., from the simple form of the chordula. We have previously +considered the origin of this four-layered embryonic form from the two-layered +gastrula. The two primary germinal layers, which form the entire body of the +gastrula, and the two middle layers of the cœlomula that develop between them, +are the four simple cell-strata, or epithelia, which alone go to the formation +of the complex body of man and the higher animals. It is so difficult to +understand this construction that we will first seek a companion who may help +us out of many difficulties. +</p> + +<p> +This helpful associate is the science of comparative anatomy. Its task is, by +comparing the fully-developed bodily forms in the various groups of animals, to +learn the general laws of organisation according to which the body is +constructed; at the same time, it has to determine the affinities of the +various groups by critical appreciation of the degrees of difference between +them. Formerly, this work was conceived in a teleological sense, and it was +sought to find traces of the plan of the Creator in the actual purposive +organisation of animals. But comparative anatomy has gone much deeper since the +establishment of the theory of descent; its philosophic aim now is to explain +the variety of organic forms by adaptation, and their similarity by heredity. +At the same time, it has to recognise in the shades of difference in form the +degree of blood-relationship, and make an effort to construct the ancestral +tree of the animal world. In this way, comparative anatomy enters into the +closest relations with comparative embryology on the one hand, and with the +science of classification on the other. +</p> + +<p> +Now, when we ask what position man occupies among the other organisms according +to the latest teaching of comparative anatomy and classification, and how +man’s place in the zoological system is determined by comparison of the +mature bodily forms, we get a very definite and significant reply; and this +reply gives us extremely important conclusions that enable us to understand the +embryonic development and its evolutionary purport. Since Cuvier and Baer, +since the immense progress that was effected in the early decades of the +nineteenth century by these two great zoologists, the opinion has generally +prevailed that the whole animal kingdom may be distributed in a small number of +great divisions or types. They are called types because a certain typical or +characteristic structure is constantly preserved within each of these large +sections. Since we applied the theory of descent to this doctrine of types, we +have learned that this common type is an outcome of heredity; all the animals +of one type are blood-relatives, or members of one stem, and can be traced to a +common ancestral form. Cuvier and Baer set up four of these types: the +vertebrates, articulates, molluscs, and radiates. The first three of these are +still retained, and may be conceived as natural phylogenetic unities, as stems +or <i>phyla</i> in the sense of the theory of descent. It is quite otherwise +with the fourth type—the radiata. These animals, little known as yet at +the beginning of the nineteenth century, were made to form a sort of +lumber-room, into which were cast all the lower animals that did not belong to +the other three types. As we obtained a closer acquaintance with them in the +course of the last sixty years, it was found that we must distinguish among +them from four to eight different types. In this way the total number of animal +stems or phyla has been raised to eight or twelve (cf. Chapter XX). +</p> + + +<p> +<span class='pagenum'><a name="Page_104" id="Page_104"></a></span> +These twelve stems of the animal kingdom are, however, by no means co-ordinate +and independent types, but have definite relations, partly of subordination, to +each other, and a very different phylogenetic meaning. Hence they must not be +arranged simply in a row one after the other, as was generally done until +thirty years ago, and is still done in some manuals. We must distribute them in +three subordinate principal groups of very different value, and arrange the +various stems phylogenetically on the principles which I laid down in my +<i>Monograph on the Sponges,</i> and developed in the <i>Study of the Gastræa +Theory.</i> We have first to distinguish the unicellular animals +(<i>protozoa</i>) from the multicellular tissue-forming (<i>metazoa</i>). Only +the latter exhibit the important processes of segmentation and gastrulation; +and they alone have a primitive gut, and form germinal layers and tissues. +</p> + +<p> +The metazoa, the tissue-animals or gut-animals, then sub-divide into two main +sections, according as a body-cavity is or is not developed between the primary +germinal layers. We may call these the <i>cœlenteria</i> and <i>cœlomaria,</i> +the former are often also called <i>zoophytes</i> or <i>cœlenterata,</i> and +the latter <i>bilaterals.</i> This division is the more important as the +cœlenteria (without cœlom) have no blood and blood-vessels, nor an anus. The +cœlomaria (with body-cavity) have generally an anus, and blood and +blood-vessels. There are four stems belonging to the cœlenteria: the gastræads +(“primitive-gut animals”), sponges, cnidaria, and platodes. Of the +cœlomaria we can distinguish six stems: the vermalia at the bottom represent +the common stem-group (derived from the platodes) of these, the other five +typical stems of the cœlomaria—the molluscs, echinoderms, articulates, +tunicates, and vertebrates—being evolved from them. +</p> + +<p> +Man is, in his whole structure, a true vertebrate, and develops from an +impregnated ovum in just the same characteristic way as the other vertebrates. +There can no longer be the slightest doubt about this fundamental fact, nor of +the fact that all the vertebrates form a natural phylogenetic unity, a single +stem. The whole of the members of this stem, from the amphioxus and the +cyclostoma to the apes and man, have the same characteristic disposition, +connection, and development of the central organs, and arise in the same way +from the common embryonic form of the chordula. Without going into the +difficult question of the origin of this stem, we must emphasise the fact that +the vertebrate stem has no direct affinity whatever to five of the other ten +stems; these five isolated phyla are the sponges, cnidaria, molluscs, +articulates, and echinoderms. On the other hand, there are important and, to an +extent, close phylogenetic relations to the other five stems—the protozoa +(through the amœbæ), the gastræads (through the blastula and gastrula), the +platodes and vermalia (through the cœlomula), and the tunicates (through the +chordula). +</p> + +<p> +How we are to explain these phylogenetic relations in the present state of our +knowledge, and what place is assigned to the vertebrates in the animal +ancestral tree, will be considered later (Chapter XX). For the present our task +is to make plainer the vertebrate character of man, and especially to point out +the chief peculiarities of organisation by which the vertebrate stem is +profoundly separated from the other eleven stems of the animal kingdom. Only +after these comparative-anatomical considerations shall we be in a position to +attack the difficult question of our embryology. The development of even the +simplest and lowest vertebrate from the simple chordula (Figs. 83–86) is +so complicated and difficult to follow that it is necessary to understand the +organic features of the fully-formed vertebrate in order to grasp the course of +its embryonic evolution. But it is equally necessary to confine our attention, +in this general anatomic description of the vertebrate-body, to the essential +facts, and pass by all the unessential. Hence, in giving now an ideal anatomic +description of the chief features of the vertebrate and its internal +organisation, I omit all the subordinate points, and restrict myself to the +most important characteristics. +</p> + +<p> +Much, of course, will seem to the reader to be essential that is only of +subordinate and secondary interest, or even not essential at all, in the light +of comparative anatomy and embryology. For instance, the skull and vertebral +column and the extremities are non-essential in this sense. It is true that +these parts are very important <i>physiologically</i>; but for the +<i>morphological</i> conception of the vertebrate they are not essential, +because they are only found in the higher, not the lower, vertebrates. The +lowest vertebrates have +<span class='pagenum'><a name="Page_105" id="Page_105"></a></span> +neither skull nor vertebræ, and no extremities or limbs. Even the human embryo +passes through a stage in which it has no skull or vertebræ; the trunk is quite +simple, and there is yet no trace of arms and legs. At this stage of +development man, like every other higher vertebrate, is essentially similar to +the simplest vertebrate form, which we now find in only one living specimen. +This one lowest vertebrate that merits the closest study—undoubtedly the +most interesting of all the vertebrates after man—is the famous lancelet +or amphioxus, to which we have already often referred. As we are going to study +it more closely later on (Chapters XVI and XVII), I will only make one or two +passing observations on it here. +</p> + +<p> +The amphioxus lives buried in the sand of the sea, is about one or two inches +in length, and has, when fully developed, the shape of a very simple, longish, +lancet-like leaf; hence its name of the lancelet. The narrow body is compressed +on both sides, almost equally pointed at the fore and hind ends, without any +trace of external appendages or articulation of the body into head, neck, +breast, abdomen, etc. Its whole shape is so simple that its first discoverer +thought it was a naked snail. It was not until much later—half a century +ago—that the tiny creature was studied more carefully, and was found to +be a true vertebrate. More recent investigations have shown that it is of the +greatest importance in connection with the comparative anatomy and ontogeny of +the vertebrates, and therefore with human phylogeny. The amphioxus reveals the +great secret of the origin of the vertebrates from the invertebrate vermalia, +and in its development and structure connects directly with certain lower +tunicates, the ascidia. +</p> + +<p> +When we make a number of sections of the body of the amphioxus, firstly +vertical longitudinal sections through the whole body from end to end, and +secondly transverse sections from right to left, we get anatomic pictures of +the utmost instructiveness (cf. Figs. 98–102). In the main they +correspond to the ideal which we form, with the aid of comparative anatomy and +ontogeny, of the primitive type or build of the vertebrate—the +long-extinct form to which the whole stem owes its origin. As we take the +phylogenetic unity of the vertebrate stem to be beyond dispute, and assume a +common origin from a primitive stem-form for all the vertebrates, from +amphioxus to man, we are justified in forming a definite morphological idea of +this primitive vertebrate (<i>Prospondylus</i> or <i>Vertebræa</i>). We need +only imagine a few slight and unessential changes in the real sections of the +amphioxus in order to have this ideal anatomic figure or diagram of the +primitive vertebrate form, as we see in Figs. 98–102. The amphioxus +departs so little from this primitive form that we may, in a certain sense, +describe it as a modified “primitive vertebrate.”<a href="#linknote-24" name="linknoteref-24" id="linknoteref-24"><sup>[24]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-24" id="linknote-24"></a> <a href="#linknoteref-24">[24]</a> +The ideal figure of the vertebrate as given in Figs. 98–102 is a +hypothetical scheme or diagram, that has been chiefly constructed on the lines +of the amphioxus, but with a certain attention to the comparative anatomy and +ontogeny of the ascidia and appendicularia on the one hand, and of the +cyclostoma and selachii on the other. This diagram has no pretension whatever +to be an “exact picture,” but merely an attempt to reconstruct +hypothetically the unknown and long extinct vertebrate stem-form, an ideal +“archetype.” +</p> + +<p> +The outer form of our hypothetical primitive vertebrate was at all events very +simple, and probably more or less similar to that of the lancelet. The +bilateral or bilateral-symmetrical body is stretched out lengthways and +compressed at the sides (Figs. 98–100), oval in section (Figs. 101, 102). +There are no external articulation and no external appendages, in the shape of +limbs, legs, or fins. On the other hand, the division of the body into two +sections, head and trunk, was probably clearer in <i>Prospondylus</i> than it +is in its little-changed ancestor, the amphioxus. In both animals the fore or +head-half of the body contains different organs from the trunk, and different +on the dorsal from on the ventral side. As this important division is found +even in the sea-squirt, the remarkable invertebrate stem-relative of the +vertebrates, we may assume that it was also found in the prochordonia, the +common ancestors of both stems. It is also very pronounced in the young larvæ +of the cyclostoma; this fact is particularly interesting, as this palingenetic +larva-form is in other respects also an important connecting-link between the +higher vertebrates and the acrania. +</p> + +<p> +The head of the acrania, or the anterior half of the body (both of the real +amphioxus and the ideal prospondylus), contains the branchial (gill) gut and +heart in the ventral section and the brain and sense-organs in the dorsal +section. The trunk, or posterior half of the body, contains the hepatic (liver) +gut and sexual-glands +<span class='pagenum'><a name="Page_106" id="Page_106"></a></span> +<span class='pagenum'><a name="Page_107" id="Page_107"></a></span> +in the ventral part, and the spinal marrow and most of the muscles in +the dorsal part. +</p> + + +<div class="fig" style="width:100%;"> +<a name="illus98"></a> +<img src="images/fig98.gif" width="406" height="616" alt="Figs. 98-102. The ideal primitive +vertebrate (prospondylus). Diagram." /> +<p class="caption">Figs. +98–102.—<b>The ideal primitive vertebrate (prospondylus). +Diagram.</b> Fig. 98 side-view (from the left). Fig. 99 back-view. Fig. 100 +front view. Fig. 101 transverse section through the head (to the left through +the gill-pouches, to the right through the gill-clefts). Fig. 102 transverse +section of the trunk (to the right a pro-renal canal is affected). <i>a</i> +aorta, <i>af</i> anus, <i>au</i> eye, <i>b</i> lateral furrow (primitive renal +process), <i>c</i> cœloma (body-cavity), <i>d</i> small intestine, <i>e</i> +parietal eye (epiphysis), <i>f</i> fin border of the skin, <i>g</i> auditory +vesicle, <i>gh</i> brain, <i>h</i> heart, <i>i</i> muscular cavity (dorsal +cœlom-pouch), <i>k</i> gill-gut, <i>ka</i> gill-artery, <i>kg</i> gill-arch, +<i>ks</i> gill-folds, <i>l</i> liver, <i>ma</i> stomach, <i>md</i> mouth, +<i>ms</i> muscles, <i>na</i> nose (smell pit), <i>n</i> renal canals, <i>u</i> +apertures of same, <i>o</i> outer skin, <i>p</i> gullet, <i>r</i> spinal +marrow, a sexual glands (gonads), <i>t</i> corium, <i>u</i> kidney-openings +(pores of the lateral furrow), <i>v</i> visceral vein (chief vein). <i>x</i> +chorda, <i>y</i> hypophysis (urinary appendage), <i>z</i> gullet-groove or +gill-groove (hypobranchial groove).</p> +</div> + +<p> +In the longitudinal section of the ideal vertebrate (Fig. 98) we have in the +middle of the body a thin and flexible, but stiff, cylindrical rod, pointed at +both ends (<i>ch</i>). It goes the whole length through the middle of the body, +and forms, as the central skeletal axis, the original structure of the later +vertebral column. This is the axial rod, or <i>chorda dorsalis,</i> also called +<i>chorda vertebralis,</i> vertebral cord, axial cord, dorsal cord, +<i>notochorda,</i> or, briefly, <i>chorda.</i> This solid, but flexible and +elastic, axial rod consists of a cartilaginous mass of cells, and forms the +inner axial skeleton or central frame of the body; it is only found in +vertebrates and tunicates, not in any other animals. As the first structure of +the spinal column it has the same radical significance in all vertebrates, from +the amphioxus to man. But it is only in the amphioxus and the cyclostoma that +the axial rod retains its simplest form throughout life. In man and all the +higher vertebrates it is found only in the earlier embryonic period, and is +afterwards replaced by the articulated vertebral column. +</p> + +<p> +The axial rod or chorda is the real solid chief axis of the vertebrate body, +and at the same time corresponds to the ideal long-axis, and serves to direct +us with some confidence in the orientation of the principal organs. We +therefore take the vertebrate-body in its original, natural disposition, in +which the long-axis lies horizontally, the dorsal side upward and the ventral +side downward (Fig. 98). When we make a vertical section through the whole +length of this long axis, the body divides into two equal and symmetrical +halves, right and left. In each half we have <i>originally</i> the same organs +in the same disposition and connection; only their disposal in relation to the +vertical plane of section, or median plane, is exactly reversed: the left half +is the reflection of the right. We call the two halves <i>antimera</i> +(opposed-parts). In the vertical plane of section that divides the two halves +the sagittal (“arrow”) axis, or “dorsoventral axis,” +goes from the back to the belly, corresponding to the sagittal seam of the +skull. But when we make a horizontal longitudinal section through the chorda, +the whole body divides into a dorsal and a ventral half. The line of section +that passes through the body from right to left is the transverse, frontal, or +lateral axis. +</p> + +<p> +The two halves of the vertebrate body that are separated by this horizontal +transverse axis and by the chorda have quite different characters. The dorsal +half is mainly the animal part of the body, and contains the greater part of +what are called the animal organs, the nervous system, muscular system, osseous +system, etc.—the instruments of movement and sensation. The ventral half +is essentially the vegetative half of the body, and contains the greater part +of the vertebrate’s vegetal organs, the visceral and vascular systems, +sexual system, etc.—the instruments of nutrition and reproduction. Hence +in the construction of the dorsal half it is chiefly the outer, and in the +construction of the ventral half chiefly the inner, germinal layer that is +engaged. Each of the two halves develops in the shape of a tube, and encloses a +cavity in which another tube is found. The dorsal half contains the narrow +spinal-column cavity or vertebral canal <i>above</i> the chorda, in which lies +the tube-shaped central nervous system, the medullary tube. The ventral half +contains the much more spacious visceral cavity or body-cavity +<i>underneath</i> the chorda, in which we find the alimentary canal and all its +appendages. +</p> + +<p> +The medullary tube, as the central nervous system or psychic organ of the +vertebrate is called in its first stage, consists, in man and all the higher +vertebrates, of two different parts: the large brain, contained in the skull, +and the long spinal cord which stretches from there over the whole dorsal part +of the trunk. Even in the primitive vertebrate this composition is plainly +indicated. The fore half of the body, which corresponds to the head, encloses a +knob-shaped vesicle, the brain (<i>gh</i>); this is prolonged backwards into +the thin cylindrical tube of the spinal marrow (<i>r</i>). Hence we find here +this very important psychic organ, which accomplishes sensation, will, and +thought, in the vertebrates, in its simplest form. The thick wall of the +nerve-tube, which runs through the long axis of the body immediately over the +axial rod, encloses a narrow central canal filled with fluid (Figs. +98–102 <i>r</i>). We still find the medullary tube in this very simple +form for a time in the embryo of all the vertebrates, and it retains this form +in the amphioxus throughout life; +<span class='pagenum'><a name="Page_108" id="Page_108"></a></span> +only in the latter case the cylindrical medullary tube barely indicates the +separation of brain and spinal cord. The lancelet’s medullary tube runs +nearly the whole length of the body, above the chorda, in the shape of a long +thin tube of almost equal diameter throughout, and there is only a slight +swelling of it right at the front to represent the rudiment of a cerebral lobe. +It is probable that this peculiarity of the amphioxus is connected with the +partial atrophy of its head, as the ascidian larvæ on the one hand and the +young cyclostoma on the other clearly show a division of the vesicular brain, +or head marrow, from the thinner, tubular spinal marrow. +</p> + +<p> +Probably we must trace to the same phylogenetic cause the defective nature of +the sense organs of the amphioxus, which we will describe later (Chapter XVI). +Prospondylus, on the other hand, probably had three pairs of sense-organs, +though of a simple character, a pair of, or a single olfactory depression, +right in front (Figs. 98, 99, <i>na</i>), a pair of eyes (<i>au</i>) in the +lateral walls of the brain, and a pair of simple auscultory vesicles (<i>g</i>) +behind. There was also, perhaps, a single parietal or “pineal” eye +at the top of the skull (<i>epiphysis, e</i>). +</p> + +<p> +In the vertical median plane (or middle plane, dividing the bilateral body into +right and left halves) we have in the acrania, underneath the chorda, the +mesentery and visceral tube, and above it the medullary tube; and above the +latter a membranous partition of the two halves of the body. With this +partition is connected the mass of connective tissue which acts as a sheath +both for the medullary tube and the underlying chorda, and is, therefore, +called the chord-sheath (<i>perichorda</i>); it originates from the dorsal and +median part of the cœlom-pouches, which we shall call the skeleton plate or +“sclerotom” in the craniote embryo. In the latter the chief part of +the skeleton—the vertebral column and skull—develops from this +chord-sheath; in the acrania it retains its simple form as a soft connective +matter, from which are formed the membranous partitions between the various +muscular plates or myotomes (Figs. 98, 99 <i>ms</i>). +</p> + +<p> +To the right and left of the cord-sheath, at each side of the medullary tube +and the underlying axial rod, we find in all the vertebrates the large masses +of muscle that constitute the musculature of the trunk and effect its +movements. Although these are very elaborately differentiated and connected in +the developed vertebrate (corresponding to the various parts of the bony +skeleton), in our ideal primitive vertebrate we can distinguish only two pairs +of these principal muscles, which run the whole length of the body parallel to +the chorda. These are the upper (dorsal) and lower (ventral) lateral muscles of +the trunk. The upper (dorsal) muscles, or the original dorsal muscles (Fig. 102 +<i>ms</i>), form the thick mass of flesh on the back. The lower (ventral) +muscles, or the original muscles of the belly, form the fleshy wall of the +abdomen. Both sets are segmented, and consist of a double row of muscular +plates (Figs. 98, 99 <i>ms</i>); the number of these myotomes determines the +number of joints in the trunk, or metamera. The myotomes are also developed +from the thick wall of the cœlom-pouches (Fig. 102 <i>i</i>). +</p> + +<p> +Outside this muscular tube we have the external envelope of the vertebrate +body, which is known as the corium or cutis. This strong and thick envelope +consists, in its deeper strata, chiefly of fat and loose connective tissue, and +in its upper layers of cutaneous muscles and firmer connective tissue. It +covers the whole surface of the fleshy body, and is of considerable thickness +in all the craniota. But in the acrania the corium is merely a thin plate of +connective tissue, an insignificant “corium-plate” (<i>lamella +corii,</i> Figs. 98–102 <i>t</i>). +</p> + +<p> +Immediately above the corium is the outer skin (<i>epidermis, o</i>), the +general covering of the whole outer surface. In the higher vertebrates the +hairs, nails, feathers, claws, scales, etc., grow out of this epidermis. It +consists, with all its appendages and products, of simple cells, and has no +blood-vessels. Its cells are connected with the terminations of the sensory +nerves. Originally, the outer skin is a perfectly simple covering of the outer +surface of the body, composed only of homogeneous cells—a permanent +horn-plate. In this simplest form, as a one-layered epithelium, we find it, at +first, in all the vertebrates, and throughout life in the acrania. It +afterwards grows thicker in the higher vertebrates, and divides into two +strata—an outer, firmer corneous (horn) layer and an inner, softer +mucus-layer; also a number of external and internal appendages grow out of it: +outwardly, the hairs, nails, claws, etc., and +<span class='pagenum'><a name="Page_109" id="Page_109"></a></span> +inwardly, the sweat-glands, fat-glands, etc. +</p> + +<p> +It is probable that in our primitive vertebrate the skin was raised in the +middle line of the body in the shape of a vertical fin border (<i>f</i>). A +similar fringe, going round the greater part of the body, is found to-day in +the amphioxus and the cyclostoma; we also find one in the tail of fish-larvæ +and tadpoles. +</p> + +<p> +Now that we have considered the external parts of the vertebrate and the animal +organs, which mainly lie in the dorsal half, above the chorda, we turn to the +vegetal organs, which lie for the most part in the ventral half, below the +axial rod. Here we find a large body-cavity or visceral cavity in all the +craniota. The spacious cavity that encloses the greater part of the +<i>viscera</i> corresponds to only a part of the original cœloma, which we +considered in Chapter X; hence it nay be called the <i>metacœloma.</i> As a +rule, it is still briefly called the cœloma; formerly it was known in anatomy +as the pleuroperitoneal cavity. In man and the other mammals (but only in +these) this cœloma divides, when fully developed, into two different cavities, +which are separated by a transverse partition—the muscular diaphragm. The +fore or pectoral cavity (pleura-cavity) contains the œsophagus (gullet), heart, +and lungs; the hind or peritoneal or abdominal cavity contains the stomach, +small and large intestines, liver, pancreas, kidneys, etc. But in the +vertebrate embryo, before the diaphragm is developed, the two cavities form a +single continuous body-cavity, and we find it thus in all the lower vertebrates +throughout life. This body-cavity is clothed with a delicate layer of cells, +the cœlom-epithelium. In the acrania the cœlom is segmented both dorsally and +ventrally, as their muscular pouches and primitive genital organs plainly show +(Fig. 102). +</p> + +<p> +The chief of the viscera in the body-cavity is the alimentary canal, the organ +that represents the whole body in the gastrula. In all the vertebrates it is a +long tube, enclosed in the body-cavity and more or less differentiated in +length, and has two apertures—a mouth for taking in food (Figs. 98, 100 +<i>md</i>) and an anus for the ejection of unusable matter or excrements +(<i>af</i>). With the alimentary canal a number of glands are connected which +are of great importance for the vertebrate body, and which all grow out of the +canal. Glands of this kind are the salivary glands, the lungs, the liver, and +many smaller glands. Nearly all these glands are wanting in the acrania; +probably there were merely a couple of simple hepatic tubes (Figs. 98, 100 +<i>l</i>) in the vertebrate stem-form. The wall of the alimentary canal and all +its appendages consists of two different layers; the inner, cellular clothing +is the gut-gland-layer, and the outer, fibrous envelope consists of the +gut-fibre-layer; it is mainly composed of muscular fibres which accomplish the +digestive movements of the canal, and of connective-tissue fibres that form a +firm envelope. We have a continuation of it in the mesentery, a thin, +bandage-like layer, by means of which the alimentary canal is fastened to the +ventral side of the chorda, originally the dorsal partition of the two +cœlom-pouches. The alimentary canal is variously modified in the vertebrates +both as a whole and in its several sections, though the original structure is +always the same, and is very simple. As a rule, it is longer (often several +times longer) than the body, and therefore folded and winding within the +body-cavity, especially at the lower end. In man and the higher vertebrates it +is divided into several sections, often separated by valves—the mouth, +pharynx, œsophagus, stomach, small and large intestine, and rectum. All these +parts develop from a very simple structure, which originally (throughout life +in the amphioxus) runs from end to end under the chorda in the shape of a +straight cylindrical canal. +</p> + +<p> +As the alimentary canal may be regarded morphologically as the oldest and most +important organ in the body, it is interesting to understand its essential +features in the vertebrate more fully, and distinguish them from unessential +features. In this connection we must particularly note that the alimentary +canal of every vertebrate shows a very characteristic division into two +sections—a fore and a hind chamber. The fore chamber is the head-gut or +branchial gut (Figs. 98–100 <i>p, k</i>), and is chiefly occupied with +respiration. The hind section is the trunk-gut or hepatic gut, which +accomplishes digestion (<i>ma, d</i>). In all vertebrates there are formed, at +an early stage, to the right and left in the fore-part of the head-gut, certain +special clefts that have an intimate connection with the original respiratory +apparatus of +<span class='pagenum'><a name="Page_110" id="Page_110"></a></span> +the vertebrate—the branchial (gill) clefts (<i>ks</i>). All the lower +vertebrates, the lancelets, lampreys, and fishes, are constantly taking in +water at the mouth, and letting it out again by the lateral clefts of the +gullet. This water serves for breathing. The oxygen contained in it is inspired +by the blood-canals, which spread out on the parts between the gill-clefts, the +gill-arches (<i>kg</i>). These very characteristic branchial clefts and arches +are found in the embryo of man and all the higher vertebrates at an early stage +of development, just as we find them throughout life in the lower vertebrates. +However, these clefts and arches never act as respiratory organs in the +mammals, birds, and reptiles, but gradually develop into quite different parts. +Still, the fact that they are found at first in the same form as in the fishes +is one of the most interesting proofs of the descent of these three higher +classes from the fishes. +</p> + +<p> +Not less interesting and important is an organ that develops from the ventral +wall in all vertebrates—the gill-groove or hypobranchial groove. In the +acrania and the ascidiæ it consists throughout life of a glandular ciliated +groove, which runs down from the mouth in the ventral middle line of the +gill-gut, and takes small particles of food to the stomach (Fig. 101 <i>z</i>). +But in the craniota the thyroid gland (<i>thyreoidea</i>) is developed from it, +the gland that lies in front of the larynx, and which, when pathologically +enlarged, forms goitre (<i>struma</i>). +</p> + +<p> +From the head-gut we get not only the gills, the organs of water-breathing in +the lower vertebrates, but also the lungs, the organs of atmospheric breathing +in the five higher classes. In these cases a vesicular fold appears in the +gullet of the embryo at an early stage, and gradually takes the shape of two +spacious sacs, which are afterwards filled with air. These sacs are the two +air-breathing lungs, which take the place of the water-breathing gills. But the +vesicular invagination, from which the lungs arise, is merely the familiar +air-filled vesicle, which we call the floating-bladder of the fish, and which +alters its specific weight, acting as hydrostatic organ or floating apparatus. +This structure is not found in the lowest vertebrate classes—the acrania +and cyclostoma. We shall see more of it in Volume II. +</p> + +<p> +The second chief section of the vertebrate-gut, the trunk or liver-gut, which +accomplishes digestion, is of very simple construction in the acrania. It +consists of two different chambers. The first chamber, immediately behind the +gill-gut, is the expanded stomach (<i>ma</i>); the second, narrower and longer +chamber, is the straight small intestine (<i>d</i>): it issues behind on the +ventral side by the anus (<i>af</i>). Near the limit of the two chambers in the +visceral cavity we find the liver, in the shape of a simple tube or blind sac +(<i>l</i>); in the amphioxus it is single; in the prospondylus it was probably +double (Figs. 98, 100 <i>l</i>). +</p> + +<p> +Closely related morphologically and physiologically to the alimentary canal is +the vascular system of the vertebrate, the chief sections of which develop from +the fibrous gut-layer. It consists of two different but directly connected +parts, the system of blood-vessels and that of lymph-vessels. In the passages +of the one we find red blood, and in the other colourless lymph. To the +lymphatic system belong, first of all, the lymphatic canals proper or absorbent +veins, which are distributed among all the organs, and absorb the used-up +juices from the tissues, and conduct them into the venous blood; but besides +these there are the chyle-vessels, which absorb the white chyle, the milky +fluid prepared by the alimentary canal from the food, and conduct this also to +the blood. +</p> + +<p> +The blood-vessel system of the vertebrate has a very elaborate construction, +but seems to have had a very simple form in the primitive vertebrate, as we +find it to-day permanently in the annelids (for instance, earth-worms) and the +amphioxus. We accordingly distinguish first of all as essential, original parts +of it two large single blood-canals, which lie in the fibrous wall of the gut, +and run along the alimentary canal in the median plane of the body, one above +and the other underneath the canal. These principal canals give out numerous +branches to all parts of the body, and pass into each other by arches before +and behind; we will call them the primitive artery and the primitive vein. The +first corresponds to the dorsal vessel, the second to the ventral vessel, of +the worms. The primitive or principal artery, usually called the aorta (Fig. 98 +<i>a</i>), lies above the gut in the middle line of its dorsal side, and +conducts oxidised or arterial blood from the gills to the body. The primitive +or principal vein (Fig. 100 <i>v</i>) lies below the +<span class='pagenum'><a name="Page_111" id="Page_111"></a></span> +gut, in the middle line of its ventral side, and is therefore also called the +vena subintestinalis; it conducts carbonised or venous blood back from the body +to the gills. At the branchial section of the gut in front the two canals are +connected by a number of branches, which rise in arches between the +gill-clefts. These “branchial vascular arches” (<i>kg</i>) run +along the gill-arches, and have a direct share in the work of respiration. The +anterior continuation of the principal vein which runs on the ventral wall of +the gill-gut, and gives off these vascular arches upwards, is the branchial +artery (<i>ka</i>). At the border of the two sections of the ventral vessel it +enlarges into a contractile spindle-shaped tube (Figs. 98, 100 <i>h</i>). This +is the first outline of the heart, which afterwards becomes a four-chambered +pump in the higher vertebrates and man. There is no heart in the amphioxus, +probably owing to degeneration. In prospondylus the ventral gill-heart probably +had the simple form in which we still find it in the ascidia and the embryos of +the craniota (Figs. 98, 100 <i>h</i>). +</p> + +<p> +The kidneys, which act as organs of excretion or urinary organs in all +vertebrates, have a very different and elaborate construction in the various +sections of this stem; we will consider them further in Chapter 2.29. Here I +need only mention that in our hypothetical primitive vertebrate they probably +had the same form as in the actual amphioxus—the primitive kidneys +(<i>protonephra</i>). These are originally made up of a double row of little +canals, which directly convey the used-up juices or the urine out of the +body-cavity (Fig. 102 <i>n</i>). The inner aperture of these pronephridial +canals opens with a ciliated funnel into the body-cavity; the external aperture +opens in lateral grooves of the epidermis, a couple of longitudinal grooves in +the lateral surface of the outer skin (Fig. 102 <i>b</i>). The pronephridial +duct is formed by the closing of this groove to the right and left at the +sides. In all the craniota it develops at an early stage in the horny plate; in +the amphioxus it seems to be converted into a wide cavity, the atrium, or +peribranchial space. +</p> + +<p> +Next to the kidneys we have the sexual organs of the vertebrate. In most of the +members of this stem the two are united in a single urogenital system; it is +only in a few groups that the urinary and sexual organs are separated (in the +amphioxus, the cyclostoma, and some sections of the fish-class). In man and all +the higher vertebrates the sexual apparatus is made up of various parts, which +we will consider in Chapter XXIX. But in the two lowest classes of our stem, +the acrania and cyclostoma, they consist merely of simple sexual glands or +gonads, the ovaries of the female sex and the testicles (<i>spermaria</i>) of +the male; the former provide the ova, the latter the sperm. In the craniota we +always find only one pair of gonads; in the amphioxus several pairs, arranged +in succession. They must have had the same form in our hypothetical +prospondylus (Figs. 98, 100 <i>s</i>). These segmental pairs of gonads are the +original ventral halves of the cœlom-pouches. +</p> + +<p> +The organs which we have now enumerated in this general survey, and of which we +have noted the characteristic disposition, are those parts of the organism that +are found in all vertebrates without exception in the same relation to each +other, however much they may be modified. We have chiefly had in view the +transverse section of the body (Figs. 101, 102), because in this we see most +clearly the distinctive arrangement of them. But to complete our picture we +must also consider the segmentation or metamera-formation of them, which has +yet been hardly noticed, and which is seen best in the longitudinal section. In +man and all the more advanced vertebrates the body is made up of a series or +chain of similar members, which succeed each other in the long axis of the +body—the segments or metamera of the organism. In man these homogeneous +parts number thirty-three in the trunk, but they run to several hundred in many +of the vertebrates (such as serpents or eels). As this internal articulation or +metamerism is mainly found in the vertebral column and the surrounding muscles, +the sections or metamera were formerly called pro-vertebræ. As a fact, the +articulation is by no means chiefly determined and caused by the skeleton, but +by the muscular system and the segmental arrangement of the kidneys and gonads. +However, the composition from these pro-vertebræ or internal metamera is +usually, and rightly, put forward as a prominent character of the vertebrate, +and the manifold division or differentiation of them is of great importance in +the various groups of the vertebrates. But as far as our present +<span class='pagenum'><a name="Page_112" id="Page_112"></a></span> +task—the derivation of the simple body of the primitive vertebrate from +the chordula—is concerned, the articulate parts or metamera are of +secondary interest, and we need not go into them just now. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus103"></a> +<img src="images/fig103.gif" width="355" height="337" alt="Fig.103, A, B. C, D. Instances of +redundant mammary glands and nipples (hypermastism)." /> +<p class="caption">Fig. +103 <i>A, B, C, D.</i>—<b>Instances of redundant mammary glands and +nipples</b> (<i>hypermastism</i>). <i>A</i> a pair of small redundant breasts +(with two nipples on the left) above the large normal ones; from a 45-year-old +Berlin woman, who had had children 17 times (twins twice). (From +<i>Hansemann.</i>) <i>B</i> the highest number: ten nipples (all giving milk), +three pairs above, one pair below, the large normal breasts; from a 22-year-old +servant at Warschau. (From <i>Neugebaur.</i>) <i>C</i> three pairs of nipples: +two pairs on the normal glands and one pair above; from a 19-year-old Japanese +girl. <i>D</i> four pairs of nipples: one pair above the normal and two pairs +of small accessory nipples underneath; from a 22-year-old Bavarian soldier. +(From <i>Wiedersheim.</i>)</p> +</div> + +<p> +The characteristic composition of the vertebrate body develops from the +embryonic structure in the same way in man as in all the other vertebrates. As +all competent experts now admit the monophyletic origin of the vertebrates on +the strength of this significant agreement, and this “common descent of +all the vertebrates from one original stem-form” is admitted as an +historical fact, we have found the answer to “the question of +questions.” We may, moreover, point out that this answer is just as +certain and precise in the case of the origin of man from the mammals. This +advanced vertebrate class is also monophyletic, or has evolved from one common +stem-group of lower vertebrates (reptiles, and, earlier still, amphibia). This +follows from the fact that the mammals are clearly distinguished from the other +classes of the stem, not merely in one striking particular, but in a whole +group of distinctive characters. +</p> + +<p> +It is only in the mammals that we find the skin covered with hair, the +breast-cavity separated from the abdominal cavity by a complete diaphragm, and +the larynx provided with an epiglottis. The +<span class='pagenum'><a name="Page_113" id="Page_113"></a></span> +mammals alone have three small auscultory bones in the tympanic cavity—a +feature that is connected with the characteristic modification of their +maxillary joint. Their red blood-cells have no nucleus, whereas this is +retained in all other vertebrates. Finally, it is only in the mammals that we +find the remarkable function of the breast structure which has given its name +to the whole class—the feeding of the young by the mother’s milk. +The mammary glands which serve this purpose are interesting in so many ways +that we may devote a few lines to them here. +</p> + +<p> +As is well known, the lower mammals, especially those which beget a number of +young at a time, have several mammary glands at the breast. Hedgehogs and sows +have five pairs, mice four or five pairs, dogs and squirrels four pairs, cats +and bears three pairs, most of the ruminants and many of the rodents two pairs, +each provided with a teat or nipple (<i>mastos</i>). In the various genera of +the half-apes (lemurs) the number varies a good deal. On the other hand, the +bats and apes, which only beget one young at a time as a rule, have only one +pair of mammary glands, and these are found at the breast, as in man. +</p> + +<p> +These variations in the number or structure of the mammary apparatus +(<i>mammarium</i>) have become doubly interesting in the light of recent +research in comparative anatomy. It has been shown that in man and the apes we +often find redundant mammary glands (<i>hyper-mastism</i>) and corresponding +teats (<i>hyper-thelism</i>) in both sexes. Fig. 103 shows four cases of this +kind—<i>A, B,</i> and <i>C</i> of three women, and <i>D</i> of a man. +They prove that all the above-mentioned numbers may be found occasionally in +man. Fig. 103 <i>A</i> shows the breast of a Berlin woman who had had children +seventeen times, and who has a pair of small accessory breasts (with two +nipples on the left one) above the two normal breasts; this is a common +occurrence, and the small soft pad above the breast is not infrequently +represented in ancient statues of Venus. In Fig. 103 <i>C</i> we have the same +phenomenon in a Japanese girl of nineteen, who has two nipples on each breast +besides (three pairs altogether). Fig. 103 <i>D</i> is a man of twenty-two with +four pairs of nipples (as in the dog), a small pair above and two small pairs +beneath the large normal teats. The maximum number of five pairs (as in the sow +and hedgehog) was found in a Polish servant of twenty-two who had had several +children; milk was given by each nipple; there were three pairs of redundant +nipples above and one pair underneath the normal and very large breasts (Fig. +103 <i>B</i>). +</p> + +<p> +A number of recent investigations (especially among recruits) have shown that +these things are not uncommon in the male as well as the female sex. They can +only be explained by evolution, which attributes them to atavism and latent +heredity. The earlier ancestors of all the primates (including man) were lower +placentals, which had, like the hedgehog (one of the oldest forms of the living +placentals), several mammary glands (five or more pairs) in the abdominal skin. +In the apes and man only a couple of them are normally developed, but from time +to time we get a development of the atrophied structures. Special notice should +be taken of the arrangement of these accessory mammæ; they form, as is clearly +seen in Fig. 103 <i>B</i> and <i>D,</i> two long rows, which diverge forward +(towards the arm-pit), and converge behind in the middle line (towards the +loins). The milk-glands of the polymastic lower placentals are arranged in +similar lines. +</p> + +<p> +The phylogenetic explanation of polymastism, as given in comparative anatomy, +has lately found considerable support in ontogeny. Hans Strahl, E. Schmitt, and +others, have found that there are always in the human embryo at the sixth week +(when it is three-fifths of an inch long) the microscopic traces of five pairs +of mammary glands, and that they are arranged at regular distances in two +lateral and divergent lines, which correspond to the mammary lines. Only one +pair of them—the central pair—are normally developed, the others +atrophying. Hence there is for a time in the human embryo a normal +hyperthelism, and this can only be explained by the descent of man from lower +primates (lemurs) with several pairs. +</p> + +<p> +But the milk-gland of the mammal has a great morphological interest from +another point of view. This organ for feeding the young in man and the higher +mammals is, as is known, found in both sexes. However, it is usually active +only in the female sex, and yields the valuable “mother’s +milk”; in the male sex it is +<span class='pagenum'><a name="Page_114" id="Page_114"></a></span> +small and inactive, a real rudimentary organ of no physiological interest. +Nevertheless, in certain cases we find the breast as fully developed in man as +in woman, and it may give milk for feeding the young. +</p> + +<p> +We have a striking instance of this gynecomastism (large milk-giving breasts in +a male) in Fig. 104. I owe the photograph (taken from life) to the kindness of +Dr. Ornstein, of Athens, a German physician, who has rendered service by a +number of anthropological observations, (for instance, in several cases of +tailed men). The gynecomast in question is a Greek recruit in his twentieth +year, who has both normally developed male organs and very pronounced female +breasts. It is noteworthy that the other features of his structure are in +accord with the softer forms of the female sex. It reminds us of the marble +statues of hermaphrodites which the ancient Greek and Roman sculptors often +produced. But the man would only be a real hermaphrodite if he had ovaries +internally besides the (externally visible) testicles. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus104"></a> +<img src="images/fig104.gif" width="333" height="244" alt="Fig.104. A Greek gynecomast." /> +<p class="caption">Fig. 104—<b>A Greek gynecomast.</b></p> +</div> + +<p> +I observed a very similar case during my stay in Ceylon (at Belligemma) in +1881. A young Cinghalese in his twenty-fifth year was brought to me as a +curious hermaphrodite, half-man and half-woman. His large breasts gave plenty +of milk; he was employed as “male nurse” to suckle a new-born +infant whose mother had died at birth. The outline of his body was softer and +more feminine than in the Greek shown in Fig. 104. As the Cinghalese are small +of stature and of graceful build, and as the men often resemble the women in +clothing (upper part of the body naked, female dress on the lower part) and the +dressing of the hair (with a comb), I first took the beardless youth to be a +woman. The illusion was greater, as in this remarkable case gynecomastism was +associated with <i>cryptorchism</i>—that is to say, the testicles had +kept to their original place in the visceral cavity, and had not travelled in +the normal way down into the scrotum. (Cf. Chapter XXIX.) Hence the latter was +very small, soft, and empty. Moreover, one could feel nothing of the testicles +in the inguinal canal. On the other hand, the male organ was very small, but +normally developed. It was +<span class='pagenum'><a name="Page_115" id="Page_115"></a></span> +clear that this apparent hermaphrodite also was a real male. +</p> + +<p> +Another case of practical gynecomastism has been described by Alexander von +Humboldt. In a South American forest he found a solitary settler whose wife had +died in child-birth. The man had laid the new-born child on his own breast in +despair; and the continuous stimulus of the child’s sucking movements had +revived the activity of the mammary glands. It is possible that nervous +suggestion had some share in it. Similar cases have been often observed in +recent years, even among other male mammals (such as sheep and goats). +</p> + +<p> +The great scientific interest of these facts is in their bearing on the +question of heredity. The stem-history of the mammarium rests partly on its +embryology (Chapter XXIV.) and partly on the facts of comparative anatomy and +physiology. As in the lower and higher mammals (the monotremes, and most of the +marsupials) the whole lactiferous apparatus is only found in the female; and as +there are traces of it in the male only in a few younger marsupials, there can +be no doubt that these important organs were originally found only in the +female mammal, and that they were acquired by these through a special +adaptation to habits of life. +</p> + +<p> +Later, these female organs were communicated to both sexes by heredity; and +they have been maintained in all persons of either sex, although they are not +physiologically active in the males. This normal permanence of the female +lactiferous organs in <i>both</i> sexes of the higher mammals and man is +independent of any selection, and is a fine instance of the much-disputed +“inheritance of acquired characters.” +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap12"></a>Chapter XII.<br/> +EMBRYONIC SHIELD AND GERMINATIVE AREA</h2> + +<p> +The three higher classes of vertebrates which we call the amniotes—the +mammals, birds, and reptiles—are notably distinguished by a number of +peculiarities of their development from the five lower classes of the +stem—the animals without an amnion (the <i>anamnia</i>). All the amniotes +have a distinctive embryonic membrane known as the amnion (or +“water-membrane”), and a special embryonic appendage—the +allantois. They have, further, a large yelk-sac, which is filled with food-yelk +in the reptiles and birds, and with a corresponding clear fluid in the mammals. +In consequence of these later-acquired structures, the original features of the +development of the amniotes are so much altered that it is very difficult to +reduce them to the palingenetic embryonic processes of the lower amnion-less +vertebrates. The gastræa theory shows us how to do this, by representing the +embryology of the lowest vertebrate, the skull-less amphioxus, as the original +form, and deducing from it, through a series of gradual modifications, the +gastrulation and cœlomation of the craniota. +</p> + +<p> +It was somewhat fatal to the true conception of the chief embryonic processes +of the vertebrate that all the older embryologists, from Malpighi (1687) and +Wolff (1750) to Baer (1828) and Remak (1850), always started from the +investigation of the hen’s egg, and transferred to man and the other +vertebrates the impressions they gathered from this. This classical object of +embryological research is, as we have seen, a source of dangerous errors. The +large round food-yelk of the bird’s egg causes, in the first place, a +flat discoid expansion of the small gastrula, and then so distinctive a +development of this thin round embryonic disk that the controversy as to its +significance occupies a large part of embryological literature. +</p> + +<p> +One of the most unfortunate errors that this led to was the idea of an original +<span class='pagenum'><a name="Page_116" id="Page_116"></a></span> +antithesis of germ and yelk. The latter was regarded as a foreign body, +extrinsic to the real germ, whereas it is properly a part of it, an embryonic +organ of nutrition. Many authors said there was no trace of the embryo until a +later stage, and outside the yelk; sometimes the two-layered embryonic disk +itself, at other times only the central portion of it (as distinguished from +the germinative area, which we will describe presently), was taken to be the +first outline of the embryo. In the light of the gastræa theory it is hardly +necessary to dwell on the defects of this earlier view and the erroneous +conclusions drawn from it. In reality, the first segmentation-cell, and even +the stem-cell itself and all that issues therefrom, belong to the embryo. As +the large original yelk-mass in the undivided egg of the bird only represents +an inclosure in the greatly enlarged ovum, so the later contents of its +embryonic yelk-sac (whether yet segmented or not) are only a part of the +entoderm which forms the primitive gut. This is clearly shown by the ova of the +amphibia and cyclostoma, which explain the transition from the yelk-less ova of +the amphioxus to the large yelk-filled ova of the reptiles and birds. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus105"></a> +<img src="images/fig105.gif" width="397" height="272" alt="Fig.105. Severance of the discoid +mammal embryo from the yelk-sac, in transverse section (diagrammatic)." /> +<p class="caption">Fig. +105—<b>Severance of the discoid mammal embryo from the yelk-sac, in +transverse section</b> (diagrammatic). <i>A</i> The germinal disk (<i>h, +hf</i>) lies flat on one side of the branchial-gut vesicle (<i>kb</i>). +<i>B</i> In the middle of the germinal disk we find the medullary groove +(<i>mr</i>), and underneath it the chorda (<i>ch</i>). <i>C</i> The +gut-fibre-layer (<i>df</i>) has been enclosed by the gut-gland-layer +(<i>dd</i>). <i>D</i> The skin-fibre-layer (<i>hf</i>) and gut-fibre-layer +(<i>df</i>) divide at the periphery; the gut (<i>d</i>) begins to separate from +the yelk-sac or umbilical vesicle (<i>nb</i>). <i> E</i> The medullary tube +(<i>mr</i>) is closed; the body-cavity (<i>c</i>) begins to form. <i>F</i> The +provertebræ (<i>w</i>) begin to grow round the medullary tube (<i>mr</i>) and +the chorda (<i>ch</i>): the gut (<i>d</i>) is cut off from the umbilical +vesicle (<i>nb</i>). <i>H</i> The vertebræ (<i>w</i>) have grown round the +medullary tube (<i>mr</i>) and chorda; the body-cavity is closed, and the +umbilical vesicle has disappeared. The amnion and serous membrane are omitted. +The letters have the same meaning throughout: <i>h</i> horn-plate, <i> mr</i> +medullary tube, <i>hf</i> skin-fibre-layer, <i>w</i> provertebræ, <i>ch</i> +chorda, <i>c</i> body-cavity or cœloma, <i>df</i> gut-fibre-layer, <i>dd</i> +gut-gland-layer, <i>d</i> gut-cavity, <i>nb</i> umbilical vesicle.</p> +</div> + +<p> +It is precisely in the study of these difficult features that we see the +incalculable value of phylogenetic considerations in explaining complex +ontogenetic facts, and the need of separating cenogenetic phenomena from +palingenetic. This is particularly clear as regards the comparative embryology +of the vertebrates, because here the phylogenetic unity of the stem has been +already established by the well-known facts of paleontology and comparative +anatomy. If this unity of the stem, on the basis of the amphioxus, were always +borne in mind, we should not have these errors constantly recurring. +</p> + +<p> +In many cases the cenogenetic relation of the embryo to the food-yelk has until +now given rise to a quite wrong idea of +<span class='pagenum'><a name="Page_117" id="Page_117"></a></span> +the first and most important embryonic processes in the higher vertebrates, and +has occasioned a number of false theories in connection with them. Until thirty +years ago the embryology of the higher vertebrates always started from the +position that the first structure of the embryo is a flat, leaf-shaped disk; it +was for this reason that the cell-layers that compose this germinal disk (also +called germinative area) are called “germinal layers.” This flat +germinal disk, which is round at first and then oval, and which is often +described as the tread or cicatricula in the laid hen’s egg, is found at +a certain part of the surface of the large globular food-yelk. I am convinced +that it is nothing else than the discoid, flattened gastrula of the birds. At +the beginning of germination the flat embryonic disk curves outwards, and +separates on the inner side from the underlying large yelk-ball. In this way +the flat layers are converted into tubes, their edges folding and joining +together (Fig. 105). As the embryo grows at the expense of the food-yelk, the +latter becomes smaller and smaller; it is completely surrounded by the germinal +layers. Later still, the remainder of the food-yelk only forms a small round +sac, the yelk-sac or umbilical vesicle (Fig. 105 <i>nb</i>). This is enclosed +by the visceral layer, is connected by a thin stalk, the yelk-duct, with the +central part of the gut-tube, and is finally, in most of the vertebrates, +entirely absorbed by this (<i>H</i>). The point at which this takes place, and +where the gut finally closes, is the visceral navel. In the mammals, in which +the remainder of the yelk-sac remains without and atrophies, the yelk-duct at +length penetrates the outer ventral wall. At birth the umbilical cord proceeds +from here, and the point of closure remains throughout life in the skin as the +navel. +</p> + +<p> +As the older embryology of the higher vertebrates was mainly based on the +chick, and regarded the antithesis of embryo (or formative-yelk) and food-yelk +(or yelk-sac) as original, it had also to look upon the flat leaf-shaped +structure of the germinal disk as the primitive embryonic form, and emphasise +the fact that hollow grooves were formed of these flat layers by folding, and +closed tubes by the joining together of their edges. +</p> + +<p> +This idea, which dominated the whole treatment of the embryology of the higher +vertebrates until thirty years ago, was totally false. The gastræa theory, +which has its chief application here, teaches us that it is the very reverse of +the truth. The cup-shaped gastrula, in the body-wall of which the two primary +germinal layers appear from the first as closed tubes, is the original +embryonic form of all the vertebrates, and all the multicellular invertebrates; +and the flat germinal disk with its superficially expanded germinal layers is a +later, secondary form, due to the cenogenetic formation of the large food-yelk +and the gradual spread of the germ-layers over its surface. Hence the actual +folding of the germinal layers and their conversion into tubes is not an +original and primary, but a much later and tertiary, evolutionary process. In +the phylogeny of the vertebrate embryonic process we may distinguish the +following three stages:— +</p> + +<table class="text" border="1" cellspacing="0" cellpadding="4" summary= +"Primary, secondary and tertiary stages in the phylogeny of the vertebrate +embryonic process."> +<tr> +<td align="center">A. First stage:<br/> <b>Primary</b><br/> +(palingenic)<br/> embryonic process.</td> <td align="center">B. +Second stage:<br/> <b>Secondary</b><br/> (cenogenetic)<br/> embryonic +process.</td> <td align="center">C. Third stage:<br/> +<b>Tertiary</b><br/> (cenogenetic)<br/> embryonic process.</td> </tr> + +<tr> +<td align="justify" valign="top">The germinal layers form from the first closed +tubes, the one-layered blastula being converted into the two-layered gastrula +by invagination.<br/> No food-yelk.<br/> + (<i>Amphioxus.</i>)</td> <td align="justify" +valign="top">The germinal layers spread out leaf-wise, food-yelk gathering in +the ventral entoderm, and a large yelk-sac being formed from the middle of the +gut-tube.<br/> (<i>Amphibia.</i>)</td> <td +align="justify" valign="top">The germinal layers form a flat germinal disk, the +borders of which join together and form closed tubes, separating from the +central yelk-sac.<br/> (<i>Amniotes.</i>)</td> </tr> +</table> + +<p> +As this theory, a logical conclusion from the gastræa theory, has been fully +substantiated by the comparative study of gastrulation in the last few decades, +we must exactly reverse the hitherto prevalent mode of treatment. The yelk-sac +is not to be treated, as was done formerly, as if it were originally antithetic +to the embryo, but as an essential part of it, a part of its visceral tube. The +primitive gut of the gastrula has, on this view, been divided into two parts in +the higher animals as a result of the cenogenetic formation of the +food-yelk—the permanent gut (<i>metagaster</i>), or permanent alimentary +canal, and the yelk-sac (<i>lecithoma</i>), or umbilical vesicle. This is very +clearly shown by the comparative ontogeny of the fishes and amphibia. In these +cases the whole yelk undergoes cleavage at first, and forms a yelk-gland, +composed of yelk-cells, in the ventral wall +<span class='pagenum'><a name="Page_118" id="Page_118"></a></span> +of the primitive gut. But it afterwards becomes so large that a part of the +yelk does not divide, and is used up in the yelk-sac that is cut off outside. +</p> + +<p> +When we make a comparative study of the embryology of the amphioxus, the frog, +the chick, and the rabbit, there cannot, in my opinion, be any further doubt as +to the truth of this position, which I have held for thirty years. Hence in the +light of the gastræa theory we must regard the features of the amphioxus as the +only and real primitive structure among all the vertebrates, departing very +little from the palingenetic embryonic form. In the cyclostoma and the frog +these features are, on the whole, not much altered cenogenetically, but they +are very much so in the chick, and most of all in the rabbit. In the +bell-gastrula of the amphioxus and in the hooded gastrula of the lamprey and +the frog the germinal layers are found to be closed tubes or vesicles from the +first. On the other hand, the chick-embryo (in the new laid, but not yet +hatched, egg) is a flat circular disk, and it was not easy to recognise this as +a real gastrula. Rauber and Goette have, however, achieved this. As the discoid +gastrula grows round the large globular yelk, and the permanent gut then +separates from the outlying yelk-sac, we find all the processes which we have +shown (diagrammatically) in Figure 1.108—processes that were hitherto +regarded as principal acts, whereas they are merely secondary. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus106"></a> +<img src="images/fig106.gif" width="317" height="181" alt="Figs. 106 and 107. The visceral embryonnic +vesicle (blastocystis or gastrocystis) of a rabbit." /> +<p class="caption">Fig. 106—<b>The +visceral embryonic vesicle</b> (<i>blastocystis</i> or <i>gastrocystis</i>) of +a rabbit (the “blastula” or <i>vesicula blastodermica</i> of other +writers), <i>a</i> outer envelope (ovolemma), <i>b</i> skin-layer or ectoderm, +forming the entire wall of the yelk-vesicle, <i>c</i> groups of dark cells, +representing the visceral layer or entoderm.<br/> Fig. 107—<b>The +same</b> in section. Letters as above. <i> d</i> cavity of the vesicle. (From +<i>Bischoff.</i>)</p> +</div> + +<p> +The oldest, oviparous mammals, the monotremes, behave in the same way as the +reptiles and birds. But the corresponding embryonic processes in the viviparous +mammals, the marsupials and placentals, are very elaborate and distinctive. +They were formerly quite misinterpreted; it was not until the publication of +the studies of Edward van Beneden (1875) and the later research of Selenka, +Kuppfer, Rabl, and others, that light was thrown on them, and we were in a +position to bring them into line with the principles of the gastræa theory and +trace them to the embryonic forms of the lower vertebrates. Although there is +no independent food-yelk, apart from the formative yelk, in the mammal ovum, +and although its segmentation is total on that account, nevertheless a large +yelk-sac is formed in their embryos, and the “embryo proper” +spreads leaf-wise over its surface, as in the reptiles and birds, which have a +large food-yelk and partial segmentation. In the mammals, as well as in the +latter, the flat, leaf-shaped germinal disk separates from the yelk-sac, and +its edges join together and form tubes. +</p> + +<p> +How can we explain this curious anomaly? Only as a result of very +characteristic and peculiar cenogenetic modifications of the embryonic process, +the real causes of which must be sought in the change in the rearing of the +young on the part of the viviparous mammals. These are clearly connected with +the fact that the ancestors of the viviparous mammals were oviparous amniotes +like the present monotremes, and only gradually became viviparous. This can no +longer be questioned now that it has been shown (1884) that the monotremes, the +lowest and oldest of the mammals, still lay eggs, and that these develop like +the ova of the reptiles and birds. Their nearest descendants, the marsupials, +formed the habit of retaining the eggs, and developing them in the +<span class='pagenum'><a name="Page_119" id="Page_119"></a></span> +oviduct; the latter was thus converted into a womb (uterus). A nutritive fluid +that was secreted from its wall, and passed through the wall of the blastula, +now served to feed the embryo, and took the place of the food-yelk. In this way +the original food-yelk of the monotremes gradually atrophied, and at last +disappeared so completely that the partial ovum-segmentation of their +descendants, the rest of the mammals, once more became total. From the +<i>discogastrula</i> of the former was evolved the distinctive +<i>epigastrula</i> of the latter. +</p> + +<p> +It is only by this phylogenetic explanation that we can understand the +formation and development of the peculiar, and hitherto totally misunderstood, +blastula of the mammal. The vesicular condition of the mammal embryo was +discovered 200 years ago (1677) by Regner de Graaf. He found in the uterus of a +rabbit four days after impregnation small, round, loose, transparent vesicles, +with a double envelope. However, Graaf’s discovery passed without +recognition. It was not until 1827 that these vesicles were rediscovered by +Baer, and then more closely studied in 1842 by Bischoff in the rabbit (Figs. +106, 107). They are found in the womb of the rabbit, the dog, and other small +mammals, a few days after copulation. The mature ova of the mammal, when they +have left the ovary, are fertilised either here or in the oviduct immediately +afterwards by the invading sperm-cells.<a href="#linknote-25" name="linknoteref-25" id="linknoteref-25"><sup>[25]</sup></a> (As to the womb and oviduct +see Chapter XXIX) The cleavage and formation of the gastrula take place in the +oviduct. Either here in the oviduct or after the mammal gastrula has passed +into the uterus it is converted into the globular vesicle which is shown +externally in Fig. 106, and in section in Fig. 107. The thick, outer, +structureless envelope that encloses it is the original <i> ovolemma</i> or +<i>zona pellucida,</i> modified, and clothed with a layer of albumin that has +been deposited on the outside. From this stage the envelope is called the +external membrane, the <i>primary chorion</i> or prochorion (<i>a</i>). The +real wall of the vesicle enclosed by it consists of a simple layer of +ectodermic cells (<i>b</i>), which are flattened by mutual pressure, and +generally hexagonal; a light nucleus shines through their fine-grained +protoplasm (Fig. 108). At one part (<i>c</i>) inside this hollow ball we find a +circular disc, formed of darker, softer, and rounder cells, the dark-grained +entodermic cells (Fig. 109). +</p> + +<p class="footnote"> +<a name="linknote-25" id="linknote-25"></a> <a href="#linknoteref-25">[25]</a> +In man and the other mammals the fertilisation of the ova probably takes place, +as a rule, in the oviduct; here the ova, which issue from the female ovary in +the shape of the Graafian follicle, and enter the inner aperture of the +oviduct, encounter the mobile sperm-cells of the male seed, which pass into the +uterus at copulation, and from this into the external aperture of the oviduct. +Impregnation rarely takes place in the ovary or in the womb. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus108"></a> +<a name="illus109"></a> +<img src="images/fig108.gif" width="267" height="165" alt="Fig.108. Four entodermic cells from the vesicle +of the rabbit. Fig. 109. Two entodermic cells from the embryonic vesicle of the +rabbit." /> +<p class="caption">Fig. 108—<b>Four entodermic +cells</b> from the embryonic vesicle of the rabbit.<br/> Fig. 109—<b>Two +entodermic cells</b> from the embryonic vesicle of the rabbit.</p> +</div> + +<p> +The characteristic embryonic form that the developing mammal now exhibits has +up to the present usually been called the “blastula” (Bischoff), +“sac-shaped embryo” (Baer), “vesicular embryo” +(<i>vesicula blastodermica,</i> or, briefly, <i>blastosphæra</i>). The wall of +the hollow vesicle, which consists of a single layer of cells, was called the +“blastoderm,” and was supposed to be equivalent to the cell-layer +of the same name that forms the wall of the real blastula of the amphioxus and +many of the invertebrates (such as <i>Monoxenia,</i> Fig. 29 <i>F, G</i>). +Formerly this real blastula was generally believed to be equivalent to the +embryonic vesicle of the mammal. However, this is by no means the case. What is +called the “blastula” of the mammal and the real blastula of the +amphioxus and many of the invertebrates are totally different embryonic +structures. The latter (blastula) is palingenetic, and precedes the formation +of the gastrula. The former (blastodermic vesicle) is cenogenetic, and follows +gastrulation. The globular wall of the blastula is a real blastoderm, and +consists of homogeneous (blastodermic) cells; it is not yet differentiated into +the two primary germinal layers. But the globular wall of the mammal vesicle is +the differentiated ectoderm, and at one point in it we find a circular disk of +quite different cells—the entoderm. The round +<span class='pagenum'><a name="Page_120" id="Page_120"></a></span> +cavity, filled with fluid, inside the real blastula is the segmentation-cavity. +But the similar cavity within the mammal vesicle is the yelk-sac cavity, which +is connected with the incipient gut-cavity. This primitive gut-cavity passes +directly into the segmentation-cavity in the mammals, in consequence of the +peculiar cenogenetic changes in their gastrulation, which we have considered +previously (Chapter IX). For these reasons it is very necessary to recognise +the secondary embryonic vesicle in the mammal (<i>gastrocystis</i> or +<i>blastocystis</i>) as a characteristic structure peculiar to this class, and +distinguish it carefully from the primary blastula of the amphioxus and the +invertebrates. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus110"></a> +<img src="images/fig110.gif" width="437" height="270" alt="Fig.110. Ovum of a rabbit from the +uterus, one-sixth of an inch in diameter. Fig. 111. The same ovum, seen in +profile. Fig. 112. Ovum of a rabbit from the uterus, one-fourth of an inch in +diameter. Fig. 113. The same ovum, seen in profile. Fig. 114. Ovum of a rabbit +from the uterus, one-third of an inch in diameter." /> +<p class="caption">Fig. 110—<b>Ovum of +a rabbit</b> from the uterus, one sixth of an inch in diameter. The embryonic +vesicle (<i>b</i>) has withdrawn a little from the smooth ovolemma (<i>a</i>). +In the middle of the ovolemma we see the round germinal disk (blastodiscus, +<i>c</i>), at the edge of which (at <i>d</i>) the inner layer of the embryonic +vesicle is already beginning to expand. (Figs. 110–114 from <i> +Bischoff.</i><br/> Fig. 111—<b>The same ovum,</b> seen in profile. +Letters as in Fig. 110.<br/> Fig. 112—<b>Ovum of a rabbit from the +uterus,</b> one-fourth of an inch in diameter. The blastoderm is already for +the most part two-layered (<i>b</i>). The ovolemma, or outer envelope, is +tufted (<i>a</i>).<br/> Fig. 113—<b>The same ovum,</b> seen in profile. +Letters as in Fig. 112.<br/> Fig. 114—<b>Ovum of a rabbit from the +uterus,</b> one-third of an inch in diameter. The embryonic vesicle is now +nearly everywhere two-layered (<i>k</i>) only remaining one-layered below (at +<i>d</i>).</p> +</div> + +<p> +<span class='pagenum'><a name="Page_121" id="Page_121"></a></span> +The small, circular, whitish, and opaque spot which the gastric disk (Fig. 106) +forms at a certain part of the surface of the clear and transparent embryonic +vesicle has long been known to science, and compared to the germinal disk of +the birds and reptiles. Sometimes it has been called the germinal disk, +sometimes the germinal spot, and usually the germinative area. From the area +the further development of the embryo proceeds. However, the larger part of the +embryonic vesicle of the mammal is not directly used for building up the later +body, but for the construction of the temporary umbilical vesicle. The embryo +separates from this in proportion as it grows at its expense; the two are only +connected by the yelk-duct (the stalk of the yelk-sac), and this maintains the +direct communication between the cavity of the umbilical vesicle and the +forming visceral cavity (Fig. 105). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus115"></a> +<img src="images/fig115.gif" width="329" height="234" alt="Fig.115. Round germinative area of +the rabbit. Fig. 116. Oval area, with the opaque whitish border of the dark +area without." /> +<p class="caption">Fig. 115—<b>Round germinative area of the rabbit,</b> divided +into the central light area (<i>area pellucida</i>) and the peripheral dark +area (<i>area opaca</i>). The light area seems darker on account of the dark +ground appearing through it.<br/> Fig. 116—<b>Oval area,</b> with the +opaque whitish border of the dark area without.</p> +</div> + +<p> +The germinative area or gastric disk of the animal consists at first (like the +germinal disk of birds and reptiles) merely of the two primary germinal layers, +the ectoderm and entoderm. But soon there appears in the middle of the circular +disk between the two a third stratum of cells, the rudiment of the middle layer +or fibrous layer (<i>mesoderm</i>). This middle germinal layer consists from +the first, as we have seen in Chapter X, of two separate epithelial plates, the +two layers of the cœlom-pouches (parietal and visceral). However, in all the +amniotes (on account of the large formation of yelk) these thin middle plates +are so firmly pressed together that they seem to represent a single layer. It +is thus peculiar to the amniotes that the middle of the germinative area is +composed of four germinal layers, the two limiting (or primary) layers and the +middle layers between them (Figs. 96, 97). These four secondary germinal layers +can be clearly distinguished as soon as what is called the sickle-groove (or +“embryonic sickle”) is seen at the hind border of the germinative +area. At the borders, however, the germinative area of the mammal only consists +of two layers. The rest of the wall of the embryonic vesicle consists at first +(but only for a short time in most of the mammals) of a single layer, the outer +germinal layer. +</p> + +<p> +From this stage, however, the whole wall of the embryonic vesicle becomes +two-layered. The middle of the germinative area is much thickened by the growth +of the cells of the middle layers, and the inner layer expands at the same +time, and increases at the border of the disk all round. Lying close on the +outer layer throughout, it grows over its inner surface at all points, covers +first the upper and then the lower hemisphere, and at last closes in the middle +of the inner layer (Figs. 110–114). The wall of the embryonic vesicle now +consists throughout of two layers of cells, the ectoderm without and the +entoderm within. It is only in the centre of the circular area, which becomes +thicker and thicker through the growth of the middle layers, that it is made up +of all four layers. At the same time, small structureless tufts or warts are +deposited on the surface of the outer +<span class='pagenum'><a name="Page_122" id="Page_122"></a></span> +ovolemma or prochorion, which has been raised above the embryonic vesicle +(Figs. 112–114 <i>a</i>). +</p> + +<p> +We may now disregard both the outer ovolemma and the greater part of the +vesicle, and concentrate our attention on the germinative area and the +four-layered embryonic disk. It is here alone that we find the important +changes which lead to the differentiation of the first organs. It is immaterial +whether we examine the germinative area of the mammal (the rabbit, for +instance) or the germinal disk of a bird or a reptile (such as a lizard or +tortoise). The embryonic processes we are now going to consider are essentially +the same in all members of the three higher classes of vertebrates which we +call the amniotes. Man is found to agree in this respect with the rabbit, dog, +ox, etc.; and in all these animals the germinative area undergoes essentially +the same changes as in the birds and reptiles. They are most frequently and +accurately studied in the chick, because we can have incubated hens’ eggs +in any quantity at any stage of development. Moreover, the round germinal disk +of the chick passes immediately after the beginning of incubation (within a few +hours) from the two-layered to the four-layered stage, the two-layered mesoderm +developing from the median primitive groove between the ectoderm and entoderm +(Figs. 82–95). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus117"></a> +<img src="images/fig117.gif" width="272" height="212" alt="Fig.117. Oval germinal disk of the +rabbit, magnified. Fig. 118. Pear-shaped germinal shield of the rabbit (eight +days old), magnified." /> +<p class="caption">Fig. 117—<b>Oval germinal disk of the rabbit,</b> +magnified. As the delicate, half-transparent disk lies on a black ground, the +pellucid area looks like a dark ring, and the opaque area (lying outside it) +like a white ring. The oval shield in the centre also looks whitish, and in its +axis we see the dark medullary groove. (From <i> Bischoff.</i>)<br/> Fig. +118—<b>Pear-shaped germinal shield of the rabbit</b> (eight days old), +magnified. <i>rf</i> medullary groove. <i>pr</i> primitive groove (primitive +mouth). (From <i> Kölliker.</i></p> +</div> + +<p> +The first change in the round germinal disk of the chick is that the cells at +its edges multiply more briskly, and form darker nuclei in their protoplasm. +This gives rise to a dark ring, more or less sharply set off from the lighter +centre of the germinal disk (Fig. 115). From this point the latter takes the +name of the “light area” (<i>area pellucida</i>), and the darker +ring is called the “dark area” (<i>area opaca</i>). (In a strong +light, as in Figs. 115–117, the light area seems dark, because the dark +ground is seen through it; and the dark area seems whiter). The circular shape +of the area now changes into elliptic, and then immediately into oval (Figs. +116, 117). One end seems to be broader and blunter, the other narrower and more +pointed; the former corresponds to the anterior and the latter to the posterior +section of the subsequent body. At the same time, we can already trace the +characteristic bilateral form of the body, the antithesis of right and left, +before and behind. This will be made clearer by the “primitive +streak,” which appears at the posterior end. +</p> + +<p> +At an early stage an opaque spot is seen in the middle of the clear germinative +<span class='pagenum'><a name="Page_123" id="Page_123"></a></span> +area, and this also passes from a circular to an oval shape. At first this +shield-shaped marking is very delicate and barely perceptible; but it soon +becomes clearer, and now stands out as an oval shield, surrounded by two rings +or areas (Fig. 117). The inner and brighter ring is the remainder of the +pellucid area, and the dark outer ring the remainder of the opaque area; the +opaque shield-like spot itself is the first rudiment of the dorsal part of the +embryo. We give it briefly the name of embryonic shield or dorsal shield. In +most works this embryonic shield is described as “the first rudiment or +trace of the embryo,” or “primitive embryo.” But this is +wrong, though it rests on the authority of Baer and Bischoff. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus119"></a> +<img src="images/fig119.gif" width="376" height="337" alt="Fig.119. Median longitudinal section +of the gastrula of four vertebrates." /> +<p class="caption">Fig. 119—<b>Median longitudinal section +of the gastrula of four vertebrates.</b> (From <i>Rabl.</i>) <i>A</i> +discogastrula of a shark (<i>Pristiurus</i>). <i>B</i> amphigastrula of a +sturgeon (<i>Accipenser</i>). <i>C</i> amphigastrula of an amphibium +(<i>Triton</i>). <i>D</i> epigastrula of an amniote (diagram). <i> a</i> +ventral, <i>b</i> dorsal lip of the primitive mouth.</p> +</div> + +<p> +As a matter of fact, we already have the embryo in the stem-cell, the gastrula, +and all the subsequent stages. The embryonic shield is simply the first +rudiment of the dorsal part, which is the earliest to develop. As the older +names of “embryonic rudiment” and “germinative area” +are used in many different senses—and this has led to a fatal confusion +in embryonic literature—we must explain very clearly the real +significance of these important embryonic parts of the amniote. It will be +useful to do so in a series of formal principles:— +</p> + +<p> +1. The so-called ”first trace of the embryo” in the amniotes, or +the embryonic shield, in the centre of the pellucid area, consists merely of an +early differentiation and formation of the middle dorsal parts. +</p> + +<p> +2. Hence the best name for it is ”the dorsal shield,” as I proposed +long ago. +</p> + +<p> +3. The germinative area, in which the first embryonic blood-vessels appear at +an early stage, is not opposed as an external area to the ”embryo +proper,” but is a part of it. +</p> + +<p> +4. In the same way, the yelk-sac or the umbilical vesicle is not a foreign +external +<span class='pagenum'><a name="Page_124" id="Page_124"></a></span> +appendage of the embryo, but an outlying part of its primitive gut. +</p> + +<p> +5. The dorsal shield gradually separates from the germinative area and the +yelk-sac, its edges growing downwards and folding together to form ventral +plates. +</p> + +<p> +6. The yelk-sac and vessels of the germinative area, which soon spread over its +whole surface, are, therefore, real embryonic organs, or temporary parts of the +embryo, and have a transitory importance in connection with the nutrition of +the growing later body; the latter may be called the ”permanent +body” in contrast to them. +</p> + +<p> +The relation of these cenogenetic features of the amniotes to the palingenetic +structures of the older non-amniotic vertebrates may be expressed in the +following theses: The original gastrula, which completely passes into the +embryonic body in the acrania, cyclostoma, and amphibia, is early divided into +two parts in the amniotes—the embryonic shield, which represents the +dorsal outline of the permanent body; and the temporary embryonic organs of the +germinative area and its blood-vessels, which soon grow over the whole of the +yelk-sac. The differences which we find in the various classes of the +vertebrate stem in these important particulars can only be fully understood +when we bear in mind their phylogenetic relations on the one hand, and, on the +other, the cenogenetic modifications of structure that have been brought about +by changes in the rearing of the young and the variation in the mass of the +food-yelk. +</p> + +<p> +We have already described in Chapter IX the changes which this increase and +decrease of the nutritive yelk causes in the form of the gastrula, and +especially in the situation and shape of the primitive mouth. The primitive +mouth or prostoma is originally a simple round aperture at the lower pole of +the long axis; its dorsal lip is above and ventral lip below. In the amphioxus +this primitive mouth is a little eccentric, or shifted to the dorsal side (Fig. +39). The aperture increases with the growth of the food-yelk in the cyclostoma +and ganoids; in the sturgeon it lies almost on the equator of the round ovum, +the ventral lip (<i>a</i>) in front and the dorsal lip (<i>b</i>) behind (Fig. +119 <i>b</i>). In the wide-mouthed, circular discoid gastrula of the selachii +or primitive fishes, which spreads quite flat on the large food-yelk, the +anterior semi-circle of the border of the disk is the ventral, and the +posterior semicircle the dorsal lip (Fig. 119 <i>A</i>). The amphiblastic +amphibia are directly connected with their earlier fish-ancestors, the +dipneusts and ganoids, and further the oldest selachii (<i>Cestracion</i>); +they have retained their total unequal segmentation, and their small primitive +mouth (Fig. 119 <i> C, ab</i>), blocked up by the yelk-stopper, lies at the +limit of the dorsal and ventral surface of the embryo (at the lower pole of its +equatorial axis), and there again has an upper dorsal and a lower ventral lip +(<i>a, b</i>). The formation of a large food-yelk followed again in the +stem-forms of the amniotes, the protamniotes or proreptilia, descended from the +amphibia (Fig. 119 <i>D</i>). But here the accumulation of the food-yelk took +place only in the ventral wall of the primitive-gut, so that the narrow +primitive mouth lying behind was forced upwards, and came to lie on the back of +the discoid ”epigastrula” in the shape of the ”primitive +groove”; thus (in contrast to the case of the selachii, Fig. 119 +<i>A</i>) the dorsal lip (<i>b</i>) had to be in front, and the ventral lip +(<i>a</i>) behind (Fig. 119 <i> D</i>). This feature was transmitted to all the +amniotes, whether they retained the large food-yelk (reptiles, birds, and +monotremes), or lost it by atrophy (the viviparous mammals). +</p> + +<p> +This phylogenetic explanation of gastrulation and cœlomation, and the +comparative study of them in the various vertebrates, throw a clear and full +light on many ontogenetic phenomena, as to which the most obscure and confused +opinions were prevalent thirty years ago. In this we see especially the high +scientific value of the biogenetic law and the careful separation of +palingenetic from cenogenetic processes. To the opponents of this law the real +explanation of these remarkable phenomena is impossible. Here, and in every +other part of embryology, the true key to the solution lies in phylogeny. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap13"></a> +<span class='pagenum'><a name="Page_125" id="Page_125"></a> +</span>Chapter XIII.<br/> +DORSAL BODY AND VENTRAL BODY</h2> + +<p> +The earliest stages of the human embryo are, for the reasons already given, +either quite unknown or only imperfectly known to us. But as the subsequent +embryonic forms in man behave and develop just as they do in all the other +mammals, there cannot be the slightest doubt that the preceding stages also are +similar. We have been able to see in the cœlomula of the human embryo (Fig. +97), by transverse sections through its primitive mouth, that its two +cœlom-pouches are developed in just the same way as in the rabbit (Fig. 96); +moreover, the peculiar course of the gastrulation is just the same. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus120"></a> +<img src="images/fig120.gif" width="407" height="193" alt="Fig.120. Embryonic vesicle of a +seven-days-old rabbit with oval embryonic shield (ag)." /> +<p class="caption">Fig. 120—<b>Embryonic vesicle of a +seven-days-old rabbit with oval embryonic shield</b> (<i>ag</i>). <i>A</i> seen +from above, <i>B</i> from the side. (From <i>Kölliker.</i>) <i>ag</i> dorsal +shield or embryonic spot. In <i>B</i> the upper half of the vesicle is made up +of the two primary germinal layers, the lower (up to <i>ge</i>) only from the +outer layer.</p> +</div> + +<p> +The germinative area forms in the human embryo in the same way as in the other +mammals, and in the middle part of this we have the embryonic shield, the +purport of which we considered in Chapter XII. The next changes in the +embryonic disk, or the “embryonic spot,” take place in +corresponding fashion. These are the changes we are now going to consider more +closely. +</p> + +<p> +The chief part of the oval embryonic shield is at first the narrow hinder end; +it is in the middle line of this that the primitive streak appears (Fig. 121 +<i>ps</i>).The narrow longitudinal groove in it—the so-called +“primitive groove”—is, as we have seen, the primitive mouth +of the gastrula. In the gastrula-embryos of the mammals, which are much +modified cenogenetically, this cleft-shaped prostoma is lengthened so much that +it soon traverses the whole of the hinder half of the dorsal shield; as we find +in a rabbit embryo of six to eight days (Fig. 122 <i>pr</i>). The two swollen +parallel borders that limit this median furrow are the side lips of the +primitive mouth, right and left. In this way the bilateral-symmetrical type of +the vertebrate becomes pronounced. The subsequent head of the amniote is +developed from the broader and rounder fore-half of the dorsal shield. +</p> + +<p> +In this fore-half of the dorsal shield a median furrow quickly makes its +appearance (Fig. 123 <i>rf</i>). This is the broader dorsal furrow or medullary +groove, the first beginning of the central nervous system. The two parallel +dorsal or medullary swellings that enclose it grow +<span class='pagenum'><a name="Page_126" id="Page_126"></a></span> +together over it afterwards, and form the medullary tube. As is seen in +transverse sections, it is formed only of the outer germinal layer (Figs. 95 +and 136). The lips of the primitive mouth, however, lie, as we know, at the +important point where the outer layer bends over the inner, and from which the +two cœlom pouches grow between the primary germinal layers. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus121"></a> +<img src="images/fig121.gif" width="255" height="203" alt="Fig.121. Oval embryonic shield of the +rabbit." /> +<p class="caption">Fig. 121—<b>Oval embryonic shield of the rabbit</b> (<i>A</i> of +six days eighteen hours, <i>B</i> of eight days). (From <i>Kölliker.</i>) +<i>ps</i> primitive streak, <i>pr</i> primitive groove, <i>arg</i> area +germinalis, <i>sw</i> sickle-shaped germinal growth.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus122"></a> +<img src="images/fig122.gif" width="367" height="247" alt="Fig.122. Dorsal shield (ag) and +germinative area of a rabbit-embryo of eight days. Fig. 123. Embryonic shield +of a rabbit of eight days." /> +<p class="caption">Fig. +122—<b>Dorsal shield</b> (<i>ag</i>) <b>and germinative area of a +rabbit-embryo</b> of eight days. (From <i>Kölliker.</i>) <i>pr</i> primitive +groove, <i>rf</i> dorsal furrow.<br/> Fig. 123.—<b>Embryonic shield of a +rabbit</b> of eight days. (From <i>Van Beneden.</i>) <i>pr</i> primitive +groove, <i>cn</i> canalis neurentericus, <i>nk</i> nodus neurentericus (or +“Hensen’s ganglion”), <i>kf</i> head-process +(chorda).</p> +</div> + +<p> +Thus the median primitive furrow (<i>pr</i>) +<span class='pagenum'><a name="Page_127" id="Page_127"></a></span> +in the hind-half and the median medullary furrow (<i>Rf</i>) in the fore-half +of the oval shield are totally different structures, although the latter seems +to a superficial observer to be merely the forward continuation of the former. +Hence they were formerly always confused. This error was the more pardonable as +immediately afterwards the two grooves do actually pass into each other in a +very remarkable way. The point of transition is the remarkable neurenteric +canal (Fig. 124 <i>cn</i>). But the direct connection which is thus established +does not last long; the two are soon definitely separated by a partition. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus124"></a> +<img src="images/fig124.gif" width="248" height="145" alt="Fig.124. Longitudinal +section of the coelomula of amphioxus." /> +<p class="caption">Fig. 124—<b>Longitudinal section of the cœlomula +of amphioxus</b> (from the left). <i>i</i> entoderm, <i>d</i> primitive gut, +<i>cn</i> medullary duct, <i>n</i> nerve tube, <i>m</i> mesoderm, <i>s</i> +first primitive segment, <i>c</i> cœlom-pouches. (From +<i>Hatschek.</i>)</p> +</div> + +<p> +The enigmatic <i>neurenteric canal</i> is a very old embryonic organ, and of +great phylogenetic interest, because it arises in the same way in all the +chordonia (both tunicates and vertebrates). In every case it touches or +embraces like an arch the posterior end of the chorda, which has been developed +here in front out of the middle line of the primitive gut (between the two +cœlom-folds of the sickle groove) (“head-process,” Fig. 123 +<i>kf</i>). These very ancient and strictly hereditary structures, which have +no physiological significance to-day, deserve (as “rudimentary +organs”) our closest attention. The tenacity with which the useless +neurenteric canal has been transmitted down to man through the whole series of +vertebrates is of equal interest for the theory of descent in general, and the +phylogeny of the chordonia in particular. +</p> + +<p> +The connection which the neurenteric canal (Fig. 123 <i>cn</i>) establishes +between the dorsal nerve-tube (<i>n</i>) and the ventral gut-tube (<i>d</i>) is +seen very plainly in the amphioxus in a longitudinal section of the cœlomula, +as soon as the primitive mouth is completely closed at its hinder end. The +medullary tube has still at this stage an opening at the forward end, the +neuroporus Fig. 83 <i>np</i>). This opening also is afterwards closed. There +are then two completely closed canals over each other—the medullary tube +above and the gastric tube below, the two being separated by the chorda. The +same features as in the acrania are exhibited by the related tunicates, the +ascidiæ. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus125"></a> +<img src="images/fig125.gif" width="206" height="164" alt="Fig.125. Longitudinal +section of the chordula of a frog." /> +<p class="caption">Fig. 125—<b>Longitudinal section of the chordula +of a frog.</b> (From <i>Balfour.</i>) <i>nc</i> nerve-tube, <i>x</i> canalis +neurentericus, <i>al</i> alimentary canal, <i>yk</i> yelk-cells, <i>m</i> +mesoderm.</p> +</div> + +<p> +Again, we find the neurenteric canal in just the same form and situation in the +amphibia. A longitudinal section of a young tadpole (Fig. 125) shows how we may +penetrate from the still open primitive mouth (<i>x</i>) either into the wide +primitive gut-cavity (<i>al</i>) or the narrow overlying nerve-tube. A little +later, when the primitive mouth is closed, the narrow neurenteric canal (Fig. +126 <i>ne</i>) represents the arched connection between the dorsal medullary +canal (<i>mc</i>) and the ventral gastric canal. +</p> + +<p> +In the amniotes this original curved form of the neurenteric canal cannot be +found at first, because here the primitive mouth travels completely over to the +dorsal surface of the gastrula, and is converted into the longitudinal furrow +we call the primitive groove. Hence the primitive groove (Fig. 128 <i>pr</i>), +examined from above, appears to be the straight +<span class='pagenum'><a name="Page_128" id="Page_128"></a></span> +continuation of the fore-lying and younger medullary furrow (<i>me</i>). The +divergent hind legs of the latter embrace the anterior end of the former. +Afterwards we have the complete closing of the primitive mouth, the dorsal +swellings joining to form the medullary tube and growing over it. The +neurenteric canal then leads directly, in the shape of a narrow arch-shaped +tube (Fig. 129 <i>ne</i>), from the medullary tube (<i>sp</i>) to the gastric +tube (<i>pag</i>). Directly in front of it is the latter end of the chorda +(<i>cli</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus126"></a> +<img src="images/fig126.gif" width="242" height="147" alt="Fig.126. Longitudinal +section of a frog-embryo." /> +<p class="caption">Fig. 126—<b>Longitudinal section of a +frog-embryo.</b> (From <i>Goette.</i>) <i>m</i> mouth, <i>l</i> liver, +<i>an</i> anus, <i>ne</i> canalis neurentericus, <i>mc</i> medullary-tube, +<i>pn</i> pineal body (epiphysis), <i>ch</i> chorda.</p> +</div> + +<p> +While these important processes are taking place in the axial part of the +dorsal shield, its external form also is changing. The oval form (Fig. 117) +becomes like the sole of a shoe or sandal, lyre-shaped or finger-biscuit shaped +(Fig. 130). The middle third does not grow in width as quickly as the +posterior, and still less than the anterior third; thus the shape of the +permanent body becomes somewhat narrow at the waist. At the same time, the oval +form of the germinative area returns to a circular shape, and the inner +pellucid area separates more clearly from the opaque outer area (Fig. 131 +<i>a</i>). The completion of the circle in the area marks the limit of the +formation of blood-vessels in the mesoderm. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus127"></a> +<img src="images/fig127.gif" width="316" height="284" alt="Figs. 127 and 128. Dorsal shield of +the chick." /> +<p class="caption">Figs. +127 and 128—<b>Dorsal shield of the chick.</b> (From <i>Balfour.</i>) +The medullary furrow (<i>me</i>), which is not yet visible in Fig. 130, +encloses with its hinder end the fore end of the primitive groove (<i>pr</i>) +in Fig. 131.)</p> +</div> + +<p> +The characteristic sandal-shape of the dorsal shield, which is determined by +the narrowness of the middle part, and which is compared to a violin, lyre, or +shoe-sole, persists for a long time in all the amniotes. All mammals, birds, +and reptiles have substantially the same construction at this stage, and even +for a longer or shorter +<span class='pagenum'><a name="Page_129" id="Page_129"></a></span> +period after the division of the primitive segments into the cœlom-folds has +begun (Fig. 132). The human embryonic shield assumes the sandal-form in the +second week of development; towards the end of the week our sole-shaped embryo +has a length of about one-twelfth of an inch (Fig. 133). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus129"></a> +<img src="images/fig129.gif" width="267" height="157" alt="Fig.129. Longitudinal +section of the hinder end of a chick." /> +<p class="caption">Fig. 129—<b>Longitudinal section of the hinder end +of a chick.</b> (From <i>Balfour.</i>) <i>sp</i> medullary tube, connected with +the terminal gut (<i>pag</i>) by the neurenteric canal (<i>ne</i>), <i>ch</i> +chorda, <i>pr</i> neurenteric (or Hensen’s) ganglion, <i>al</i> +allantois, <i>ep</i> ectoderm, <i>hy</i> entoderm, <i>so</i> parietal layer, +<i>sp</i> visceral layer, <i>an</i> anus-pit, <i>am</i> amnion.</p> +</div> + +<p> +The complete bilateral symmetry of the vertebrate body is very early indicated +in the oval form of the embryonic shield (Fig. 117) by the median primitive +streak; in the sandal-form it is even more pronounced (Figs. 131–135). In +the lateral parts of the embryonic shield a darker central and a lighter +peripheral zone become more obvious; the former is called the stem-zone (Fig. +134 <i>stz</i>), and the latter the parietal zone (<i>pz</i>); from the first +we get the dorsal and from the second the ventral half of the body-wall. The +stem-zone of the amniote embryo would be called more appropriately the dorsal +zone or dorsal shield; from it develops the whole of the dorsal half of the +later body (or permanent body)—that is to say, the dorsal body +(<i>episoma</i>). Again, it would be better to call the “parietal +zone” the ventral zone or ventral shield; from it develop the ventral +“lateral plates,” which afterwards separate from the embryonic +vesicle and form the ventral body (<i>hyposoma</i>)—that is to say, the +ventral half of the permanent body, together with the body-cavity and the +gastric canal that it encloses. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus130"></a> +<img src="images/fig130.gif" width="182" height="192" alt="Fig.130. Germinal area +or germinal disk of the rabbit, with sole-shaped embryonic shield." /> +<p class="caption">Fig. 130—<b>Germinal area or germinal disk of the +rabbit, with sole-shaped embryonic shield,</b> magnified. The clear circular +field (<i>d</i>) is the opaque area. The pellucid area (<i>c</i>) is +lyre-shaped, like the embryonic shield itself (<i>b</i>). In its axis is seen +the dorsal furrow or medullary furrow (<i>a</i>). (From +<i>Bischoff</i>.</p> +</div> + +<p> +The sole-shaped germinal shields of all the amniotes are still, at the stage of +construction which Fig. 134 illustrates in the rabbit and Fig. 135 in the +opossum, so like each other that we can either not distinguish them at all or +only by means of quite subordinate peculiarities in the size of the various +parts. Moreover, the human sandal-shaped embryo cannot at this stage be +distinguished from those of other mammals, and it particularly resembles that +of the rabbit. On the other hand, the outer form of these flat sandal-shaped +embryos is very different from the corresponding form of the lower animals, +especially the acrania (amphioxus). Nevertheless, the body is just the same in +the essential features of its structure as that we find in the chordula of the +latter (Figs. 83–86), and in the embryonic forms which immediately +develop from it. The striking external difference is here again due to the fact +that in the palingenetic embryos of the amphioxus (Figs. 83, 84) and the +amphibia (Figs. 85, 86) the gut-wall and body-wall form closed tubes from the +first, whereas in the cenogenetic embryos of the amniotes they are forced to +expand leaf-wise on the surface owing to the great extension of the food-yelk. +</p> + +<p> +<span class='pagenum'><a name="Page_130" id="Page_130"></a></span>It is all +the more notable that the early separation of dorsal and ventral +halves takes place in the same rigidly hereditary fashion in all the +vertebrates. In both the acrania and the craniota the dorsal body is about this +period separated from the ventral body. In the middle part of the body this +division has already taken place by the construction of the chorda between the +dorsal nerve-tube and the ventral canal. But in the outer or lateral part of +the body it is only brought about by the division of the coelom-pouches into +two sections—a dorsal <i>episomite</i> (dorsal segment or provertebra) +and a ventral <i>hyposomite</i> (or ventral segment) by a frontal constriction. +In the amphioxus each of the former makes a muscular pouch, and each of the +latter a sex-pouch or gonad. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus131"></a> +<a name="illus132"></a> +<img src="images/fig131.gif" width="412" height="306" alt="Fig.131. Embryo of the opossum, sixty +hours old. Fig. 132. Sandal-shaped embryonic shield of a rabbit of eight +days." /> +<p class="caption">Fig. 131—<b>Embryo of the opossum,</b> sixty hours old, +one-sixth of an inch in diameter. (From <i>Selenka</i>) <i>b</i> the globular +embryonic vesicle, <i>a</i> the round germinative area, <i>b</i> limit of the +ventral plates, <i>r</i> dorsal shield, <i>v</i> its fore part, <i>u</i> the +first primitive segment, <i>ch</i> chorda, <i>chr</i> its fore-end, <i>pr</i> +primitive groove (or mouth).<br/> Fig. 132—<b>Sandal-shaped embryonic +shield of a rabbit of eight days,</b> with the fore part of the germinative +area (<i>ao</i> opaque, <i>ap</i> pellucid area). (From <i>Kölliker.</i>) +<i>rf</i> dorsal furrow, in the middle of the medullary plate, <i>h, pr</i> +primitive groove (mouth), <i>stz</i> dorsal (stem) zone, <i>pz</i> ventral +(parietal) zone. In the narrow middle part the first three primitive segments +may be seen.</p> +</div> + +<p> +These important processes of differentiation in the mesoderm, which we will +consider more closely in the next chapter, proceed step by step with +interesting changes in the ectoderm, while the entoderm changes little at +first. We can study these processes best in transverse sections, made +vertically to the surface through the sole-shaped embryonic shield. Such a +transverse section of a chick embryo, at the end of the first day of +incubation, shows the gut-gland layer as a very simple epithelium, which is +spread like a leaf over the outer surface of the food-yelk (Fig. 92). The +chorda (<i>ch</i>) has separated from the dorsal middle line of the entoderm; +to the right and left of it are the two halves of the mesoderm, or the two +cœlom-folds. A narrow cleft in the latter indicates the body-cavity +(<i>uwh</i>); this separates the two plates of the cœlom-pouches, the lower +(visceral) and upper (parietal). The broad dorsal furrow (<i>rf</i>) formed by +the medullary plate (<i>m</i>) is still wide open, but is divided from the +lateral horn-plate +<span class='pagenum'><a name="Page_131" id="Page_131"></a></span> +(<i>h</i>) by the parallel medullary swellings, which eventually close. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus133"></a> +<a name="illus134"></a> +<img src="images/fig133.gif" width="436" height="343" alt="Fig.133. Human embryo at the +sandal-stage. Fig. 134. Sandal-shaped embryonic shield of a rabbit of nine +days." /> +<p class="caption">Fig. 133—<b>Human embryo at the sandal-stage,</b> one-twelfth of +an inch long, from the end of the second week, magnified. (From <i>Count +Spee.</i>)<br/> Fig. 134—<b>Sandal-shaped embryonic shield of a rabbit of +nine days.</b> (From <i>Kölliker.</i>) (Back view from above.) <i>stz</i> +stem-zone or dorsal shield (with eight pairs of primitive segments), <i>pz</i> +parietal or ventral zone, <i>ap</i> pellucid area, <i>af</i> amnion-fold, +<i>h</i> heart, <i>ph</i> pericardial cavity, <i>vo</i> omphalo-mesenteric +vein, <i>ab</i> eye-vesicles, <i>vh</i> fore brain, <i>mh</i> middle brain, +<i>hh</i> hind brain, <i>uw</i> primitive segments (or vertebræ).</p> +</div> + +<p> +During these processes important changes are taking place in the outer germinal +layer (the “skin-sense layer”). The continued rise and growth of +the dorsal swellings causes their higher parts to bend together at their free +borders, approach nearer and nearer (Fig. 136 <i>w</i>), and finally unite. +Thus in the end we get from the open dorsal furrow, the upper cleft of which +becomes narrower and narrower, a closed cylindrical tube (Fig. 137 <i>mr</i>). +This tube is of the utmost importance; it is the beginning of the central +nervous system, the brain and spinal marrow, the <i>medullary tube.</i> This +embryonic fact was formerly looked upon as very mysterious. We shall see +presently that in the light of the theory of descent it is a thoroughly natural +process. The phylogenetic explanation of it is that the central nervous system +is the organ by means of which all intercourse with the outer world, all +psychic action and sense-perception, are accomplished; hence it was bound to +develop originally from the outer and upper surface of the body, or from the +outer skin. The medullary tube afterwards separates completely from the outer +germinal layer, and is surrounded by the middle parts of the provertebræ and +forced inwards (Fig. 146).The remaining portion of the skin-sense layer (Fig. +93 <i>h</i>) is now called the horn-plate or horn-layer, because from it is +developed the whole of the outer skin or epidermis, with all its horny +appendages (nails, hair, etc.). +</p> + +<p> +A totally different organ, the <i>prorenal</i> +<span class='pagenum'><a name="Page_132" id="Page_132"></a></span> +(primitive kidney) <i>duct</i> (<i>ung</i>), is found to be developed at an +early stage from the ectoderm. This is originally a quite simple, tube-shaped, +lengthy duct, or straight canal, which runs from front to rear at each side of +the provertebræ (on the outer side, Fig. 93 <i>ung</i>). It originates, it +seems, out of the horn-plate at the side of the medullary tube, in the gap that +we find between the provertebral and the lateral plates. The prorenal duct is +visible in this gap even at the time of the severance of the medullary tube +from the horn-plate. Other observers think that the first trace of it does not +come from the skin-sense layer, but the skin-fibre layer. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus135"></a> +<img src="images/fig135.gif" width="299" height="354" alt="Fig.135. Sandal-shaped embryonic +shield of an opossum." /> +<p class="caption">Fig. 135—<b>Sandal-shaped embryonic shield +of an opossum</b> (<i>Didelphys</i>), three days old. (From <i>Selenka.</i>) +(Back view from above.) <i>stz</i> stem-zone or dorsal shield (with eight pairs +of primitive segments), <i>pz</i> parietal or ventral zone, <i>ap</i> pellucid +area, <i>ao</i> opaque area, <i>hh</i> halves of the heart, <i>v</i> fore-end, +<i>h</i> hind-end. In the median line we see the chorda (<i>ch</i>) through the +transparent medullary tube (<i>m</i>). <i>u</i> primitive segment, <i>pr</i> +primitive streak (or primitive mouth).</p> +</div> + +<p> +The inner germinal layer, or the gut-fibre layer (Fig. 93 <i>dd</i>), remains +unchanged during these processes. A little later, however, it shows a quite +flat, groove-like depression in the middle line of the embryonic shield, +directly under the chorda. This depression is called the gastric groove or +furrow. This at once indicates the future lot of this germinal layer. As this +ventral groove gradually deepens, and its lower edges bend towards each other, +it is formed into a closed tube, +<span class='pagenum'><a name="Page_133" id="Page_133"></a></span> +the alimentary canal, in the same way as the medullary groove grows into the +medullary tube. The gut-fibre layer (Fig. 137 <i>f</i>), which lies on the +gut-gland layer (<i>d</i>), naturally follows it in its folding. Moreover, the +incipient gut-wall consists from the first of two layers, internally the +gut-gland layer and externally the gut-fibre layer. +</p> + +<p> +The formation of the alimentary canal resembles that of the medullary tube to +this extent—in both cases a straight groove or furrow arises first of all +in the middle line of a flat layer. The edges of this furrow then bend towards +each other, and join to form a tube (Fig. 137). But the two processes are +really very different. The medullary tube closes in its whole length, and forms +a cylindrical tube, whereas the alimentary canal remains open in the middle, +and its cavity continues for a long time in connection with the cavity of the +embryonic vesicle. The open connection between the two cavities is only closed +at a very late stage, by the construction of the navel. The closing of the +medullary tube is effected from both sides, the edges of the groove joining +together from right and left. But the closing of the alimentary canal is not +only effected from right and left, but also from front and rear, the edges of +the ventral groove growing together from every side towards the navel. +Throughout the three higher classes of vertebrates the whole of this process of +the construction of the gut is closely connected with the formation of the +navel, or with the separation of the embryo from the yelk-sac or umbilical +vesicle. +</p> + +<p> +In order to get a clear idea of this, we must understand carefully the relation +of the embryonic shield to the germinative area and the embryonic vesicle. This +is done best by a comparison of the five stages which are shown in longitudinal +section in Figs. 138–142. The embryonic shield (<i>c</i>), which at first +projects very slightly over the surface of the germinative area, soon begins to +rise higher above it, and to separate from the embryonic vesicle. At this point +the embryonic shield, looked at from the dorsal surface, shows still the +original simple sandal-shape (Figs. 133–135). We do not yet see any trace +of articulation into head, neck, trunk, etc., or limbs. But the embryonic +shield has increased greatly in thickness, especially in the anterior part. It +now has the appearance of a thick, oval swelling, strongly curved over the +surface of the germinative area. It begins to sever completely from the +embryonic vesicle, with which it is connected at the ventral surface. As this +severance proceeds, the back bends more and more; in proportion as the embryo +grows the embryonic vesicle decreases, and at last it merely hangs as a small +vesicle from the belly of the embryo (Fig. 142 <i>ds</i>). In consequence of +the growth-movements which cause this severance, a groove-shaped depression is +formed at the surface of the vesicle, the <i>limiting furrow,</i> which +surrounds the vesicle in the shape of a pit, and a circular mound or dam (Fig. +139 ks) is formed at the outside of this pit by the elevation of the contiguous +parts of the germinal vesicle. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus136"></a> +<img src="images/fig136.gif" width="282" height="83" alt="Fig.136. Transverse +section of the embryonic disk of a chick at the end of the first day of +incubation." /> +<p class="caption">Fig. 136—<b>Transverse section of the embryonic +disk of a chick</b> at the end of the first day of incubation, magnified. The +edges of the medullary plate (<i>m</i>), the medullary swellings (<i>w</i>), +which separate the medullary from the horn-plate (<i>h</i>), are bending +towards each other. At each side of the chorda (<i>ch</i>) the primitive +segment plates (<i>u</i>) have separated from the lateral plates (<i>sp</i>). A +gut-gland layer. (From <i>Remak.</i>)</p> +</div> + +<p> +In order to understand clearly this important process, we may compare the +embryo to a fortress with its surrounding rampart and trench. The ditch +consists of the outer part of the germinative area, and comes to an end at the +point where the area passes into the vesicle. The important fold of the middle +germinal layer that brings about the formation of the body-cavity spreads +beyond the borders of the embryo over the whole germinative area. At first this +middle layer reaches as far as the germinative area; the whole of the rest of +the embryonic vesicle consists in the beginning only of the two original +limiting layers, the outer and inner germinal layers. Hence, as far as the +germinative area extends the germinal layer splits into the two plates we have +already recognised in it, the outer skin-fibre layer and the inner gut-fibre +layer. These two plates diverge considerably, a clear fluid gathering between +them (Fig. 140 <i>am</i>). The inner plate, the gut-fibre layer, remains on the +inner layer of the embryonic vesicle (on the gut-gland layer). The +<span class='pagenum'><a name="Page_134" id="Page_134"></a></span> +outer plate, the skin-fibre layer, lies close on the outer layer of the +germinative area, or the skin-sense layer, and separates together with this +from the embryonic vesicle. From these two united outer plates is formed a +continuous membrane. This is the circular mound that rises higher and higher +round the whole embryo, and at last joins above it (Figs. 139–142 +<i>am</i>). To return to our illustration of the fortress, we must imagine the +circular rampart to be extraordinarily high and towering far above the +fortress. Its edges bend over like the combs of an overhanging wall of rock +that would enclose the fortress; they form a deep hollow, and at last join +together above. In the end the fortress lies entirely within the hollow that +has been formed by the growth of the edges of this large rampart. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus137"></a> +<img src="images/fig137.gif" width="382" height="163" alt="Fig.137. Three diagrammatic +transverse sections of the embryonic disk of the higher vertebrate, to show the +origin of the tubular organs from the bending germinal layers." /> +<p class="caption">Fig. 137—<b>Three diagrammatic +transverse sections of the embryonic disk</b> of the higher vertebrate, to show +the origin of the tubular organs from the bending germinal layers. In Fig. +<i>A</i> the medullary tube (<i>n</i>) and the alimentary canal (<i>a</i>) are +still open grooves. In Fig. <i>B</i> the medullary tube (<i>n</i>) and the +dorsal wall are closed, but the alimentary canal (<i>a</i>) and the ventral +wall are open; the prorenal ducts (<i>u</i>) are cut off from the horn-plate +(<i>h</i>) and internally connected with segmental prorenal canals. In Fig. +<i>C</i> both the medullary tube and the dorsal wall above and the alimentary +canal and ventral wall below are closed. All the open grooves have become +closed tubes; the primitive kidneys are directed inwards. The letters have the +same meaning in all three figures: <i>h</i> skin-sense layer, <i>n</i> +medullary tube, <i>u</i> prorenal ducts, <i>x</i> axial rod, <i>s</i> +primitive-vertebra, <i>r</i> dorsal wall, <i>b</i> ventral wall, <i>c</i> +body-cavity or cœloma, <i>f</i> gut-fibre layer, <i>t</i> primitive artery +(aorta), <i>v</i> primitive vein (subintestinal vein), <i>d</i> gut-fibre +layer, <i>a</i> alimentary canal.</p> +</div> + +<p> +As the two outer layers of the germinative area thus rise in a fold about the +embryo, and join above it, they come at last to form a spacious sac-like +membrane about it. This envelope takes the name of the germinative membrane, or +water-membrane, or <i>amnion</i> (Fig. 142 <i>am</i>). The embryo floats in a +watery fluid, which fills the space between the embryo and the amnion, and is +called the amniotic fluid (Figs. 141, 142 <i>ah</i>). We will deal with this +remarkable formation and with the allantois later on (Chapter XV). In front of +the allantois the yelk-sac or umbilical vesicle (<i>ds</i>), the remainder of +the original embryonic vesicle, starts from the open belly of the embryo (Fig. +138 <i>kh</i>). In more advanced embryos, in which the gastric wall and the +ventral wall are nearly closed, it hangs out of the navel-opening in the shape +of a small vesicle with a stalk (Figs. 141, 142 <i>ds</i>). The more the embryo +grows, the smaller becomes the vitelline (yelk) sac. At first the embryo looks +like a small appendage of the large embryonic vesicle. Afterwards it is the +yelk-sac, or the remainder of the embryonic vesicle, that seems a small +pouch-like appendage of the embryo (Fig. 142 <i>ds</i>). It ceases to have any +significance in the end. The very wide opening, through which the gastric +cavity at first communicates with the umbilical vesicle, becomes narrower and +narrower, and at last disappears altogether. The <i>navel,</i> the small +pit-like depression that we find in the developed man in the middle of the +abdominal wall, is the spot at which the remainder of the embryonic vesicle +(the umbilical vesicle) originally entered into the ventral cavity, and joined +on to the growing gut. +</p> + +<p> +The origin of the navel coincides with the complete closing of the external +ventral wall. In the amniotes the ventral wall originates in the same way as +the dorsal wall. Both are formed substantially from the skin-fibre layer, and +externally covered with the horn-plate, the border section of the skin-sense +layer. Both come into +<span class='pagenum'><a name="Page_135" id="Page_135"></a></span> +<span class='pagenum'><a name="Page_136" id="Page_136"></a></span> +existence by the conversion of the four flat germinal layers of the embryonic +shield into a double tube by folding from opposite directions; above, at the +back, we have the vertebral canal which encloses the medullary tube, and below, +at the belly, the wall of the body-cavity which contains the alimentary canal +(Fig. 137). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus138"></a> +<img src="images/fig138.gif" width="374" height="505" alt="Figs. 138 to 142. Five diagrammatic +longitudinal sections of the maturing mammal embryo and its envelopes." /> +<p class="caption">Figs. 138–142—<b>Five diagrammatic longitudinal +sections of the maturing mammal embryo and its envelopes.</b> In Figs. +138–141 the longitudinal section passes through the sagittal or middle +plane of the body, dividing the right and left halves; in Fig. 142 the embryo +is seen from the left side. In Fig. 138 the tufted it prochorion +(<i>dd′</i>) encloses the germinal vesicle, the wall of which consists +of the two primary layers. Between the outer (<i>a</i>) and inner (<i>i</i>) +layer the middle layer (<i>m</i>) has been developed in the region of the +germinative area. In Fig. 139 the embryo (<i>e</i>) begins to separate from the +embryonic vesicle (<i>ds</i>), while the wall of the amnion-fold rises about it +(in front as head-sheath, <i>ks,</i> behind as tail-sheath, <i>ss</i>). In Fig. +140 the edges of the amniotic fold (<i>am</i>) rise together over the back of +the embryo, and form the amniotic cavity (<i>ah</i>); as the embryo separates +more completely from the embryonic vesicle (<i>ds</i>) the alimentary canal +(<i>dd</i>) is formed, from the hinder end of which the allantois grows +(<i>al</i>). In Fig. 141 the allantois is larger; the yelk-sac (<i>ds</i>) +smaller. In Fig. 142 the embryo shows the gill-clefts and the outline of the +two legs; the chorion has formed branching villi (tufts.) In all four figures +<i>e</i>=embryo, <i>a</i> outer germinal layer, <i>m</i> middle germinal layer, +<i>i</i> inner germinal layer, <i>am</i> amnion (<i>ks</i> head-sheath, +<i>ss</i> tail-sheath), <i>ah</i> amniotic cavity, <i>as</i> amniotic sheath of +the umbilical cord, <i>kh</i> embryonic vesicle, <i>ds</i> yelk-sac (umbilical +vesicle), <i>dg</i> vitelline duct, <i>df</i> gut-fibre layer, <i>dd</i> +gut-gland layer, <i>al</i> allantois, <i>vl=hh</i> place of heart, <i>d</i> +vitelline membrane (ovolemma or prochorion), <i>d′</i> tufts or villi of +same, <i>sh</i> serous membrane (serolemma), <i>sz</i> tufts of same, <i>ch</i> +chorion, <i>chz</i> tufts or villi, <i>st</i> terminal vein, <i>r</i> pericœlom +or serocœlom (the space, filled with fluid, between the amnion and chorion). +(From <i>Kölliker.</i>)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus143"></a> +<img src="images/fig143.gif" width="277" height="322" alt="Figs. 143 and 144. Transverse sections of embryos +(of chicks)." /> +<p class="caption">Figs. 143–144—<b>Transverse +sections of embryos</b> (of chicks). Fig. 143 of the second, Fig. 144 of the +third, Fig. 145 of the fourth, and Fig. 146 of the fifth day of incubation. +Fig. 143–145 from <i>Kölliker,</i> magnified; Fig. 146 from <i>Remak,</i> +magnified. <i>h</i> horn-plate, <i>mr</i> medullary tube, <i>ung</i> prorenal +duct, <i>un</i> prorenal vesicles, <i>hp</i> skin-fibre layer, <i>m=mu=mp</i> +muscle-plate, <i>uw</i> provertebral plate (<i>wh</i> cutaneous rudiment of the +body of the vertebra, <i>wb</i> of the arch of the vertebra, <i>wq</i> the rib +or transverse continuation), <i>uwh</i> provertebral cavity, <i>ch</i> axial +rod or chorda, <i>sh</i> chorda-sheath, <i>bh</i> ventral wall, <i>g</i> hind +and <i>v</i> fore root of the spinal nerves, <i>a=af=am</i> amniotic fold, +<i>p</i> body-cavity or cœloma, <i>df</i> gut-fibre layer, <i>ao</i> primitive +aortas, <i>sa</i> secondary aorta, <i>vc</i> cardinal veins, <i>d=dd</i> +gut-gland layer, <i>dr</i> gastric groove. In Fig. 143 the larger part of the +right half, in Fig. 144 the larger part of the left half, of the section is +omitted. Of the yelk-sac or remainder of the embryonic vesicle only a small +piece of the wall is indicated below.</p> +</div> + +<p> +We will consider the formation of the dorsal wall first, and that of the +ventral wall afterwards (Figs. 143–147). In the middle of the dorsal +surface of the embryo there is originally, as we already know, the medullary +(<i>mr</i>) tube directly underneath the horn-plate (<i>h</i>), from the middle +part of which it has been developed. Later, however, the provertebral plates +(<i>uw</i>) grow over from the right and left between these originally +connected parts (Figs. 145, 146). The upper and inner edges of the two +provertebral plates push between the horn-plate and medullary tube, force them +away from each other, and finally join between them in a seam that corresponds +to the middle line of the back. The coalescence of these two dorsal plates and +the closing in the middle of the dorsal wall take place in the same way as the +medullary tube, which is henceforth enclosed by the vertebral tube. Thus is +formed the dorsal wall, and the medullary tube takes up a position inside the +body. In the same way the provertebral mass grows afterwards round the chorda, +and forms the vertebral column. Below this the inner and outer edge of the +provertebral plate splits on each side into two horizontal plates, of which the +upper pushes between the chorda and medullary tube, and the lower between the +chorda and gastric tube. As the plates meet from both sides above and below the +chorda, they completely enclose it, and so form the tubular, outer +chord-sheath, the sheath from which the vertebral column is formed +(<i>perichorda,</i> Fig. 137 <i>C, s</i>; Figs. 145 <i>uwh,</i> 146). +</p> + +<p> +We find in the construction of the ventral wall precisely the same processes +<span class='pagenum'><a name="Page_137" id="Page_137"></a></span> +as in the formation of the dorsal wall (Fig. 137 <i>B,</i> Fig. 144 <i>hp,</i> +Fig. 146 <i>bh</i>). It is formed on the flat embryonic shield of the amniotes +from the upper plates of the parietal zone. The right and left parietal plates +bend downwards towards each other, and grow round the gut in the same way as +the gut itself closes. The outer part of the lateral plates forms the ventral +wall or the lower wall of the body, the two lateral plates bending considerably +on the inner side of the amniotic fold, and growing towards each other from +right and left. While the alimentary canal is closing, the body-wall also +closes on all sides. Hence the ventral wall, which encloses the whole ventral +cavity below, consists of two parts, two lateral plates that bend towards each +other. These approach each other all along, and at last meet at the navel. We +ought, therefore, really to distinguish two navels, an inner and an outer one. +The internal or intestinal navel is the definitive point of the closing of the +gut wall, which puts an end to the open communication between the ventral +cavity and the cavity of the yelk-sac (Fig. 105). The external navel in the +skin is the definitive point of the closing of the ventral wall; this is +visible in the developed body as a small depression. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus145"></a> +<img src="images/fig145.gif" width="263" height="512" alt="Figs. 145 and 146. Transverse sections of embryos +(of chicks)." /> +<p class="caption">Figs. 145–146—<b>Transverse +sections of embryos</b> (of chicks). Fig. 143 of the second, Fig. 144 of the +third, Fig. 145 of the fourth, and Fig. 146 of the fifth day of incubation. +Fig. 143–145 from <i>Kölliker,</i> magnified; Fig. 146 from <i>Remak,</i> +magnified. <i>h</i> horn-plate, <i>mr</i> medullary tube, <i>ung</i> prorenal +duct, <i>un</i> prorenal vesicles, <i>hp</i> skin-fibre layer, <i>m=mu=mp</i> +muscle-plate, <i>uw</i> provertebral plate (<i>wh</i> cutaneous rudiment of the +body of the vertebra, <i>wb</i> of the arch of the vertebra, <i>wq</i> the rib +or transverse continuation), <i>uwh</i> provertebral cavity, <i>ch</i> axial +rod or chorda, <i>sh</i> chorda-sheath, <i>bh</i> ventral wall, <i>g</i> hind +and <i>v</i> fore root of the spinal nerves, <i>a=af=am</i> amniotic fold, +<i>p</i> body-cavity or cœloma, <i>df</i> gut-fibre layer, <i>ao</i> primitive +aortas, <i>sa</i> secondary aorta, <i>vc</i> cardinal veins, <i>d=dd</i> +gut-gland layer, <i>dr</i> gastric groove. In Fig. 143 the larger part of the +right half, in Fig. 144 the larger part of the left half, of the section is +omitted. Of the yelk-sac or remainder of the embryonic vesicle only a small +piece of the wall is indicated below.</p> +</div> + +<p> +<span class='pagenum'><a name="Page_138" id="Page_138"></a></span> +With the formation of the internal navel and the closing of the alimentary +canal is connected the formation of two cavities, which we call the capital and +the pelvic sections of the visceral cavity. As the embryonic shield lies flat +on the wall of the embryonic vesicle at first, and only gradually separates +from it, its fore and hind ends are independent in the beginning; on the other +hand, the middle part of the ventral surface is connected with the yelk-sac by +means of the vitelline or umbilical duct (Fig. 147 <i>m</i>). This leads to a +notable curving of the dorsal surface; the head-end bends downwards towards the +breast and the tail-end towards the belly. We see this very clearly in the +excellent old diagrammatic illustration given by Baer (Fig. 147), a median +longitudinal section of the embryo of the chick, in which the dorsal body or +episoma is deeply shaded. The embryo seems to be trying to roll up, like a +hedgehog protecting itself from its pursuers. This pronounced curve of the back +is due to the more rapid growth of the convex dorsal surface, and is directly +connected with the severance of the embryo from the yelk-sac. The further +bending of the embryo leads to the formation of the “head-cavity” +of the gut (Fig. 148 above <i>D</i>) and a similar one at the tail, known as +its “pelvic cavity.” +</p> + +<div class="fig" style="width:100%;"> +<a name="illus147"></a> +<img src="images/fig147.gif" width="328" height="227" alt="Fig.147. Median longitudinal section of the +embryo of a chick (fifth day of incubation), seen from the right side (head to +the right, tail to the left)." /> +<p class="caption">Fig. 147—<b>Median longitudinal section of the embryo of a +chick</b> (fifth day of incubation), seen from the right side (head to the +right, tail to the left). Dorsal body dark, with convex outline. <i>d</i> gut, +<i>o</i> mouth, <i>a</i> anus, <i>l</i> lungs, <i>h</i> liver, <i>g</i> +mesentery, <i>v</i> auricle of the heart, <i>k</i> ventricle of the heart, +<i>b</i> arch of the arteries, <i>t</i> aorta, <i>c</i> yelk-sac, <i>m</i> +vitelline (yelk) duct, <i>u</i> allantois, <i>r</i> pedicle (stalk) of the +allantois, <i>n</i> amnion, <i>w</i> amniotic cavity, <i>s</i> serous membrane. +(From <i>Baer.</i>)</p> +</div> + +<p> +As a result of these processes the embryo attains a shape that may be compared +to a wooden shoe, or, better still, to an overturned canoe. Imagine a canoe or +boat with both ends rounded and a small covering before and behind; if this +canoe is turned upside down, so that the curved keel is uppermost, we have a +fair picture of the canoe-shaped embryo (Fig. 147). The upturned convex keel +corresponds to the middle line of the back; the small chamber underneath the +fore-deck represents the capital cavity, and the small chamber under the +rear-deck the pelvic chamber of the gut (cf. Fig. 140). +</p> + +<p> +The embryo now, as it were, presses into the outer surface of the embryonic +vesicle with its free ends, while it moves away from it with its middle part. +As a result of this change the yelk-sac becomes henceforth only a pouch-like +outer appendage at the middle of the ventral wall. The ventral appendage, +growing smaller and smaller, is afterwards called the umbilical (navel) +vesicle. The cavity of the yelk-sac or umbilical vesicle communicates with the +corresponding visceral cavity by a wide opening, which gradually contracts into +a narrow and long canal, the vitelline (yelk) duct (<i>ductus vitellinus,</i> +Fig. 147 <i>m</i>). Hence, if we were to imagine ourselves in +<span class='pagenum'><a name="Page_139" id="Page_139"></a></span> +the cavity of the yelk-sac, we could get from it through the yelk-duct into the +middle and still wide open part of the alimentary canal. If we were to go +forward from there into the head-part of the embryo, we should reach the +capital cavity of the gut, the fore-end of which is closed up. +</p> + +<p> +The reader will ask: “Where are the mouth and the anus?” These are +not at first present in the embryo. The whole of the primitive gut-cavity is +completely closed, and is merely connected in the middle by the vitelline duct +with the equally closed cavity of the embryonic vesicle (Fig. 140). The two +later apertures of the alimentary canal—the anus and the mouth—are +secondary constructions, formed from the outer skin. In the horn-plate, at the +spot where the mouth is found subsequently, a pit-like depression is formed, +and this grows deeper and deeper, pushing towards the blind fore-end of the +capital cavity; this is the mouth-pit. In the same way, at the spot in the +outer skin where the anus is afterwards situated a pit-shaped depression +appears, grows deeper and deeper, and approaches the blind hind-end of the +pelvic cavity; this is the anus-pit. In the end these pits touch with their +deepest and innermost points the two blind ends of the primitive alimentary +canal, so that they are now only separated from them by thin membranous +partitions. This membrane finally disappears, and henceforth the alimentary +canal opens in front at the mouth and in the rear by the anus (Figs. 141, 147). +Hence at first, if we penetrate into these pits from without, we find a +partition cutting them off from the cavity of the alimentary canal, which +gradually disappears. The formation of mouth and anus is secondary in all the +vertebrates. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus148"></a> +<img src="images/fig148.gif" width="200" height="262" alt="Fig.148. Longitudinal +section of the fore half of a chick-embryo at the end of the first day of +incubation (seen from the left side)." /> +<p class="caption">Fig. 148—<b>Longitudinal section of the fore half +of a chick-embryo</b> at the end of the first day of incubation (seen from the +left side). <i>k</i> head-plates, <i>ch</i> chorda. Above it is the blind +fore-end of the ventral tube (<i>m</i>); below it the capital cavity of the +gut. <i>d</i> gut-gland layer, <i>df</i> gut-fibre layer, <i>h</i> horn plate, +<i>hh</i> cavity of the heart, <i>hk</i> heart-capsule, <i>ks</i> head-sheath, +<i>kk</i> head-capsule. (From <i>Remak.</i>)</p> +</div> + +<p> +During the important processes which lead to the formation of the navel, and of +the intestinal wall and ventral wall, we find a number of other interesting +changes taking place in the embryonic shield of the amniotes. These relate +chiefly to the prorenal ducts and the first blood-vessels. The prorenal +(primitive kidney) ducts, which at first lie quite flat under the horn-plate or +epiderm (Fig. 93 <i>ung</i>), soon back towards each other in consequence of +special growth movements (Figs. 143–145 <i>ung</i>). They depart more and +more from their point of origin, and approach the gut-gland layer. In the end +they lie deep in the interior, on either side of the mesentery, underneath the +chorda, (Fig. 145 <i>ung</i>). At the same time, the two primitive aortas +change their position (cf. Figs. 138–145 <i>ao</i>); they travel inwards +underneath the chorda, and there coalesce at last to form a single secondary +aorta, which is found under the rudimentary vertebral column (Fig. 145 +<i>ao</i>). The cardinal veins, the first venous blood-vessels, also back +towards each other, and eventually unite immediately above the rudimentary +kidneys (Figs. 145 <i>vc,</i> 152 <i>cav</i>). In the same spot, at the inner +side of the fore-kidneys, we soon see the first trace of the sexual organs. The +most important part of this apparatus (apart from all its appendages) is the +ovary in the female and the testicle +<span class='pagenum'><a name="Page_140" id="Page_140"></a></span> +<span class='pagenum'><a name="Page_141" id="Page_141"></a></span> +in the male. Both develop from a small part of the cell-lining of the +body-cavity, at the spot where the skin-fibre layer and gut-fibre layer touch. +The connection of this embryonic gland with the prorenal ducts, which lie close +to it and assume most important relations to it, is only secondary. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus149"></a> +<img src="images/fig149.gif" width="415" height="275" alt="Fig.149. Longitudinal section of a +human embryo of the fourth week, one-fifth of an inch long." /> +<p class="caption">Fig. 149—<b>Longitudinal +section of a human embryo</b> of the fourth week, one-fifth of an inch long, +magnified. (From <i>Kollmann.</i>)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus150"></a> +<a name="illus151"></a> +<img src="images/fig150.gif" width="347" height="310" alt="Fig.150. Transverse +section of a human embryo of fourteen days. Fig. 151. Transverse section of a +shark-embryo (or young selachius)." /> +<p class="caption">Fig. +150—<b>Transverse section of a human embryo</b> of fourteen days. +<i>mr</i> medullary tube, <i>ch</i> chorda. <i>vu</i> umbilical vein, <i>mt</i> +myotome, <i>mp</i> middle plate, <i>ug</i> prorenal duct, <i>lh</i> +body-cavity, <i>e</i> ectoderm, <i>bh</i> ventral skin, <i>hf</i> skin-fibre +layer, <i>df</i> gut-fibre layer. (From <i>Kollmann.</i>)<br/> +Fig. 151—<b>Transverse section of a +shark-embryo</b> (or young selachius). <i>mr</i> medullary tube, <i>ch</i> +chorda, <i>a</i> aorta, <i>d</i> gut, <i>vp</i> principal (or subintestinal) +vein, <i>mt</i> myotome, <i>mm</i> muscular mass of the provertebra, <i>mp</i> +middle plate, <i>ug</i> prorenal duct, <i>lh</i> body-cavity, <i>e</i> ectoderm +of the rudimentary extremities, <i>mz</i> mesenchymic cells, <i>z</i> point +where the myotome and nephrotome separate. (From <i>H. E. +Ziegler.</i>)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus152"></a> +<img src="images/fig152.gif" width="341" height="269" alt="Fig.152. Transverse section of a +duck-embryo with twenty-four primitive segments." /> +<p class="caption">Fig. 152—<b>Transverse section of a duck-embryo with twenty-four +primitive segments.</b> (From <i>Balfour.</i>) From a dorsal lateral joint of +the medullary tube (<i>spc</i>) the spinal ganglia (<i>spg</i>) grow out +between it and the horn-plate. <i>ch</i> chorda, <i>ao</i> double aorta, +<i>hy</i> gut-gland layer, <i>sp</i> gut-fibre layer, with blood-vessels in +section, <i>ms</i> muscle plate, in the dorsal wall of the myocœl (episomite). +Below the cardinal vein (<i>cav</i>) is the prorenal duct (<i>wd</i>) and a +segmental prorenal canal (<i>st</i>). The skin-fibre layer of the body-wall +(<i>so</i>) is continued in the amniotic fold (<i>am</i>). Between the four +secondary germinal layers and the structures formed from them there is formed +embryonic connective matter with stellate cells and vascular structures +(Hertwig’s “mesenchym”).</p> +</div> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap14"></a>Chapter XIV.<br/> +THE ARTICULATION OF THE BODY<a href="#linknote-26" name="linknoteref-26" id="linknoteref-26"><sup>[26]</sup></a></h2> + +<p class="footnote"> +<a name="linknote-26" id="linknote-26"></a> <a href="#linknoteref-26">[26]</a> +The term articulation is used in this chapter to denote both +“segmentation” and “articulation” in the ordinary +sense.—Translator. +</p> + +<p> +The vertebrate stem, to which our race belongs as one of the latest and most +advanced outcomes of the natural development of life, is rightly placed at the +head of the animal kingdom. This privilege must be accorded to it, not only +because man does in point of fact soar far above all other animals, and has +been lifted to +<span class='pagenum'><a name="Page_142" id="Page_142"></a></span> +the position of “lord of creation”; but also because the vertebrate +organism far surpasses all the other animal-stems in size, in complexity of +structure, and in the advanced character of its functions. From the point of +view of both anatomy and physiology, the vertebrate stem outstrips all the +other, or invertebrate, animals. +</p> + +<p> +There is only one among the twelve stems of the animal kingdom that can in many +respects be compared with the vertebrates, and reaches an equal, if not a +greater, importance in many points. This is the stem of the articulates, +composed of three classes: 1, the annelids (earth-worms, leeches, and cognate +forms); 2, the crustacea (crabs, etc.); 3, the tracheata (spiders, insects, +etc.). The stem of the articulates is superior not only to the vertebrates, but +to all other animal-stems, in variety of forms, number of species, +elaborateness of individuals, and general importance in the economy of nature. +</p> + +<p> +When we have thus declared the vertebrates and the articulates to be the most +important and most advanced of the twelve stems of the animal kingdom, the +question arises whether this special position is accorded to them on the ground +of a peculiarity of organisation that is common to the two. The answer is that +this is really the case; it is their segmental or transverse articulation, +which we may briefly call metamerism. In all the vertebrates and articulates +the developed individual consists of a series of successive members (segments +or metamera = “parts”); in the embryo these are called primitive +segments or somites. In each of these segments we have a certain group of +organs reproduced in the same arrangement, so that we may regard each segment +as an individual unity, or a special “individual” subordinated to +the entire personality. +</p> + +<p> +The similarity of their segmentation, and the consequent physiological advance +in the two stems of the vertebrates and articulates, has led to the assumption +of a direct affinity between them, and an attempt to derive the former directly +from the latter. The annelids were supposed to be the direct ancestors, not +only of the crustacea and tracheata, but also of the vertebrates. We shall see +later (Chapter XX) that this annelid theory of the vertebrates is entirely +wrong, and ignores the most important differences in the organisation of the +two stems. The internal articulation of the vertebrates is just as profoundly +different from the external metamerism of the articulates as are their skeletal +structure, nervous system, vascular system, and so on. The articulation has +been developed in a totally different way in the two stems. The unarticulated +chordula (Figs. 83–86), which we have recognised as one of the chief +palingenetic embryonic forms of the vertebrate group, and from which we have +inferred the existence of a corresponding ancestral form for all the +vertebrates and tunicates, is quite unthinkable as the stem-form of the +articulates. +</p> + +<p> +All articulated animals came originally from unarticulated ones. This +phylogenetic principle is as firmly established as the ontogenetic fact that +every articulated animal-form develops from an unarticulated embryo. But the +organisation of the embryo is totally different in the two stems. The +chordula-embryo of all the vertebrates is characterised by the dorsal medullary +tube, the neurenteric canal, which passes at the primitive mouth into the +alimentary canal, and the axial chorda between the two. None of the +articulates, either annelids or arthropods (crustacea and tracheata), show any +trace of this type of organisation. Moreover, the development of the chief +systems of organs proceeds in the opposite way in the two stems. Hence the +segmentation must have arisen independently in each. This is not at all +surprising; we find analogous cases in the stalk-articulation of the higher +plants and in several groups of other animal stems. +</p> + +<p> +The characteristic internal articulation of the vertebrates and its importance +in the organisation of the stem are best seen in the study of the skeleton. Its +chief and central part, the cartilaginous or bony vertebral column, affords an +obvious instance of vertebrate metamerism; it consists of a series of +cartilaginous or bony pieces, which have long been known as <i>vertebræ</i> (or +<i>spondyli</i>). Each vertebra is directly connected with a special section of +the muscular system, the nervous system, the vascular system, etc. Thus most of +the “animal organs” take part in this vertebration. But we saw, +when we were considering our own vertebrate character (in Chapter XI), that the +same internal articulation is also found in the lowest primitive vertebrates, +the acrania, although here the whole skeleton consists merely of the simple +chorda, and is not at all articulated. +<span class='pagenum'><a name="Page_143" id="Page_143"></a></span> +Hence the articulation does not proceed primarily from the skeleton, but from +the muscular system, and is clearly determined by the more advanced +swimming-movements of the primitive chordonia-ancestors. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus153"></a> +<img src="images/fig153.gif" width="383" height="384" alt="Figs. 153-155. Sole-shaped embryonic +disk of the chick, in three successive stages of development, looked at from +the dorsal surface, magnified, somewhat diagrammatic." /> +<p class="caption">Figs. +153–155—<b>Sole-shaped embryonic disk of the chick,</b> in +three successive stages of development, looked at from the dorsal surface, +magnified, somewhat diagrammatic. Fig. 153 with six pairs of somites. Brain a +simple vesicle (<i>hb</i>). Medullary furrow still wide open from <i>x</i>; +greatly widened at <i>z. mp</i> medullary plates, <i>sp</i> lateral plates, +<i>y</i> limit of gullet-cavity (<i>sh</i>) and fore-gut (<i>vd</i>). Fig. 154 +with ten pairs of somites. Brain divided into three vesicles: <i>v</i> +fore-brain, <i>m</i> middle-brain, <i>h</i> hind-brain, <i>c</i> heart, +<i>dv</i> vitelline-veins. Medullary furrow still wide open behind (<i>z</i>). +<i>mp</i> medullary plates. Fig. 155 with sixteen pairs of somites. Brain +divided into five vesicles: <i>v</i> fore-brain, <i>z</i> intermediate-brain, +<i>m</i> middle-brain, <i>h</i> hind-brain, <i>n</i> after-brain, <i>a</i> +optic vesicles, <i>g</i> auditory vesicles, <i>c</i> heart, <i> dv</i> +vitelline veins, <i>mp</i> medullary plate, <i>uw</i> primitive vertebra.</p> +</div> + +<p> +It is, therefore, wrong to describe the first rudimentary segments in the +vertebrate embryo as primitive vertebræ or provertebræ; the fact that they have +been so called for some time has led to much error and misunderstanding. Hence +we shall give the name of “somites” or primitive segments to these +so-called “primitive vertebræ.” If the latter name is retained at +all, it should only be used of the sclerotom—i.e., the small part of the +somites from which the later vertebra does actually develop. +</p> + +<p> +Articulation begins in all vertebrates at a very early embryonic stage, and +this indicates the considerable phylogenetic age of the process. When the +chordula (Figs. 83–86) has completed its characteristic composition, +often even a little earlier, we find in the amniotes, in the +<span class='pagenum'><a name="Page_144" id="Page_144"></a></span> +middle of the sole-shaped embryonic shield, several pairs of dark square spots, +symmetrically distributed on both sides of the chorda (Figs. +131–135).Transverse sections (Fig. 93 <i>uw</i>) show that they belong to +the stem-zone (episoma) of the mesoderm, and are separated from the parietal +zone (hyposoma) by the lateral folds; in section they are still quadrangular, +almost square, so that they look something like dice. These pairs of +“cubes” of the mesoderm are the first traces of the primitive +segments or somites, the so-called “protovertebræ.” (Figs. +153–155 <i>uw</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus156"></a> +<img src="images/fig156.gif" width="149" height="198" alt="Fig.156. Embryo of the +amphioxus, sixteen hours old, seen from the back." /> +<p class="caption">Fig. 156—<b>Embryo of the amphioxus, sixteen hours +old,</b> seen from the back. (From <i>Hatschek.</i>) <i>d</i> primitive gut, +<i>u</i> primitive mouth, <i>p</i> polar cells of the mesoderm, <i> c</i> +cœlom-pouches, <i>m</i> their first segment, <i>n</i> medullary tube, <i>i</i> +entoderm, <i>e</i> ectoderm, <i>s</i> first segment-fold.</p> +</div> + +<p> +Among the mammals the embryos of the marsupials have three pairs of somites +(Fig. 131) after sixty hours, and eight pairs after seventy-two hours (Fig. +135). They develop more slowly in the embryo of the rabbit; this has three +somites on the eighth day (Fig. 132), and eight somites a day later (Fig. 134). +In the incubated hen’s egg the first somites make their appearance thirty +hours after incubation begins (Fig. 153). At the end of the second day the +number has risen to sixteen or eighteen (Fig. 155). The articulation of the +stem-zone, to which the somites owe their origin, thus proceeds briskly from +front to rear, new transverse constrictions of the “protovertebral +plates” forming continuously and successively. The first segment, which +is almost half-way down in the embryonic shield of the amniote, is the foremost +of all; from this first somite is formed the first cervical vertebra with its +muscles and skeletal parts. It follows from this, firstly, that the +multiplication of the primitive segments proceeds backwards from the front, +with a constant lengthening of the hinder end of the body; and, secondly, that +at the beginning of segmentation nearly the whole of the anterior half of the +sole-shaped embryonic shield of the amniote belongs to the later head, while +the whole of the rest of the body is formed from its hinder half. We are +reminded that in the amphioxus (and in our hypothetic primitive vertebrate, +Figs. 98–102) nearly the whole of the fore half corresponds to the head, +and the hind half to the trunk. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus157"></a> +<img src="images/fig157.gif" width="249" height="147" alt="Fig.157. Embryo of the +amphioxus, twenty hours old, with five somites." /> +<p class="caption">Fig. 157—<b>Embryo of the amphioxus, twenty hours old, +with five somites.</b> (Right view; for left view see Fig. 124.) (From +<i>Hatschek.</i>) <i> V</i> fore end, <i>H</i> hind end. <i>ak, mk, ik</i> +outer, middle, and inner germinal layers; <i>dh</i> alimentary canal, <i>n</i> +neural tube, <i>cn</i> canalis neurentericus, <i>ush</i> cœlom-pouches (or +primitive-segment cavities), <i>us1</i> first (and foremost) primitive +segment.</p> +</div> + +<p> +The number of the metamera, and of the embryonic somites or primitive segments +from which they develop, varies considerably in the vertebrates, according as +the hind part of the body is short or is lengthened by a tail. In the developed +man the trunk (including the rudimentary tail) consists of thirty-three +metamera, the solid centre of which is formed by that number of vertebræ in the +vertebral column (seven cervical, twelve dorsal, five lumbar, five sacral, and +four caudal). To these we must add at least nine head-vertebræ, which +originally (in all the craniota) constitute the skull. Thus the total number of +the primitive segments of the human +<span class='pagenum'><a name="Page_145" id="Page_145"></a></span> +body is raised to at least forty-two; it would reach forty-five to forty-eight +if (according to recent investigations) the number of the original segments of +the skull is put at twelve to fifteen. In the tailless or anthropoid apes the +number of metamera is much the same as in man, only differing by one or two; +but it is much larger in the long-tailed apes and most of the other mammals. In +long serpents and fishes it reaches several hundred (sometimes 400). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus158"></a> +<img src="images/fig158.gif" width="387" height="330" alt="Figs. 158-160. Embryo of the +amphioxus, twenty four hours old, with eight somites." /> +<p class="caption">Figs. 158–160—<b>Embryo of +the amphioxus, twenty four hours old, with eight somites.</b> (From +<i>Hatschek.</i>) Figs. 158 and 159 lateral view (from left). Fig. 160 seen +from back. In Fig. 158 only the outlines of the eight primitive segments are +indicated, in Fig. 159 their cavities and muscular walls. <i>V</i> fore end, +<i>H</i> hind end, <i>d</i> gut, <i>du</i> under and <i> dd</i> upper wall of +the gut, <i>ne</i> canalis neurentericus, <i> nv</i> ventral, <i>nd</i> dorsal +wall of the neural tube, <i>np</i> neuroporus, <i>dv</i> fore pouch of the gut, +<i>ch</i> chorda, <i> mf</i> mesodermic fold, <i>pm</i> polar cells of the +mesoderm (<i>ms</i>), <i>e</i> ectoderm.</p> +</div> + +<p> +In order to understand properly the real nature and origin of articulation in +the human body and that of the higher vertebrates, it is necessary to compare +it with that of the lower vertebrates, and bear in mind always the genetic +connection of all the members of the stem. In this the simple development of +the invaluable amphioxus once more furnishes the key to the complex and +cenogenetically modified embryonic processes of the craniota. The articulation +of the amphioxus begins at an early stage—earlier than in the craniotes. +The two cœlom-pouches have hardly grown out of the primitive gut (Fig. 156 +<i>c</i>) when the blind fore part of it (farthest away from the primitive +mouth, <i>u</i>) begins to separate by a transverse fold (<i>s</i>): this is +the first primitive segment. Immediately afterwards the hind part of the +cœlom-pouches begins to divide into a series of pieces by new transverse folds +(Fig. 157). The foremost of these primitive segments (<i>us</i>1) is the first +and oldest; in Figs. 124 and 157 there are already five formed. They separate +so rapidly, one behind the other, that eight pairs are formed within +twenty-four hours of the beginning of development, and seventeen pairs +twenty-four hours later. The number increases as the embryo grows and extends +<span class='pagenum'><a name="Page_146" id="Page_146"></a></span> +backwards, and new cells are formed constantly (at the primitive mouth) from +the two primitive mesodermic cells (Figs. 159–160). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus161"></a> +<img src="images/fig161.gif" width="395" height="284" alt="Figs. 161 and 162. Transverse section +of shark-embryos (through the region of the kidneys)." /> +<p class="caption">Figs. 161 and +162—<b>Transverse section of shark-embryos</b> (through the region of +the kidneys). (From <i>Wijhe</i> and <i>Hertwig.</i>) In Fig. 162 the dorsal +segment-cavities (<i>h</i>) are already separated from the body-cavity +(<i>lh</i>), but they are connected a little earlier (Fig. 161), <i>nr</i> +neural tube, <i>ch</i> chorda, <i>sch</i> subchordal string, <i>ao</i> aorta, +<i>sk</i> skeletal-plate, <i>mp</i> muscle-plate, <i>cp</i> cutis-plate, <i> +w</i> connection of latter (growth-zone), <i>vn</i> primitive kidneys, +<i>ug</i> prorenal duct, <i>uk</i> prorenal canals, <i> us</i> point where they +are cut off, <i>tr</i> prorenal funnel, <i> mk</i> middle germ-layer +(<i>mk</i><sub>1</sub> parietal, <i> mk</i><sub>2</sub> visceral), <i>ik</i> +inner germ-layer (gut-gland layer).</p> +</div> + +<p> +This typical articulation of the two cœlom-sacs begins very early in the +lancelet, before they are yet severed from the primitive gut, so that at first +each segment-cavity (<i>us</i>) still communicates by a narrow opening with the +gut, like an intestinal gland. But this opening soon closes by complete +severance, proceeding regularly backwards. The closed segments then extend +more, so that their upper half grows upwards like a fold between the ectoderm +(<i>ak</i>) and neural tube (<i>n</i>), and the lower half between the ectoderm +and alimentary canal (<i>ch</i>; Fig. 82 <i>d,</i> left half of the figure). +Afterwards the two halves completely separate, a lateral longitudinal fold +cutting between them (<i>mk,</i> right half of Fig. 82). The dorsal segments +(<i>sd</i>) provide the muscles of the trunk the whole length of the body +(159): this cavity afterwards disappears. On the other hand, the ventral parts +give rise, from their uppermost section, to the pronephridia or +primitive-kidney canals, and from the lower to the segmental rudiments of the +sexual glands or gonads. The partitions of the muscular dorsal pieces +(<i>myotomes</i>) remain, and determine the permanent articulation of the +vertebrate organism. But the partitions of the large ventral pieces +(<i>gonotomes</i>) become thinner, and afterwards disappear in part, so that +their cavities run together to form the metacœl, or the simple permanent +body-cavity. +</p> + +<p> +The articulation proceeds in substantially the same way in the other +vertebrates, the craniota, starting from the cœlom-pouches. But whereas in the +former case there is first a transverse division of the cœlom-sacs (by vertical +folds) and then the dorso-ventral division, the procedure is reversed in the +craniota; in their case each of the long cœlom-pouches first divides into a +dorsal (primitive segment plates) and a ventral (lateral plates) section by a +lateral longitudinal fold. Only the former are then broken up into primitive +segments by the subsequent vertical folds; while the latter (segmented +<span class='pagenum'><a name="Page_147" id="Page_147"></a></span> +for a time in the amphioxus) remain undivided, and, by the divergence of their +parietal and visceral plates, form a body-cavity that is unified from the +first. In this case, again, it is clear that we must regard the features of the +younger craniota as cenogenetically modified processes that can be traced +palingenetically to the older acrania. +</p> + +<p> +We have an interesting intermediate stage between the acrania and the fishes in +these and many other respects in the cyclostoma (the hag and the lamprey, cf. +Chapter XXI). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus163"></a> +<img src="images/fig163.gif" width="160" height="121" alt="Fig.163. Frontal (or +horizontal-longitudinal) section of a triton-embryo with three pairs of +primitive segments." /> +<p class="caption">Fig. 163—<b>Frontal (or horizontal-longitudinal) +section of a triton-embryo</b> with three pairs of primitive segments. +<i>ch</i> chorda, <i>us</i> primitive segments, <i>ush</i> their cavity, <i> +ak</i> horn plate.</p> +</div> + +<p> +Among the fishes the selachii, or primitive fishes, yield the most important +information on these and many other phylogenetic questions (Figs. 161 and 162). +The careful studies of Rückert, Van Wijhe, H. E. Ziegler, and others, have +given us most valuable results. The products of the middle germinal layer are +partly clear in these cases at the period when the dorsal primitive segment +cavities (or myocœls, <i>h</i>) are still connected with the ventral +body-cavity (<i>lh</i>; Fig. 161). In Fig. 162, a somewhat older embryo, these +cavities are separated. The outer or lateral wall of the dorsal segment yields +the cutis-plate (<i>cp</i>), the foundation of the connective corium. From its +inner or median wall are developed the muscle-plate (<i>mp,</i> the rudiment of +the trunk-muscles) and the skeletal plate, the formative matter of the +vertebral column (<i>sk</i>). +</p> + +<p> +In the amphibia, also, especially the water-salamander (<i>Triton</i>), we can +observe very clearly the articulation of the cœlom-pouches and the rise of the +primitive segments from their dorsal half (cf. Fig. 91, <i>A, B, C</i>). A +horizontal longitudinal section of the salamander-embryo (Fig. 163) shows very +clearly the series of pairs of these vesicular dorsal segments, which have been +cut off on each side from the ventral side-plates, and lie to the right and +left of the chorda. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus164"></a> +<img src="images/fig164.gif" width="323" height="125" alt="Fig.164. The third cervical vertebra +(human)> Fig. 165. The sixth dorsal vertebra (human). Fig. 166. The second +lumbar vertebra (human)." /> +<p class="caption">Fig. 164—<b>The third cervical vertebra</b> +(human).<br/> Fig. 165—<b>The sixth dorsal vertebra</b> (human).<br/> +Fig. 166—<b>The second lumbar vertebra</b> (human).</p> +</div> + +<p> +The metamerism of the amniotes agrees in all essential points with that of the +three lower classes of vertebrates we have considered; but it varies +considerably in detail, in consequence of cenogenetic disturbances that are due +in the first place (like the degeneration of the cœlom-pouches) to the large +development of the food-yelk. As the pressure of this seems to force the two +middle layers together from the start, and as the solid structure of the +mesoderm apparently belies the original hollow character of the sacs, the two +sections of the mesoderm, which are at that time divided by the lateral +fold—the dorsal segment-plates and ventral side-plates—have the +appearance at first of solid layers of cells (Figs. 94–97). And when the +articulation of the somites begins in the sole-shaped embryonic shield, and a +couple of protovertebræ are developed in succession, constantly increasing in +number towards the rear, these cube-shaped somites (formerly called +protovertebræ, or primitive vertebræ) have the appearance of solid dice, made +up of mesodermic cells (Fig. 93). Nevertheless, there is for a time a ventral +cavity, or provertebral cavity, even in these solid +<span class='pagenum'><a name="Page_148" id="Page_148"></a></span> +“protovertebræ” (Fig. 143 <i>uwh</i>). This vesicular condition of +the provertebra is of the greatest phylogenetic interest; we must, according to +the cœlom theory, regard it as an hereditary reproduction of the hollow dorsal +somites of the amphioxus (Figs. 156–160) and the lower vertebrates (Fig. +161–163). This rudimentary “provertebral cavity” has no +physiological significance whatever in the amniote-embryo; it soon disappears, +being filled up with cells of the muscular plate. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus167"></a> +<img src="images/fig167.gif" width="372" height="293" alt="Fig.167. Head of a shark embryo." /> +<p class="caption">Fig. 167—<b>Head +of a shark embryo</b> (<i>Pristiurus</i>), one-third of an inch long, +magnified. (From <i>Parker.</i>) Seen from the ventral side.</p> +</div> + +<p> +The innermost median part of the primitive segment plates, which lies +immediately on the chorda (Fig. 145 <i>ch</i>) and the medullary tube +(<i>m</i>), forms the vertebral column in all the higher vertebrates (it is +wanting in the lowest); hence it may be called the <i>skeleton</i> plate. In +each of the provertebræ it is called the “sclerotome” (in +opposition to the outlying muscular plate, the “myotome”). From the +phylogenetic point of view the myotomes are much older than the sclerotomes. +The lower or ventral part of each sclerotome (the inner and lower edge of the +cube-shaped provertebra) divides into two plates, which grow round the chorda, +and thus form the foundation of the body of the vertebra (<i>wh</i>). The upper +plate presses between the chorda and the medullary tube, the lower between the +chorda and the alimentary canal (Fig. 137 <i>C</i>). As the plates of two +opposite provertebral pieces unite from the right and left, a circular sheath +is formed round this part of the chorda. From this develops the <i>body</i> of +a vertebra—that is to say, the massive lower or ventral half of the bony +ring, which is called the “vertebra” proper and surrounds the +medullary tube (Figs. 164–166). The upper or dorsal half of this bony +ring, the vertebral arch (Fig. 145 <i>wb</i>), arises in just the same way from +the upper part of the skeletal plate, and therefore from the inner and upper +edge of the cube-shaped primitive vertebra. As the upper edges of two opposing +somites grow together over the medullary tube from right and left, the +vertebra-arch becomes closed. </p> + +<p> +The whole of the secondary vertebra, which is thus formed from the union of the +skeletal plates of two provertebral pieces +<span class='pagenum'><a name="Page_149" id="Page_149"></a></span> +and encloses a part of the chorda in its body, consists at first of a rather +soft mass of cells; this afterwards passes into a firmer, cartilaginous stage, +and finally into a third, permanent, bony stage. These three stages can +generally be distinguished in the greater part of the skeleton of the higher +vertebrates; at first most parts of the skeleton are soft, tender, and +membranous; they then become cartilaginous in the course of their development, +and finally bony. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus168"></a> +<img src="images/fig168.gif" width="202" height="135" alt="Figs. 168 and 169. Head +of a chick embryo, of the third day." /> +<p class="caption">Fig. 168 and 169—<b>Head of a chick embryo,</b> of +the third day. Fig. 168 from the front, Fig. 169 from the right. <i>n</i> +rudimentary nose (olfactory pit), <i>l</i> rudimentary eye (optic pit, +lens-cavity), <i>g</i> rudimentary ear (auditory pit), <i>v</i> fore-brain, +<i>gl</i> eye-cleft. Of the three pairs of gill-arches the first has passed +into a process of the upper jaw (<i>o</i>) and of the lower jaw (<i>u</i>). +(From <i>Kölliker.</i>)</p> +</div> + +<p> +At the head part of the embryo in the amniotes there is not generally a +cleavage of the middle germinal layer into provertebral and lateral plates, but +the dorsal and ventral somites are blended from the first, and form what are +called the “head-plates” (Fig. 148 <i>k</i>). From these are formed +the skull, the bony case of the brain, and the muscles and corium of the body. +The skull develops in the same way as the membranous vertebral column. The +right and left halves of the head curve over the cerebral vesicle, enclose the +foremost part of the chorda below, and thus finally form a simple, soft, +membranous capsule about the brain. This is afterwards converted into a +cartilaginous primitive skull, such as we find permanently in many of the +fishes. Much later this cartilaginous skull becomes the permanent bony skull +with its various parts. The bony skull in man and all the other amniotes is +more highly differentiated and modified than that of the lower vertebrates, the +amphibia and fishes. But as the one has arisen phylogenetically from the other, +we must assume that in the former no less than the latter the skull was +originally formed from the sclerotomes of a number of (at least nine) +head-somites. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus170"></a> +<img src="images/fig170.gif" width="170" height="172" alt="Fig.170. Head of a dog +embryo, seen from the front." /> +<p class="caption">Fig. 170—<b>Head of a dog embryo,</b> seen from the +front. <i> a</i> the two lateral halves of the foremost cerebral vesicle, <i> +b</i> rudimentary eye, <i>c</i> middle cerebral vesicle, <i>de</i> first pair +of gill-arches (<i>e</i> upper-jaw process, <i>d</i> lower-jaw process), <i>f, +f′, f″,</i> second, third, and fourth pairs of gill-arches, <i>g +h i k</i> heart (<i>g</i> right, <i> h</i> left auricle; <i>i</i> left, +<i>k</i> right ventricle), <i> l</i> origin of the aorta with three pairs of +arches, which go to the gill-arches. (From <i>Bischoff.</i>)</p> +</div> + +<p> +While the articulation of the vertebrate body is always obvious in the +<i>episoma</i> or dorsal body, and is clearly expressed in the segmentation of +the muscular plates and vertebræ, it is more latent in the <i>hyposoma</i> or +ventral body. Nevertheless, the hyposomites of the vegetal half of the body are +not less important than the episomites of the animal half. The segmentation in +the ventral cavity affects the following principal systems of organs: 1, the +gonads or sex-glands (gonotomes); 2, the nephridia or kidneys (nephrotomes); +and 3, the head-gut with its gill-clefts (branchiotomes). +</p> + +<p> +The metamerism of the hyposoma is less conspicuous because in all the craniotes +the cavities of the ventral segments, in the walls of which the sexual products +are developed, have long since coalesced, and formed a single large +body-cavity, owing to the disappearance of the partition. This cenogenetic +process is so old that the cavity seems to be unsegmented from the first in all +the craniotes, and the rudiment of the gonads also is almost always +unsegmented. It is the more interesting to learn that, according to the +important discovery of Rückert, this sexual structure is at first segmental +even in the actual selachii, and the several +<span class='pagenum'><a name="Page_150" id="Page_150"></a></span> +gonotomes only blend into a simple sexual gland on either side secondarily. +</p> + +<p> +Amphioxus, the sole surviving representative of the acrania, once more yields +us most interesting information; in this case the sexual glands remain +segmented throughout life. The sexually mature lancelet has, on the right and +left of the gut, a series of metamerous sacs, which are filled with ova in the +female and sperm in the male. These segmental gonads are originally nothing +else than the real gonotomes, separate body-cavities, formed from the +hyposomites of the trunk. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus171"></a> +<img src="images/fig171.gif" width="450" height="385" alt="Fig.171. Human embryo of the fourth +week (twenty-six days old)." /> +<p class="caption">Fig. 171—<b>Human embryo of the fourth week</b> +(twenty-six days old), one-fourth of an inch in length, magnified. (From +<i>Moll.</i>) The rudiments of the cerebral nerves and the roots of the spinal +nerves are especially marked. Underneath the four gill-arches (left side) is +the heart (with auricle, <i>V,</i> and ventricle, <i>K</i>), under this again +the liver (<i>L</i>).</p> +</div> + +<p> +The gonads are the most important segmental organs of the hyposoma, in the +sense that they are phylogenetically the oldest. We find sexual glands (as +pouch-like appendages of the gastro-canal system) in most of the lower animals, +even in the medusæ, etc., which have no kidneys. The latter appear first (as a +pair of excretory tubes) in the platodes (turbellaria), and have probably been +inherited from these by the articulates +<span class='pagenum'><a name="Page_151" id="Page_151"></a></span> +(annelids) on the one hand and the unarticulated prochordonia on the other, and +from these passed to the articulated vertebrates. The oldest form of the kidney +system in this stem are the segmental pronephridia or prorenal canals, in the +same arrangement as Boveri found them in the amphioxus. They are small canals +that lie in the frontal plane, on each side of the chorda, between the episoma +and hyposoma (Fig. 102 <i>n</i>); their internal funnel-shaped opening leads +into the various body-cavities, their outer opening is the lateral furrow of +the epidermis. Originally they must have had a double function, the carrying +away of the urine from the episomites and the release of the sexual cells from +the hyposomites. +</p> + +<p> +The recent investigations of Ruckert and Van Wijhe on the mesodermic segments +of the trunk and the excretory system of the selachii show that these +“primitive fishes” are closely related to the amphioxus in this +further respect. The transverse section of the shark-embryo in Fig. 161 shows +this very clearly. +</p> + +<p> +In other higher vertebrates, also, the kidneys develop (though very differently +formed later on) from similar structures, which have been secondarily derived +from the segmental pronephridia of the acrania. The parts of the mesoderm at +which the first traces of them are found are usually called the middle or +mesenteric plates. As the first traces of the gonads make their appearance in +the lining of these middle plates nearer inward (or the middle) from the inner +funnels of the nephro-canals, it is better to count this part of the mesoderm +with the hyposoma. +</p> + +<p> +The chief and oldest organ of the vertebrate hyposoma, the alimentary canal, is +generally described as an unsegmented organ. But we could just as well say that +it is the oldest of all the segmented organs of the vertebrate; the double row +of the cœlom-pouches grows out of the dorsal wall of the gut, on either side of +the chorda. In the brief period during which these segmental cœlom-pouches are +still openly connected with the gut, they look just like a double chain of +segmented visceral glands. But apart from this, we have originally in all +vertebrates an important articulation of the fore-gut, that is wanting in the +lower gut, the segmentation of the branchial (gill) gut. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus172"></a> +<img src="images/fig172.gif" width="219" height="231" alt="Fig.172. Transverse +section of the shoulder and fore-limb (wing) of a chick-embryo of the fourth +day." /> +<p class="caption">Fig. 172—<b>Transverse section of the shoulder</b> +and fore-limb (wing) of a chick-embryo of the fourth day, magnified about +twenty times. Beside the medullary tube we can see on each side three clear +streaks in the dark dorsal wall, which advance into the rudimentary fore-limb +or wing (<i>e</i>). The uppermost of them is the muscular plate; the middle is +the hind and the lowest the fore root of a spinal nerve. Under the chorda in +the middle is the single aorta, at each side of it a cardinal vein, and below +these the primitive kidneys. The gut is almost closed. The ventral wall +advances into the amnion, which encloses the embryo. (From <i> Remak.</i>)</p> +</div> + +<p> +The gill-clefts, which originally in the older acrania pierced the wall of the +fore-gut, and the gill-arches that separated them, were presumably also +segmental, and distributed among the various metamera of the chain, like the +gonads in the after-gut and the nephridia. In the amphioxus, too, they are +still segmentally formed. Probably there was a division of labour of the +hyposomites in the older (and long extinct) acrania, in such wise that those of +the fore-gut took over the function of breathing and those of the after-gut +that of reproduction. The former developed into gill-pouches, the latter into +sex-pouches. There may have been primitive kidneys in both. Though the gills +have lost their function in the higher animals, certain parts of them have been +generally maintained in the embryo by a tenacious heredity. At a very early +stage we notice in the embryo of man and the other amniotes, at each side of +the head, the remarkable and important structures which we call the gill-arches +and gill-clefts (Figs. 167–170 <i>f</i>). They belong to the +characteristic and inalienable organs of the amniote-embryo, and are found +always in the same +<span class='pagenum'><a name="Page_152" id="Page_152"></a></span> +spot and with the same arrangement and structure. There are formed to the right +and left in the lateral wall of the fore-gut cavity, in its foremost part, +first a pair and then several pairs of sac-shaped inlets, that pierce the whole +thickness of the lateral wall of the head. They are thus converted into clefts, +through which one can penetrate freely from without into the gullet. The wall +thickens between these branchial folds, and changes into an arch-like or +sickle-shaped piece—the gill, or gullet-arch. In this the muscles and +skeletal parts of the branchial gut separate; a blood-vessel arch rises +afterwards on their inner side (Fig. 98 <i>ka</i>). The number of the branchial +arches and the clefts that alternate with them is four or five on each side in +the higher vertebrates (Fig. 170 <i>d, f, f′, f″</i>). In some of +the fishes (selachii) and in the cyclostoma we find six or seven of them +permanently. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus173"></a> +<img src="images/fig173.gif" width="255" height="267" alt="Fig.173. Transverse +section of the pelvic region and hind legs of a chick-embryo of the fourth +day." /> +<p class="caption">Fig. 173—<b>Transverse section of the pelvic +region</b> and hind legs of a chick-embryo of the fourth day, magnified. +<i>h</i> horn-plate, <i>w</i> medullary tube, <i>n</i> canal of the tube, +<i>u</i> primitive kidneys, <i>x</i> chorda, <i>e</i> hind legs, <i>b</i> +allantoic canal in the ventral wall, <i>t</i> aorta, <i> v</i> cardinal veins, +<i>a</i> gut, <i>d</i> gut-gland layer, <i> f</i> gut-fibre layer, <i>g</i> +embryonic epithelium, <i>r</i> dorsal muscles, <i>c</i> body-cavity or cœloma. +(From <i> Waldeyer.</i>)</p> +</div> + +<p> +These remarkable structures had originally the function of respiratory +organs—gills. In the fishes the water that serves for breathing, and is +taken in at the mouth, still always passes out by the branchial clefts at the +sides of the gullet. In the higher vertebrates they afterwards disappear. The +branchial arches are converted partly into the jaws, partly into the bones of +the tongue and the ear. From the first gill-cleft is formed the tympanic cavity +of the ear. +</p> + +<p> +There are few parts of the vertebrate organism that, like the outer covering or +integument of the body, are not subject to metamerism. The outer skin +(<i>epidermis</i>) is unsegmented from the first, and proceeds from the +continuous horny plate. Moreover, the underlying <i>cutis</i> is also not +metamerous, although it develops from the segmental structure of the +cutis-plates (Figs. 161, 162 <i>cp</i>). The vertebrates are strikingly and +profoundly different from the articulates in these respects also. +</p> + +<p> +Further, most of the vertebrates still have a number of unarticulated organs, +which have arisen locally, by adaptation of particular parts of the body to +certain special functions. Of this character are the sense-organs in the +episoma, and the limbs, the heart, the spleen, and the large visceral +glands—lungs, liver, pancreas, etc.—in the hyposoma. The heart is +originally only a local spindle-shaped enlargement of the large ventral +blood-vessel or principal vein, at the point where the subintestinal passes +into the branchial artery, at the limit of the head and trunk (Figs. 170, 171). +The three higher sense-organs—nose, eye, and ear—were originally +developed in the same form in all the craniotes, as three pairs of small +depressions in the skin at the side of the head. +</p> + +<p> +The organ of smell, the nose, has the appearance of a pair of small pits above +the mouth-aperture, in front of the head (Fig. 169 <i>n</i>). The organ of +sight, the eye, is found at the side of the head, also in the shape of a +depression (Figs. 169 <i>l</i>, 170 <i>b</i>), to which corresponds a large +outgrowth of the foremost cerebral vesicle on each side. Farther behind, at +each side of the head, there is a third depression, the first trace of the +organ of hearing (Fig. 169 <i>g</i>). As yet we can see nothing of the later +elaborate structure of these organs, nor of the characteristic build of the +face. +</p> + +<p> +When the human embryo has reached +<span class='pagenum'><a name="Page_153" id="Page_153"></a></span> +When the human embryo has reached this stage of development, it can still +scarcely be distinguished from that of any other higher vertebrate. All the +chief parts of the body are now laid down: the head with the primitive skull, +the rudiments of the three higher sense-organs and the five cerebral vesicles, +and the gill-arches and clefts; the trunk with the spinal cord, the rudiment of +the vertebral column, the chain of metamera, the heart and chief blood-vessels, +and the kidneys. At this stage man is a higher vertebrate, but shows no +essential morphological difference from the embryos of the mammals, the birds, +the reptiles, etc. This is an ontogenetic fact of the utmost significance. From +it we can gather the most important phylogenetic conclusions. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus174"></a> +<img src="images/fig174.gif" width="449" height="401" alt="Fig.174. Development +of the lizard’s legs." /> +<p class="caption">Fig. +174—<b>Development of the lizard’s legs</b> (<i>Lacerta +agilis</i>), with special relation to their blood-vessels. <i>1, 3, 5, 7, 9, +11</i> right fore-leg; <i>13, 15</i> left fore-leg; <i>2, 4, 6, 8, 10, 12</i> +right hind-leg; <i> 14, 16</i> left hind-leg; <i>SRV</i> lateral veins of the +trunk, <i>VU</i> umbilical vein. (From <i>F. Hochstetter.</i>)</p> +</div> + +<p> +There is still no trace of the limbs. Although head and trunk are separated and +all the principal internal organs are laid down, there is no indication +whatever of the “extremities” at this stage; they are formed later +on. Here again we have a fact of the utmost interest. It proves that the older +vertebrates had no feet, as we find to be the case in the lowest living +vertebrates (amphioxus and the cyclostoma). The descendants of these ancient +footless vertebrates only acquired extremities—two fore-legs and two +hind-legs—at a much later stage of development. +<span class='pagenum'><a name="Page_154" id="Page_154"></a></span> +These were at first all alike, though they afterwards vary considerably in +structure—becoming fins (of breast and belly) in the fishes, wings and +legs in the birds, fore and hind legs in the creeping animals, arms and legs in +the apes and man. All these parts develop from the same simple original +structure, which forms secondarily from the trunk-wall (Figs. 172, 173). They +have always the appearance of two pairs of small buds, which represent at first +simple roundish knobs or plates. Gradually each of these plates becomes a large +projection, in which we can distinguish a small inner part and a broader outer +part. The latter is the rudiment of the foot or hand, the former that of the +leg or arm. The similarity of the original rudiment of the limbs in different +groups of vertebrates is very striking. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus175"></a> +<img src="images/fig175.gif" width="401" height="436" alt="Fig.175. Human embryo, five weeks +old, half an inch long, seen from the right." /> +<p class="caption">Fig. 175—<b>Human embryo,</b> five weeks +old, half an inch long, seen from the right, magnified. (From <i>Russel +Bardeen</i> and <i>Harmon Lewis.</i>) In the undissected head we see the eye, +mouth, and ear. In the trunk the skin and part of the muscles have been +removed, so that the cartilaginous vertebral column is free; the dorsal root of +a spinal nerve goes out from each vertebra (towards the skin of the back). In +the middle of the lower half of the figure part of the ribs and intercostal +muscles are visible. The skin and muscles have also been removed from the right +limbs; the internal rudiments of the five fingers of the hand, and five toes of +the foot, are clearly seen within the fin-shaped plate, and also the strong +network of nerves that goes from the spinal cord to the extremities. The tail +projects under the foot, and to the right of it is the first part of the +umbilical cord.</p> +</div> + +<p> +How the five fingers or toes with their +<span class='pagenum'><a name="Page_155" id="Page_155"></a></span> +blood-vessels gradually differentiate within the simple fin-like structure of +the limbs can be seen in the instance of the lizard in Fig. 174. They are +formed in just the same way in man: in the human embryo of five weeks the five +fingers can clearly be distinguished within the fin-plate (Fig. 175). +</p> + +<p> +The careful study and comparison of human embryos with those of other +vertebrates at this stage of development is very instructive, and reveals more +mysteries to the impartial student than all the religions in the world put +together. For instance, if we compare attentively the three successive stages +of development that are represented, in twenty different amniotes we find a +remarkable likeness. When we see that as a fact twenty different amniotes of +such divergent characters develop from the same embryonic form, we can easily +understand that they may all descend from a common ancestor. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus176"></a> +<img src="images/fig176.gif" width="466" height="210" alt="Figs. 176-178. Embryos of the bat +(Vespertilio murinus) at three different stages." /> +<p class="caption">Figs. +176–178—<b>Embryos of the bat</b> (<i>Vespertilio murinus</i>) +at three different stages. (From <i>Oscar Schultze.</i>) Fig. 176: Rudimentary +limbs (<i>v</i> fore-leg, <i> h</i> hind-leg). <i>l</i> lenticular depression, +<i>r</i> olfactory pit, <i>ok</i> upper jaw, <i>uk</i> lower jaw, <i> +k</i><sub>2</sub>, <i>k</i><sub>3</sub>, <i>k</i><sub>4</sub> first, second and +third gill-arches, <i>a</i> amnion, <i>n</i> umbilical vessel, <i>d</i> +yelk-sac. Fig. 177: Rudiment of flying membrane, membranous fold between fore +and hind leg. <i>n</i> umbilical vessel, <i>o</i> ear-opening, <i>f</i> flying +membrane. Fig. 178: The flying membrane developed and stretched across the +fingers of the hands, which cover the face.</p> +</div> + +<p> +In the first stage of development, in which the head with the five cerebral +vesicles is already clearly indicated, but there are no limbs, the embryos of +all the vertebrates, from the fish to man, are only incidentally or not at all +different from each other. In the second stage, which shows the limbs, we begin +to see differences between the embryos of the lower and higher vertebrates; but +the human embryo is still hardly distinguishable from that of the higher +mammals. In the third stage, in which the gill-arches have disappeared and the +face is formed, the differences become more pronounced. These are facts of a +significance that cannot be exaggerated.<a href="#linknote-27" name="linknoteref-27" id="linknoteref-27"><sup>[27]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-27" id="linknote-27"></a> <a href="#linknoteref-27">[27]</a> +Because they show how the most diverse structures may be developed from a +common form. As we actually see this in the case of the embryos, we have a +right to assume it in that of the stem-forms. Nevertheless, this resemblance, +however great, is never a real identity. Even the embryos of the different +individuals of one species are usually not really identical. If the reader can +consult the complete edition of this work at a library, he will find six plates +illustrating these twenty embryos. +</p> + +<p> +<span class='pagenum'><a name="Page_156" id="Page_156"></a></span> +If there is an intimate causal connection between the processes of embryology +and stem-history, as we must assume in virtue of the laws of heredity, several +important phylogenetic conclusions follow at once from these ontogenetic facts. +The profound and remarkable similarity in the embryonic development of man and +the other vertebrates can only be explained when we admit their descent from a +common ancestor. As a fact, this common descent is now accepted by all +competent scientists; they have substituted the natural evolution for the +supernatural creation of organisms. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap15"></a>Chapter XV.<br/> +FŒTAL MEMBRANES AND CIRCULATION</h2> + +<p> +Among the many interesting phenomena that we have encountered in the course of +human embryology, there is an especial importance in the fact that the +development of the human body follows from the beginning just the same lines as +that of the other viviparous mammals. As a fact, all the embryonic +peculiarities that distinguish the mammals from other animals are found also in +man; even the ovum with its distinctive membrane (<i>zona pellucida,</i> Fig. +14) shows the same typical +<span class='pagenum'><a name="Page_157" id="Page_157"></a></span> +structure in all mammals (apart from the older oviparous monotremes). It has +long since been deduced from the structure of the developed man that his +natural place in the animal kingdom is among the mammals. Linné (1735) placed +him in this class with the apes, in one and the same order (<i>primates</i>), +in his <i>Systema Naturæ.</i> This position is fully confirmed by comparative +embryology. We see that man entirely resembles the higher mammals, and most of +all the apes, in embryonic development as well as in anatomic structure. And if +we seek to understand this ontogenetic agreement in the light of the biogenetic +law, we find that it proves clearly and necessarily the descent of man from a +series of other mammals, and proximately from the primates. The common origin +of man and the other mammals from a single ancient stem-form can no longer be +questioned; nor can the immediate blood-relationship of man and the ape. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus179"></a> +<img src="images/fig179.gif" width="303" height="315" alt="Fig.179. Human embryos from the +second to the fifteenth week, seen from the left." /> +<p class="caption">Fig. 179—<b>Human embryos from +the second to the fifteenth week,</b> seen from the left, the curved back +turned towards the right. (Mostly from <i> Ecker.</i>) II of fourteen days. III +of three weeks. IV of four weeks. V of five weeks. VI of six weeks. VII of +seven weeks. VIII of eight weeks. XII of twelve weeks. XV of fifteen +weeks.</p> +</div> + +<p> +The essential agreement in the whole bodily form and inner structure is still +visible in the embryo of man and the other mammals at the late stage of +development at which the mammal-body can be recognised as such. But at a +somewhat earlier stage, in which the limbs, gill-arches, sense-organs, etc., +are already outlined, we cannot yet recognise the mammal embryos as such, or +distinguish them from those of birds and reptiles. When we consider still +earlier stages of development, we are unable to discover any essential +difference in bodily structure between the embryos of these higher vertebrates +and those of the lower, the amphibia and fishes. If, in fine, we go back to the +construction of the body out of the four germinal layers, we are astonished to +perceive that these four layers are the same in all vertebrates, and everywhere +take a similar part in the building-up of the fundamental organs of the body. +If we inquire as to the origin of these four secondary layers, we learn that +they always arise in the same way from the two primary layers; and the latter +have the same significance in all the metazoa (<i>i.e.,</i> all animals except +the unicellulars). Finally, we see that the cells which make up the primary +germinal layers owe their origin in every case to the repeated cleavage of a +single simple cell, the stem-cell or fertilised ovum. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus180"></a> +<img src="images/fig180.gif" width="251" height="267" alt="Fig.180. Very young +human embryo of the fourth week, one-fourth of an inch long." /> +<p class="caption">Fig. 180—<b>Very young human embryo of the fourth +week,</b> one-fourth of an inch long (taken from the womb of a suicide eight +hours after death). (From <i>Rabl.</i>) <i>n</i> nasal pits, <i> a</i> eye, +<i>u</i> lower jaw, <i>z</i> arch of hyoid bone, <i> k<sub>3</sub></i> and +<i>k<sub>4</sub></i> third and fourth gill-arch, <i>h</i> heart; <i>s</i> +primitive segments, <i>vg</i> fore-limb (arm), <i>hg</i> hind-limb (leg), +between the two the ventral pedicle.</p> +</div> + +<p> +It is impossible to lay too much stress on this remarkable agreement in the +chief embryonic features in man and the other animals. We shall make use of it +later on for our monophyletic theory of descent—the hypothesis of a +common descent of man and all the metazoa from the gastræa. The first rudiments +of the principal parts of the body, especially the oldest organ, the alimentary +canal, are the same everywhere; they have always the same extremely simple +form. All the peculiarities that distinguish the various groups of animals from +each other only appear gradually in the course of embryonic development; and +the closer the relation of the various groups, the later they are found. We may +formulate this phenomenon in a definite law, which may in a sense be regarded +as an appendix to our biogenetic law. This is the law of the ontogenetic +connection of related animal forms. It runs: The closer the +<span class='pagenum'><a name="Page_158" id="Page_158"></a></span> +relation of two fully-developed animals in respect of their whole bodily +structure, and the nearer they are connected in the classification of the +animal kingdom, the longer do their embryonic forms retain their identity, and +the longer is it impossible (or only possible on the ground of subordinate +features) to distinguish between their embryos. This law applies to all animals +whose embryonic development is, in the main, an hereditary summary of their +ancestral history, or in which the original form of development has been +faithfully preserved by heredity. When, on the other hand, it has been altered +by cenogenesis, or disturbance of development, we find a limitation of the law, +which increases in proportion to the introduction of new features by adaptation +(cf. Chapter I, pp. 4–6). Thus the apparent exceptions to the law can +always be traced to cenogenesis. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus181"></a> +<img src="images/fig181.gif" width="255" height="255" alt="Fig.181. Human embryo +of the middle of the fifth week, one-third of an inch long." /> +<p class="caption">Fig. 181—<b>Human embryo of the middle of the +fifth week,</b> one-third of an inch long. (From <i>Rabl.</i>) Letters as in +Fig. 180, except <i>sk</i> curve of skull, <i>ok</i> upper jaw, <i> hb</i> +neck-indentation.</p> +</div> + +<p> +When we apply to man this law of the ontogenetic connection of related forms, +and run rapidly over the earliest stages of human development with an eye to +it, we notice first of all the structural identity of the ovum in man and the +other mammals at the very beginning (Figs. 1, 14). The human ovum possesses all +the distinctive features of the ovum of the viviparous mammals, especially the +characteristic formation of its membrane (<i>zona pellucida</i>), which clearly +distinguishes it from the ovum of all other animals. When the human fœtus has +attained the age of fourteen days, it forms a round vesicle (or +“embryonic vesicle”) about a quarter of an inch in diameter. A +thicker part of its border forms a simple sole-shaped embryonic shield +one-twelfth of an inch long (Fig. 133). On its dorsal side we find in the +middle line the straight medullary furrow, bordered by the two parallel dorsal +or medullary swellings. Behind, it passes by the neurenteric canal into the +primitive gut or primitive groove. From this the folding of the two +cœlom-pouches proceeds in the same way as in the other mammals (cf. Fig. 96, +97). In the middle of the sole-shaped embryonic shield the first primitive +segments immediately begin to make their appearance. At this age the human +embryo cannot be distinguished from that of other mammals, such as the hare or +dog. +</p> + +<p> +A week later (or after the twenty-first day) the human embryo has doubled its +length; it is now about one-fifth of an inch long, and, when seen from the +side, shows the characteristic bend of the back, the swelling of the head-end, +the first outline of the three higher sense-organs, and the rudiments of the +gill-clefts, which pierce the sides of the neck (Fig. 179, III). The allantois +has grown out of the gut behind. The embryo is already entirely enclosed in the +amnion, and is only connected in the middle of the belly by the vitelline duct +with the embryonic vesicle, which changes into the yelk-sac. There are no +extremities or limbs at this stage, no trace of arms or legs. The head-end has +been strongly differentiated from the tail-end; and the first outlines of the +cerebral vesicles in front, and the heart below, under the fore-arm, are +already more or less clearly seen. There is as yet no real face. Moreover, we +seek in vain at this stage a special character that may distinguish the human +embryo from that of other mammals. +</p> + +<p> +A week later (after the fourth week, on the twenty-eighth to thirtieth day of +development) the human embryo has +<span class='pagenum'><a name="Page_159" id="Page_159"></a></span> +reached a length of about one-third of an inch (Fig 179 IV). We can now clearly +distinguish the head with its various parts; inside it the five primitive +cerebral vesicles (fore-brain, middle-brain, intermediate-brain, hind-brain, +and after-brain); under the head the gill-arches, which divide the gill-clefts; +at the sides of the head the rudiments of the eyes, a couple of pits in the +outer skin, with a pair of corresponding simple vesicles growing out of the +lateral wall of the fore-brain (Figs. 180, 181 <i>a</i>). Far behind the eyes, +over the last gill-arches, we see a vesicular rudiment of the auscultory organ. +The rudimentary limbs are now clearly outlined—four simple buds of the +shape of round plates, a pair of fore (<i>vg</i>) and a pair of hind legs +(<i>hg</i>), the former a little larger than the latter. The large head bends +over the trunk, almost at a right angle. The latter is still connected in the +middle of its ventral side with the embryonic vesicle; but the embryo has still +further severed itself from it, so that it already hangs out as the yelk-sac. +The hind part of the body is also very much curved, so that the pointed +tail-end is directed towards the head. The head and face-part are sunk entirely +on the still open breast. The bend soon increases so much that the tail almost +touches the forehead (Fig. 179 V.; Fig. 181). We may then distinguish three or +four special curves on the round dorsal surface—namely, a skull-curve in +the region of the second cerebral vesicle, a neck-curve at the beginning of the +spinal cord, and a tail-curve at the fore-end. This pronounced curve is only +shared by man and +<span class='pagenum'><a name="Page_160" id="Page_160"></a></span> +the higher classes of vertebrates (the amniotes); it is much slighter, or not +found at all, in the lower vertebrates. At this age (four weeks) man has a +considerable tail, twice as long as his legs. A vertical longitudinal section +through the middle plane of this tail (Fig. 182) shows that the hinder end of +the spinal marrow extends to the point of the tail, as also does the underlying +chorda (<i>ch</i>), the terminal continuation of the vertebral column. Of the +latter, the rudiments of the seven coccygeal (or lowest) vertebræ are +visible—thirty-two indicates the third and thirty-six the seventh of +these. Under the vertebral column we see the hindmost ends of the two large +blood-vessels of the tail, the principal artery (<i>aorta caudalis</i> or +<i>arteria sacralis media, Ao</i>), and the principal vein (<i>vena +caudalis</i> or <i>sacralis media</i>). Underneath is the opening of the anus +(<i>an</i>) and the urogenital sinus (<i>S.ug</i>). From this anatomic +structure of the human tail it is perfectly clear that it is the rudiment of an +ape-tail, the last hereditary relic of a long hairy tail, which has been handed +down from our tertiary primate ancestors to the present day. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus182"></a> +<img src="images/fig182.gif" width="284" height="308" alt="Fig.182. Median longitudinal section +of the tail of a human embryo, two-thirds of an inch long." /> +<p class="caption">Fig. 182—<b>Median +longitudinal section of the tail of a human embryo,</b> two-thirds of an inch +long. (From <i>Ross Granville Harrison.</i>) <i>Med</i> medullary tube, +<i>Ca.fil</i> caudal filament, <i>ch</i> chorda, <i>ao</i> caudal artery, +<i>V.c.i</i> caudal vein, <i>an</i> anus, <i>S.ug</i> sinus urogenitalis.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus183"></a> +<img src="images/fig183.gif" width="186" height="308" alt="Fig.183. Human embryo, +four weeks old, opened on the ventral side." /> +<p class="caption">Fig. 183—<b>Human embryo, four weeks old,</b> opened +on the ventral side. Ventral and dorsal walls are cut away, so as to show the +contents of the pectoral and abdominal cavities. All the appendages are also +removed (amnion, allantois, yelk-sac), and the middle part of the gut. <i>n</i> +eye, <i>3</i> nose, <i>4</i> upper jaw, <i>5</i> lower jaw, <i>6</i> second, +<i>6″</i> third gill-arch, <i>ov</i> heart (<i>o</i> right, +<i>o′</i> left auricle; <i>v</i> right, <i>v′</i> left +ventricle), <i>b</i> origin of the aorta, <i>f</i> liver (<i>u</i> umbilical +vein), <i>e</i> gut (with vitelline artery, cut off at <i>a′</i>), +<i>j′</i> vitelline vein, <i>m</i> primitive kidneys, <i>t</i> +rudimentary sexual glands, <i> r</i> terminal gut (cut off at the mesentery +<i>z</i>), <i>n</i> umbilical artery, <i>u</i> umbilical vein, <i>9</i> +fore-leg, <i> 9′</i> hind-leg. (From <i>Coste.</i>)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus184"></a> +<img src="images/fig184.gif" width="186" height="387" alt="Human embryo, five weeks old, opened from the +ventral side." /> +<p class="caption">Fig. 184—<b>Human embryo, five weeks old,</b> opened +from the ventral side (as in Fig. 183). Breast and belly-wall and liver are +removed. <i>3</i> outer nasal process, <i>4</i> upper jaw, <i>5</i> lower jaw, +<i>z</i> tongue, <i>v</i> right, <i>v′</i> left ventricle of heart, +<i>o′</i> left auricle, <i>b</i> origin of aorta, <i>b′, +b″, b‴</i> first, second, and third aorta-arches, <i>c, +c′, c″</i> vena cava, <i>ae</i> lungs (<i>y</i> pulmonary +artery), <i>e</i> stomach, <i>m</i> primitive kidneys (<i>j</i> left vitelline +vein, <i>s</i> cystic vein, <i>a</i> right vitelline artery, <i>n</i> umbilical +artery, <i>u</i> umbilical vein), <i> x</i> vitelline duct, <i>i</i> rectum, +<i>8</i> tail, <i>9</i> fore-leg, <i>9′</i> hind-leg. (From +<i>Coste.</i>)</p> +</div> + +<p> +It sometimes happens that we find even external relics of this tail growing. +According to the illustrated works of +<span class='pagenum'><a name="Page_161" id="Page_161"></a></span> +Surgeon-General Bernhard Ornstein, of Greece, these tailed men are not +uncommon; it is not impossible that they gave rise to the ancient fables of the +satyrs. A great number of such cases are given by Max Bartels in his essay on +“Tailed Men” (1884, in the <i>Archiv für Anthropologie,</i> Band +XV), and critically examined. These atavistic human tails are often mobile; +sometimes they contain only muscles and fat, sometimes also rudiments of caudal +vertebræ. They have a length of eight to ten inches and more. Granville +Harrison has very carefully studied one of these cases of +“pigtail,” which he removed by operation from a six months old +child in 1901. The tail moved briskly when the child cried or was excited, and +was drawn up when at rest. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus185"></a> +<img src="images/fig185.gif" width="159" height="137" alt="Fig.185. The head of Miss Julia Pastrana." /> +<p class="caption">Fig. 185—<b>The head of Miss Julia Pastrana.</b> +(From a photograph by <i>Hintze.</i>)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus186"></a> +<img src="images/fig186.gif" width="150" height="165" alt="Human ovum of twelve to thirteen days." /> +<p class="caption">Fig. 186—<b>Human ovum of twelve to thirteen days +(?).</b> (From <i>Allen Thomson.</i>) 1. Not opened. 2. Opened and magnified. +Within the outer chorion the tiny curved fœtus lies on the large embryonic +vesicle, to the left above.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus187"></a> +<img src="images/fig187.gif" width="192" height="165" alt="Fig.187. Human ovum of +ten days. Fig. 188. Human foetus of ten days, taken from the preceding ovum, magnified." /> +<p class="caption">Fig. 187—<b>Human ovum of ten days.</b> (From +<i>Allen Thomson.</i>) Opened; the small fœtus in the right half, above.<br/> +<a name="illus188"></a>Fig. 188—<b>Human fœtus of ten days,</b> taken +from the preceding ovum, magnified, <i>a</i> yelk-sac, <i>b</i> neck (the +medullary groove already closed), <i> c</i> head (with open medullary groove), +<i>d</i> hind part (with open medullary groove), <i>e</i> a shred of the +amnion.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus189"></a> +<img src="images/fig189.gif" width="192" height="292" alt="Fig.189. Human ovum of +twenty to twenty-two days. Fig. 190. Human foetus of twenty to twenty-two days, +taken from the preceding ovum, magnified." /> +<p class="caption">Fig. 189—<b>Human ovum</b> of twenty to twenty-two +days. (From <i>Allen Thomson.</i>) Opened. The chorion forms a spacious +vesicle, to the inner wall of which the small fœtus (to the right above) is +attached by a short umbilical cord.<br/> +<a name="illus190"></a>Fig. 190—<b>Human fœtus</b> of twenty to +twenty-two days, taken from the preceding ovum, magnified. <i>a</i> amnion, +<i>b</i> yelk-sac, <i>c</i> lower-jaw process of the first gill-arch, <i>d</i> +upper-jaw process of same, <i>e</i> second gill-arch (two smaller ones behind). +Three gill-clefts are clearly seen. <i>f</i> rudimentary fore-leg, <i> g</i> +auditory vesicle, <i>h</i> eye, <i>i</i> heart.</p> +</div> + +<p> +In the opinion of some travellers and anthropologists, the atavistic +tail-formation is hereditary in certain isolated tribes (especially in +south-eastern Asia and the archipelago), so that we might speak of a special +race or “species” of tailed men +<span class='pagenum'><a name="Page_162" id="Page_162"></a></span> +(<i>Homo caudatus</i>). Bartels has “no doubt that these tailed men will +be discovered in the advance of our geographical and ethnographical knowledge +of the lands in question” (<i>Archiv für Anthropologie,</i> Band XV, p. +129). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus191"></a> +<img src="images/fig191.gif" width="371" height="384" alt="Fig.191. Human embryo of sixteen to +eighteen days." /> +<p class="caption">Fig. +191—<b>Human embryo of sixteen to eighteen days.</b> (From +<i>Coste.</i>) Magnified. The embryo is surrounded by the amnion, (<i>a</i>), +and lies free with this in the opened embryonic vesicle. The belly is drawn up +by the large yelk-sac (<i>d</i>), and fastened to the inner wall of the +embryonic membrane by the short and thick pedicle (<i>b</i>). Hence the normal +convex curve of the back (Fig. 190) is here changed into an abnormal concave +surface. <i>h</i> heart, <i> m</i> parietal mesoderm. The spots on the outer +wall of the serolemma are the roots of the branching chorion-villi, which are +free at the border.</p> +</div> + +<p> +When we open a human embryo of one month (Fig. 183), we find the alimentary +canal formed in the body-cavity, and for the most part cut off from the +embryonic vesicle. There are both mouth and anus apertures. But the +mouth-cavity is not yet separated from the nasal cavity, and the face not yet +shaped. The heart shows all its four sections; it is very large, and almost +fills the whole of the pectoral cavity (Fig. 183 <i>ov</i>). Behind it are the +very small rudimentary lungs. The primitive kidneys (<i>m</i>) are very large; +they fill the greater part of the abdominal cavity, and extend from the liver +(<i>f</i>) to the pelvic gut. Thus at the end of the first month all the chief +organs are already outlined. But there are at this stage no features by which +the human embryo materially differs from that of the dog, the hare, the ox, or +the horse—in a word, of any other higher mammal. All these embryos have +the same, or at least a very similar, form; they can at the most be +<span class='pagenum'><a name="Page_163" id="Page_163"></a></span> +distinguished from the human embryo by the total size of the body or some other +insignificant difference in size. Thus, for instance, in man the head is larger +in proportion to the trunk than in the ox. The tail is rather longer in the dog +than in man. These are all negligible differences. On the other hand, the whole +internal organisation and the form and arrangement of the various organs are +essentially the same in the human embryo of four weeks as in the embryos of the +other mammals at corresponding stages. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus192"></a> +<img src="images/fig192.gif" width="367" height="238" alt="Fig.192. Human embryo of the fourth +week, one-third of an inch long, lying in the dissected chorion." /> +<p class="caption">Fig. 192—<b>Human embryo</b> of the fourth week, +one-third of an inch long, lying in the dissected chorion.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus193"></a> +<img src="images/fig193.gif" width="219" height="239" alt="Fig.193. Human embryo +of the fourth week, with its membranes, like Fig. 192, but a little older." /> +<p class="caption">Fig. 193—<b>Human embryo</b> of the fourth week, +with its membranes, like Fig. 192, but a little older. The yelk-sac is rather +smaller, the amnion and chorion larger.</p> +</div> + +<p> +It is otherwise in the second month of human development. Fig. 179 represents a +human embryo of six weeks (VI), one of seven weeks (VII), and one of eight +weeks (VIII), at natural size. The differences which mark off the human embryo +from that of the dog and the lower mammals now begin to be more pronounced. We +can see important differences at the sixth, and still more at the eighth week, +especially in the formation of the head. The size of the various sections of +the brain is greater in man, and the tail is shorter. +</p> + +<p> +Other differences between man and the lower mammals are found in the relative +size of the internal organs. But even at this stage the human embryo differs +very little from that of the nearest related mammals—the apes, especially +the anthropomorphic apes. +</p> + +<p> +The features by means of which we distinguish between them are not clear until +later on. Even at a much more advanced stage of development, when we can +distinguish the human fœtus from that of the ungulates at a glance, it still +closely resembles that of the higher apes. At last we get the distinctive +features, and +<span class='pagenum'><a name="Page_164" id="Page_164"></a></span> +we can distinguish the human embryo confidently at the first glance from that +of all other mammals during the last four months of fœtal life—from the +sixth to the ninth month of pregnancy. Then we begin to find also the +differences between the various races of men, especially in regard to the +formation of the skull and the face. (Cf. Chapter XXIII.) +</p> + +<div class="fig" style="width:100%;"> +<a name="illus194"></a> +<img src="images/fig194.gif" width="270" height="238" alt="Fig.194. Human embryo +with its membranes, six weeks old." /> +<p class="caption">Fig. 194—<b>Human embryo with its membranes,</b> six +weeks old. The outer envelope of the whole ovum is the chorion, thickly covered +with its branching villi, a product of the serous membrane. The embryo is +enclosed in the delicate amnion-sac. The yelk-sac is reduced to a small +pear-shaped umbilical vesicle; its thin pedicle, the long vitelline duct, is +enclosed in the umbilical cord. In the latter, behind the vitelline duct, is +the much shorter pedicle of the allantois, the inner lamina of which (the +gut-gland layer) forms a large vesicle in most of the mammals, while the outer +lamina is attached to the inner wall of the outer embryonic coat, and forms the +placenta there. (Half diagrammatic.)</p> +</div> + +<p> +The striking resemblance that persists so long between the embryo of man and of +the higher apes disappears much earlier in the lower apes. It naturally remains +longest in the large anthropomorphic apes (gorilla, chimpanzee, orang, and +gibbon). The physiognomic similarity of these animals, which we find so great +in their earlier years, lessens with the increase of age. On the other hand, it +remains throughout life in the remarkable long-nosed ape of Borneo (<i>Nasalis +larvatus</i>). Its finely-shaped nose would be regarded with envy by many a man +who has too little of that organ. If we compare the face of the long-nosed ape +with that of abnormally ape-like human beings (such as the famous Miss Julia +Pastrana, Fig. 185), it will be admitted to represent a higher stage of +development. There are still people among us who look especially to the face +for the “image of God in man.” The long-nosed ape would have more +claim to this than some of the stumpy-nosed human individuals one meets. +</p> + +<p> +This progressive divergence of the human from the animal form, which is based +on the law of the ontogenetic connection between related forms, is found in the +structure of the internal organs as well as in external form. It is also +expressed in the construction of the envelopes and appendages that we find +surrounding the fœtus externally, and that we will now consider more closely. +Two of these appendages—the amnion and the allantois—are only found +in the three higher classes of vertebrates, while the third, the yelk-sac, is +found in most of the vertebrates. This is a circumstance of great importance, +and it gives us valuable data for constructing man’s genealogical tree. +</p> + +<p> +As regards the external membrane that encloses the ovum in the mammal womb, +<span class='pagenum'><a name="Page_165" id="Page_165"></a></span> +we find it just the same in man as in the higher mammals. The ovum is, the +reader will remember, first surrounded by the transparent structureless +<i>ovolemma</i> or <i>zona pellucida</i> (Figs. 1, 14). But very soon, even in +the first week of development, this is replaced by the permanent chorion. This +is formed from the external layer of the amnion, the <i>serolemma,</i> or +“serous membrane,” the formation of which we shall consider +presently; it surrounds the fœtus and its appendages as a broad, completely +closed sac; the space between the two, filled with clear watery fluid, is the +<i>serocœlom,</i> or interamniotic cavity (“extra-embryonic +body-cavity”). But the smooth surface of the sac is quickly covered with +numbers of tiny tufts, which are really hollow outgrowths like the fingers of a +glove (Figs. 186, 191, 198 <i>chz</i>). They ramify and push into the +corresponding depressions that are formed by the tubular glands of the mucous +membrane of the maternal womb. Thus, the ovum secures its permanent seat (Fig. +186–194). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus195"></a> +<img src="images/fig195.gif" width="232" height="228" alt="Fig.195. Diagram of the +embryonic organs of the mammal (foetal membranes and appendages)." /> +<p class="caption">Fig. 195—<b>Diagram of the embryonic organs of the +mammal</b> (fœtal membranes and appendages). (From <i>Turner.</i>) <i>E, M, +H</i> outer, middle, and inner germ layer of the embryonic shield, which is +figured in median longitudinal section, seen from the left. <i>am</i> amnion. +<i>AC</i> amniotic cavity, <i>UV</i> yelk-sac or umbilical vesicle, <i>ALC</i> +allantois, <i>al</i> pericœlom or serocœlom (inter-amniotic cavity), <i> sz</i> +serolemma (or serous membrane), <i>pc</i> prochorion (with villi).)</p> +</div> + +<p> +In human ova of eight to twelve days this external membrane, the chorion, is +already covered with small tufts or villi, and forms a ball or spheroid of +one-fourth to one-third of an inch in diameter (Figs. 186–188). As a +large quantity of fluid gathers inside it, the chorion expands more and more, +so that the embryo only occupies a small part of the space within the vesicle. +The villi of the chorion grow larger and more numerous. They branch out more +and more. At first the villi cover the whole surface, but they afterwards +disappear from the greater part of it; they then develop with proportionately +greater vigour at a spot where the placenta is formed from the allantois. +</p> + +<p> +When we open the chorion of a human embryo of three weeks, we find on the +ventral side of the fœtus a large round sac, filled with fluid. This is the +yelk-sac, or “umbilical vesicle,” the origin of which we have +considered previously. The larger the embryo becomes the smaller we find the +yelk-sac. In the end we find the remainder of it in the shape of a small +pear-shaped vesicle, fastened to a long thin stalk (or pedicle), and hanging +from the open belly of the fœtus (Fig. 194). This pedicle is the vitelline +duct, and is separated from the body at the closing of the navel. +</p> + +<p> +Behind the yelk-sac a second appendage, +<span class='pagenum'><a name="Page_166" id="Page_166"></a></span> +of much greater importance, is formed at an early stage at the belly of the +mammal embryo. This is the allantois or “primitive urinary sac,” an +important embryonic organ, only found in the three higher classes of +vertebrates. In all the amniotes the allantois quickly appears at the hinder +end of the alimentary canal, growing out of the cavity of the pelvic gut (Fig. +147 <i>r, u,</i> Fig. 195 <i> ALC</i>). +</p> + +<p> +The further development of the allantois varies considerably in the three +sub-classes of the mammals. The two lower sub-classes, monotremes and +marsupials, retain the simpler structure of their ancestors, the reptiles. The +wall of the allantois and the enveloping serolemma remains smooth and without +villi, as in the birds. But in the third sub-class of the mammals the serolemma +forms, by invagination at its outer surface, a number of hollow tufts or villi, +from which it takes the name of the <i>chorion</i> or <i>mallochorion.</i> The +gut-fibre layer of the allantois, richly supplied with branches of the +umbilical vessel, presses into these tufts of the primary chorion, and forms +the “secondary chorion.” Its embryonic blood-vessels are closely +correlated to the contiguous maternal blood-vessels of the environing womb, and +thus is formed the important nutritive apparatus of the embryo which we call +the placenta. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus196"></a> +<img src="images/fig196.gif" width="217" height="198" alt="Fig.196. Diagrammatic +frontal section of the pregnant human womb." /> +<p class="caption">Fig. 196—<b>Diagrammatic frontal section of the +pregnant human womb.</b> (From <i>Longet.</i>) The embryo hangs by the +umbilical cord, which encloses the pedicle of the allantois (<i>al</i>). <i> +nb</i> umbilical vessel, <i>am</i> amnion, <i>ch</i> chorion, <i> ds</i> +decidua serotina, <i>dv</i> decidua vera, <i>dr</i> decidua reflexa, <i>z</i> +villi of the placenta, <i>c</i> cervix uteri, <i> u</i> uterus.)</p> +</div> + +<p> +The pedicle of the allantois, which connects the embryo with the placenta and +conducts the strong umbilical vessels from the former to the latter, is covered +by the amnion, and, with this amniotic sheath and the pedicle of the yelk-sac, +forms what is called the <i>umbilical cord</i> (Fig. 196 <i>al</i>). As the +large and blood-filled vascular network of the fœtal allantois attaches itself +closely to the mucous lining of the maternal womb, and the partition between +the blood-vessels of mother and child becomes much thinner, we get that +remarkable nutritive apparatus of the fœtal body which is characteristic of the +placentalia (or choriata). We shall return afterwards to the closer +consideration of this (cf. Chapter XXIII). +</p> + +<p> +In the various orders of mammals the placenta undergoes many modifications, and +these are in part of great evolutionary importance and useful in +classification. There is only one of these that need be specially +mentioned—the important fact, established by Selenka in 1890, that the +distinctive human placentation is confined to the anthropoids. In this most +advanced group of the mammals the allantois is very small, soon loses its +cavity, and then, in common with the amnion, undergoes certain peculiar +changes. The umbilical cord develops in this case from what is called the +“ventral pedicle.” Until very recently this was regarded as a +structure peculiar to man. We now know from Selenka that the much-discussed +ventral pedicle is merely the pedicle of the allantois, combined with the +pedicle of the amnion and the rudimentary pedicle of the yelk-sac. It has just +the same structure in the orang and gibbon (Fig. 197) and very probably in the +chimpanzee and gorilla, as in man; it is, therefore, not a <i>disproof,</i> but +a striking fresh proof, of the blood-relationship of man and the anthropoid +apes. +</p> + +<p> +We find only in the anthropoid apes—the gibbon and orang of Asia and the +chimpanzee and gorilla of Africa—the peculiar and elaborate formation of +the placenta that characterises man (Fig. 198). +<span class='pagenum'><a name="Page_167" id="Page_167"></a></span> +In this case there is at an early stage an intimate blending of the chorion of +the embryo and the part of the mucous lining of the womb to which it attaches. +The villi of the chorion with the blood-vessels they contain grow so completely +into the tissue of the uterus, which is rich in blood, that it becomes +impossible to separate them, and they form together a sort of cake. This comes +away as the “afterbirth” at parturition; at the same time, the part +of the mucous lining of the womb that has united inseparably with the chorion +is torn away; hence it is called the <i>decidua</i> (“falling-away +membrane”), and also the “sieve-membrane,” because it is +perforated like a sieve. We find a decidua of this kind in most of the higher +placentals; but it is only in man and the anthropoid apes that it divides into +three parts—the outer, inner, and placental decidua. The external or true +decidua (Fig. 196 <i>du,</i> Fig. 199 <i>g</i>) is the part of the mucous +lining of the womb that clothes the inner surface of the uterine cavity +wherever it is not connected with the placenta. The placental or spongy decidua +(<i>placentalis</i> or <i>serotina,</i> Fig. 196 <i>ds,</i> Fig. 199 <i>d</i>) +is really the placenta itself, or the maternal part of it (<i>placenta +uterina</i>)—namely, that part of the mucous lining of the womb which +unites intimately with the chorion-villi of the fœtal placenta. The internal or +false decidua (<i>interna</i> or <i>reflexa,</i> Fig. 196 <i>dr,</i> Fig. 199 +<i>f</i>) is that part of the mucous lining of the womb which encloses the +remaining surface of the ovum, the smooth chorion (<i>chorion læve</i>), in the +shape of a special thin membrane. The origin of these three different deciduous +membranes, in regard to which quite erroneous views (still retained in their +names) formerly prevailed, is now quite clear, The external <i> decidua +vera</i> is the specially modified and subsequently detachable superficial +stratum of the original mucous lining of the womb. The placental <i>decidua +serotina</i> is that part of the preceding which is completely transformed by +the ingrowth of the chorion-villi, and is used for constructing the placenta. +The inner <i>decidua reflexa</i> is formed by the rise of a circular fold of +the mucous lining (at the border of the <i>decidua vera</i> and <i> +serotina</i>), which grows over the fœtus (like the anmnion) to the end. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus197"></a> +<img src="images/fig197.gif" width="381" height="253" alt="Fig.197. Male embryo of the +Siamang-gibbon (Hylobates siamanga) of Sumatra." /> +<p class="caption">Fig. 197—<b>Male embryo of the +Siamang-gibbon</b> (<i>Hylobates siamanga</i>) of Sumatra; to the left the +dissected uterus, of which only the dorsal half is given. The embryo has been +taken out, and the limbs folded together; it is still connected by the +umbilical cord with the centre of the circular placenta which is attached to +the inside of the womb. This embryo takes the head-position in the womb, and +this is normal in man also.</p> +</div> + +<p> +The peculiar anatomic features that characterise the human fœtal membranes are +found in just the same way in the higher +<span class='pagenum'><a name="Page_168" id="Page_168"></a></span> +apes. Until recently it was thought that the human embryo was distinguished by +its peculiar construction of a solid allantois and a special ventral pedicle, +and that the umbilical cord developed from this in a different way than in the +other mammals. The opponents of the unwelcome “ape-theory” laid +great stress on this, and thought they had at last discovered an important +indication that separated man from all the other placentals. But the remarkable +discoveries published by the distinguished zoologist Selenka in 1890 proved +that man shares these peculiarities of placentation with the anthropoid apes, +though they are not found in the other apes. Thus the very feature which was +advanced by our critics as a disproof became a most important piece of evidence +in favour of our pithecoid origin.) +</p> + +<div class="fig" style="width:100%;"> +<a name="illus198"></a> +<img src="images/fig198.gif" width="395" height="347" alt="Fig.198. Frontal +section of the pregnant human womb." /> +<p class="caption">Fig. 198—<b>Frontal section of the pregnant human womb.</b> +(From <i>Turner.</i>) The embryo (a month old) hangs in the middle of the +amniotic cavity by the ventral pedicle or umbilical cord, which connects it +with the placenta (above).</p> +</div> + +<p> +Of the three vesicular appendages of the amniote embryo which we have now +described the amnion has no blood-vessels at any moment of its existence. But +the other two vesicles, the yelk-sac and the allantois, are equipped with large +blood-vessels, and these effect the nourishment of the embryonic body. We may +take the opportunity to make a few general observations on the first +circulation in the embryo and its central organ, the heart. The first +blood-vessels, the heart, and the first blood itself, are formed from the +gut-fibre layer. Hence it was called by earlier embryologists the +“vascular layer.” In a sense the term is quite correct. But it must +not be understood as if all the blood-vessels in the body came from this layer, +or as if the whole of this layer were taken up only with the formation of +blood-vessels. Neither of these suppositions is true. Blood-vessels may be +formed independently in other parts, especially in the various products of the +skin-fibre layer. +<span class='pagenum'><a name="Page_169" id="Page_169"></a></span> +</p> + +<div class="fig" style="width:100%;"> +<a name="illus199"></a> +<img src="images/fig199.gif" width="241" height="276" alt="Fig.199. Human foetus, twelve weeks +old, with its membranes." /> +<p class="caption">Fig. 199—<b>Human fœtus, twelve weeks old, with its +membranes.</b> The umbilical cord goes from its navel to the placenta. <i>b</i> +amnion, <i>c</i> chorion, <i>d</i> placenta, <i>d</i> apostrophe, relics of +villi on smooth chorion, <i>f</i> internal or reflex decidua, <i>g</i> external +or true decidua. (From <i>B. Schultze.</i>)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus200"></a> +<img src="images/fig200.gif" width="281" height="195" alt="Fig.200. Mature human foetus +(at the end of the pregnancy, in its natural position, taken out of the uterine +cavity)." /> +<p class="caption">Fig. +200—<b>Mature human fœtus</b> (at the end of pregnancy, in its +natural position, taken out of the uterine cavity). On the inner surface of the +latter (to the left) is the placenta, which is connected by the umbilical cord +with the child’s navel. (From <i>Bernhard Schultze.</i>)</p> +</div> + +<p> +<span class='pagenum'><a name="Page_170" id="Page_170"></a></span> +The first blood-vessels of the mammal embryo have been considered by us +previously, and we shall study the development of the heart in the second +volume. +</p> + +<p> +In every vertebrate it lies at first in the ventral wall of the fore-gut, or in +the ventral (or cardiac) mesentery, by which it is connected for a time with +the wall of the body. But it soon severs itself from the place of its origin, +and lies freely in a cavity—the cardiac cavity. For a short time it is +still connected with the former by the thin plate of the mesocardium. +Afterwards it lies quite free in the cardiac cavity, and is only directly +connected with the gut-wall by the vessels which issue from it. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus201"></a> +<img src="images/fig201.gif" width="288" height="283" alt="Fig.201. Vitelline vessels in the +germinative area of a chick-embryo, at the close of the third day of +incubation." /> +<p class="caption">Fig. +201—<b>Vitelline vessels in the germinative area of a +chick-embryo,</b> at the close of the third day of incubation. (From +<i>Balfour.</i>) The detached germinative area is seen from the ventral side: +the arteries are dark, the veins light. <i>H</i> heart, <i>AA</i> aorta-arches, +<i>Ao</i> aorta, <i>R.of.A</i> right omphalo-mesenteric artery, <i>S.T.</i> +sinus terminalis, <i> L.Of</i> and <i>R.Of</i> right and left +omphalo-mesenteric veins, <i>S.V.</i> sinus venosus, <i>D.C.</i> ductus +Cuvieri, <i> S.Ca.V.</i> and <i>V.Ca.</i> fore and hind cardinal veins.</p> +</div> + +<p> +The fore-end of the spindle-shaped tube, which soon bends into an S-shape +(Figure 1.202), divides into a right and left branch. These tubes are bent +upwards arch-wise, and represent the first arches of the aorta. They rise in +the wall of the fore-gut, which they enclose in a sense, and then unite above, +in the upper wall of the fore gut-cavity, to form a large single artery, that +runs backward immediately under the chorda, and is called the aorta (Fig. 201 +<i>Ao</i>). The first pair of aorta-arches rise on the inner wall of the first +pair of gill-arches, and so lie between the first gill-arch (<i>k</i>) and the +fore-gut (<i>d</i>), just as we find them throughout life in the fishes. The +single aorta, which results from the conjunction of these two first vascular +arches, divides again immediately into two parallel branches, which run +backwards on either side of the chorda. These are the primitive aortas which we +have already mentioned; they are also called the posterior vertebral arteries. +These two arteries now give off at each side, behind, at right angles, four or +five branches, and these pass from the embryonic body to the germinative area, +they +<span class='pagenum'><a name="Page_171" id="Page_171"></a></span> +are called omphalo-mesenteric or vitelline arteries. They represent the first +beginning of a fœtal circulation. Thus, the first blood-vessels pass over the +embryonic body and reach as far as the edge of the germinative area. At first +they are confined to the dark or “vascular” area. But they +afterwards extend over the whole surface of the embryonic vesicle. In the end, +the whole of the yelk-sac is covered with a vascular net-work. These vessels +have to gather food from the contents of the yelk-sac and convey it to the +embryonic body. This is done by the veins, which pass first from the +germinative area, and afterwards from the yelk-sac, to the farther end of the +heart. They are called vitelline, or, frequently, omphalo-mesenteric, veins. +</p> + +<p> +These vessels naturally atrophy with the degeneration of the umbilical vesicle, +and the vitelline circulation is replaced by a second, that of the allantois. +Large blood-vessels are developed in the wall of the urinary sac or the +allantois, as before, from the gut-fibre layer. These vessels grow larger and +larger, and are very closely connected with the vessels that develop in the +body of the embryo itself. Thus, the secondary, allantoic circulation gradually +takes the place of the original vitelline circulation. When the allantois has +attached itself to the inner wall of the chorion and been converted into the +placenta, its blood-vessels alone effect the nourishment of the embryo. They +are called umbilical vessels, and are originally double—a pair of +umbilical arteries and a pair of umbilical veins. The two umbilical veins (Fig. +183 <i>u</i>), which convey blood from the placenta to the heart, open it first +into the united vitelline veins. The latter then disappear, and the right +umbilical vein goes with them, so that henceforth a single large vein, the left +umbilical vein, conducts all the blood from the placenta to the heart of the +embryo. The two arteries of the allantois, or the umbilical arteries (Figs. 183 +<i>n</i>, 184 <i>n</i>), are merely the ultimate terminations of the primitive +aortas, which are strongly developed afterwards. This umbilical circulation is +retained until the nine months of embryonic life are over, and the human embryo +enters into the world as the independent individual. The umbilical cord (Fig. +196 <i>al</i>), in which these large blood-vessels pass from the embryo to the +placenta, comes away, together with the latter, in the after-birth, and with +the use of the lungs begins an entirely new form of circulation, which is +confined to the body of the infant. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus202"></a> +<img src="images/fig202.gif" width="222" height="271" alt="Fig.202. Boat-shaped +embryo of the dog, from the ventral side, magnified." /> +<p class="caption">Fig. 202—<b>Boat-shaped embryo of the dog,</b> +from the ventral side, magnified. In front under the forehead we can see the +first pair of gill-arches; underneath is the S-shaped heart, at the sides of +which are the auditory vesicles. The heart divides behind into the two +vitelline veins, which expand in the germinative area (which is torn off all +round). On the floor of the open belly lie, between the protovertebræ, the +primitive aortas, from which five pairs of vitelline arteries are given off. +(From <i> Bischoff.</i>)</p> +</div> + +<p> +There is a great phylogenetic significance in the perfect agreement which we +find between man and the anthropoid apes in these important features of +embryonic circulation, and the special construction of the placenta and the +umbilical cord. We must infer from it a close blood-relationship of man and the +anthropomorphic apes—a common descent of them from one and the same +extinct group of lower apes. Huxley’s +“pithecometra-principle” applies to these ontogenetic features as +much as to any other morphological relations: “The differences in +construction of any part of the body are less between man and the anthropoid +apes than between the latter and the lower apes.” +</p> + +<p> +This important Huxleian law, the chief consequence of which is “the +descent of man from the ape,” has lately been confirmed in an interesting +and unexpected way from the side of the experimental +<span class='pagenum'><a name="Page_172" id="Page_172"></a></span> +physiology of the blood. The experiments of Hans Friedenthal at Berlin have +shown that human blood, mixed with the blood of lower apes, has a poisonous +effect on the latter; the serum of the one destroys the blood-cells of the +other. But this does not happen when human blood is mixed with that of the +anthropoid ape. As we know from many other experiments that the mixture of two +different kinds of blood is only possible without injury in the case of two +closely related animals of the same family, we have another proof of the close +blood-relationship, in the literal sense of the word, of man and the anthropoid +ape. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus203"></a> +<img src="images/fig203.gif" width="338" height="415" alt="Fig.203. Lar or white-handed gibbon +(Hylobates lar or albimanus), from the Indian mainland." /> +<p class="caption">Fig. 203—<b>Lar or white-handed gibbon</b> +(<i>Hylobates lar</i> or <i>albimanus</i>), from the Indian mainland (From +<i>Brehm.</i>)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus204"></a> +<img src="images/fig204.gif" width="301" height="481" alt="Fig.204. Young orang (Satyrus +orang), asleep." /> +<p class="caption">Fig. 204—<b>Young orang</b> (<i>Satyrus +orang</i>), asleep.</p> +</div> + +<p> +<span class='pagenum'><a name="Page_173" id="Page_173"></a></span> +The existing anthropoid apes are only a small remnant of a large family of +eastern apes (or <i>Catarrhinæ</i>), from which man was evolved about the end +of the Tertiary period. They fall into two geographical groups—the +Asiatic and the African anthropoids. In each group we can distinguish two +genera. The oldest of these four genera is the gibbon <i>Hylobates,</i> Fig. +203); there are from eight to twelve species of it in the East Indies. I made +observations of four of them during my voyage in the East Indies (1901), and +had a specimen of the ash-grey gibbon (<i>Hylobates leuciscus</i>) living for +several months in the garden of my house in Java. I have described the +interesting habits of this ape (regarded by the Malays as the wild descendant +of men who had lost their way) in my <i>Malayischen</i> +<span class='pagenum'><a name="Page_174" id="Page_174"></a></span> +<i>Reisebriefen</i> (chap. xi). Psychologically, he showed a good deal of +resemblance to the children of my Malay hosts, with whom he played and formed a +very close friendship. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus205"></a> +<img src="images/fig205.gif" width="414" height="411" alt="Fig.205. Wild orang (Dyssatyrus +auritus)." /> +<p class="caption">Fig. 205—<b>Wild orang</b> (<i>Dyssatyrus +auritius</i>). (From <i>R. Fick</i> and <i>Leutemann.</i>).</p> +</div> + +<p> +The second, larger and stronger, genus of Asiatic anthropoid ape is the orang +(<i>Satyrus</i>); he is now found only in the islands of Borneo and Sumatra. +Selenka, who has published a very thorough <i>Study of the Development and +Cranial Structure of the Anthropoid Apes</i> (1899), distinguishes ten races of +the orang, which may, however, also be regarded as “local varieties or +species.” They fall into two sub-genera or genera: one group, +<i>Dyssatyrus</i> (orang-bentang, Fig. 205), is distinguished for the strength +of its limbs, and the formation of very peculiar and salient cheek-pads in the +elderly male; these are wanting in the other group, the ordinary orang-outang +(<i>Eusatyrus</i>). +</p> + +<p> +Several species have lately been distinguished in the two genera of the black +African anthropoid apes (chimpanzee and gorilla). In the genus +<i>Anthropithecus</i> (or <i>Anthropopithecus,</i> formerly +<i>Troglodytes</i>), the bald-headed chimpanzee, <i>A. calvus</i> (Fig. 206), +and the gorilla-like <i>A. mafuca</i> differ very strikingly from the ordinary +<i>Anthropithecus niger</i> (Fig. 207), not only in the size and proportion of +many parts of the body, but also in the peculiar shape of the head, especially +the ears and lips, and in the hair and colour. The controversy that still +continues as to whether these different forms of +<span class='pagenum'><a name="Page_175" id="Page_175"></a></span> +<span class='pagenum'><a name="Page_176" id="Page_176"></a></span> +chimpanzee and orang are “merely local varieties” or “true +species” is an idle one; as in all such disputes of classifiers there is +an utter absence of clear ideas as to what a species really is. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus206"></a> +<img src="images/fig206.gif" width="362" height="445" alt="Fig.206. The bald-headed chimpanzee +(Anthropithecus calvus). Female." /> +<p class="caption">Fig. 206—<b>The bald-headed chimpanzee</b> +(<i>Anthropithecus calvus</i>). Female. This fresh species, described by Frank +Beddard in 1897 as Troglodytes calvus, differs considerably from the ordinary +<i>A. niger</i> Fig. 207) in the structure of the head, the colouring, and the +absence of hair in parts.</p> +</div> + +<p> +Of the largest and most famous of all the anthropoid apes, the gorilla, Paschen +has lately discovered a giant-form in the interior of the Cameroons, which +seems to differ from the ordinary species (<i>Gorilla gina</i> Fig. 208), not +only by its unusual size and strength, but also by a special formation of the +skull. This giant gorilla (<i>Gorilla gigas,</i> Fig. 209) is six feet eight +inches long; the span of its great arms is about nine feet; its powerful chest +is twice as broad as that of a strong man. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus207"></a> +<img src="images/fig207.gif" width="248" height="327" alt="Fig.207. Female chimpanzee +(Anthropithecus niger)." /> +<p class="caption">Fig. 207—<b>Female chimpanzee</b> +(<i>Anthropithecus niger</i>). (From <i> Brehm.</i>)</p> +</div> + +<p> +The whole structure of this huge anthropoid ape is not merely very similar to +that of man, but it is substantially the same. “The same 200 bones, +arranged in the same way, form our internal skeleton; the same 300 muscles +effect our movements; the same hair covers our skin; the same groups of +ganglionic cells compose the ingenious mechanism of our brain; the same +four-chambered heart is the central pump of our circulation.” The really +existing differences in the shape and size of the various parts are explained +by differences in their growth, due to adaptation to different habits of life +and unequal use of the various organs. This of itself proves morphologically +the descent of man from the ape. We will return to the point in Chapter XXIII. +But I wanted to point already to this important solution of “the question +of questions,” because that agreement +<span class='pagenum'><a name="Page_177" id="Page_177"></a></span> +in the formation of the embryonic membranes and in fœtal circulation which I +have described affords a particularly weighty proof of it. It is the more +instructive as even cenogenetic structures may in certain circumstances have a +high phylogenetic value. In conjunction with the other facts, it affords a +striking confirmation of our biogenetic law. +<span class='pagenum'><a name="Page_178" id="Page_178"></a></span> +</p> + +<div class="fig" style="width:100%;"> +<a name="illus208"></a> +<img src="images/fig208.gif" width="230" height="353" alt="Fig.208. Female gorilla." /> +<p class="caption">Fig. 208—<b>Female gorilla.</b> (From +<i>Brehm</i>).</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus209"></a> +<img src="images/fig209.gif" width="327" height="523" alt="Fig.209. Male giant-gorilla (Gorilla +gigas), from Yaunde, in the interior of the Cameroons. Killed by H. Paschen, +stuffed by Umlauff." /> +<p class="caption">Fig. 209—<b>Male giant-gorilla</b> (<i>Gorilla +gigas</i>), from Yaunde, in the interior of the Cameroons. Killed by H. +Paschen, stuffed by Umlauff.</p> +</div> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap16"></a> +<span class='pagenum'><a name="Page_179" id="Page_179"></a></span> +Chapter XVI.<br/> +STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT</h2> + +<p> +In turning from the embryology to the phylogeny of man—from the +development of the individual to that of the species—we must bear in mind +the direct causal connection that exists between these two main branches of the +science of human evolution. This important causal nexus finds its simplest +expression in “the fundamental law of organic development,” the +content and purport of which we have fully considered in the first chapter. +According to this biogenetic law, ontogeny is a brief and condensed +recapitulation of phylogeny. If this compendious reproduction were complete in +all cases, it would be very easy to construct the whole story of evolution on +an embryonic basis. When we wanted to know the ancestors of any higher +organism, and, therefore, of man—to know from what forms the race as a +whole has been evolved we should merely have to follow the series of forms in +the development of the individual from the ovum; we could then regard each of +the successive forms as the representative of an extinct ancestral form. +However, this direct application of ontogenetic facts to phylogenetic ideas is +possible, without limitations, only in a very small section of the animal +kingdom. There are, it is true, still a number of lower invertebrates (for +instance, some of the Zoophyta and Vermalia) in which we are justified in +recognising at once each embryonic form as the historical reproduction, or +silhouette, as it were, of an extinct ancestor. But in the great majority of +the animals, and in the case of man, this is impossible, because the embryonic +forms themselves have been modified through the change of the conditions of +existence, and have lost their original character to some extent. During the +immeasurable course of organic history, the many millions of years during which +life was developing on our planet, secondary changes of the embryonic forms +have taken place in most animals. The young of animals (not only detached +larvæ, but also the embryos enclosed in the womb) may be modified by the +influence of the environment, just as well as the mature organisms are by +adaptation to the conditions of life; even species are altered during the +embryonic development. Moreover, it is an advantage for all higher organisms +(and the advantage is greater the more advanced they are) to curtail and +simplify the original course of development, and thus to obliterate the traces +of their ancestors. The higher the individual organism is in the animal +kingdom, the less completely does it reproduce in its embryonic development the +series of its ancestors, for reasons that are as yet only partly known to us. +The fact is easily proved by comparing the different developments of higher and +lower animals in any single stem. +</p> + +<p> +In order to appreciate this important feature, we have distributed the +embryological phenomena in two groups, <i> palingenetic</i> and +<i>cenogenetic.</i> Under palingenesis we count those facts of embryology that +we can directly regard as a faithful synopsis of the corresponding +stem-history. By cenogenesis we understand those embryonic processes which we +cannot directly correlate with corresponding evolutionary processes, but must +regard as modifications or falsifications of them. With this careful +discrimination between palingenetic and cenogenetic phenomena, our biogenetic +law assumes the following more precise shape:—The rapid and brief +development of the individual (ontogeny) is a condensed synopsis of the long +and slow history of the stem (phylogeny): this synopsis is the more faithful +and complete in proportion as the original features have been preserved by +heredity, and modifications have not been introduced by adaptation. +</p> + +<p> +<span class='pagenum'><a name="Page_180" id="Page_180"></a></span> +In order to distinguish correctly between palingenetic and cenogenetic +phenomena in embryology, and deduce sound conclusions in connection with +stem-history, we must especially make a comparative study of the former. In +doing this it is best to employ the methods that have long been used by +geologists for the purpose of establishing the succession of the sedimentary +rocks in the crust of the earth. This solid crust, which encloses the glowing +central mass like a thin shell, is composed of different kinds of rocks: there +are, firstly, the volcanic rocks which were formed directly by the cooling at +the surface of the molten mass of the earth; secondly, there are the +sedimentary rocks, that have been made out of the former by the action of +water, and have been laid in successive strata at the bottom of the sea. Each +of these sedimentary strata was at first a soft layer of mud; but in the course +of thousands of years it condensed into a solid, hard mass of stone (sandstone, +limestone, marl, etc.), and at the same time permanently preserved the solid +and imperishable bodies that had chanced to fall into the soft mud. Among these +bodies, which were either fossilised or left characteristic impressions of +their forms in the soft slime, we have especially the more solid parts of the +animals and plants that lived and died during the deposit of the slimy strata. +</p> + +<p> +Hence each of the sedimentary strata has its characteristic fossils, the +remains of the animals and plants that lived during that particular period of +the earth’s history. When we make a comparative study of these strata, we +can survey the whole series of such periods. All geologists are now agreed that +we can demonstrate a definite historical succession in the strata, and that the +lowest of them were deposited in very remote, and the uppermost in +comparatively recent, times. However, there is no part of the earth where we +find the series of strata in its entirety, or even approximately complete. The +succession of strata and of corresponding historical periods generally given in +geology is an ideal construction, formed by piecing together the various +partial discoveries of the succession of strata that have been made at +different points of the earth’s surface (cf. Chapter XVIII). +</p> + +<p> +We must act in this way in constructing the phylogeny of man. We must try to +piece together a fairly complete picture of the series of our ancestors from +the various phylogenetic fragments that we find in the different groups of the +animal kingdom. We shall see that we are really in a position to form an +approximate picture of the evolution of man and the mammals by a proper +comparison of the embryology of very different animals—a picture that we +could never have framed from the ontogeny of the mammals alone. As a result of +the above-mentioned cenogenetic processes—those of disturbed and +curtailed heredity—whole series of lower stages have dropped out in the +embryonic development of man and the other mammals especially from the earliest +periods, or been falsified by modification. But we find these lower stages in +their original purity in the lower vertebrates and their invertebrate +ancestors. Especially in the lowest of all the vertebrates, the lancelet or +Amphioxus, we have the oldest stem-forms completely preserved in the embryonic +development. We also find important evidence in the fishes, which stand between +the lower and higher vertebrates, and throw further light on the course of +evolution in certain periods. Next to the fishes come the amphibia, from the +embryology of which we can also draw instructive conclusions. They represent +the transition to the higher vertebrates, in which the middle and older stages +of ancestral development have been either distorted or curtailed, but in which +we find the more recent stages of the phylogenetic process well preserved in +ontogeny. We are thus in a position to form a fairly complete idea of the past +development of man’s ancestors within the vertebrate stem by putting +together and comparing the embryological developments of the various groups of +vertebrates. And when we go below the lowest vertebrates and compare their +embryology with that of their invertebrate relatives, we can follow the +genealogical tree of our animal ancestors much farther, down to the very lowest +groups of animals. +</p> + +<p> +In entering the obscure paths of this phylogenetic labyrinth, clinging to the +Ariadne-thread of the biogenetic law and guided by the light of comparative +anatomy, we will first, in accordance with the methods we have adopted, +discover and arrange those fragments from the manifold embryonic developments +of very different animals from which the stem-history of man can be composed. I +would call attention particularly to the fact that +<span class='pagenum'><a name="Page_181" id="Page_181"></a></span> +we can employ this method with the same confidence and right as the geologist. +No geologist has ever had ocular proof that the vast rocks that compose our +Carboniferous or Jurassic or Cretaceous strata were really deposited in water. +Yet no one doubts the fact. Further, no geologist has ever learned by direct +observation that these various sedimentary formations were deposited in a +certain order; yet all are agreed as to this order. This is because the nature +and origin of these rocks cannot be rationally understood unless we assume that +they were so deposited. These hypotheses are universally received as safe and +indispensable “geological theories,” because they alone give a +rational explanation of the strata. +</p> + +<p> +Our evolutionary hypotheses can claim the same value, for the same reasons. In +formulating them we are acting on the same inductive and deductive methods, and +with almost equal confidence, as the geologist. We hold them to be correct, and +claim the status of “biological theories” for them, because we +cannot understand the nature and origin of man and the other organisms without +them, and because they alone satisfy our demand for a knowledge of causes. And +just as the geological hypotheses that were ridiculed as dreams at the +beginning of the nineteenth century are now universally admitted, so our +phylogenetic hypotheses, which are still regarded as fantastic in certain +quarters, will sooner or later be generally received. It is true that, as will +soon appear, our task is not so simple as that of the geologist. It is just as +much more difficult and complex as man’s organisation is more elaborate +than the structure of the rocks. +</p> + +<p> +When we approach this task, we find an auxiliary of the utmost importance in +the comparative anatomy and embryology of two lower animal-forms. One of these +animals is the lancelet (<i>Amphioxus</i>), the other the sea-squirt +(<i>Ascidia</i>). Both of these animals are very instructive. Both are at the +border between the two chief divisions of the animal kingdom—the +vertebrates and invertebrates. The vertebrates comprise the already mentioned +classes, from the Amphioxus to man (acrania, lampreys, fishes, dipneusts, +amphibia, reptiles, birds, and mammals). Following the example of Lamarck, it +is usual to put all the other animals together under the head of invertebrates. +But, as I have often mentioned already, the group is composed of a number of +very different stems. Of these we have no interest just now in the echinoderms, +molluscs, and articulates, as they are independent branches of the animal-tree, +and have nothing to do with the vertebrates. On the other hand, we are greatly +concerned with a very interesting group that has only recently been carefully +studied, and that has a most important relation to the ancestral tree of the +vertebrates. This is the stem of the Tunicates. One member of this group, the +sea-squirt, very closely approaches the lowest vertebrate, the Amphioxus, in +its essential internal structure and embryonic development. Until 1866 no one +had any idea of the close connection of these apparently very different +animals; it was a very fortunate accident that the embryology of these related +forms was discovered just at the time when the question of the descent of the +vertebrates from the invertebrates came to the front. In order to understand it +properly, we must first consider these remarkable animals in their +fully-developed forms and compare their anatomy. +</p> + +<p> +We begin with the lancelet—after man the most important and interesting +of all animals. Man is at the highest summit, the lancelet at the lowest root, +of the vertebrate stem. +</p> + +<p> +It lives on the flat, sandy parts of the Mediterranean coast, partly buried in +the sand, and is apparently found in a number of seas.<a href="#linknote-28" name="linknoteref-28" id="linknoteref-28"><sup>[28]</sup></a> It has been +found in the North Sea (on the British and Scandinavian coasts and in +Heligoland), and at various places on the Mediterranean (for instance, at Nice, +Naples, and Messina). It is also found on the coast of Brazil and in the most +distant parts of the Pacific Ocean (the coast of Peru, Borneo, China, +Australia, etc.). Recently eight to ten species of the amphioxus have been +determined, distributed in two or three genera. +</p> + +<p class="footnote"> +<a name="linknote-28" id="linknote-28"></a> <a href="#linknoteref-28">[28]</a> +See the ample monograph by Arthur Willey, <i> Amphioxus and the Ancestry of the +Vertebrates</i>; Boston, 1894. +</p> + +<p> +Johannes Müller classed the lancelet with the fishes, although he pointed out +that the differences between this simple vertebrate and the lowest fishes are +much greater than between the fishes and the amphibia. But this was far from +expressing the real significance of the animal. We may confidently lay down the +following principle: The Amphioxus differs more from the fishes than the fishes +do from +<span class='pagenum'><a name="Page_182" id="Page_182"></a></span> +man and the other vertebrates. As a matter of fact, it is so different from all +the other vertebrates in its whole organisation that the laws of logical +classification compel us to distinguish two divisions of this stem: 1, the +Acrania (Amphioxus and its extinct relatives); and 2, the Craniota (man and the +other vertebrates). The first and lower division comprises the vertebrates that +have no vertebræ or skull +<span class='pagenum'><a name="Page_183" id="Page_183"></a></span> +(<i>cranium</i>). Of these the only living representatives are the Amphioxus +and Paramphioxus, though there must have been a number of different species at +an early period of the earth’s history. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus210"></a> +<a name="illus211"></a> +<img src="images/fig210.gif" width="367" height="473" alt="Fig.210. The lancelet (Amphioxus +lanceolatus), left view. Fig. 211. Transverse section of the head of the +Amphioxus." /> +<p class="caption">Fig. 210—<b>The lancelet</b> (<i>Amphioxus +lanceolatus</i>), left view. The long axis is vertical; the mouth-end is above, +the tail-end below; <i>a</i> mouth, surrounded by threads of beard; <i>b</i> +anus, <i>c</i> gill-opening (<i>porus branchialis</i>), <i>d</i> gill-crate, +<i> e</i> stomach, <i>f</i> liver, <i>g</i> small intestine, <i>h</i> branchial +cavity, <i>i</i> chorda (axial rod), underneath it the aorta; <i>k</i> aortic +arches, <i>l</i> trunk of the branchial artery, <i>m</i> swellings on its +branches, <i>n</i> vena cava, <i> o</i> visceral vein.<br/> +Fig. 211—<b>Transverse section of the head of the +Amphioxus.</b> (From <i>Boveri.</i>) Above the branchial gut (<i>kd</i>) is the +chorda, above this the neural tube (in which we can distinguish the inner grey +and the outer white matter); above again is the dorsal fin (<i>fh</i>). To the +right and left above (in the episoma) are the thick muscular plates (<i>m</i>); +below (in the hyposoma) the gonads (<i>g</i>). <i> ao</i> aorta (here double), +<i>c</i> corium, <i>ec</i> endostyl, <i>f</i> fascie, <i>gl</i> glomerulus of +the kidneys, <i>k</i> branchial vessel, <i>ld</i> partition between the cœloma +(<i>sc</i>) and atrium (<i>p</i>), <i>mt</i> transverse ventral muscle, +<i>n</i> renal canals, of upper and <i>uf</i> lower canals in the mantle-folds, +<i>p</i> peribranchial cavity, (atrium), <i> sc</i> cœloma (subchordal +body-cavity), <i>si</i> principal (or subintestinal) vein, <i>sk</i> perichorda +(skeletal layer).</p> +</div> + +<p> +Opposed to the Acrania is the second division of the vertebrates, which +comprises all the other members of the stem, from the fishes up to man. All +these vertebrates have a head quite distinct from the trunk, with a skull +(<i>cranium</i>) and brain; all have a centralised heart, fully-formed kidneys, +etc. Hence they are called the <i>Craniota.</i> These Craniotes are, however, +without a skull in their earlier period. As we already know from embryology, +even man, like every other mammal, passes in the earlier course of his +development through the important stage which we call the chordula; at this +lower stage the animal has neither vertebræ nor skull nor limbs (Figs. +83–86). And even after the formation of the primitive vertebræ has begun, +the segmented fœtus of the amniotes still has for a long time the simple form +of a lyre-shaped disk or a sandal, without limbs or extremities. When we +compare this embryonic condition, the sandal-shaped fœtus, with the developed +lancelet, we may say that the amphioxus is, in a certain sense, a permanent +sandal-embryo, or a permanent embryonic form of the Acrania; it never rises +above a low grade of development which we have long since passed. +</p> + +<p> +The fully-developed lancelet (Fig. 210) is about two inches long, is colourless +or of a light red tint, and has the shape of a narrow lancet-formed leaf. The +body is pointed at both ends, but much compressed at the sides. There is no +trace of limbs. The outer skin is very thin and delicate, naked, transparent, +and composed of two different layers, a simple external stratum of cells, the +epidermis, and a thin underlying cutis-layer. Along the middle line of the back +runs a narrow fin-fringe which expands behind into an oval tail-fin, and is +continued below in a short anus-fin. The fin-fringe is supported by a number of +square elastic fin-plates. +</p> + +<p> +In the middle of the body we find a thin string of cartilage, which goes the +whole length of the body from front to back, and is pointed at both ends (Fig. +210 <i>i</i>). This straight, cylindrical rod (somewhat compressed for a time) +is the axial rod or the <i>chorda dorsalis</i>; in the lancelet this is the +only trace of a vertebral column. The chorda develops no further, but retains +its original simplicity throughout life. It is enclosed by a firm membrane, the +chorda-sheath or <i>perichorda.</i> The real features of this and of its +dependent formations are best seen in the transverse section of the Amphioxus +(Fig. 211). The perichorda forms a cylindrical tube immediately over the +chorda, and the central nervous system, the medullary tube, is enclosed in it. +This important psychic organ also remains in its simplest shape throughout +life, as a cylindrical tube, terminating with almost equal plainness at either +end, and enclosing a narrow canal in its thick wall. However, the fore end is a +little rounder, and contains a small, almost imperceptible bulbous swelling of +the canal. This must be regarded as the beginning of a rudimentary brain. At +the foremost end of it there is a small black pigment-spot, a rudimentary eye; +and a narrow canal leads to a superficial sense-organ. In the vicinity of this +optic spot we find at the left side a small ciliated depression, the single +olfactory organ. There is no organ of hearing. This defective development of +the higher sense-organs is probably, in the main, not an original feature, but +a result of degeneration. +</p> + +<p> +Underneath the axial rod or chorda runs a very simple alimentary canal, a tube +that opens on the ventral side of the animal by a mouth in front and anus +behind. The oval mouth is surrounded by a ring of cartilage, on which there are +twenty to thirty cartilaginous threads (organs of touch, Fig. 210 <i>a</i>). +The alimentary canal divides into sections of about equal length by a +constriction in the middle. The fore section, or head-gut, serves for +respiration; the hind section, or trunk-gut, for digestion. The limit of the +two alimentary regions is also the limit of the two parts of the body, the head +and the trunk. The head-gut or branchial gut forms a broad gill-crate, the +grilled wall of which is pierced by numbers of gill-clefts (Fig. 210 <i>d</i>). +The fine bars of the gill-crate between the clefts are strengthened with firm +parallel rods, and these are connected in pairs by cross-rods. The water that +enters the mouth of the Amphioxus passes through these clefts into the large +surrounding branchial cavity or <i> atrium,</i> and then pours out behind +through a hole in it, the respiratory pore (<i>porus branchialis,</i> Fig. 210 +<i>c</i>). Below, on the ventral side of the gill-crate, there is in the middle +<span class='pagenum'><a name="Page_184" id="Page_184"></a></span> +line a ciliated groove with a glandular wall (the hypobranchial groove), which +is also found in the Ascidia and the larvæ of the Cyclostoma. It is interesting +because the thyroid gland in the larynx of the higher vertebrates (underneath +the “Adam’s apple”) has been developed from it. +</p> + +<p> +Behind the respiratory part of the gut we have the digestive section, the trunk +or liver (hepatic) gut. The small particles that the Amphioxus takes in with +the water—infusoria, diatoms, particles of decomposed plants and animals, +etc.—pass from the gill-crate into the digestive part of the canal, and +are used up as food. From a somewhat enlarged portion, that corresponds to the +stomach (Fig. 210 <i>e</i>), a long, pouch-like blind sac proceeds straight +forward (<i>f</i>); it lies underneath on the left side of the gill-crate, and +ends blindly about the middle of it. This is the liver of the Amphioxus, the +simplest kind of liver that we meet in any vertebrate. In man also the liver +develops, as we shall see, in the shape of a pouch-like blind sac, that forms +out of the alimentary canal behind the stomach. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus212"></a> +<a name="illus213"></a> +<img src="images/fig212.gif" width="252" height="170" alt="Fig.212. Transverse section of an +Amphioxus-larva, with five gill-clefts, through the middle of the body. Fig. +213. Diagram of the preceding." /> +<p class="caption">Fig. +212—<b>Transverse section of an Amphioxus-larva,</b> with five +gill-clefts, through the middle of the body.<br/> Fig. 213—<b>Diagram of +the preceding.</b> (From <i> Hatschek.</i>) <i>A</i> epidermis, <i>B</i> +medullary tube, <i> C</i> chorda, <i>C</i><sub>1</sub> inner chorda-sheath, +<i>D</i> visceral epithelium, <i>E</i> sub-intestinal vein. <i>1</i> cutis, +<i>2</i> muscle-plate (myotome), <i>3</i> skeletal plate (sclerotome), <i>4</i> +cœloseptum (partition between dorsal and ventral cœloma), <i>5</i> skin-fibre +layer, <i>6</i> gut-fibre layer, <i>I</i> myocœl (dorsal body-cavity), <i> +II</i> splanchnocœl (ventral body-cavity).)</p> +</div> + +<p> +The formation of the circulatory system in this animal is not less interesting. +All the other vertebrates have a compressed, thick, pouch-shaped heart, which +develops from the wall of the gut at the throat, and from which the +blood-vessels proceed; in the Amphioxus there is no special centralised heart, +driving the blood by its pulsations. This movement is effected, as in the +annelids, by the thin blood-vessels themselves, which discharge the function of +the heart, contracting and pulsating in their whole length, and thus driving +the colourless blood through the entire body. On the under-side of the +gill-crate, in the middle line, there is the trunk of a large vessel that +corresponds to the heart of the other vertebrates and the trunk of the +branchial artery that proceeds from it; this drives the blood into the gills +(Fig. 210 <i>l</i>). A number of small vascular arches arise on each side from +this branchial artery, and form little heart-shaped swellings or <i> +bulbilla</i> (<i>m</i>) at their points of departure; they advance along the +branchial arches, between the gill-clefts and the fore-gut, and unite, as +branchial veins, above the gill-crate in a large trunk blood-vessel that runs +under the chorda dorsalis. This is the principal artery or primitive aorta +(Fig. 214 <i>D</i>). The branches which it gives off to all parts of the body +unite again in a larger venous vessel at the underside of the gut, called the +subintestinal vein (Figs. 210 <i>o,</i> 212 <i>E</i>). This single main vessel +of the Amphioxus goes like a closed circular water-conduit along the alimentary +canal through the whole body, and pulsates in its whole length above and below. +When the upper tube contracts the lower one is filled with blood, and <i>vice +versa.</i> In the upper tube the blood flows from front to rear, then back from +rear to front in the lower vessel. The whole of the long tube that runs along +the ventral side of the alimentary canal and contains venous blood may be +called the “principal vein,” and may be compared to the ventral +vessel in the worms. On the other hand, the long +<span class='pagenum'><a name="Page_185" id="Page_185"></a></span> +straight vessel that runs along the dorsal line of the gut above, between it +and the chorda, and contains arterial blood, is clearly identical with the +aorta or principal artery of the other vertebrates; and on the other side it +may be compared to the dorsal vessel in the worms. +</p> + +<p> +The cœloma or body-cavity has some very important and distinctive features in +the Amphioxus. The embryology of it is most instructive in connection with the +stem-history of the body-cavity in man and the other vertebrates. As we have +already seen (Chapter X), in these the two cœlom-pouches are divided at an +early stage by transverse constrictions into a double row of primitive segments +(Fig. 124), and each of these subdivides, by a frontal or lateral constriction, +into an upper (dorsal) and lower (ventral) pouch. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus214"></a> +<a name="illus215"></a> +<img src="images/fig214.gif" width="325" height="241" alt="Fig.214. Transverse section of a +young Amphioxus, immediately after metamorphosis. Fig. 215. Diagram of +preceding." /> +<p class="caption">Fig. 214—<b>Transverse section of a young +Amphioxus,</b> immediately after metamorphosis, through the hindermost third +(between the atrium-cavity and the anus).<br/> Fig. 215—<b>Diagram of +preceding.</b> (From <i>Hatschek.</i>) <i>A</i> epidermis, <i>B</i> medullary +tube, <i>C</i> chorda, <i> D</i> aorta, <i>E</i> visceral epithelium, <i>F</i> +subintestinal vein. <i>1</i> corium-plate, <i>2</i> muscle-plate, <i>3</i> +fascie-plate, <i>4</i> outer chorda-sheath, <i>5</i> myoseptum, <i> 6</i> +skin-fibre plate, <i>7</i> gut-fibre plate, <i>I</i> myocœl, <i>II</i> +splanchnocœl, <i>I</i><sub>1</sub> dorsal fin, <i>I</i><sub>2</sub> +anus-fin.)</p> +</div> + +<p> +These important structures are seen very clearly in the trunk of the amphioxus +(the latter third, Figs. 212–215), but it is otherwise in the head, the +foremost third (Fig. 216). Here we find a number of complicated structures that +cannot be understood until we have studied them on the embryological side in +the next chapter (cf. Fig. 81). The branchial gut lies free in a spacious +cavity filled with water, which was wrongly thought formerly to be the +body-cavity (Fig. 216 <i>A</i>). As a matter of fact, this atrium (commonly +called the peribranchial cavity) is a secondary structure formed by the +development of a couple of lateral mantle-folds or gill-covers +(<i>M</i><sub>1</sub>, <i>U</i>). The real body-cavity (<i>Lh</i>) is very +narrow and entirely closed, lined with epithelium. The peribranchial cavity +(<i>A</i>) is full of water, and its walls are lined with the skin-sense layer; +it opens outwards in the rear through the respiratory pore (Fig. 210 <i> +c</i>). +</p> + +<p> +On the inner surface of these mantle-folds (<i>M</i><sub>1</sub>), in the +ventral half of the wide mantle cavity (atrium), we find the sex-organs of the +Amphioxus. At each side of the branchial gut there are between twenty and +thirty roundish four-cornered sacs, which can clearly be seen from without with +the naked eye, as they shine through the thin transparent body-wall. These sacs +are the sexual glands they are the same size and shape in both sexes, only +differing in contents. In the female they contain a quantity of simple ova +(Fig. 219 <i>g</i>); in the male a number of much smaller cells that change +into mobile ciliated cells (sperm-cells). Both sacs lie on the inner wall of +the atrium, and have no special outlets. When the ova of the female and the +sperm of the male are ripe, they fall into the atrium, pass through the +gill-clefts into the +<span class='pagenum'><a name="Page_186" id="Page_186"></a></span> +fore-gut, and are ejected through the mouth. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus216"></a> +<img src="images/fig216.gif" width="298" height="466" alt="Fig.216. Transverse section of the lancelet, in +the fore half." /> +<p class="caption">Fig. 216—<b>Transverse section of +the lancelet,</b> in the fore half. (From <i>Ralph.</i>) The outer covering is +the simple cell-layer of the epidermis (<i>E</i>). Under this is the thin +corium, the subcutaneous tissue of which is thickened; it sends +connective-tissue partitions between the muscles (<i>M</i><sub>1</sub>) and to +the chorda-sheath. (<i>N</i> medullary tube, <i>Ch</i> chorda, <i>Lh</i> +body-cavity, <i>A</i> atrium, <i>L</i> upper wall of same, <i>E</i><sub>1</sub> +inner wall, <i>E</i><sub>2</sub> outer wall, <i>Lh</i><sub>1</sub> ventral +remnant of same, <i>Kst</i> gill-reds, <i>M</i> ventral muscles, <i>R</i> seam +of the joining of the ventral folds (gill-covers), <i>G</i> sexual glands.</p> +</div> + +<p> +Above the sexual glands, at the dorsal angle of the atrium, we find the +kidneys. These important excretory organs could not be found in the Amphioxus +for a long time, on account of their remote position and their smallness; they +were discovered in 1890 by Theodor Boveri (Fig. 217 <i>x</i>). They are short +segmented canals; corresponding to the primitive kidneys of the other +vertebrates (Fig. 218 <i>B</i>). Their internal aperture (Fig. 217 <i>B</i>) +opens into the body-cavity; their outer aperture into the atrium (<i>C</i>). +The prorenal canals lie in the middle of the line of the head, outwards from +the uppermost section of the gill-arches, and have important relations to the +branchial vessels (<i>H</i>). For this reason, and in their whole arrangement, +the primitive kidneys of the Amphioxus +<span class='pagenum'><a name="Page_187" id="Page_187"></a></span> +show clearly that they are equivalent to the prorenal canals of the Craniotes +(Fig. 218 <i>B</i>). The prorenal duct of the latter (Fig. 218 <i>C</i>) +corresponds to the branchial cavity or atrium of the former (Fig. 217 +<i>C</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus217"></a> +<a name="illus218"></a> +<img src="images/fig217.gif" width="434" height="357" alt="Fig.217. Transverse section through +the middle of the Amphioxus. Fig. 218. Transverse section of a primitive fish +embryo." /> +<p class="caption">Fig. 217—<b>Transverse section through the middle +of the Amphioxus.</b> (From <i>Boveri.</i>) On the left a gill-rod has been +struck, and on the right a gill-cleft; consequently on the left we see the +whole of a prorenal canal (<i>x</i>), on the right only the section of its +fore-leg. <i>A</i> genital chamber (ventral section of the gonocœl), <i>x</i> +pronephridium, <i>B</i> its cœlom-aperture, <i>C</i> atrium, <i>D</i> +body-cavity, <i>E</i> visceral cavity, <i>F</i> subintestinal vein, <i>G</i> +aorta (the left branch connected by a branchial vessel with the subintestinal +vein), <i>H</i> renal vessel.<br/> Fig. 218—<b>Transverse section of a +primitive fish embryo</b> (Selachii-embryo, from <i>Boveri.</i>). To the left +pronephridia (<i>B</i>), the right primitive kidneys (<i>A</i>). The dotted +lines on the right indicate the later opening of the primitive kidney canals +(<i>A</i>) into the prorenal duct (<i>C</i>). <i> D</i> body-cavity, <i>E</i> +visceral cavity, <i>F</i> subintestinal vein, <i>G</i> aorta, <i>H</i> renal +vessel.</p> +</div> + +<p> +If we sum up the results of our anatomic study of the Amphioxus, and compare +them with the familiar organisation of man, we shall find an immense distance +between the two. As a fact, the highest summit of the vertebrate organisation +which man represents is in every respect so far above the lowest stage, at +which the lancelet remains, that one would at first scarcely believe it +possible to class both animals in the same division of the animal kingdom. +Nevertheless, this classification is indisputably just. Man is only a more +advanced stage of the vertebral type that we find unmistakably in the Amphioxus +in its characteristic features. We need only recall the picture of the ideal +Primitive Vertebrate given in a former chapter, and compare it with the lower +stages of human embryonic development, to convince ourselves of our close +relationship to the lancelet. (Cf. Chapter XI) +</p> + +<p> +It is true that the Amphioxus is far below all other living vertebrates. It is +true that it has no separate head, no developed brain or skull, the +characteristic feature of the other vertebrates. +<span class='pagenum'><a name="Page_188" id="Page_188"></a></span> +It is (probably as a result of degeneration) without the auscultory organ and +the centralised heart that all the others have; and it has no fully-formed +kidneys. Every single organ in it is simpler and less advanced than in any of +the others. Yet the characteristic connection and arrangement of all the organs +is just the same as in the other vertebrates. All these, moreover, pass, during +their embryonic development, through a stage in which their whole organisation +is no higher than that of the Amphioxus, but is substantially identical with +it. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus219"></a> +<img src="images/fig219.gif" width="211" height="296" alt="Fig.219. Transverse +section of the head of the Amphioxus." /> +<p class="caption">Fig. 219—<b>Transverse section of the head of the +Amphioxus</b> (at the limit of the first and second third of the body). (From +<i>Boveri</i>) <i>a</i> aorta (here double), <i>b</i> atrium, <i>c</i> chorda, +<i>co</i> umlaut cœloma (body-cavity), <i>e</i> endostyl (hypobranchial +groove), <i>g</i> gonads (ovaries), <i>kb</i> gill-arches, <i>kd</i> branchial +gut, <i>l</i> liver-tube (on the right, one-sided), <i>m</i> muscles, <i>n</i> +renal canals, <i>r</i> spinal cord, <i>sn</i> spinal nerves, <i>sp</i> +gill-clefts.</p> +</div> + +<p> +In order to see this quite clearly, it is particularly useful to compare the +Amphioxus with the youthful forms of those vertebrates that are classified next +to it. This is the class of the Cyclostoma. There are to-day only a few species +of this once extensive class, and these may be distributed in two groups. One +group comprises the hag-fishes or Myxinoides. The other group are the +Petromyzontes, or lampreys, which are a familiar delicacy in their marine form. +These Cyclostoma are usually classified with the fishes. But they are far below +the true fishes, and form a very interesting connecting-group between them and +the lancelet. One can see how closely they approach the latter by comparing a +young lamprey with the Amphioxus. The chorda is of the same simple character in +both; also the medullary tube, that lies above the chorda, and the alimentary +canal below it. However, in the lamprey the spinal cord swells in front into a +simple pear-shaped cerebral vesicle, and at each side of it there are a very +simple eye and a rudimentary auditory vesicle. The nose is a single pit, as in +the Amphioxus. The two sections of the gut are also just the same and very +rudimentary in the lamprey. On the other hand, we see a great advance in the +structure of the heart, which is found underneath the gills in the shape of a +centralised muscular tube, and is divided into an auricle and a ventricle. +Later on the lamprey advances still further, and gets a skull, five cerebral +vesicles, a series of independent gill-pouches, etc. This makes all the more +interesting the striking resemblance of its immature larva to the developed and +sexually mature Amphioxus. +</p> + +<p> +While the Amphioxus is thus connected through the Cyclostoma with the fishes, +and so with the series of the higher vertebrates, it is, on the other hand, +very closely related to a lowly invertebrate marine animal, from which it seems +to be entirely remote at first glance. This remarkable animal is the sea-squirt +or Ascidia, which was formerly thought to be closely related to the mussel, and +so classed in the molluscs. But since the remarkable embryology of these +animals was discovered in 1866, there can be no question that they have nothing +to do with the molluscs. To the great astonishment of zoologists, they were +found, in their whole individual development, to be closely related to the +vertebrates. When fully developed the Ascidiæ are shapeless lumps that would +not, at first sight, be taken for animals at all. The oval body, frequently +studded with knobs or uneven and lumpy, in which we can discover no special +external organs, is attached at one end to marine plants, rocks, or the floor +of the sea. Many species look like potatoes, others like melon-cacti, others +like prunes. Many of the Ascidiæ form transparent crusts or +<span class='pagenum'><a name="Page_189" id="Page_189"></a></span> +deposits on stones and marine plants. Some of the larger species are eaten like +oysters. Fishermen, who know them very well, think they are not animals, but +plants. They are sold in the fish markets of many of the Italian coast-towns +with other lower marine animals under the name of “sea-fruit” +(<i>frutti di mare</i>). There is nothing about them to show that they are +animals. When they are taken out of the water with the net the most one can +perceive is a slight contraction of the body that causes water to spout out in +two places. The bulk of the Ascidiæ are very small, at the most a few inches +long. A few species are a foot or more in length. There are many species of +them, and they are found in every sea. As in the case of the Acrania, we have +no fossilised remains of the class, because they have no hard and fossilisable +parts. However, they must be of great antiquity, and must go back to the +primordial epoch. +</p> + +<p> +The name of “Tunicates” is given to the whole class to which the +Ascidiæ belong, because the body is enclosed in a thick and stiff covering like +a mantle (<i>tunica</i>). This mantle—sometimes soft like jelly, +sometimes as tough as leather, and sometimes as stiff as cartilage—has a +number of peculiarities. The most remarkable of them is that it consists of a +woody matter, cellulose—the same vegetal substance that forms the stiff +envelopes of the plant-cells, the substance of the wood. The tunicates are the +only class of animals that have a real cellulose or woody coat. Sometimes the +cellulose mantle is brightly coloured, at other times colourless. Not +infrequently it is set with needles or hairs, like a cactus. Often we find a +mass of foreign bodies—stone, sand, fragments of mussel-shells, +etc.—worked into the mantle. This has earned for the Ascidia the name of +“the microcosm.” +</p> + +<div class="fig" style="width:100%;"> +<a name="illus220"></a> +<img src="images/fig220.gif" width="192" height="352" alt="Fig.220. Organisation +of an Ascidia (left view)." /> +<p class="caption">Fig. 220—<b>Organisation of an Ascidia</b> (left +view); the dorsal side is turned to the right and the ventral side to the left, +the mouth (<i>o</i>) above; the ascidia is attached at the tail end. The +branchial gut (<i>br</i>), which is pierced by a number of clefts, continues +below in the visceral gut. The rectum opens through the anus (<i>a</i>) into +the atrium (<i>cl</i>), from which the excrements are ejected with the +respiratory water through the mantle-hole or cloaca (<i>a</i>); <i>m</i> +mantle. (From <i> Gegenbaur.</i></p> +</div> + +<p> +The hind end, which corresponds to the tail of the Amphioxus, is usually +attached, often by means of regular roots. The dorsal and ventral sides differ +a good deal internally, but frequently cannot be distinguished externally. If +we open the thick tunic or mantle in order to examine the internal +organisation, we first find a spacious cavity filled with water—the +mantle-cavity or respiratory cavity (Fig. 220 <i>cl</i>). It is also called the +branchial cavity and the cloaca, because it receives the excrements and sexual +products as well as the respiratory water. The greater part of the respiratory +cavity is occupied by the large grated branchial sac (<i>br</i>). This is so +like the gill-crate of the Amphioxus in its whole arrangement that the +resemblance was pointed out by the English naturalist Goodsir, years ago, +before anything was known of the relationship of the two animals. As a fact, +even in the Ascidia the mouth (<i>o</i>) opens first into this wide branchial +sac. The respiratory water passes through the lattice-work of the branchial sac +into the branchial cavity, and is ejected from this by the respiratory pore +(<i>a</i>′). Along the ventral side of the branchial sac runs a ciliated +groove—the hypobranchial groove which we have previously found at the +same spot in the Amphioxus. The food of the Ascidia also +<span class='pagenum'><a name="Page_190" id="Page_190"></a></span> +consists of tiny organisms, infusoria, diatoms, parts of decomposed marine +plants and animals; etc. These pass with the water into the gill-crate and the +digestive part of the gut at the end of it, at first into an enlargement of it +that represents the stomach. The adjoining small intestine usually forms a +loop, bends forward, and opens by an anus (Fig. 220 <i>a</i>), not directly +outwards, but first into the mantle cavity; from this the excrements are +ejected by a common outlet (<i>a</i>′) together with the used-up water +and the sexual products. The outlet is sometimes called the branchial pore, and +sometimes the cloaca or ejection-aperture. In many of the Ascidiæ a glandular +mass opens into the gut, and this represents the liver. In some there is +another gland besides the liver, and this is taken to represent the kidneys. +The body-cavity proper, or cœloma, which is filled with blood and encloses the +hepatic gut, is very narrow in the Ascidia, as in the Amphioxus, and is here +also usually confounded with the wide atrium, or peribranchial cavity, full of +water. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus221"></a> +<img src="images/fig221.gif" width="151" height="299" alt="Organisation of an +Ascidia (as in Fig. 220, seen from the left)." /> +<p class="caption">Fig. 221—<b>Organisation of an Ascidia</b> (as in +Fig. 220, seen from the left). <i>sb</i> branchial sac, <i>v</i> stomach, +<i>i</i> small intestine, <i>c</i> heart, <i>t</i> testicle, <i>vd</i> +sperm-duct, <i>o</i> ovary, <i> o</i>′ ripe ova in the branchial cavity. +The two small arrows indicate the entrance and exit of the water through the +openings of the mantle. (From <i>Milne-Edwards.</i>)</p> +</div> + +<p> +There is no trace in the fully-developed Ascidia of a chorda dorsalis, or +internal axial skeleton. It is the more interesting that the young animal that +emerges from the ovum <i>has</i> a chorda, and that there is a rudimentary +medullary tube above it. The latter is wholly atrophied in the developed +Ascidia, and looks like a small nerve-ganglion in front above the gill-crate. +It corresponds to the upper “gullet-ganglion” or “primitive +brain” in other vermalia. Special sense-organs are either wanting +altogether or are only found in a very rudimentary form, as simple optic spots +and touch-corpuscles or tentacles that surround the mouth. The muscular system +is very slightly and irregularly developed. Immediately under the thin corium, +and closely connected with it, we find a thin muscle tube, as in the worms. On +the other hand, the Ascidia has a centralised heart, and in this respect it +seems to be more advanced than the Amphioxus. On the ventral side of the gut, +some distance behind the gill-crate, there is a spindle-shaped heart. It +retains permanently the simple tubular form that we find temporarily as the +first structure of the heart in the vertebrates. This simple heart of the +Ascidia has, however, a remarkable peculiarity. It contracts in alternate +directions. In all other animals the beat of the heart is always in the same +direction (generally from rear to front); it changes in the Ascidia to the +reverse direction. The heart contracts first from the rear to the front, stands +still for a minute, and then begins to beat the opposite way, now driving the +blood from front to rear; the two large vessels that start from either end of +the heart act alternately as arteries and veins. This feature is found in the +Tunicates alone. +</p> + +<p> +Of the other chief organs we have still to mention the sexual glands, which lie +right behind in the body-cavity. All the Ascidiæ are hermaphrodites. Each +individual has a male and a female gland, and so is able to fertilise itself. +The ripe ova (Fig. 221 <i>o</i>′) fall directly from the ovary (<i>o</i>) +into the mantle-cavity. The male sperm is conducted into this cavity from the +testicle (<i>t</i>) by a special duct (<i>vd</i>). Fertilisation is +accomplished here, and in many of the Ascidiæ developed embryos are found. +These are then ejected +<span class='pagenum'><a name="Page_191" id="Page_191"></a></span> +with the breathing-water through the cloaca (<i>q</i>), and so “born +alive.” +</p> + +<p> +If we now glance at the entire structure of the simple Ascidia (especially +<i>Phallusia, Cynthia,</i> etc.) and compare it with that of the Amphioxus, we +shall find that the two have few points of contact. It is true that the +fully-developed Ascidia resembles the Amphioxus in several important features +of its internal structure, and especially in the peculiar character of the +gill-crate and gut. But in most other features of organisation it is so far +removed from it, and is so unlike it in external appearance, that the really +close relationship of the two was not discovered until their embryology was +studied. We will now compare the embryonic development of the two animals, and +find to our great astonishment that the same embryonic form develops from the +ovum of the Amphioxus as from that of the Ascidia—a typical <i> +chordula.</i> +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap17"></a>Chapter XVII.<br/> +EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT</h2> + +<p> +The structural features that distinguish the vertebrates from the invertebrates +are so prominent that there was the greatest difficulty in the earlier stages +of classification in determining the affinity of these two great groups. When +scientists began to speak of the affinity of the various animal groups in more +than a figurative—in a genealogical—sense, this question came at +once to the front, and seemed to constitute one of the chief obstacles to the +carrying-out of the evolutionary theory. Even earlier, when they had studied +the relations of the chief groups, without any idea of real genealogical +connection, they believed they had found here and there among the invertebrates +points of contact with the vertebrates: some of the worms, especially, seemed +to approach the vertebrates in structure, such as the marine arrow-worm +(<i>Sagitta</i>). But on closer study the analogies proved untenable. When +Darwin gave an impulse to the construction of a real stem-history of the animal +kingdom by his reform of the theory of evolution, the solution of this problem +was found to be particularly difficult. When I made the first attempt in my +<i>General Morphology</i> (1866) to work out the theory and apply it to +classification, I found no problem of phylogeny that gave me so much trouble as +the linking of the vertebrates with the invertebrates. +</p> + +<p> +But just at this time the true link was discovered, and at a point where it was +least expected. Towards the end of 1866 two works of the Russian zoologist, +Kowalevsky, who had lived for some time at Naples, and studied the embryology +of the lower animals, were issued in the publications of the St. Petersburg +Academy. A fortunate accident had directed the attention of this able observer +almost simultaneously to the embryology of the lowest vertebrate, the +Amphioxus, and that of an invertebrate, the close affinity of which to the +Amphioxus had been least suspected, the Ascidia. To the extreme astonishment of +all zoologists who were interested in this important question, there turned out +to be the utmost resemblance in structure from the commencement of development +between these two very different animals—the lowest vertebrate and the +mis-shaped, sessile invertebrate. With this undeniable identity of ontogenesis, +which can be demonstrated to an astounding extent, we had, in virtue of the +biogenetic law, discovered the long-sought genealogical link, and definitely +identified the invertebrate group that represents the nearest blood-relatives +of the vertebrates. +<span class='pagenum'><a name="Page_192" id="Page_192"></a></span> +The discovery was confirmed by other zoologists, and there can no longer be any +doubt that of all the classes of invertebrates that of the Tunicates is most +closely related to the vertebrates, and of the Tunicates the nearest are the +Ascidiæ. We cannot say that the vertebrates are descended from the +Ascidiæ—and still less the reverse—but we can say that of all the +invertebrates it is the Tunicates, and, within this group, the Ascidiæ, that +are the nearest blood-relatives of the ancient stem-form of the vertebrates. We +must assume as the common ancestral group of both stems an extinct family of +the extensive vermalia-stem, the <i>Prochordonia</i> or <i>Prochordata</i> +(“primitive chorda-animals”). +</p> + +<p> +In order to appreciate fully this remarkable fact, and especially to secure the +sound basis we seek for the genealogical tree of the vertebrates, it is +necessary to study thoroughly the embryology of both these animals, and compare +the individual development of the Amphioxus step by step with that of the +Ascidia. We begin with the ontogeny of the Amphioxus. +</p> + +<p> +From the concordant observations of Kowalevsky at Naples and Hatschek at +Messina, it follows, firstly, that the ovum-segmentation and gastrulation of +the Amphioxus are of the simplest character. They take place in the same way as +we find them in many of the lower animals of different invertebrate stems, +which we have already described as original or primordial; the development of +the Ascidia is of the same type. Sexually mature specimens of the Amphioxus, +which are found in great quantities at Messina from April or May onwards, begin +as a rule to eject their sexual products in the evening; if you catch them +about the middle of a warm night and put them in a glass vessel with seawater, +they immediately eject through the mouth their accumulated sexual products, in +consequence of the disturbance. The males give out masses of sperm, and the +females discharge ova in such quantity that many of them stick to the fibrils +about their mouths. Both kinds of cells pass first into the mantle-cavity after +the opening of the gonads, proceed through the gill-clefts into the branchial +gut, and are discharged from this through the mouth. +</p> + +<p> +The ova are simply round cells. They are only 1/250 of an inch in diameter, and +thus are only half the size of the mammal ova, and have no distinctive +features. The clear protoplasm of the mature ovum is made so turbid by the +numbers of dark granules of food-yelk or deutoplasm scattered in it that it is +difficult to follow the process of fecundation and the behaviour of the two +nuclei during it (p. 51). The active elements of the male sperm, the +cone-shaped spermatozoa, are similar to those of most other animals (cf. Fig. +20). Fecundation takes place when these lively ciliated cells of the sperm +approach the ovum, and seek to penetrate into the yelk-matter or the cellular +substance of the ovum with their head-part—the thicker part of the cell +that encloses the nucleus. Only one spermatozoon can bore its way into the yelk +at one pole of the ovum-axis; its head or nucleus coalesces with the female +nucleus, which remains after the extrusion of the directive bodies from the +germinal vesicle. Thus is formed the “stem-nucleus,” or the nucleus +of the “stem-cell” (cytula, Fig. 2). This now undergoes total +segmentation, dividing into two, four, eight, sixteen, thirty-two cells, and so +on. In this way we get the spherical, mulberry-shaped body, which we call the +<i> morula.</i> +</p> + +<p> +The segmentation of the Amphioxus is not entirely regular, as was supposed +after the first observations of Kowalevsky (1866). It is not completely equal, +but a little unequal. As Hatschek afterwards found (1879), the +segmentation-cells only remain equal up to the morula-stage, the spherical body +of which consists of thirty-two cells. Then, as always happens in unequal +segmentation, the more sluggish vegetal cells are outstripped in the cleavage. +At the lower or vegetal pole of the ovum a crown of eight large entodermic +cells remains for a long time unchanged, while the other cells divide, owing to +the formation of a series of horizontal circles, into an increasing number of +crowns of sixteen cells each. Afterwards the segmentation-cells get more or +less irregularly displaced, while the segmentation-cavity enlarges in the +centre of the morula; in the end the former all lie on the surface of the +latter, so that the fœtus attains the familiar blastula shape and forms a +hollow ball, the wall of which consists of a single stratum of cells (Fig. 38 +<i> A–C</i>). This layer is the blastoderm, the simple epithelium from +the cells of which all the tissues of the body proceed. +</p> + +<p> +<span class='pagenum'><a name="Page_193" id="Page_193"></a></span> +These important early embryonic processes take place so quickly in the +Amphioxus that four or five hours after fecundation, or about midnight, the +spherical blastula is completed. A pit-like depression is then formed at the +vegetal pole of it, and in consequence of this the hollow sphere doubles on +itself (Fig. 38 <i>D</i>). This pit becomes deeper and deeper (Fig. 38 <i>E, +F</i>); at last the invagination (or doubling) is complete, and the inner or +folded part of the blastula-wall lies on the inside of the outer wall. We thus +get a hollow hemisphere, the thin wall of which is made up of two layers of +cells (Fig. 38 <i>E</i>). From hemispherical the body soon becomes almost +spherical once more, and then oval, the internal cavity enlarging considerably +and its mouth growing narrower (Fig. 213). The form which the Amphioxus-embryo +has thus reached is a real “cup-larva” or <i>gastrula,</i> of the +original simple type that we have previously described as the +“bell-gastrula” or <i>archigastrula</i> (Figs. 29–35). +</p> + +<p> +As in all the other animals that form an archigastrula, the whole body is +nothing but a simple gastric sac or stomach; its internal cavity is the +primitive gut (<i>progaster</i> or <i> archenteron,</i> Fig. 38 <i>g,</i> 35 +<i>d</i>), and its aperture the primitive mouth (<i>prostoma</i> or +<i>blastoporus, o</i>). The wall is at once gut-wall and body-wall. It is +composed of two simple cell-layers, the familiar primary germinal layers. The +inner layer or the invaginated part of the blastoderm, which immediately +encloses the gut-cavity is the entoderm, the inner or vegetal germ-layer, from +which develop the wall of the alimentary canal and all its appendages, the +cœlom-pouches, etc. (Figs. 35, 36 <i> i</i>). The outer stratum of cells, or +the non-invaginated part of the blastoderm, is the ectoderm, the outer or +animal germ-layer, which provides the outer skin (epidermis) and the nervous +system (<i>e</i>). The cells of the entoderm are much larger, darker, and more +fatty than those of the ectoderm, which are clearer and less rich in fatty +particles. Hence before and during invagination there is an increasing +differentiation of the inner from the outer layer. The animal cells of the +outer layer soon develop vibratory hairs; the vegetal cells of the inner layer +do so much later. A thread-like process grows out of each cell, and effects +continuous vibratory movements. By the vibrations of these slender hairs the +gastrula of the Amphioxus swims about in the sea, when it has pierced the thin +ovolemma, like the gastrula of many other animals (Fig. 36). As in many other +lower animals, the cells have only one whip-like hair each, and so are called +<i>flagellate</i> (whip) cells (in contrast with the <i>ciliated</i> cells, +which have a number of short lashes or cilia). +</p> + +<p> +In the further course of its rapid development the roundish bell-gastrula +becomes elongated, and begins to flatten on one side, parallel to the long +axis. The flattened side is the subsequent dorsal side; the opposite or ventral +side remains curved. The latter grows more quickly than the former, with the +result that the primitive mouth is forced to the dorsal side (Fig. 39). In the +middle of the dorsal surface a shallow longitudinal groove or furrow is formed +(Fig. 79), and the edges of the body rise up on each side of this groove in the +shape of two parallel swellings. This groove is, of course, the dorsal furrow, +and the swellings are the dorsal or medullary swellings; they form the first +structure of the central nervous system, the medullary tube. The medullary +swellings now rise higher; the groove between them becomes deeper and deeper. +The edges of the parallel swellings curve towards each other, and at last +unite, and the medullary tube is formed (Figs. 83 <i>m,</i> 84 <i>m</i>). Hence +the formation of a medullary tube out of the outer skin takes place in the +naked dorsal surface of the free-swimming larva of the Amphioxus in just the +same way as we have found in the embryo of man and the higher animals within +the fœtal membranes. +</p> + +<p> +Simultaneously with the construction of the medullary tube we have in the +Amphioxus-embryo the formation of the chorda, the cœlom-pouches, and the +mesoderm proceeding from their wall. These processes also take place with +characteristic simplicity and clearness, so that they are very instructive to +compare with the vermalia on the one hand and with the higher vertebrates on +the other. While the medullary groove is sinking in the middle line of the flat +dorsal side of the oval embryo, and its parallel edges unite to form the +ectodermic neural tube, the single chorda is formed directly underneath them, +and on each side of this a parallel longitudinal fold, from the dorsal wall of +the primitive gut. These longitudinal folds of the entoderm proceed from the +primitive mouth, or from its lower +<span class='pagenum'><a name="Page_194" id="Page_194"></a></span> +and hinder edge. Here we see at an early stage a couple of large entodermic +cells, which are distinguished from all the others by their great size, round +form, and fine-grained protoplasm; they are the two promesoblasts, or polar +cells of the mesoderm (Fig. 83 <i>p</i>). They indicate the original +starting-point of the two cœlom-pouches, which grow from this spot between the +inner and outer germinal layers, sever themselves from the primitive gut, and +provide the cellular material for the middle layer. +</p> + +<p> +Immediately after their formation the two cœlom-pouches of the Amphioxus are +divided into several parts by longitudinal and transverse folds. Each of the +primary pouches is divided into an upper dorsal and a lower ventral section by +a couple of lateral longitudinal folds (Fig. 82). But these are again divided +by several parallel transverse folds into a number of successive sacs, the +primitive segments or somites (formerly called by the unsuitable name of +“primitive vertebræ”). They have a different future above and +below. The upper or dorsal segments, the <i>episomites,</i> lose their cavity +later on, and form with their cells the muscular plates of the trunk. The lower +or ventral segments, the <i>hyposomites,</i> corresponding to the lateral +plates of the craniote-embryo, fuse together in the upper part owing to the +disappearance of their lateral walls, and thus form the later body-cavity +(metacœl); in the lower part they remain separate, and afterwards form the +segmental gonads. +</p> + +<p> +In the middle, between the two lateral cœlom-folds of the primitive gut, a +single central organ detaches from this at an early stage in the middle line of +its dorsal wall. This is the dorsal chorda (Figs. 83, 84 <i>ch</i>). This axial +rod, which is the first foundation of the later vertebral column in all the +vertebrates, and is the only representative of it in the Amphioxus, originates +from the entoderm. +</p> + +<p> +In consequence of these important folding-processes in the primitive gut, the +simple entodermic tube divides into four different sections:— I, +underneath, at the ventral side, the permanent alimentary canal or permanent +gut; II, above, at the dorsal side, the axial rod or chorda; and III, the two +cœlom-sacs, which immediately sub-divide into two +structures:—III<small>A</small>, above, on the dorsal side, the +<i>episomites,</i> the double row of primitive or muscular segments; and +III<small>B</small>, below, on each side of the gut, the <i>hyposomites,</i> +the two lateral plates that give rise to the sex-glands, and the cavities of +which partly unite to form the body-cavity. At the same time, the neural or +medullary tube is formed above the chorda, on the dorsal surface, by the +closing of the parallel medullary swellings. All these processes, which outline +the typical structure of the vertebrate, take place with astonishing rapidity +in the embryo of the Amphioxus; in the afternoon of the first day, or +twenty-four hours after fertilisation, the young vertebrate, the typical +embryo, is formed; it then has, as a rule, six to eight somites. +</p> + +<p> +The chief occurrence on the second day of development is the construction of +the two permanent openings of the gut—the mouth and anus. In the earlier +stages the alimentary tube is found to be entirely closed, after the closing of +the primitive mouth; it only communicates behind by the neurenteric canal with +the medullary tube. The permanent mouth is a secondary formation, at the +opposite end. Here, at the end of the second day, we find a pit-like depression +in the outer skin, which penetrates inwards into the closed gut. The anus is +formed behind in the same way a few hours later (in the vicinity of the +additional gastrula-mouth). In man and the higher vertebrates also the mouth +and anus are formed, as we have seen, as flat pits in the outer skin; they then +penetrate inwards, gradually becoming connected with the blind ends of the +closed gut-tube. During the second day the Amphioxus-embryo undergoes few other +changes. The number of primitive segments increases, and generally amounts to +fourteen, some forty-eight to fifty hours after impregnation. +</p> + +<p> +Almost simultaneously with the formation of the mouth the first gill-cleft +breaks through in the fore section of the Amphioxus-embryo (generally forty +hours after the commencement of development). It now begins to nourish itself +independently, as the food material stored up in the ovum is completely used +up. The further development of the free larvæ takes place very slowly, and +extends over several months. The body becomes much longer, and is compressed at +the sides, the head-end being broadened in a sort of triangle. Two rudimentary +sense-organs are developed in it. Inside we find the first blood-vessels, an +upper or dorsal vessel, corresponding to the aorta, between the gut and the +dorsal cord, and a lower or ventral +<span class='pagenum'><a name="Page_195" id="Page_195"></a></span> +vessel, corresponding to the subintestinal vein, at the lower border of the +gut. Now, the gills or respiratory organs also are formed at the fore-end of +the alimentary canal. The whole of the anterior or respiratory section of the +gut is converted into a gill-crate, which is pierced trellis-wise by numbers of +branchial-holes, as in the ascidia. This is done by the foremost part of the +gut-wall joining star-wise with the outer skin, and the formation of clefts at +the point of connection, piercing the wall and leading into the gut from +without. At first there are very few of these branchial clefts; but there are +soon a number of them—first in one, then in two, rows. The foremost +gill-cleft is the oldest. In the end we have a sort of lattice work of fine +gill-clefts, supported on a number of stiff branchial rods; these are connected +in pairs by transverse rods. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus222"></a> +<img src="images/fig222.gif" width="452" height="261" alt="Figs. 222-224. Transverse sections of +young Amphioxus-larvae." /> +<p class="caption">Figs. 222–224—<b>Transverse sections of +young Amphioxus-larvæ</b> (diagrammatic, from <i> Ralph.</i>) (Cf. also Fig. +216.) In Fig. 222 there is free communication from without with the gut-cavity +(<i>D</i>) through the gill-clefts (<i>K</i>). In Fig. 223 the lateral folds of +the body-wall, or the gill-covers, which grow downwards, are formed. In Fig. +224 these lateral folds have united underneath and joined their edges in the +middle line of the ventral side (<i>R</i> seam). The respiratory water now +passes from the gut-cavity (<i>D</i>) into the mantle-cavity (<i>A</i>). The +letters have the same meaning throughout: <i>N</i> medullary tube, <i>Ch</i> +chorda, <i> M</i> lateral muscles, <i>Lh</i> body-cavity, <i>G</i> part of the +body-cavity in which the sexual organs are subsequently formed. <i> D</i> +gut-cavity, clothed with the gut-gland layer (<i>a</i>). A mantle-cavity, +<i>K</i> gill-clefts, <i>b</i>=<i>E</i> epidermis, <i>E</i><sub>1</sub> the +same as visceral epithelium of the mantle-cavity, <i>E</i><sub>2</sub> as +parietal epithelium of the mantle-cavity.</p> +</div> + +<p> +At an early stage of embryonic development the structure of the Amphioxus-larva +is substantially the same as the ideal picture we have previously formed of the +“Primitive Vertebrate” (Figs. 98–102). But the body +afterwards undergoes various modifications, especially in the fore-part. These +modifications do not concern us, as they depend on special adaptations, and do +not affect the hereditary vertebrate type. When the free-swimming +Amphioxus-larva is three months old, it abandons its pelagic habits and changes +into the young animal that lives in the sand. In spite of its smallness +(one-eighth of an inch), it has substantially the same structure as the adult. +As regards the remaining organs of the Amphioxus, we need only mention that the +gonads or sexual glands are developed very late, immediately out of the inner +cell-layer of the +<span class='pagenum'><a name="Page_196" id="Page_196"></a></span> +body-cavity. Although we can find afterwards no continuation of the body-cavity +(Fig. 216 <i>U</i>) in the lateral walls of the mantle-cavity, in the +gill-covers or mantle-folds (Fig. 224 <i>U</i>), there is one present in the +beginning (Fig. 224 <i>Lh</i>). The sexual cells are formed below, at the +bottom of this continuation (Fig. 224 <i>S</i>). For the rest, the subsequent +development into the adult Amphioxus of the larva we have followed is so simple +that we need not go further into it here. +</p> + +<p> +We may now turn to the embryology of the Ascidia, an animal that seems to stand +so much lower and to be so much more simply organised, remaining for the +greater part of its life attached to the bottom of the sea like a shapeless +lump. It was a fortunate accident that Kowalevsky first examined just those +larger specimens of the Ascidiæ that show most clearly the relationship of the +vertebrates to the invertebrates, and the larvæ of which behave exactly like +those of the Amphioxus in the first stages of development. This resemblance is +so close in the main features that we have only to repeat what we have already +said of the ontogenesis of the Amphioxus. +</p> + +<p> +The ovum of the larger Ascidia (<i>Phallusia, Cynthia,</i> etc.) is a simple +round cell of 1/250 to 1/125 of an inch in diameter. In the thick fine-grained +yelk we find a clear round germinal vesicle of about 1/750 of an inch in +diameter, and this encloses a small embryonic spot or nucleolus. Inside the +membrane that surrounds the ovum, the stem-cell of the Ascidia, after +fecundation, passes through just the same metamorphoses as the stem-cell of the +Amphioxus. It undergoes total segmentation; it divides into two, four, eight, +sixteen, thirty-two cells, and so on. By continued total cleavage the morula, +or mulberry-shaped cluster of cells, is formed. Fluid gathers inside it, and +thus we get once more a globular vesicle (the blastula); the wall of this is a +single stratum of cells, the blastoderm. A real gastrula (a simple +bell-gastrula) is formed from the blastula by invagination, in the same way as +in the amphioxus. +</p> + +<p> +Up to this there is no definite ground in the embryology of the Ascidiæ for +bringing them into close relationship with the Vertebrates; the same gastrula +is formed in the same way in many other animals of different stems. But we now +find an embryonic process that is peculiar to the Vertebrates, and that proves +irrefragably the affinity of the Ascidiæ to the Vertebrates. From the epidermis +of the gastrula a <i>medullary tube</i> is formed on the dorsal side, and, +between this and the primitive gut, a <i>chorda</i>; these are the organs that +are otherwise only found in Vertebrates. The formation of these very important +organs takes place in the Ascidia-gastrula in precisely the same way as in that +of the Amphioxus. In the Ascidia (as in the other case) the oval gastrula is +first flattened on one side—the subsequent dorsal side. A groove or +furrow (the medullary groove) is sunk in the middle line of the flat surface, +and two parallel longitudinal swellings arise on either side from the skin +layer. These medullary swellings join together over the furrow, and form a +tube; in this case, again, the neural or medullary tube is at first open in +front, and connected with the primitive gut behind by the neurenteric canal. +Further, in the Ascidia-larva also the two permanent apertures of the +alimentary canal only appear later, as independent and new formations. The +permanent mouth does not develop from the primitive mouth of the gastrula; this +primitive mouth closes up, and the later anus is formed near it by invagination +from without, on the hinder end of the body, opposite to the aperture of the +medullary tube. +</p> + +<p> +During these important processes, that take place in just the same way in the +Amphioxus, a tail-like projection grows out of the posterior end of the +larva-body, and the larva folds itself up within the round ovolemma in such a +way that the dorsal side is curved and the tail is forced on to the ventral +side. In this tail is developed—starting from the primitive gut—a +cylindrical string of cells, the fore end of which pushes into the body of the +larva, between the alimentary canal and the neural canal, and is no other than +the chorda dorsalis. This important organ had hitherto been found only in the +Vertebrates, not a single trace of it being discoverable in the Invertebrates. +At first the chorda only consists of a single row of large entodermic cells. It +is afterwards composed of several rows of cells. In the Ascidia-larva, also, +the chorda develops from the dorsal middle part of the primitive gut, while the +two cœlom-pouches detach themselves from it on both sides. The simple +body-cavity is formed by the coalescence of the two. +</p> + +<p> +When the Ascidia-larva has attained +<span class='pagenum'><a name="Page_197" id="Page_197"></a></span> +this stage of development it begins to move about in the ovolemma. This causes +the membrane to burst. The larva emerges from it, and swims about in the sea by +means of its oar-like tail. These free-swimming larvæ of the Ascidia have been +known for a long time. They were first observed by Darwin during his voyage +round the world in 1833. They resemble tadpoles in outward appearance, and use +their tails as oars, as the tadpoles do. However, this lively and +highly-developed condition does not last long. At first there is a progressive +development; the foremost part of the medullary tube enlarges into a brain, and +inside this two single sense-organs are developed, a dorsal auditory vesicle +and a ventral eye. Then a heart is formed on the ventral side of the animal, or +the lower wall of the gut, in the same simple form and at the same spot at +which the heart is developed in man and all the other vertebrates. In the lower +muscular wall of the gut we find a weal-like thickening, a solid, +spindle-shaped string of cells, which becomes hollow in the centre; it begins +to contract in different directions, now forward and now backward, as is the +case with the adult Ascidia. In this way the sanguineous fluid accumulated in +the hollow muscular tube is driven in alternate directions into the +blood-vessels, which develop at both ends of the cardiac tube. One principal +vessel runs along the dorsal side of the gut, another along its ventral side. +The former corresponds to the aorta and the dorsal vessel in the worms. The +other corresponds to the subintestinal vein and the ventral vessel of the +worms. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus225"></a> +<img src="images/fig225.gif" width="147" height="391" alt="Fig.225. An +Appendicaria (Copelata), seen from the left." /> +<p class="caption">Fig. 225—<b>An Appendicaria (Copelata),</b> seen +from the left. <i>m</i> mouth, <i>k</i> branchial gut, <i>o</i> gullet, <i> +v</i> stomach, <i>a</i> anus, <i>n</i> brain (ganglion above the gullet), +<i>g</i> auditory vesicle, <i>f</i> ciliated groove under the gills, <i>h</i> +heart, <i>t</i> testicles, <i>e</i> ovary, <i> c</i> chorda, <i>s</i> +tail.</p> +</div> + +<p> +With the formation of these organs the progressive development of the Ascidia +comes to an end, and degeneration sets in. The free-swimming larva sinks to the +floor of the sea, abandons its locomotive habits, and attaches itself to +stones, marine plants, mussel-shells, corals, and other objects; this is done +with the part of the body that was foremost in movement. The attachment is +effected by a number of out-growths, usually three, which can be seen even in +the free-swimming larva. The tail is lost, as there is no further use for it. +It undergoes a fatty degeneration, and disappears with the chorda dorsalis. The +tailless body changes into an unshapely tube, and, by the atrophy of some parts +and the modification of others, gradually assumes the appearance we have +already described. +</p> + +<p> +Among the living Tunicates there is a very interesting group of small animals +that remain throughout life at the stage of development of the tailed, free +Ascidia-larva, and swim about briskly in the sea by means of their broad +oar-tail. These are the remarkable Copelata (<i>Appendicaria</i> and +<i>Vexillaria,</i> Fig. 225). They are the only living Vertebrates that have +throughout life a chorda dorsalis and a neural string above it; the latter must +be regarded as the prolongation of the cerebral ganglion and the equivalent of +the medullary tube. Their branchial gut also opens directly outwards by a pair +of +<span class='pagenum'><a name="Page_198" id="Page_198"></a></span> +branchial clefts. These instructive Copelata, comparable to permanent +Ascidia-larvæ, come next to the extinct Prochordonia, those ancient worms which +we must regard as the common ancestors of the Tunicates and Vertebrates. The +chorda of the Appendicaria is a long, cylindrical string (Fig. 225 <i> c</i>), +and serves as an attachment for the muscles that work the flat oar-tail. +</p> + +<p> +Among the various modifications which the Ascidia-larva undergoes after its +establishment at the sea-floor, the most interesting (after the loss of the +axial rod) is the atrophy of one of its chief organs, the medullary tube. In +the Amphioxus the spinal marrow continues to develop, but in the Ascidia the +tube soon shrinks into a small and insignificant nervous ganglion that lies +above the mouth and the gill-crate, and is in accord with the extremely slight +mental power of the animal. This insignificant relic of the medullary tube +seems to be quite beyond comparison with the nervous centre of the vertebrate, +yet it started from the same structure as the spinal cord of the Amphioxus. The +sense-organs that had been developed in the fore part of the neural tube are +also lost; no trace of which can be found in the adult Ascidia. On the other +hand, the alimentary canal becomes a most extensive organ. It divides presently +into two sections—a wide fore or branchial gut that serves for +respiration, and a narrower hind or hepatic gut that accomplishes digestion. +The branchial or head-gut of the Ascidia is small at first, and opens directly +outwards only by a couple of lateral ducts or gill-clefts—a permanent +arrangement in the Copelata. The gill-clefts are developed in the same way as +in the Amphioxus. As their number greatly increases we get a large gill-crate, +pierced like lattice work. In the middle line of its ventral side we find the +hypobranchial groove. The mantle or cloaca-cavity (the atrium) that surrounds +the gill-crate is also formed in the same way in the Ascidia as in the +Amphioxus. The ejection-opening of this peribranchial cavity corresponds to the +branchial pore of the Amphioxus. In the adult Ascidia the branchial gut and the +heart on its ventral side are almost the only organs that recall the original +affinity with the vertebrates. +</p> + +<p> +The further development of the Ascidia in detail has no particular interest for +us, and we will not go into it. The chief result that we obtain from its +embryology is the complete agreement with that of the Amphioxus in the earliest +and most important embryonic stages. They do not begin to diverge until after +the medullary tube and alimentary canal, and the axial rod with the muscles +between the two, have been formed. The Amphioxus continues to advance, and +resembles the embryonic forms of the higher vertebrates; the Ascidia +degenerates more and more, and at last, in its adult condition, has the +appearance of a very imperfect invertebrate. +</p> + +<p> +If we now look back on all the remarkable features we have encountered in the +structure and the embryonic development of the Amphioxus and the Ascidia, and +compare them with the features of man’s embryonic development which we +have previously studied, it will be clear that I have not exaggerated the +importance of these very interesting animals. It is evident that the Amphioxus +from the vertebrate side and the Ascidia from the invertebrate form the bridge +by which we can span the deep gulf that separates the two great divisions of +the animal kingdom. The radical agreement of the lancelet and the sea-squirt in +the first and most important stages of development shows something more than +their close anatomic affinity and their proximity in classification; it shows +also their real blood-relationship and their common origin from one and the +same stem-form. In this way, it throws considerable light on the oldest roots +of man’s genealogical tree. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap18"></a> +<span class='pagenum'><a name="Page_199" id="Page_199"></a></span> +Chapter XVIII.<br/> +DURATION OF THE HISTORY OF OUR STEM</h2> + +<p> +Our comparative investigation of the anatomy and ontogeny of the Amphioxus and +Ascidia has given us invaluable assistance. We have, in the first place, +bridged the wide gulf that has existed up to the present between the +Vertebrates and Invertebrates; and, in the second place, we have discovered in +the embryology of the Amphioxus a number of ancient evolutionary stages that +have long since disappeared from human embryology, and have been lost, in +virtue of the law of curtailed heredity. The chief of these stages are the +spherical blastula (in its simplest primary form), and the succeeding +archigastrula, the pure, original form of the <i>gastrula</i> which the +Amphioxus has preserved to this day, and which we find in the same form in a +number of Invertebrates of various classes. Not less important are the later +embryonic forms of the cœlomula, the chordula, etc. +</p> + +<p> +Thus the embryology of the Amphioxus and the Ascidia has so much increased our +knowledge of man’s stem-history that, although our empirical information +is still very incomplete, there is now no defect of any great consequence in +it. We may now, therefore, approach our proper task, and reconstruct the +phylogeny of man in its chief lines with the aid of this evidence of +comparative anatomy and ontogeny. In this the reader will soon see the immense +importance of the direct application of the biogenetic law. But before we enter +upon the work it will be useful to make a few general observations that are +necessary to understand the processes aright. +</p> + +<p> +We must say a few words with regard to the period in which the human race was +evolved from the animal kingdom. The first thought that occurs to one in this +connection is the vast difference between the duration of man’s ontogeny +and phylogeny. The individual man needs only nine months for his complete +development, from the fecundation of the ovum to the moment when he leaves the +maternal womb. The human embryo runs its whole course in the brief space of +forty weeks (as a rule, 280 days). In many other mammals the time of the +embryonic development is much the same as in man—for instance, in the +cow. In the horse and ass it takes a little longer, forty-three to forty-five +weeks; in the camel, thirteen months. In the largest mammals, the embryo needs +a much longer period for its development in the womb—a year and a half in +the rhinoceros, and ninety weeks in the elephant. In these cases pregnancy +lasts twice as long as in the case of man, or one and three-quarter years. In +the smaller mammals the embryonic period is much shorter. The smallest mammals, +the dwarf-mice, develop in three weeks; hares in four weeks, rats and marmots +in five weeks, the dog in nine, the pig in seventeen, the sheep in twenty-one +and the goat in thirty-six. Birds develop still more quickly. The chick only +needs, in normal circumstances, three weeks for its full development. The duck +needs twenty-five days, the turkey twenty-seven, the peacock thirty-one, the +swan forty-two, and the cassowary sixty-five. The smallest bird, the +humming-bird, leaves the egg after twelve days. Hence the duration of +individual development within the fœtal membranes is, in the mammals and birds, +clearly related to the absolute size of the body of the animal in question. But +this is not the only determining feature. There are a number of other +circumstances that have an influence on the period of embryonic development. In +the Amphioxus the earliest and most important embryonic processes take place so +rapidly that the blastula is formed in four hours, the gastrula in six, and the +typical vertebrate form in twenty-four. +</p> + +<p> +In every case the duration of ontogeny shrinks into insignificance when we +compare it with the enormous period that has been necessary for phylogeny, or +the gradual development of the ancestral series. This period is not measured by +years or centuries, but by thousands and millions of years. Many millions of +years had to pass before the most advanced +<span class='pagenum'><a name="Page_200" id="Page_200"></a></span> +vertebrate, man, was evolved, step by step, from his ancient unicellular +ancestors. The opponents of evolution, who declare that this gradual +development of the human form from lower animal forms, and ultimately from a +unicellular organism, is an incredible miracle, forget that the same miracle +takes place within the space of mine months in the embryonic development of +every human being. Each of us has, in the forty weeks—properly speaking, +in the first four weeks—of his development in the womb, passed through +the same series of transformations that our animal ancestors underwent in the +course of millions of years. +</p> + +<p> +It is impossible to determine even approximately, in hundreds or even thousands +of years, the real and absolute duration of the phylogenetic period. But for +some time now we have, through the research of geologists, been in a position +to assign the relative length of the various sections of the organic history of +the earth. The immediate data for determining this relative length of the +geological periods are found in the thickness of the sedimentary +strata—the strata that have been formed at the bottom of the sea or in +fresh water from the mud or slime deposited there. These successive layers of +limestone, sandstone, slate, marl, etc., which make up the greater part of the +rocks, and are often several thousand feet thick, give us a standard for +computing the relative length of the various periods. +</p> + +<p> +To make the point quite clear, I must say a word about the evolution of the +earth in general, and point out briefly the chief features of the story. In the +first place, we encounter the principle that on our planet organic life began +to exist at a definite period. That statement is no longer disputed by any +competent geologist or biologist. The organic history of the earth could not +commence until it was possible for water to settle on our planet in fluid +condition. Every organism, without exception, needs fluid water as a condition +of existence, and contains a considerable quantity of it. Our own body, when +fully formed, contains sixty to seventy per cent of water in its tissues, and +only thirty to forty per cent of solid matter. There is even more water in the +body of the child, and still more in the embryo. In the earlier stages of +development the human fœtus contains more than ninety per cent of water, and +not ten per cent of solids. In the lower marine animals, especially certain +medusæ, the body consists to the extent of more than ninety-nine per cent of +sea-water, and has not one per cent of solid matter. No organism can exist or +discharge its functions without water. No water, no life! +</p> + +<p> +But fluid water, on which the existence of life primarily depends, could not +exist on our planet until the temperature of the surface of the incandescent +sphere had sunk to a certain point. Up to that time it remained in the form of +steam. But as soon as the first fluid water could be condensed from the +envelope of steam, it began its geological action, and has continued down to +the present day to modify the solid crust of the earth. The final outcome of +this incessant action of the water—wearing down and dissolving the rocks +in the form of rain, hail, snow, and ice, as running stream or boiling +surge—is the formation of mud. As Huxley says in his admirable +<i>Lectures on the Causes of Phenomena in Organic Nature,</i> the chief +document as to the past history of our earth is mud; the question of the +history of past ages resolves itself into a question about the formation of +mud. +</p> + +<p> +As I have said, it is possible to form an approximate idea of the relative age +of the various strata by comparing them at different parts of the earth’s +surface. Geologists have long been agreed that there is a definite historical +succession of the different strata. The various superimposed layers correspond +to successive periods in the organic history of the earth, in which they were +deposited in the form of mud at the bottom of the sea. The mud was gradually +converted into stone. This was lifted out of the water owing to variations in +the earth’s surface, and formed the mountains. As a rule, four or five +great divisions are distinguished in the organic history of the earth, +corresponding to the larger and smaller groups of the sedimentary strata. The +larger periods are then sub-divided into a series of smaller ones, which +usually number from twelve to fifteen. The comparative thickness of the groups +of strata enables us to make an approximate calculation of the relative length +of these various periods of time. We cannot say, it is true, “In a +century a stratum of a certain thickness (about two feet) is formed on the +average; therefore, a layer 1000 feet thick must be 500,000 years old.” +Different strata of the same thickness may need very different periods for +their formation. But from +<span class='pagenum'><a name="Page_201" id="Page_201"></a></span> +the thickness or size of the stratum we can draw some conclusion as to the +<i>relative</i> length of the period. +</p> + +<p> +The first and oldest of the four or five chief divisions of the organic history +of the earth is called the primordial, archaic, or archeozoic period. If we +compute the total average thickness of the sedimentary strata at about 130,000 +feet, this first period comprises 70,000 feet, or the greater part of the +whole. For this and other reasons we may at once conclude that the +corresponding primordial or archeolithic period must have been in itself much +longer than the whole of the remaining periods together, from its close to the +present day. It was probably much longer than the figures I have quoted (7:6) +indicate—possibly 9:6. Of late years the thickness of the archaic rocks +has been put at 90,000 feet. +</p> + +<p class="center"> +<b> SYNOPSIS OF THE PALEONTOLOGICAL FORMATIONS,<br/> +OR THE FOSSILIFEROUS STRATA OF THE CRUST</b> +</p> + +<table border="1" cellspacing="0" +cellpadding="4" summary="Column 1 Groups; Column 2 Systems, Column 3 +Formations, Column 4 Synonyms of Formations."> +<tr> +<td align="center" valign="bottom"> <b>Groups</b> </td> <td align="center" +valign="bottom"> <b>Systems</b> </td> <td align="center" valign="bottom"> +<b>Formations</b> </td> <td align="center" valign="bottom"> <b> Synonyms of +<br/> Formations </b> </td> </tr> + +<tr> +<td align="center" rowspan="2" valign="middle"> + +V. Anthropolithic <br/> groups, or <br/> anthropozoic <br/> (quaternary) <br/> +groups of strata. </td> <td align="center"> XIV. Recent <br/> (alluvium). </td> +<td> 38. Present <br/> 37. Recent </td> <td> Upper +alluvial <br/> Lower alluvial </td> </tr> + +<tr> +<td align="center"> XIII. Pleistocene <br/> (diluvium) </td> <td> +36. Post-glacial <br/> 35. Glacial </td> <td> Upper diluvial <br/> +Lower diluvial </td> </tr> + +<tr> +<td align="center" rowspan="4" valign="middle"> + +IV. Cenolithic <br/> groups, or <br/> cenozoic <br/> (tertiary) <br/> groups of +strata. </td> <td align="center"> XII. Pliocene <br/> (neo-tertiary) </td> <td +align="left"> 34. Arverne <br/> 33. Subapennine </td> <td> Upper +pliocene <br/> Lower pliocene </td> </tr> + +<tr> +<td align="center"> XI. Miocene <br/> (middle tertiary) </td> <td> +32. Falun <br/> 31. Limbourg </td> <td> Upper miocene <br/> Lower +miocene </td> </tr> + +<tr> +<td align="center"> Xb. Oligocene <br/> (old tertiary) </td> <td> +30. Aquitaine <br/> 29. Ligurium </td> <td> Upper oligocene <br/> +Lower oligocene </td> </tr> + +<tr> +<td align="center" valign="middle"> Xa. Eocene <br/> (primitive tertiary) </td> +<td> 28. Gypsum <br/> 27. Coarse chalk <br/> 26. London clay </td> +<td> Upper eocene <br/> Middle eocene <br/> Lower eocene </td> +</tr> + +<tr> +<td align="center" rowspan="3" valign="middle"> + +III. Mesolithic <br/> groups, or <br/> mesozoic <br/> (secondary) <br/> groups +of strata. </td> <td align="center" valign="middle"> IX. Chalk <br/> +(cretaceous) </td> <td> 25. White chalk <br/> 24. Green sand <br/> +23. Neoconian <br/> 22. Wealden </td> <td> Upper cretaceous <br/> +Middle cretaceous <br/> Lower cretaceous <br/> Weald formation </td> </tr> + +<tr> +<td align="center" valign="middle">VIII. Jurassic</td> <td> 21. +Portland <br/> 20. Oxford <br/> 19. Bath <br/> 18. Lias </td> <td> +Upper oolithic <br/> Middle oolithic <br/> Lower oolithic <br/> Liassic </td> +</tr> + +<tr> +<td align="center" valign="middle">VII. Triassic</td> <td> 17. +Keuper <br/> 16. Muschelkalk <br/> 15. Bunter </td> <td> Upper +triassic <br/> Middle triassic <br/> Lower triassic </td> </tr> + +<tr> +<td align="center" rowspan="4" valign="middle"> + +II. Paleolithic <br/> groups, or <br/> paleozoic <br/> (primary) <br/> groups +of strata. </td> <td align="center">VIb. Permian</td> <td> 14. +Zechstein <br/> 13. Neurot sand </td> <td> Upper permian <br/> +Lower permian </td> </tr> + +<tr> +<td align="center" valign="middle"> VIa. Carboniferous <br/> coal-measures) +</td> <td> 12. Carboniferous <br/> + sandstone <br/> 11. Carboniferous <br/> + limestone </td> <td> Upper +carboniferous <br/> <br/> Lower carboniferous </td> </tr> + +<tr> +<td align="center">V. Devonian</td> <td> 10. Pilton <br/> + 9. Ilfracombe <br/> 8. Linton </td> <td> +Upper devonian <br/> Middle devonian <br/> Lower devonian </td> </tr> + +<tr> +<td align="center" valign="middle">IV. Silurian</td> <td> + 7. Ludlow <br/> 6. Wenlock <br/> 5. +Llandeilo </td> <td> Upper silurian <br/> Middle silurian <br/> +Lower silurian </td> </tr> + +<tr> +<td align="center" rowspan="2" valign="middle"> + +I. Archeolithic <br/> groups, or <br/> archeozoic <br/> (primordial) <br/> +groups of strata. </td> <td align="center" valign="middle">III. Cambrian</td> +<td> 4. Potsdam <br/> 3. Longmynd </td> +<td> Upper cambrian <br/> Lower cambrian </td> </tr> + +<tr> +<td align="center" valign="middle"> II. Huronian <br/> I. Laurentian </td> <td +align="left"> 2. Labrador <br/> 1. Ottawa </td> <td +align="left"> Upper laurentian <br/> Lower laurentian </td> </tr> + +</table> +<p> +The primordial period falls into three subordinate sections—the +Laurentian, Huronian, and Cambrian, corresponding to the three chief groups of +rocks that comprise the archaic formation. The immense period during which +these rocks were forming in the primitive ocean probably comprises more than +50,000,000 years. At the commencement of it the oldest and simplest organisms +were formed by spontaneous generation—the Monera, with which the history +of life on our planet opened. From these were first developed unicellular +organisms of the simplest character, the Protophyta +<span class='pagenum'><a name="Page_202" id="Page_202"></a></span> +and Protozoa (paulotomea, amœbæ, rhizopods, infusoria, and other Protists). +During this period the whole of the invertebrate ancestors of the human race +were evolved from the unicellular organisms. We can deduce this from the fact +that we already find remains of fossilised fishes (Selachii and Ganoids) +towards the close of the following Silurian period. These are much more +advanced and much younger than the lowest vertebrate, the Amphioxus, and the +numerous skull-less vertebrates, related to the Amphioxus, that must have lived +at that time. The whole of the invertebrate ancestors of the human race must +have preceded these. +</p> + +<p> +The primordial age is followed by a much shorter division, the <i>paleozoic</i> +or Primary age. It is divided into four long periods, the Silurian, Devonian, +Carboniferous, and Permian. The Silurian strata are particularly interesting +because they contain the first fossil traces of vertebrates—teeth and +scales of Selachii ( <i>Palæodus</i>) in the lower, and Ganoids ( +<i>Pteraspis</i>) in the upper Silurian. During the Devonian period the +“old red sandstone” was formed; during the Carboniferous period +were deposited the vast coal-measures that yield us our chief combustive +material; in the Permian (or the Dyas), in fine, the new red sandstone, the +Zechstein (magnesian limestone), and the Kupferschiefer (marl-slate) were +formed. The collective depth of these strata is put at 40,000 to 45,000 feet. +In any case, the paleozoic age, taken as a whole, was much shorter than the +preceding and much longer than the subsequent periods. The strata that were +deposited during this primary epoch contain a large number of fossils; besides +the invertebrate species there are a good many vertebrates, and the fishes +preponderate. There were so many fishes, especially primitive fishes (of the +shark type) and plated fishes, during the Devonian, and also during the +Carboniferous and Permian periods, that we may describe the whole paleozoic +period as “the age of fishes.” Among the paleozoic plated fishes or +Ganoids the Crossopterygii and the Ctenodipterina (dipneusts) are of great +importance. +</p> + +<p> +During this period some of the fishes began to adapt themselves to living on +land, and so gave rise to the class of the amphibia. We find in the +Carboniferous period fossilised remains of five-toed amphibia, the oldest +terrestrial, air-breathing vertebrates. These amphibia increase in variety in +the Permian epoch. Towards the close of it we find the first Amniotes, the +ancestors of the three higher classes of Vertebrates. These are lizard-like +animals; the first to be discovered was the <i>Proterosaurus,</i> from the marl +at Eisenach. The rise of the earliest Amniotes, among which must have been the +common ancestor of the reptiles, birds, and mammals, is put back towards the +close of the paleozoic age by the discovery of these reptile remains. The +ancestors of our race during this period were at first represented by true +fishes, then by dipneusts and amphibia, and finally by the earliest Amniotes, +or the Protamniotes. +</p> + +<p> +The third chief section of the organic history of the earth is the +<i>Mesozoic</i> or Secondary period. This again is subdivided into three +divisions Triassic, Jurassic, and Cretaceous. The thickness of the strata that +were deposited in this period, from the beginning of the Triassic to the end of +the Cretaceous period, is altogether about 15,000 feet, or not half as much as +the paleozoic deposits. During this period there was a very brisk and manifold +development in all branches of the animal kingdom. There were especially a +number of new and interesting forms evolved in the vertebrate stem. Bony fishes +( <i>Teleostei</i>) make their first appearance. Reptiles are found in +extraordinary variety and number; the extinct giant-serpents (dinosauria), the +sea-serpents (halisauria), and the flying lizards (pterosauria) are the most +remarkable and best known of these. On account of this predominance of the +reptile-class, the period is called “the age of reptiles.” But the +bird-class was also evolved during this period; they certainly originated from +some division of the lizard-like reptiles. This is proved by the embryological +identity of the birds and reptiles and their comparative anatomy, and, among +other features, from the circumstance that in this period there were birds with +teeth in their jaws and with tails like lizards (Archeopteryx, Odontornis). +</p> + +<p> +Finally, the most advanced and (for us) the most important class of the +vertebrates, the mammals, made their appearance during the mesozoic period. The +earliest fossil remains of them were found in the latest Triassic +strata—lower jaws of small ungulates and marsupials. More numerous +remains are found a little later +<span class='pagenum'><a name="Page_203" id="Page_203"></a></span> +in the Jurassic, and some in the Cretaceous. All the mammal remains that we +have from this section belong to the lower promammals and marsupials; among +these were most certainly the ancestors of the human race. On the other hand, +we have not found a single indisputable fossil of any higher mammal (a +placental) in the whole of this period. This division of the mammals, which +includes man, was not developed until later, towards the close of this or in +the following period. +</p> + +<p> +The fourth section of the organic history of the earth, the Tertiary or +<i>Cenozoic</i> age, was much shorter than the preceding. The strata that were +deposited during this period have a collective thickness of only about 3,000 +feet. It is subdivided into four sections—the Eocene, Oligocene, Miocene, +and Pliocene. During these periods there was a very varied development of +higher plant and animal forms; the fauna and flora of our planet approached +nearer and nearer to the character that they bear to-day. In particular, the +most advanced class, the mammals, began to preponderate. Hence the Tertiary +period may be called “the age of mammals.” The highest section of +this class, the placentals, now made their appearance; to this group the human +race belongs. The first appearance of man, or, to be more precise, the +development of man from some closely-related group of apes, probably falls in +either the miocene or the pliocene period, the middle or the last section of +the Tertiary period. Others believe that man properly so-called—man +endowed with speech—was not evolved from the non-speaking ape-man ( +<i>Pithecanthropus</i>) until the following, the anthropozoic, age. +</p> + +<p> +In this fifth and last section of the organic history of the earth we have the +full development and dispersion of the various races of men, and so it is +called the <i>Anthropozoic</i> as well as the <i>Quaternary</i> period. In the +imperfect condition of paleontological and ethnographical science we cannot as +yet give a confident answer to the question whether the evolution of the human +race from some extinct ape or lemur took place at the beginning of this or +towards the middle or the end of the Tertiary period. However, this much is +certain: the development of civilisation falls in the anthropozoic age, and +this is merely an insignificant fraction of the vast period of the whole +history of life. When we remember this, it seems ridiculous to restrict the +word “history” to the civilised period. If we divide into a hundred +equal parts the whole period of the history of life, from the spontaneous +generation of the first Monera to the present day, and if we then represent the +relative duration of the five chief sections or ages, as calculated from the +average thickness of the strata they contain, as percentages of this, we get +something like the following relation:— +</p> + +<table class="text" border="0" cellspacing="0" cellpadding="0" +summary="Relative duration of the five chief sections or ages, as calculated +from the average thickness of the strata they contain."> +<tr> +<td align="right" valign="top"> I. <br/> +II. <br/> III. <br/> +IV. <br/> V. </td> <td +align="left" valign="top"> Archeolithic or archeozoic (primordial) age <br/> +Paleolithic or paleozoic (primary) age <br/> Mesolithic or mesozoic (secondary) +age <br/> Cenolithic or cenozoic (tertiary) age <br/> Anthropolithic or +anthropozoic (quaternary) age </td> <td align="right"> 53.6 <br/> 32.1 <br/> +11.5 <br/> 2.3 <br/> 0.5 <br/> ——— <br/> 100.0 </td> </tr> +</table> + +<p> +In any case, the “historical period” is an insignificant quantity +compared with the vast length of the preceding ages, in which there was no +question of human existence on our planet. Even the important Cenozoic or +Tertiary period, in which the first placentals or higher mammals appear, +probably amounts to little over two per cent of the whole organic age. +</p> + +<p> +Before we approach our proper task, and, with the aid of our ontogenetic +acquirements and the biogenetic law, follow step by step the paleontological +development of our animal ancestors, let us glance for a moment at another, and +apparently quite remote, branch of science, a general consideration of which +will help us in the solving of a difficult problem. I mean the science of +comparative philology. Since Darwin gave new life to biology by his theory of +selection, and raised the question of evolution on all sides, it has often been +pointed out that there is a remarkable analogy between the development of +languages and the evolution of species. The comparison is perfectly just and +very instructive. We could hardly find a better analogy when we are dealing +with some of the difficult and obscure features of the evolution of species. In +both cases we find the action of the same natural laws. +</p> + +<p> +All philologists of any competence in their science now agree that all human +languages have been gradually evolved from very rudimentary beginnings. The +<span class='pagenum'><a name="Page_204" id="Page_204"></a></span> +idea that speech is a gift of the gods—an idea held by distinguished +authorities only fifty years ago—is now generally abandoned, and only +supported by theologians and others who admit no natural development whatever. +Speech has been developed simultaneously with its organs, the larynx and +tongue, and with the functions of the brain. Hence it will be quite natural to +find in the evolution and classification of languages the same features as in +the evolution and classification of organic species. The various groups of +languages that are distinguished in philology as primitive, fundamental, +parent, and daughter languages, dialects, etc., correspond entirely in their +development to the different categories which we classify in zoology and botany +as stems, classes, orders, families, genera, species, and varieties. The +relation of these groups, partly co-ordinate and partly subordinate, in the +general scheme is just the same in both cases; and the evolution follows the +same lines in both. +</p> + +<p> +When, with the assistance of this tree, we follow the formation of the various +languages that have been developed from the common root of the ancient +Indo-Germanic tongue, we get a very clear idea of their phylogeny. We shall see +at the same time how analogous this is to the development of the various groups +of vertebrates that have arisen from the common stem-form of the primitive +vertebrate. The ancient Indo-Germanic root-language divided first into two +principal stems—the Slavo-Germanic and the Aryo-Romanic. The +Slavo-Germanic stem then branches into the ancient Germanic and the ancient +Slavo-Lettic tongues; the Aryo-Romanic into the ancient Aryan and the ancient +Greco-Roman. If we still follow the genealogical tree of these four +Indo-Germanic tongues, we find that the ancient Germanic divides into three +branches—the Scandinavian, the Gothic, and the German. From the ancient +German came the High German and Low German; to the latter belong the Frisian, +Saxon, and modern Low-German dialects. The ancient Slavo-Lettic divided first +into a Baltic and a Slav language. The Baltic gave rise to the Lett, +Lithuanian, and old-Prussian varieties; the Slav to the Russian and South-Slav +in the south-east, and to the Polish and Czech in the west. +</p> + +<p> +We find an equally prolific branching of its two chief stems when we turn to +the other division of the Indo-Germanic languages. The Greco-Roman divided into +the Thracian (Albano-Greek) and the Italo-Celtic. From the latter came the +divergent branches of the Italic (Roman and Latin) in the south, and the Celtic +in the north: from the latter have been developed all the British (ancient +British, ancient Scotch, and Irish) and Gallic varieties. The ancient Aryan +gave rise to the numerous Iranian and Indian languages. +</p> + +<p> +This “comparative anatomy” and evolution of languages admirably +illustrates the phylogeny of species. It is clear that in structure and +development the primitive languages, mother and daughter languages, and +varieties, correspond exactly to the classes, orders, genera, and species of +the animal world. In both cases the “natural” system is +phylogenetic. As we have been convinced from comparative anatomy and ontogeny, +and from paleontology, that all past and living vertebrates descend from a +common ancestor, so the comparative study of dead and living Indo-Germanic +tongues proves beyond question that they are all modifications of one primitive +language. This view of their origin is now accepted by all the chief +philologists who have worked in this branch and are unprejudiced. +</p> + +<p> +But the point to which I desire particularly to draw the reader’s +attention in this comparison of the Indo-Germanic languages with the branches +of the vertebrate stem is, that one must never confuse direct descendants with +collateral branches, nor extinct forms with living. This confusion is very +common, and our opponents often make use of the erroneous ideas it gives rise +to for the purpose of attacking evolution generally. When, for instance, we say +that man descends from the ape, this from the lemur, and the lemur from the +marsupial, many people imagine that we are speaking of the living species of +these orders of mammals that they find stuffed in our museums. Our opponents +then foist this idea on us, and say, with more astuteness than intelligence, +that it is quite impossible; or they ask us, by way of physiological +experiment, to turn a kangaroo into a lemur, a lemur into a gorilla, and a +gorilla into a man! The demand is childish, and the idea it rests on erroneous. +All these living forms have diverged more or less from the ancestral form; none +of them could engender the +<span class='pagenum'><a name="Page_205" id="Page_205"></a></span> +same posterity that the stem-form really produced thousands of years ago. +</p> + +<p> +It is certain that man has descended from some extinct mammal; and we should +just as certainly class this in the order of apes if we had it before us. It is +equally certain that this primitive ape descended in turn from an unknown +lemur, and this from an extinct marsupial. But it is just as clear that all +these extinct ancestral forms can only be claimed as belonging to the living +order of mammals in virtue of their essential internal structure and their +resemblance in the decisive anatomic characteristics of each <i>order.</i> In +external appearance, in the characteristics of the <i>genus</i> or +<i>species,</i> they would differ more or less, perhaps very considerably, from +all living representatives of those orders. It is a universal and natural +procedure in phylogenetic development that the stem-forms themselves, with +their specific peculiarities, have been extinct for some time. The forms that +approach nearest to them among the living species are more or +less—perhaps very substantially—different from them. Hence in our +phylogenetic inquiry and in the comparative study of the living, divergent +descendants, there can only be a question of determining the greater or less +remoteness of the latter from the ancestral form. Not a single one of the older +stem-forms has continued unchanged down to our time. +</p> + +<p> +We find just the same thing in comparing the various dead and living languages +that have developed from a common primitive tongue. If we examine our +genealogical tree of the Indo-Germanic languages in this light, we see at once +that all the older or parent tongues, of which we regard the living varieties +of the stem as divergent daughter or grand-daughter languages, have been +extinct for some time. The Aryo-Romanic and the Slavo-Germanic tongues have +completely disappeared; so also the Aryan, the Greco-Roman, the Slavo-Lettic, +and the ancient Germanic. Even their daughters and grand-daughters have been +lost; all the living Indo-Germanic languages are only related in the sense that +they are divergent descendants of common stem-forms. Some forms have diverged +more, and some less, from the original stem-form. +</p> + +<p> +This easily demonstrable fact illustrates very well the analogous case of the +origin of the vertebrate species. Phylogenetic comparative philology here +yields a strong support to phylogenetic comparative zoology. But the one can +adduce more direct evidence than the other, as the paleontological material of +philology—the old monuments of the extinct tongue—have been +preserved much better than the paleontological material of zoology, the +fossilised bones and imprints of vertebrates. +</p> + +<p> +We may, however, trace man’s genealogical tree not only as far as the +lower mammals, but much further—to the amphibia, to the shark-like +primitive fishes, and, in fine, to the skull-less vertebrates that closely +resembled the Amphioxus. But this must not be understood in the sense that the +existing Amphioxus, or the sharks or amphibia of to-day, can give us any idea +of the external appearance of these remote stem-forms. Still less must it be +thought that the Amphioxus or any actual shark, or any living species of +amphibia, is a real ancestral form of the higher vertebrates and man. The +statement can only rationally mean that the living forms I have referred to are +<i>collateral lines</i> that are much more closely related to the extinct +stem-forms, and have retained the resemblance much better, than any other +animals we know. They are still so like them in regard to their distinctive +internal structure that we should put them in the same class with the extinct +forms if we had these before us. But no direct descendants of these earlier +forms have remained unchanged. Hence we must entirely abandon the idea of +finding direct ancestors of the human race in their characteristic <i>external +form</i> among the living species of animals. The essential and distinctive +features that still connect living forms more or less closely with the extinct +common stem-forms lie in the internal structure, not the external appearance. +The latter has been much modified by adaptation. The former has been more or +less preserved by heredity. +</p> + +<p> +Comparative anatomy and ontogeny prove beyond question that man is a true +vertebrate, and, therefore, man’s special genealogical tree must be +connected with that of the other Vertebrates, which spring from a common root +with him. But we have also many important grounds in comparative anatomy and +ontogeny for assuming a common origin for all the Vertebrates. If the general +theory of +<span class='pagenum'><a name="Page_206" id="Page_206"></a></span> +evolution is correct, all the Vertebrates, including man, come from a single +common ancestor, a long-extinct “Primitive Vertebrate.” Hence the +genealogical tree of the Vertebrates is at the same time that of the human +race. +</p> + +<p> +Our task, therefore, of constructing man’s genealogy becomes the larger +aim of discovering the genealogy of the entire vertebrate stem. As we now know +from the comparative anatomy and ontogeny of the Amphioxus and the Ascidia, +this is in turn connected with the genealogical tree of the Invertebrates +(directly with that of the Vermalia), but has no direct connection with the +independent stems of the Articulates, Molluscs, and Echinoderms. If we do thus +follow our ancestral tree through various stages down to the lowest worms, we +come inevitably to the <i>Gastræa,</i> that most instructive form that gives +the clearest possible picture of an animal with two germinal layers. The +Gastræa itself has originated from the simple multicellular vesicle, the +<i>Blastæa,</i> and this in turn must have been evolved from the lowest circle +of unicellular animals, to which we give the name of Protozoa. We have already +considered the most important primitive type of these, the unicellular +<i>Amœba,</i> which is extremely instructive when compared with the human ovum. +With this we reach the lowest of the solid data to which we are to apply our +biogenetic law, and by which we may deduce the extinct ancestor from the +embryonic form. The amœboid nature of the young ovum and the unicellular +condition in which (as stem-cell or cytula) every human being begins its +existence justify us in affirming that the earliest ancestors of the human race +were simple amœboid coils. +</p> + +<p> +But the further question now arises: “Whence came these first amœbæ with +which the history of life began at the commencement of the Laurentian +epoch?” There is only one answer to this. The earliest unicellular +organisms can only have been evolved from the simplest organisms we know, the +<i>Monera.</i> These are the simplest living things that we can conceive. Their +whole body is nothing but a particle of plasm, a granule of living albuminous +matter, discharging of itself all the essential vital functions that form the +material basis of life. Thus we come to the last, or, if you prefer, the first, +question in connection with evolution—the question of the origin of the +Monera. This is the real question of the origin of life, or of spontaneous +generation. +</p> + +<p> +We have neither space nor occasion to go further in this Chapter into the +question of spontaneous generation. For this I must refer the reader to the +fifteenth chapter of the <i>History of Creation,</i> and especially to the +second book of the <i>General Morphology,</i> or to the essay on “The +Monera and Spontaneous Generation” in my <i>Studies of the Monera and +other Protists.</i><a href="#linknote-29" name="linknoteref-29" id="linknoteref-29"><sup>[29]</sup></a> I have given there fully my own view of this +important question. The famous botanist Nägeli afterwards (1884) developed the +same ideas. I will only say a few words here about this obscure question of the +origin of life, in so far as our main subject, organic evolution in general, is +affected by it. Spontaneous generation, in the definite and restricted sense in +which I maintain it, and claim that it is a necessary hypothesis in explaining +the origin of life, refers solely to the evolution of the Monera from inorganic +carbon-compounds. When living things made their first appearance on our planet, +the very complex nitrogenous compound of carbon that we call <i>plasson,</i> +which is the earliest material embodiment of vital action, must have been +formed in a purely chemical way from inorganic carbon-compounds. The first +Monera were formed in the sea by spontaneous generation, as crystals are formed +in the mother-water. Our demand for a knowledge of causes compels us to assume +this. If we believe that the whole inorganic history of the earth has proceeded +on mechanical principles without any intervention of a Creator, and that the +history of life also has been determined by the same mechanical laws; if we see +that there is no need to admit creative action to explain the origin of the +various groups of organisms; it is utterly irrational to assume such creative +action in dealing with the first appearance of organic life on the earth. +</p> + +<p class="footnote"> +<a name="linknote-29" id="linknote-29"></a> <a href="#linknoteref-29">[29]</a> +The English reader will find a luminous and up-to-date chapter on the subject +in Haeckel’s recently written and translated <i>Wonders of +Life.</i>—Translator. +</p> + +<p> +This much-disputed question of “spontaneous generation” seems so +obscure, because people have associated with the term a mass of very different, +and often very absurd, ideas, and have attempted to solve the difficulty by the +crudest experiments. The real doctrine of the spontaneous generation of life +cannot possibly be refuted by experiments. +<span class='pagenum'><a name="Page_207" id="Page_207"></a></span> +Every experiment that has a negative result only proves that no organism has +been formed out of inorganic matter in the conditions—highly artificial +conditions—we have established. On the other hand, it would be +exceedingly difficult to prove the theory by way of experiment; and even if +Monera were still formed daily by spontaneous generation (which is quite +possible), it would be very difficult, if not impossible, to find a solid proof +of it. Those who will not admit the spontaneous generation of the first living +things in our sense must have recourse to a supernatural miracle; and this is, +as a matter of fact, the desperate resource to which our “exact” +scientists are driven, to the complete abdication of reason. +</p> + +<p> +A famous English physicist, Lord Kelvin (then Sir W. Thomson), attempted to +dispense with the hypothesis of spontaneous generation by assuming that the +organic inhabitants of the earth were developed from germs that came from the +inhabitants of other planets, and that chanced to fall on our planet on +fragments of their original home, or meteorites. This hypothesis found many +supporters, among others the distinguished German physicist, Helmholtz. +However, it was refuted in 1872 by the able physicist, Friedrich Zöllner, of +Leipzig, in his work, <i>On the Nature of Comets.</i> He showed clearly how +unscientific this hypothesis is; firstly in point of logic, and secondly in +point of scientific content. At the same time he pointed out that our +hypothesis of spontaneous generation is “a necessary condition for +understanding nature according to the law of causality.” +</p> + +<p> +I repeat that we must call in the aid of the hypothesis only as regards the +Monera, the structureless “organisms without organs.” Every complex +organism must have been evolved from some lower organism. We must not assume +the spontaneous generation of even the simplest cell, for this itself consists +of at least two parts—the internal, firm nuclear substance, and the +external, softer cellular substance or the protoplasm of the cell-body. These +two parts must have been formed by differentiation from the indifferent plasson +of a moneron, or a cytode. For this reason the natural history of the Monera is +of great interest; here alone can we find the means to overcome the chief +difficulties of the problem of spontaneous generation. The actual living Monera +are specimens of such organless or structureless organisms, as they must have +boon formed by spontaneous generation at the commencement of the history of +life. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap19"></a>Chapter XIX.<br/> +OUR PROTIST ANCESTORS</h2> + +<p> +Under the guidance of the biogenetic law, and on the basis of the evidence we +have obtained, we now turn to the interesting task of determining the series of +man’s animal ancestors. Phylogeny us a whole is an inductive science. +From the totality of the biological processes in the life of plants, animals, +and man we have gathered a confident inductive idea that the whole organic +population of our planet has been moulded on a harmonious law of evolution. All +the interesting phenomena that we meet in ontogeny and paleontology, +comparative anatomy and dysteleology, the distribution and habits of +organisms—all the important general laws that we abstract from the +phenomena of these sciences, and combine in harmonious unity—are the +broad bases of our great biological induction. +</p> + +<p> +But when we come to the application of this law, and seek to determine with its +aid the origin of the various species of organisms, we are compelled to frame +<span class='pagenum'><a name="Page_208" id="Page_208"></a></span> +hypotheses that have essentially a <i>deductive</i> character, and are +inferences from the general law to particular cases. But these special +deductions are just as much justified and necessitated by the rigorous laws of +logic as the inductive conclusions on which the whole theory of evolution is +built. The doctrine of the animal ancestry of the human race is a special +deduction of this kind, and follows with logical necessity from the general +inductive law of evolution. +</p> + +<p> +I must point out at once, however, that the certainty of these evolutionary +hypotheses, which rest on clear special deductions, is not always equally +strong. Some of these inferences are now beyond question; in the case of others +it depends on the knowledge and the competence of the inquirer what degree of +certainty he attributes to them. In any case, we must distinguish between the +<i> absolute</i> certainty of the general (inductive) theory of descent and the +<i>relative</i> certainty of special (deductive) evolutionary hypotheses. We +can never determine the whole ancestral series of an organism with the same +confidence with which we hold the general theory of evolution as the sole +scientific explanation of organic modifications. The special indication of +stem-forms in detail will always be more or less incomplete and hypothetical. +This is quite natural. The evidence on which we build is imperfect, and always +will be imperfect; just as in comparative philology. +</p> + +<p> +The first of our documents, paleontology, is exceedingly incomplete. We know +that all the fossils yet discovered are only an insignificant fraction of the +plants and animals that have lived on our planet. For every single species that +has been preserved for us in the rocks there are probably hundreds, perhaps +thousands, of extinct species that have left no trace behind them. This extreme +and very unfortunate incompleteness of the paleontological evidence, which +cannot be pointed out too often, is easily explained. It is absolutely +inevitable in the circumstances of the fossilisation of organisms. It is also +due in part to the incompleteness of our knowledge in this branch. It must be +borne in mind that the great majority of the stratified rocks that compose the +crust of the earth have not yet been opened. We have only a few specimens of +the innumerable fossils that are buried in the vast mountain ranges of Asia and +Africa. Only a part of Europe and North America has been investigated +carefully. The whole of the fossils known to us certainly do not amount to a +hundredth part of the remains that are really buried in the crust of the earth. +We may, therefore, look forward to a rich harvest in the future as regards this +science. However, our paleontological evidence will (for reasons that I have +fully explained in the sixteenth chapter of the <i>History of Creation</i>) +always be defective. +</p> + +<p> +The second chief source of evidence, ontogeny, is not less incomplete. It is +the most important source of all for special phylogeny; but it has great +defects, and often fails us. We must, above all, clearly distinguish between +palingenetic and cenogenetic phenomena. We must never forget that the laws of +curtailed and disturbed heredity often make the original course of development +almost unrecognisable. The recapitulation of phylogeny by ontogeny is only +fairly complete in a few cases, and is never wholly complete. As a rule, it is +precisely the earliest and most important embryonic stages that suffer most +from alteration and condensation. The earlier embryonic forms have had to adapt +themselves to new circumstances, and so have been modified. The struggle for +existence has had just as profound an influence on the freely moving and still +immature young forms as on the adult forms. Hence in the embryology of the +higher animals, especially, palingenesis is much restricted by cenogenesis; it +is to-day, as a rule, only a faded and much altered picture of the original +evolution of the animal’s ancestors. We can only draw conclusions from +the embryonic forms to the stem-history with the greatest caution and +discrimination. Moreover, the embryonic development itself has only been fully +studied in a few species. +</p> + +<p> +Finally, the third and most valuable source of evidence, comparative anatomy, +is also, unfortunately, very imperfect; for the simple reason that the whole of +the living species of animals are a mere fraction of the vast population that +has dwelt on our planet since the beginning of life. We may confidently put the +total number of these at more than a million species. The number of animals +whose organisation has been studied up to the present in comparative anatomy is +proportionately very small. Here, again, future research will yield +incalculable treasures. +<span class='pagenum'><a name="Page_209" id="Page_209"></a></span> +But, for the present, in view of this patent incompleteness of our chief +sources of evidence, we must naturally be careful not to lay too much stress in +human phylogeny on the particular animals we have studied, or regard all the +various stages of development with equal confidence as stem-forms. +</p> + +<p> +In my first efforts to construct the series of man’s ancestors I drew up +a list of, at first ten, afterwards twenty to thirty, forms that may be +regarded more or less certainly as animal ancestors of the human race, or as +stages that in a sense mark off the chief sections in the long story of +evolution from the unicellular organism to man. Of these twenty to thirty +stages, ten to twelve belong to the older group of the Invertebrates and +eighteen to twenty to the younger division of the Vertebrates. +</p> + +<p> +In approaching, now, the difficult task of establishing the evolutionary +succession of these thirty ancestors of humanity since the beginning of life, +and in venturing to lift the veil that covers the earliest secrets of the +earth’s history, we must undoubtedly look for the first living things +among the wonderful organisms that we call the Monera; they are the simplest +organisms known to us—in fact, the simplest we can conceive. Their whole +body consists merely of a simple particle or globule of structureless plasm or +plasson. The discoveries of the last four decades have led us to believe with +increasing certainty that wherever a natural body exhibits the vital processes +of nutrition, reproduction, voluntary movement, and sensation, we have the +action of a nitrogenous carbon-compound of the chemical group of the +albuminoids; this plasm (or protoplasm) is the material basis of all vital +functions. Whether we regarded the function, in the monistic sense, as the +direct action of the material substratum, or whether we take matter and force +to be distinct things in the dualistic sense, it is certain that we have not as +yet found any living organism in which the exercise of the vital functions is +not inseparably bound up with plasm. +</p> + +<p> +The soft slimy plasson of the body of the moneron is generally called +“protoplasm,” and identified with the cellular matter of the +ordinary plant and animal cells. But we must, to be accurate, distinguish +between the plasson of the cytodes and the protoplasm of the cells. This +distinction is of the utmost importance for the purposes of evolution. As I +have often said, we must recognise two different stages of development in these +“elementary organisms,” or plastids (“builders”), that +represent the ultimate units of organic individuality. The earlier and lower +stage are the unnucleated cytodes, the body of which consists of only one kind +of albuminous matter—the homogeneous plasson or “formative +matter.” The later and higher stage are the nucleated cells, in which we +find a differentiation of the original plasson into two different formative +substances—the caryoplasm of the nucleus and the cytoplasm of the body of +the cell (cf. pp. 37 and 42). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus226"></a> +<img src="images/fig226.gif" width="282" height="91" alt="Fig.226. Chroococcus minor." /> +<p class="caption">Fig. +226—<b>Chroococcus minor</b> (<i>Nägeli</i>), magnified. A +phytomoneron, the globular plastids of which secrete a gelatinous structureless +membrane. The unnucleated globule of plasm (bluish-green in colour) increases +by simple cleavage (<i>a–d</i>).</p> +</div> + +<p> +The Monera are permanent cytodes. Their whole body consists of soft, +structureless plasson. However carefully we examine it with our finest chemical +reagents and most powerful microscopes, we can find no definite parts or no +anatomic structure in it. Hence, the Monera are literally organisms without +organs; in fact, from the philosophic point of view they are not organisms at +all, since they have no organs. They can only be called organisms in the sense +that they are capable of the vital functions of nutrition, reproduction, +sensation, and movement. If we were to try to imagine the simplest possible +organism, we should frame something like the moneron. +</p> + +<p> +The Monera that we find to-day in various forms fall into two groups according +to the nature of their nutrition—the <i> Phytomonera</i> and the +<i>Zoomonera</i>; from the physiological point of view, the former are the +simplest specimens of the plant (<i>phyton</i>) kingdom, and the latter of the +animal (<i>zoon</i>) world. The Phytomonera, especially in their simplest form, +the Chromacea (<i>Phycochromacea</i> or <i>Cyanophycea</i>), are the most +primitive and the +<span class='pagenum'><a name="Page_210" id="Page_210"></a></span> +oldest of living organisms. The typical genus <i> Chroococcus</i> (Fig. 226) is +represented by several fresh-water species, and often forms a very delicate +bluish-green deposit on stones and wood in ponds and ditches. It consists of +round, light green particles, from 1/7000 to 1/2500 of an inch in diameter. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus227"></a> +<img src="images/fig227.gif" width="228" height="250" alt="Fig.227. Aphanocapsa +primordialis." /> +<p class="caption">Fig. 227—<b>Aphanocapsa primordialis</b> +(<i>Nägeli</i>), magnified. A phytomoneron, the round plastids of which +(bluish-green in colour) secrete a shapeless gelatinous mass; in this the +unnucleated cytodes increase continually by simple cleavage.</p> +</div> + +<p>The whole life of these homogeneous globules of plasm consists +of simple growth and reproduction by cleavage. When the tiny particle has +reached a certain size by the continuous assimilation of inorganic matter, it +divides into two equal halves, by a constriction in the middle. The two +daughter-monera that are thus formed immediately begin a similar vital process. +It is the same with the brown <i>Procytella primordialis</i> (formerly called +the <i>Protococcus marinus</i>); it forms large masses of floating matter in +the arctic seas. The tiny plasma-globules of this species are of a +greenish-brown colour, and have a diameter of 1/10,000 to 1/5000 of an inch. +There is no membrane discoverable in the simplest <i>Chroococcacea,</i> but we +find one in other members of the same family; in <i>Aphanocapsa</i> (Fig. 227) +the enveloping membranes of the social plastids combine; in <i>Glœcapsa</i> +they are retained through several generations, so that the little +plasma-globules are enfolded in many layers of membrane. +</p> + +<p> +Next to the Chromacea come the Bacteria, which have been evolved from them by +the remarkable change in nutrition which gives us the simple explanation of the +differentiation of plant and animal in the protist kingdom. The Chromacea build +up their plasm directly from inorganic matter; the Bacteria feed on organic +matter. Hence, if we logically divide the protist kingdom into plasma-forming +Protophyta and plasma-consuming Protozoa, we must class the Bacteria with the +latter; it is quite illogical to describe them—as is still often +done—as <i>Schizomycetes,</i> and class them with the true fungi. The +Bacteria, like the Chromacea, have no nucleus. As is well-known, they play an +important part in modern biology as the causes of fermentation and +putrefaction, and of tuberculosis, typhus, cholera, and other infectious +diseases, and as parasites, etc. But we cannot linger now to deal with these +very interesting features; the Bacteria have no relation to man’s +genealogical tree. +</p> + +<p> +We may now turn to consider the remarkable Protamœba, or unnucleated Amœba. I +have, in the first volume, pointed out the great importance of the ordinary +Amœba in connection with several weighty questions of general biology. The tiny +Protamœbæ, which are found both in fresh and salt water, have the same +unshapely form and irregular movements of their simple naked body as the real +Amœbæ; but they differ from them very materially in having no nucleus in their +cell-body. The short, blunt, finger-like processes that are thrust out at the +surface of the creeping Protamœba serve for getting food as well as for +locomotion. They multiply by simple cleavage (Fig. 228). +</p> + +<p> +The next stage to the simple cytode-forms of the Monera in the genealogy of +mankind (and all other animals) is the simple cell, or the most rudimentary +form of the cell which we find living independently to-day as the Amœba. The +earliest process of inorganic differentiation in the structureless body of the +Monera led to its division into two different substances—the caryoplasm +and the cytoplasm. The caryoplasm is the inner and firmer part of the cell, the +substance of the nucleus. The cytoplasm is the outer and softer part, the +substance of the body of the cell. By this important differentiation of the +plasson into nucleus and cell-body, the +<span class='pagenum'><a name="Page_211" id="Page_211"></a></span> +organised cell was evolved from the structureless cytode, the nucleated from +the unnucleated plastid. That the first cells to appear on the earth were +formed from the Monera by such a differentiation seems to us the only possible +view in the present condition of science. We have a direct instance of this +earliest process of differentiation to-day in the ontogeny of many of the lower +Protists (such as the Gregarinæ). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus228"></a> +<img src="images/fig228.gif" width="224" height="108" alt="Fig.228. A moneron (Protamoeba) in the act of +reproduction." /> +<p class="caption">Fig. 228—<b>A moneron +(Protamœba)</b> in the act of reproduction. <i>A</i> The whole moneron, moving +like an ordinary amœba by thrusting out changeable processes. <i>B</i> It +divides into two halves by a constriction in the middle. <i>C</i> The two +halves separate, and each becomes an independent individual. (Highly +magnified.)</p> +</div> + +<p> +The unicellular form that we have in the ovum has already been described as the +reproduction of a corresponding unicellular stem-form, and to this we have +ascribed the organisation of an Amœba (cf. Chapter VI). The irregular-shaped +Amœba, which we find living independently to-day in our fresh and salt water, +is the least definite and the most primitive of all the unicellular Protozoa +(Fig. 16). As the unripe ova (the <i>protova</i> that we find in the ovaries of +animals) cannot be distinguished from the common Amœbæ, we must regard the +Amœba as the primitive form that is reproduced in the embryonic stage of the +amœboid ovum to-day, in accordance with the biogenetic law. I have already +pointed out, in proof of the striking resemblance of the two cells, that the +ova of many of the sponges were formerly regarded as parasitic Amœbæ (Figure +1.18). Large unicellular organisms like the Amœbæ were found creeping about +inside the body of the sponge, and were thought to be parasites. It was +afterwards discovered that they were really the ova of the sponge from which +the embryos were developed. As a matter of fact, these sponge-ova are so much +like many of the Amœbæ in size, shape, the character of their nucleus, and +movement of the pseudopodia, that it is impossible to distinguish them without +knowing their subsequent development. +</p> + +<p> +Our phylogenetic interpretation of the ovum, and the reduction of it to some +ancient amœboid ancestral form, supply the answer to the old problem: +“Which was first, the egg or the chick?” We can now give a very +plain answer to this riddle, with which our opponents have often tried to drive +us into a corner. The egg came a long time before the chick. We do not mean, of +course, that the egg existed from the first as a bird’s egg, but as an +indifferent amœboid cell of the simplest character. The egg lived for thousands +of years as an independent unicellular organism, the Amœba. The egg, in the +modern physiological sense of the word, did not make its appearance until the +descendants of the unicellular Protozoon had developed into multicellular +animals, and these had undergone sexual differentiation. Even then the egg was +first a gastræa-egg, then a platode-egg, then a vermalia-egg, and +chordonia-egg; later still acrania-egg, then fish-egg, amphibia-egg, +reptile-egg, and finally bird’s egg. The bird’s egg we have +experience of daily is a highly complicated historical product, the result of +countless hereditary processes that have taken place in the course of millions +of years. +</p> + +<p> +The earliest ancestors of our race were simple Protophyta, and from these our +protozoic ancestors were developed afterwards. From the morphological point of +view both the vegetal and the animal Protists were simple organisms, +individualities of the first order, or plastids. All our later ancestors are +complex organisms, or individualities of a higher order—social +aggregations of a plurality of cells. The earliest of these, the <i> +Moræada,</i> which represent the third stage in our genealogy, are very simple +associations of homogeneous, indifferent cells—undifferentiated colonies +of social Amœbæ or Infusoria. To understand the nature and origin of these +protozoa-colonies we need only follow step by step the first embryonic products +of the stem-cell. In all the Metazoa the first embryonic process is the +repeated cleavage of the stem-cell, or first segmentation-cell (Fig. 229). We +have already fully considered this process, and found that all the different +forms of it may be reduced to one type, the original equal or primordial +segmentation (cf. Chapter VIII). In the genealogical tree +<span class='pagenum'><a name="Page_212" id="Page_212"></a></span> +of the Vertebrates this palingenetic form of segmentation has been preserved in +the Amphioxus alone, all the other Vertebrates having cenogenetically modified +forms of cleavage. In any case, the latter were developed from the former, and +so the segmentation of the ovum in the Amphioxus has a great interest for us +(cf. Fig. 38). The outcome of this repeated cleavage is the formation of a +round cluster of cells, composed of homogeneous, indifferent cells of the +simplest character (Fig. 230). This is called the <i>morula</i> (= +mulberry-embryo) on account of its resemblance to a mulberry or blackberry. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus229"></a> +<img src="images/fig229.gif" width="344" height="107" alt="Fig.229. Original or primordial ovum-cleavage." /> +<p class="caption">Fig. 229—<b>Original or primordial +ovum-cleavage.</b> The stem-cell or cytula, formed by fecundation of the ovum, +divides by repeated regular cleavage first into two (<i>A</i>), then four +(<i>B</i>), then eight (<i>C</i>), and finally a large number of +segmentation-cells (<i>D</i>).</p> +</div> + +<p> +It is clear that this morula reproduces for us to-day the simple structure of +the multicellular animal that succeeded the unicellular amœboid form in the +early Laurentian period. In accordance with the biogenetic law, the morula +recalls the ancestral form of the Moræa, or simple colony of Protozoa. The +first cell-communities to be formed, which laid the early foundation of the +higher multicellular body, must have consisted of homogeneous and simple +amœboid cells. The oldest Amœbæ lived isolated lives, and even the amœboid +cells that were formed by the segmentation of these unicellular organisms must +have continued to live independently for a long time. But gradually small +communities of Amœbæ arose by the side of these eremitical Protozoa, the +sister-cells produced by cleavage remaining joined together. The advantages in +the struggle for life which these communities had over the isolated cells +favoured their formation and their further development. We find plenty of these +cell-colonies or communities to-day in both fresh and salt water. They belong +to various groups both of the Protophyta and Protozoa. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus230"></a> +<img src="images/fig230.gif" width="103" height="101" alt="Fig.230. Morula, or mulberry-shaped +embryo." /> +<p class="caption">Fig. 230—<b>Morula,</b> or <b> mulberry-shaped +embryo.</b></p> +</div> + +<p> +To have some idea of those ancestors of our race that succeeded +phylogenetically to the Moræada, we have only to follow the further embryonic +development of the morula. We then see that the social cells of the round +cluster secrete a sort of jelly or a watery fluid inside their globular body, +and they themselves rise to the surface of it (Fig. 29 <i>F, G</i>). In this +way the solid mulberry-embryo becomes a hollow sphere, the wall of which is +composed of a single layer of cells. We call this layer the <i>blastoderm,</i> +and the sphere itself the <i>blastula,</i> or embryonic vesicle. +</p> + +<p> +This interesting blastula is very important. The conversion of the morula into +a hollow ball proceeds on the same lines originally in the most diverse +stems—as, for instance, in many of the zoophytes and worms, the ascidia, +many of the echinoderms and molluscs, and in the amphioxus. Moreover, in the +animals in which we do not find a real palingenetic blastula the defect is +clearly due to cenogenetic causes, such as the formation of food-yelk and other +embryonic adaptations. We may, therefore, conclude that the ontogenetic +blastula is the reproduction of a very early phylogenetic ancestral form, and +that all the Metazoa are descended from a common stem-form, which was in the +main constructed like the blastula. In many of the lower animals the blastula +is not developed +<span class='pagenum'><a name="Page_213" id="Page_213"></a></span> +within the fœtal membranes, but in the open water. In those cases each +blastodermic cell begins at an early stage to thrust out one or more mobile +hair-like processes; the body swims about by the vibratory movement of these +lashes or whips (Fig. 29 <i>F</i>). +</p> + +<p> +We still find, both in the sea and in fresh water, various kinds of primitive +multicellular organisms that substantially resemble the blastula in structure, +and may be regarded in a sense as permanent blastula-forms—hollow +vesicles or gelatinous balls, with a wall composed of a single layer of +ciliated homogeneous cells. There are “blastæads” of this kind even +among the Protophyta—the familiar Volvocina, formerly classed with the +infusoria. The common <i>Volvox globator</i> is found in the ponds in the +spring—a small, green, gelatinous globule, swimming about by means of the +stroke of its lashes, which rise in pairs from the cells on its surface. In the +similar <i> Halosphæra viridis</i> also, which we find in the marine plancton +(floating matter), a number of green cells form a simple layer at the surface +of the gelatinous ball; but in this case there are no cilia. +</p> + +<p> +Some of the infusoria of the flagellata-class (<i>Signura, Magosphæra,</i> +etc.) are similar in structure to these vegetal clusters, but differ in their +animal nutrition; they form the special group of the <i>Catallacta.</i> In +September, 1869, I studied the development of one of these graceful animals on +the island of Gis-Oe, off the coast of Norway (<i>Magosphæra planula</i>), +Figures 2.231 and 2.232). The fully-formed body is a gelatinous ball, with its +wall composed of thirty-two to sixty-four ciliated cells; it swims about freely +in the sea. After reaching maturity the community is dissolved. Each cell then +lives independently for some time, grows, and changes into a creeping amœba. +This afterwards contracts, and clothes itself with a structureless membrane. +The cell then looks just like an ordinary animal ovum. When it has been in this +condition for some time the cell divides into two, four, eight, sixteen, +thirty-two, and sixty-four cells. These arrange themselves in a round vesicle, +thrust out vibratory lashes, burst the capsule, and swim about in the same +magosphæra-form with which we started. This completes the life-circle of the +remarkable and instructive animal. +</p> + +<p> +If we compare these permanent blastulæ with the free-swimming ciliated larvæ or +blastulæ, with similar construction, of many of the lower animals, we can +confidently deduce from them that there was a very early and long-extinct +common stem-form of substantially the same structure as the blastula. We may +call it the <i>Blastæa.</i> Its body consisted, when fully formed, of a simple +hollow ball, filled with fluid or structureless jelly, with a wall composed of +a single stratum of ciliated cells. There were probably many genera and species +of these blastæads in the Laurentian period, forming a special class of marine +protists. +</p> + +<p> +It is an interesting fact that in the plant kingdom also the simple hollow +sphere is found to be an elementary form of the multicellular organism. At the +surface and below the surface (down to a depth of 2000 yards) of the sea there +are green globules swimming about, with a wall composed of a single layer of +chlorophyll-bearing cells. The botanist Schmitz gave them the name of +<i>Halosphæra viridis</i> in 1879. +</p> + +<p> +The next stage to the <i>Blastæa,</i> and the sixth in our genealogical tree, +is the Gastræa that is developed from it. As we have already seen, this +ancestral form is particularly important. That it once existed is proved with +certainty by the gastrula, which we find temporarily in the ontogenesis of all +the Metazoa (Fig. 29 <i>J, K</i>). As we saw, the original, palingenetic form +of the gastrula is a round or oval uni-axial body, the simple cavity of which +(the primitive gut) has an aperture at one pole of its axis (the primitive +mouth). The wall of the gut consists of two strata of cells, and these are the +primary germinal layers, the animal skin-layer (ectoderm) and vegetal gut-layer +(entoderm). +</p> + +<p> +The actual ontogenetic development of the gastrula from the blastula furnishes +sound evidence as to the phylogenetic origin of the <i>Gastræa</i> from the +<i>Blastæa.</i> A pit-shaped depression appears at one side of the spherical +blastula (Fig. 29 <i>H</i>). In the end this invagination goes so far that the +outer or invaginated part of the blastoderm lies close on the inner or +non-invaginated part (Fig. 29 <i>J</i>). In explaining the phylogenetic origin +of the gastræa in the light of this ontogenetic process, we may assume that the +one-layered cell-community of the blastæa began to take in food more largely at +one particular part of its surface. Natural selection would gradually lead to +<span class='pagenum'><a name="Page_214" id="Page_214"></a></span> +the formation of a depression or pit at this alimentary spot on the surface of +the ball. The depression would grow deeper and deeper. In time the vegetal +function of taking in and digesting food would be confined to the cells that +lined this hole; the other cells would see to the animal functions of +locomotion, sensation, and protection. This was the first division of labour +among the originally homogeneous cells of the blastæa. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus231"></a> +<img src="images/fig231.gif" width="361" height="204" alt="Fig.231. The Norwegian Magosphaera +planula, swimming about by means of the lashes or cilia at its surface. Fig. +232. Section of same, showing how the pear-shaped cells in the centre of the +gelatinous ball are connected by a fibrous process." /> +<p class="caption">Fig. 231—<b>The Norwegian Magosphæra planula,</b> +swimming about by means of the lashes or cilia at its surface.<br/> Fig. +232—<b>Section of same,</b> showing how the pear-shaped cells in the +centre of the gelatinous ball are connected by a fibrous process. Each cell has +a contractile vacuole as well as a nucleus.</p> +</div> + +<p> +The effect, then, of this earliest histological differentiation was to produce +two different kinds of cells—nutritive cells in the depression and +locomotive cells on the surface outside. But this involved the severance of the +two primary germinal layers—a most important process. When we remember +that even man’s body, with all its various parts, and the body of all the +other higher animals, are built up originally out of these two simple layers, +we cannot lay too much stress on the phylogenetic significance of this +gastrulation. In the simple primitive gut or gastric cavity of the gastrula and +its rudimentary mouth we have the first real organ of the animal frame in the +morphological sense; all the other organs were developed afterwards from these. +In reality, the whole body of the gastrula is merely a “primitive +gut.” I have shown already (Chapters VIII and XIX) that the two-layered +embryos of all the Metazoa can be reduced to this typical gastrula. This +important fact justifies us in concluding, in accordance with the biogenetic +law, that their ancestors also were phylogenetically developed from a similar +stem-form. This ancient stem-form is the gastræa. +</p> + +<p> +The gastræa probably lived in the sea during the Laurentian period, swimming +about in the water by means of its ciliary coat much as free ciliated gastrulæ +do to-day. Probably it differed from the existing gastrula only in one +essential point, though extinct millions of years ago. We have reason, from +comparative anatomy and ontogeny, to believe that it multiplied by sexual +generation, not merely asexually (by cleavage, gemmation, and spores), as was +no doubt the case with the earlier ancestors. Some of the cells of the primary +germ-layers probably became ova and others fertilising sperm. We base these +hypotheses on the fact that we do to-day find the simplest form of sexual +reproduction in some of the living gastræads and other lower animals, +especially the sponges. +</p> + +<p> +The fact that there are still in existence various kinds of gastræads, or lower +Metazoa with an organisation little higher than that of the hypothetical +gastræa, is a strong point in favour of our theory. There are not very many +species of these living gastræads; but their morphological and phylogenetic +interest is so great, and their intermediate position between the Protozoa and +Metazoa so instructive, that I proposed long ago (1876) to make a special class +of them. I distinguished three orders in this class—the Gastremaria, +Physemaria, and Cyemaria (or Dicyemida). +<span class='pagenum'><a name="Page_215" id="Page_215"></a></span> +But we might also regard these three orders as so many independent classes in a +primitive gastræad stem. +</p> + +<p> +The Gastremaria and Cyemaria, the chief of these living gastræads, are small +Metazoa that live parasitically inside other Metazoa, and are, as a rule, 1/50 +to 1/25 of an inch long, often much less (Fig. 233, 1–15). Their soft +body, devoid of skeleton, consists of two simple strata of cells, the primary +germinal layers; the outer of these is thickly clothed with long hair-like +lashes, by which the parasites swim about in the various cavities of their +host. The inner germinal layer furnishes the sexual products. The pure type of +the original gastrula (or <i> archigastrula,</i> Fig. 29 <i> I</i>) is seen in +the <i>Pemmatodiscus gastrulaceus,</i> which Monticelli discovered in the +umbrella of a large medusa (<i>Pilema pulmo</i>) in 1895; the convex surface of +this gelatinous umbrella was covered with numbers of clear vesicles, of 1/25 to +1/8 inch in diameter, in the fluid contents of which the little parasites were +swimming. The cup-shaped body of the <i>Pemmatodiscus</i> (Fig. 233, <i>1</i>) +is sometimes rather flat, and shaped like a hat or cone, at other times almost +curved into a semi-circle. The simple hollow of the cup, the primitive gut +(<i>g</i>), has a narrow opening (<i>o</i>). The skin layer (<i>e</i>) consists +of long slender cylindrical cells, which bear long vibratory hairs; it is +separated by a thin structureless, gelatinous plate (<i>f</i>) from the +visceral or gut layer (<i>i</i>), the prismatic cells of which are much smaller +and have no cilia. Pemmatodiscus propagates asexually, by simple longitudinal +cleavage; on this account it has recently been regarded as the representative +of a special order of gastræads (<i>Mesogastria</i>). +</p> + +<p> +Probably a near relative of the <i>Pemmatodiscus</i> is the <i> Kunstleria +Gruveli</i> (Fig. 233, <i>2</i>). It lives in the body-cavity of Vermalia +(Sipunculida), and differs from the former in having no lashes either on the +large ectodermic cells (<i>e</i>) or the small entodermic (<i>i</i>); the +germinal layers are separated by a thick, cup-shaped, gelatinous mass, which +has been called the “clear vesicle” (<i>f</i>). The primitive mouth +is surrounded by a dark ring that bears very strong and long vibratory lashes, +and effects the swimming movements. +</p> + +<p> +<i>Pemmatodiscus</i> and <i>Kunstleria</i> may be included in the family of the +Gastremaria. To these gastræads with open gut are closely related the +Orthonectida (<i>Rhopalura,</i> Fig. 233, <i>3–5</i>). They live +parasitically in the body-cavity of echinoderms (Ophiura) and vermalia; they +are distinguished by the fact that their primitive gut-cavity is not empty, but +filled with entodermic cells, from which the sexual cells are developed. These +gastræads are of both sexes, the male (Fig. 3) being smaller and of a somewhat +different shape from the oval female (Fig. 4). +</p> + +<p> +The somewhat similar <i>Dicyemida</i> (Fig. 6) are distinguished from the +preceding by the fact that their primitive gut-cavity is occupied by a single +large entodermic cell instead of a crowded group of sexual cells. This cell +does not yield sexual products, but afterwards divides into a number of cells +(spores), each of which, without being impregnated, grows into a small embryo. +The Dicyemida live parasitically in the body-cavity, especially the renal +cavities, of the cuttle-fishes. They fall in several genera, some of which are +characterised by the possession of special polar cells; the body is sometimes +roundish, oval, or club-shaped, at other times long and cylindrical. The genus +<i>Conocyema</i> (Figs. 7–15) differs from the ordinary <i>Dicyema</i> in +having four polar pimples in the form of a cross, which may be incipient +tentacles. +</p> + +<p> +The classification of the Cyemaria is much disputed; sometimes they are held to +be parasitic infusoria (like the <i>Opalina</i>), sometimes platodes or +vermalia, related to the suctorial worms or rotifers, but having degenerated +through parasitism. I adhere to the phylogenetically important theory that I +advanced in 1876, that we have here real gastræads, primitive survivors of the +common stem-group of all the Metazoa. In the struggle for life they have found +shelter in the body-cavity of other animals. +</p> + +<p> +The small Cœlenteria attached to the floor of the sea that I have called the +Physemaria (<i>Haliphysema</i> and <i> Gastrophysema</i>) probably form a third +order (or class) of the living gastræads. The genus <i>Haliphysema</i> (Figs. +234, 235) is externally very similar to a large rhizopod (described by the same +name in 1862) of the family of the <i>Rhabdamminida,</i> which was at first +taken for a sponge. In order to avoid confusion with these, I afterwards gave +them the name of Prophysema. The whole mature body of the <i>Prophysema</i> is +a simple cylindrical or oval tube, with a two-layered wall. The hollow of the +tube is the gastric cavity, and the upper opening of it the mouth (Fig. 235 +<i>m</i>). +<span class='pagenum'><a name="Page_216" id="Page_216"></a></span> +</p> + +<div class="fig" style="width:100%;"> +<a name="illus233"></a> +<img src="images/fig233.gif" width="340" height="497" alt="Fig.233. Modern gastræads. Fig. 1. +Pemmatodiscus gastrulaceus (Monticelli), in longitudinal section. Fig. 2. +Kunstleria gruveli (Delage), in longitudinal section. (From Kunstler and +Gruvel.) Figs. 3-5. Rhopalura Giardi (Julin): Fig. 3 male, Fig. 4 female, Fig. +5 planula. Fig. 6. Dicyema macrocephala (Van Beneden). Fig. 7-15. Conocyema +polymorpha (Van Beneden): Fig. 7 the mature gastræad, Fig. 8-15 its +gastrulation." /> +<p class="caption">Fig. 233—<b>Modern gastræads.</b> Fig. 1. +<b>Pemmatodiscus gastrulaceus</b> (<i>Monticelli</i>), in longitudinal section. +Fig. 2. <b>Kunstleria gruveli</b> (<i>Delage</i>), in longitudinal section. +(From <i> Kunstler</i> and <i>Gruvel.</i>) Figs. 3–5. <b>Rhopalura +Giardi</b> (<i>Julin</i>): Fig. 3 male, Fig. 4 female, Fig. 5 planula. Fig. 6. +<b>Dicyema macrocephala</b> (<i>Van Beneden</i>). Figs. 7–15. +<b>Conocyema polymorpha</b> (<i>Van Beneden</i>): Fig. 7 the mature gastræad, +Figs. 8–15 its gastrulation. <i>d</i> primitive gut, <i>o</i> primitive +mouth, <i> e</i> ectoderm, <i>i</i> entoderm, <i>f</i> gelatinous plate between +<i>e</i> and <i>i</i> (supporting plate, blastocœl).</p> +</div> + +<p class="noindent"> +<span class='pagenum'><a name="Page_217" id="Page_217"></a></span> +The two strata of cells that form the wall of the tube are the primary germinal +layers. These rudimentary zoophytes differ from the swimming gastræads chiefly +in being attached at one end (the end opposite to the mouth) to the floor of +the sea. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus234"></a> +<img src="images/fig234.gif" width="231" height="285" alt="Figs. 234 and 235. Prophysema primordiale, a +living gastraead." /> +<p class="caption">Figs. 234 and 235—<b>Prophysema primordiale, a +living gastræad.</b> Fig. 234. The whole of the spindle-shaped animal (attached +below to the floor of the sea. Fig. 235. The same in longitudinal section. The +primitive gut (<i>d</i>) opens above at the primitive mouth (<i>m</i>). Between +the ciliated cells (<i>g</i>) are the amœboid ova (<i>e</i>). The skin-layer +(<i>h</i>) is encrusted with grains of sand below and sponge-spicules +above.</p> +</div> + +<p> +In <i>Prophysema</i> the primitive gut is a simple oval cavity, but in the +closely related <i>Gastrophysema</i> it is divided into two chambers by a +transverse constriction; the hind and smaller chamber above furnishes the +sexual products, the anterior one being for digestion. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus236"></a> +<img src="images/fig236.gif" width="187" height="211" alt="Figs. 236-237. Ascula of +gastrophysema, attached to the floor of the sea." /> +<p class="caption">Figs. 236–237—<b>Ascula of +gastrophysema,</b> attached to the floor of the sea. Fig. 236 external view, +237 longitudinal section. <i>g</i> primitive gut, <i>o</i> primitive mouth, +<i>i</i> visceral layer, <i>e</i> cutaneous layer. (Diagram.)</p> +</div> + +<p>The simplest sponges (<i>Olynthus,</i> Fig. 238) have the same +organisation as the Physemaria. The only material difference between them is +that in the sponge the thin two-layered body-wall is pierced by numbers of +pores. When these are closed they resemble the Physemaria. Possibly the +gastræads that we call Physemaria are only olynthi with the pores closed. The +<i> Ammoconida,</i> or the simple tubular sand-sponges of the deep-sea +(<i>Ammolynthus,</i> etc.), do not differ from the gastræads in any important +point when the pores are closed. In my <i> Monograph on the Sponges</i> (with +sixty plates) I endeavoured to prove analytically that all the species of this +class can be traced phylogenetically to a common stem-form +(<i>Calcolynthus</i>). +</p> + +<p> +The lowest form of the Cnidaria is also not far removed from the gastræads. In +the interesting common fresh-water polyp (<i>Hydra</i>) the whole body is +simply an oval tube with a double wall; only in this case the mouth has a crown +of tentacles. Before these develop the hydra resembles an ascula (Figs. 236, +237). Afterwards there are slight histological differentiations in its +ectoderm, though the entoderm remains +<span class='pagenum'><a name="Page_218" id="Page_218"></a></span> +a single stratum of cells. We find the first differentiation of epithelial and +stinging cells, or of muscular and neural cells, in the thick ectoderm of the +hydra. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus238"></a> +<img src="images/fig238.gif" width="104" height="160" alt="Fig.238. Olynthus, a +very rudimentary sponge." /> +<p class="caption">Fig. 238—<b>Olynthus,</b> a very rudimentary sponge. A piece cut +away in front.</p> +</div> + +<p>In all these rudimentary living cœlenteria the sexual cells of +both kinds—ova and sperm cells—are formed by the same individual; +it is possible that the oldest gastræads were hermaphroditic. It is clear from +comparative anatomy that hermaphrodism—the combination of both kinds of +sexual cells in one individual—is the earliest form of sexual +differentiation; the separation of the sexes (gonochorism) was a much later +phenomenon. The sexual cells originally proceeded from the edge of the +primitive mouth of the gastræad. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap20"></a>Chapter XX.<br/> +OUR WORM-LIKE ANCESTORS</h2> + +<p> +The gastræa theory has now convinced us that all the Metazoa or multicellular +animals can be traced to a common stem-form, the Gastræa. In accordance with +the biogenetic law, we find solid proof of this in the fact that the +two-layered embryos of all the Metazoa can be reduced to a primitive common +type, the gastrula. Just as the countless species of the Metazoa do actually +develop in the individual from the simple embryonic form of the gastrula, so +they have all descended in past time from the common stem-form of the Gastræa. +In this fact, and the fact we have already established that the Gastræa has +been evolved from the hollow vesicle of the one-layered Blastæa, and this again +from the original unicellular stem-form, we have obtained a solid basis for our +study of evolution. The clear path from the stem-cell to the gastrula +represents the first section of our human stem-history (Chapters VIII, IX, and +XIX). +</p> + +<p> +The second section, that leads from the Gastræa to the Prochordonia, is much +more difficult and obscure. By the Prochordonia we mean the ancient and +long-extinct animals which the important embryonic form of the chordula proves +to have once existed (cf. Figs. 83–86). The nearest of living animals to +this embryonic structure are the lowest Tunicates, the Copelata ( +<i>Appendicaria</i>) and the larvæ of the Ascidia. As both the Tunicates and +the Vertebrates develop from the same chordula, we may infer that there was a +corresponding common ancestor of both stems. We may call this the +<i>Chordæa,</i> and the corresponding stem-group the <i>Prochordonia</i> or +<i>Prochordata.</i> +</p> + +<p> +From this important stem-group of the unarticulated Prochordonia (or +“primitive chorda-animals”) the stems of the Tunicates and +Vertebrates have been divergently evolved. We shall see presently how this +conclusion is justified in the present condition of morphological science. +</p> + +<p> +We have first to answer the difficult and much-discussed question of the +development of the Chordæa from the Gastræa; in other words, “How and by +what transformations were the characteristic animals, resembling the embryonic +chordula, which we regard as the common stem-forms of all the Chordonia, both +<span class='pagenum'><a name="Page_219" id="Page_219"></a></span> +Tunicates and Vertebrates, evolved from the simplest two-layered +Metazoa?” +</p> + +<p> +The descent of the Vertebrates from the Articulates has been maintained by a +number of zoologists during the last thirty years with more zeal than +discernment; and, as a vast amount has been written on the subject, we must +deal with it to some extent. All three classes of Articulates in succession +have been awarded the honour of being considered the “real +ancestors” of the Vertebrates: first, the Annelids (earth-worms, leeches, +and the like), then the Crustacea (crabs, etc.), and, finally, the Tracheata +(spiders, insects, etc.). The most popular of these hypotheses was the annelid +theory, which derived the Vertebrates from the Worms. It was almost +simultaneously (1875) formulated by Carl Semper, of Würtzburg, and Anton Dohrn, +of Naples. The latter advanced this theory originally in favour of the failing +degeneration theory, with which I dealt in my work, <i>Aims and Methods of +Modern Embryology.</i> +</p> + +<p> +This interesting degeneration theory—much discussed at that time, but +almost forgotten now—was formed in 1875 with the aim of harmonising the +results of evolution and ever-advancing Darwinism with religious belief. The +spirited struggle that Darwin had occasioned by the reformation of the theory +of descent in 1859, and that lasted for a decade with varying fortunes in every +branch of biology, was drawing to a close in 1870–1872, and soon ended in +the complete victory of transformism. To most of the disputants the chief point +was not the general question of evolution, but the particular one of +“man’s place in nature”—“the question of +questions,” as Huxley rightly called it. It was soon evident to every +clear-headed thinker that this question could only be answered in the sense of +our anthropogeny, by admitting that man had descended from a long series of +Vertebrates by gradual modification and improvement. +</p> + +<p> +In this way the real affinity of man and the Vertebrates came to be admitted on +all hands. Comparative anatomy and ontogeny spoke too clearly for their +testimony to be ignored any longer. But in order still to save man’s +unique position, and especially the dogma of personal immortality, a number of +natural philosophers and theologians discovered an admirable way of escape in +the “theory of degeneration.” Granting the affinity, they turned +the whole evolutionary theory upside down, and boldly contended that “man +is not the most highly developed animal, but the animals are degenerate +men.” It is true that man is closely related to the ape, and belongs to +the vertebrate stem; but the chain of his ancestry goes upward instead of +downward. In the beginning “God created man in his own image,” as +the prototype of the perfect vertebrate; but, in consequence of original sin, +the human race sank so low that the apes branched off from it, and afterwards +the lower Vertebrates. When this theory of degeneration was consistently +developed, its supporters were bound to hold that the entire animal kingdom was +descended from the debased children of men. +</p> + +<p> +This theory was most strenuously defended by the Catholic priest and natural +philosopher, Michelis, in his <i>Hæckelogony: An Academic Protest against +Hæckel’s Anthropogeny</i> (1875). In still more “academic” +and somewhat mystic form the theory was advanced by a natural philosopher of +the older Jena school—the mathematician and physicist, Carl Snell. But it +received its chief support on the zoological side from Anton Dohrn, who +maintained the anthropocentric ideas of Snell with particular ability. The +Amphioxus, which modern science now almost unanimously regards as the real +Primitive Vertebrate, the ancient model of the original vertebrate structure, +is, according to Dohrn, a late, degenerate descendant of the stem, the +“prodigal son” of the vertebrate family. It has descended from the +Cyclostoma by a profound degeneration, and these in turn from the fishes; even +the Ascidia and the whole of the Tunicates are merely degenerate fishes! +Following out this curious theory, Dohrn came to contest the general belief +that the Cœlenterata and Worms are “lower animals”; he even +declared that the unicellular Protozoa were degenerate Cœlenterata. In his +opinion “degeneration is the great principle that explains the existence +of all the lower forms.” +</p> + +<p> +If this Michelis-Dohrn theory were true, and all animals were really degenerate +descendants of an originally perfect humanity, man would assuredly be the true +centre and goal of all terrestrial life; his anthropocentric position and his +immortality would be saved. Unfortunately, this trustful theory is in such +<span class='pagenum'><a name="Page_220" id="Page_220"></a></span> +flagrant contradiction to all the known facts of paleontology and embryology +that it is no longer worth serious scientific consideration. +</p> + +<p> +But the case is no better for the much-discussed descent of the Vertebrates +from the Annelids, which Dohrn afterwards maintained with great zeal. Of late +years this hypothesis, which raised so much dust and controversy, has been +entirely abandoned by most competent zoologists, even those who once supported +it. Its chief supporter, Dohrn, admitted in 1890 that it is “dead and +buried,” and made a blushing retraction at the end of his <i>Studies of +the Early History of the Vertebrate.</i> +</p> + +<p> +Now that the annelid-hypothesis is “dead and buried,” and other +attempts to derive the Vertebrates from Medusæ, Echinoderms, or Molluscs, have +been equally unsuccessful, there is only one hypothesis left to answer the +question of the origin of the Vertebrates—the hypothesis that I advanced +thirty-six years ago and called the “chordonia-hypothesis.” In view +of its sound establishment and its profound significance, it may very well +claim to be a <i>theory,</i> and so should be described as the chordonia or +chordæa theory. +</p> + +<p> +I first advanced this theory in a series of university lectures in 1867, from +which the <i>History of Creation</i> was composed. In the first edition of this +work (1868) I endeavoured to prove, on the strength of Kowalevsky’s +epoch-making discoveries, that “of all the animals known to us the +Tunicates are undoubtedly the nearest blood-relatives of the Vertebrates; they +are the most closely related to the Vermalia, from which the Vertebrates have +been evolved. Naturally, I do not mean that the Vertebrates have descended from +the Tunicates, but that the two groups have sprung from a common root. It is +clear that the real Vertebrates (primarily the Acrania) were evolved in very +early times from a group of Worms, from which the degenerate Tunicates also +descended in another and retrogressive direction.” This common extinct +stem-group are the Prochordonia; we still have a silhouette of them in the +chordula-embryo of the Vertebrates and Tunicates; and they still exist +independently, in very modified form, in the class of the Copelata ( +<i>Appendicaria,</i> Fig. 225). +</p> + +<p> +The chordæa-theory received the most valuable and competent support from Carl +Gegenbaur. This able comparative morphologist defended it in 1870, in the +second edition of his <i>Elements of Comparative Anatomy</i> ; at the same time +he drew attention to the important relations of the Tunicates to a curious +worm, <i>Balanoglossus</i> : he rightly regards this as the representative of a +special class of worms, which he called “gut-breathers” ( +<i>Enteropneusta</i>). Gegenbaur referred on many other occasions to the close +blood-relationship of the Tunicates and Vertebrates, and luminously explained +the reasons that justify us in framing the hypothesis of the descent of the two +stems from a common ancestor, an unsegmented worm-like animal with an axial +chorda between the dorsal nerve-tube and the ventral gut-tube. +</p> + +<p> +The theory afterwards received a good deal of support from the research made by +a number of distinguished zoologists and anatomists, especially C. Kupffer, B. +Hatschek, F. Balfour, E. Van Beneden, and Julin. Since Hatschek’s +<i>Studies of the Development of the Amphioxus</i> gave us full information as +to the embryology of this lowest vertebrate, it has become so important for our +purpose that we must consider it a document of the first rank for answering the +question we are dealing with. +</p> + +<p> +The ontogenetic facts that we gather from this sole survivor of the Acrania are +the more valuable for phylogenetic purposes, as paleontology, unfortunately, +throws no light whatever on the origin of the Vertebrates. Their invertebrate +ancestors were soft organisms without skeleton, and thus incapable of +fossilisation, as is still the case with the lowest vertebrates—the +Acrania and Cyclostoma. The same applies to the greater part of the Vermalia or +worm-like animals, the various classes and orders of which differ so much in +structure. The isolated groups of this rich stem are living branches of a huge +tree, the greater part of which has long been dead, and we have no fossil +evidence as to its earlier form. Nevertheless, some of the surviving groups are +very instructive, and give us clear indications of the way in which the +Chordonia were developed from the Vermalia, and these from the Cœlenteria. +</p> + +<p> +While we seek the most important of these palingenetic forms among the groups +of Cœlenteria and Vermalia, it is understood that not a single one of them +<span class='pagenum'><a name="Page_221" id="Page_221"></a></span> +must be regarded as an unchanged, or even little changed, copy of the extinct +stem-form. One group has retained one feature, another a different feature, of +the original organisation, and other organs have been further developed and +characteristically modified. Hence here, more than in any other part of our +genealogical tree, we have to keep before our mind the <i>full picture</i> of +development, and separate the unessential secondary phenomena from the +essential and primary. It will be useful first to point out the chief advances +in organisation by which the simple Gastræa gradually became the more developed +Chordæa. +</p> + +<p> +We find our first solid datum in the gastrula of the Amphioxus (Figure 1.38). +Its bilateral and tri-axial type indicates that the Gastræads—the common +ancestors of all the Metazoa—divided at an early stage into two divergent +groups. The uni-axial Gastræa became sessile, and gave rise to two stems, the +Sponges and the Cnidaria (the latter all reducible to simple polyps like the +hydra). But the tri-axial Gastræa assumed a certain pose or direction of the +body on account of its swimming or creeping movement, and in order to sustain +this it was a great advantage to share the burden equally between the two +halves of the body (right and left). Thus arose the typical bilateral form, +which has three axes. The same bilateral type is found in all our artificial +means of locomotion—carts, ships, etc.; it is by far the best for the +movement of the body in a certain direction and steady position. Hence natural +selection early developed this bilateral type in a section of the Gastræads, +and thus produced the stem-forms of all the bilateral animals. +</p> + +<p> +The <i>Gastræa bilateralis,</i> of which we may conceive the bilateral gastrula +of the amphioxus to be a palingenetic reproduction, represented the two-sided +organism of the earliest Metazoa in its simplest form. The vegetal entoderm +that lined their simple gut-cavity served for nutrition; the ciliated ectoderm +that formed the external skin attended to locomotion and sensation; finally, +the two primitive mesodermic cells, that lay to the right and left at the +ventral border of the primitive mouth, were sexual cells, and effected +reproduction. In order to understand the further development of the gastræa, we +must pay particular attention to: (1) the careful study of the embryonic stages +of the amphioxus that lie between the gastrula and the chordula; (2) the +morphological study of the simplest Platodes ( <i>Platodaria</i> and +<i>Turbellaria</i>) and several groups of unarticulated Vermalia ( +<i>Gastrotricha, Nemertina, Enteropneusta</i>). +</p> + +<p> +We have to consider the Platodes first, because they are on the border between +the two principal groups of the Metazoa, the Cœlenteria and the Cœlomaria. With +the former they share the lack of body-cavity, anus, and vascular system; with +the latter they have in common the bilateral type, the possession of a pair of +nephridia or renal canals, and the formation of a vertical brain or cerebral +ganglion. It is now usual to distinguish four classes of Platodes: the two +free-living classes of the primitive worms ( <i>Platodaria</i>) and the +coiled-worms ( <i>Turbellaria</i>), and the two parasitic classes of the +suctorial worms ( <i>Trematoda</i>) and the tape-worms ( <i>Cestoda</i>). We +have only to consider the first two of these classes; the other two are +parasites, and have descended from the former by adaptation to parasitic habits +and consequent degeneration. +</p> + +<p> +The primitive worms ( <i>Platodaria</i>) are very small flat worms of simple +construction, but of great morphological and phylogenetic interest. They have +been hitherto, as a rule, regarded as a special order of the +<i>Turbellaria,</i> and associated with the <i>Rhabdocœla</i> ; but they differ +considerably from these and all the other Platodes (flat worms) in the absence +of renal canals and a special central nervous system; the structure of their +tissue is also simpler than in the other Platodes. Most of the Platodes of this +group ( <i>Aphanostomum, Amphichœrus, Convoluta, Schizoprora,</i> etc.) are +very soft and delicate animals, swimming about in the sea by means of a ciliary +coat, and very small (1/10 to 1/20 inch long). Their oval body, without +appendages, is sometimes spindle-shaped or cylindrical, sometimes flat and +leaf-shaped. Their skin is merely a layer of ciliated ectodermic cells. Under +this is a soft medullary substance, which consists of entodermic cells with +vacuoles. The food passes through the mouth directly into this digestive +medullary substance, in which we do not generally see any permanent gut-cavity +(it may have entirely collapsed); hence these primitive Platodes have been +called <i>Acœla</i> (without gut-cavity or cœlom), or, more correctly, +<i>Cryptocœla,</i> or <i>Pseudocœla.</i> The sexual organs of these +hermaphroditic +<span class='pagenum'><a name="Page_222" id="Page_222"></a></span> +Platodaria are very simple—two pairs of strings of cells, the inner of +which (the ovaries, Fig. 239 <i>o</i>) produce ova, and the outer (the +spermaria, <i>s</i>) sperm-cells. These gonads are not yet independent sexual +glands, but sexually differentiated cell-groups in the medullary substance, or, +in other words, parts of the gut-wall. Their products, the sex-cells, are +conveyed out behind by two pairs of short canals; the male opening ( <i>m</i>) +lies just behind the female ( <i>f</i>). Most of the Platodaria have not the +muscular pharynx, which is very advanced in the <i>Turbellaria</i> and +<i>Trematoda.</i> On the other hand, they have, as a rule, before or behind the +mouth, a bulbous sense-organ (auditory vesicle or organ of equilibrium, +<i>g</i>), and many of them have also a couple of simple optic spots. The +cell-pit of the ectoderm that lies underneath is rather thick, and represents +the first rudiment of a neural ganglion (vertical brain or acroganglion). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus239"></a> +<img src="images/fig239.gif" width="219" height="346" alt="Fig.239. Aphanostomum +Langii (Haekel), a primitive worm of the platodaria class, of the order of +Cryptocoela or Acoela." /> +<p class="caption">Fig. 239—<b>Aphanostomum Langii</b> ( +<i>Haeckel</i>), a primitive worm of the platodaria class, of the order of +<i>Cryptocoela</i> or <i>Acoela.</i> This new species of the genus +Aphanostomum, named after Professor Arnold Lang of Zurich, was found in +September, 1899, at Ajaccio in Corsica (creeping between fucoidea). It is +one-twelfth of an inch long, one-twenty-fifth of an inch broad, and violet in +colour. <i>a</i> mouth, <i>g</i> auditory vesicle, <i>e</i> ectoderm, <i>i</i> +entoderm, <i>o</i> ovaries, <i>a</i> spermaries, <i>f</i> female aperture, +<i>m</i> male aperture.</p> +</div> + +<p> +The <i>Turbellaria,</i> with which the similar <i>Platodaria</i> were formerly +classed, differ materially from them in the more advanced structure of their +organs, and especially in having a central nervous system (vertical brain) and +excretory renal canals (nephridia); both originate from the ectoderm. But +between the two germinal layers a mesoderm is developed, a soft mass of +connective tissue, in which the organs are embedded. The <i>Turbellaria</i> are +still represented by a number of different forms, in both fresh and sea-water. +The oldest of these are the very rudimentary and tiny forms that are known as +<i>Rhabdocœla</i> on account of the simple construction of their gut; they are, +as a rule, less than a quarter of an inch long and of a simple oval or lancet +shape (Fig. 240). The surface is covered with ciliated epithelium, a stratum of +ectodermic cells. The digestive gut is still the simple primitive gut of the +gastræa ( <i>d</i>), with a single aperture that is both mouth and anus ( +<i>m</i>). There is, however, an invagination of the ectoderm at the mouth, +which has given rise to a muscular pharynx ( <i>sd</i>). It is noteworthy that +the mouth of the Turbellaria (like the primitive mouth of the Gastræa) may, in +this class, change its position considerably in the middle line of the ventral +surface; sometimes it lies behind ( <i>Opisthostomum</i>), sometimes in the +middle ( <i>Mesostomum</i>), sometimes in front ( <i>Prosostomum</i>). This +displacement of the mouth from front to rear is very interesting, because it +corresponds to a phylogenetic displacement of the mouth. This probably occurred +in the Platode ancestors of most (or all?) of the Cœlomaria; in these the +permanent mouth ( <i>metastoma</i>) lies at the fore end (oral pole), whereas +the primitive mouth ( <i>prostoma</i>) lay at the hind end of the bilateral +body. +</p> + +<p> +In most of the Turbellaria there is a narrow cavity, containing a number of +secondary organs, between the two primary germinal layers, the outer or animal +layer of which forms the epidermis and the inner vegetal layer the visceral +epithelium. The earliest of these organs are the sexual organs; they are very +variously constructed in the Platode-class; in the simplest case there are +merely two pairs of gonads or sexual glands—a pair of testicles (Fig. 241 +<i>h</i>) +<span class='pagenum'><a name="Page_223" id="Page_223"></a></span> +and a pair of ovaries ( <i>e</i>). They open externally, sometimes by a common +aperture ( <i>Monogonopora</i>), sometimes by separate ones, the female behind +the male ( <i>Digonopora,</i> Fig. 241). The sexual glands develop originally +from the two promesoblasts or primitive mesodermic cells (Fig. 83 <i>p</i>). As +these earliest mesodermic structures extended, and became spacious sexual +pouches in the later descendants of the Platodes, probably the two +cœlom-pouches were formed from them, the first trace of the real body-cavity of +the higher Metazoa ( <i>Enterocœla</i>). +</p> + +<p> +The gonads are among the oldest organs, the few other organs that we find in +the Platodes between the gut-wall and body-wall being later evolutionary +products. One of the oldest and most important of these are the kidneys or +<i>nephridia,</i> which remove unusable matter from the body (Fig. 240 +<i>nc</i>). These urinary or excretory organs were originally enlarged +skin-glands—a couple of canals that run the length of the body, and have +a separate or common external aperture ( <i>nm</i>). They often have a number +of branches. These special excretory organs are not found in the other +Cœlenteria (Gastræads, Sponges, Cnidaria) or the Cryptocœla. They are first met +in the <i>Turbellaria,</i> and have been transmitted direct from these to the +Vermalia, and from these to the higher stems. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus240"></a> +<img src="images/fig240.gif" width="241" height="394" alt="Fig.240. A simple +turbellarian (Rhabdocoelum). Fig. 241. The same, showing the other organs." /> +<p class="caption">Fig. 240—<b>A simple turbellarian</b> ( +<i>Rhabdocœlum</i>). <i>m</i> mouth, <i>sd</i> gullet epithelium, <i>sm</i> +gullet muscles, <i>d</i> gastric gut, <i>nc</i> renal canals, <i>nm</i> renal +aperture, <i>au</i> eye, <i>na</i> olfactory pit. (Diagram.)<br/> + +Fig. 241—<b>The same,</b> showing the other organs. <i>g</i> brain, +<i>au</i> eye, <i>na</i> olfactory pit, <i>n</i> nerves, <i>h</i> testicles, +<i>ma</i> male aperture, <i>fa</i> female aperture, <i>e</i> ovary, <i>f</i> +ciliated epiderm. (Diagram.)</p> +</div> + +<p> +Finally, there is a very important new organ in the Turbellaria, which we do +not find in the <i>Cryptocœla</i> (Fig. 239) and their gastræad +ancestors—the rudimentary nervous system. It consists of a couple of +simple cerebral ganglia (Fig. 241 <i>g</i>) and fine nervous fibres that +radiate from them; these are partly voluntary nerves (or motor fibres) that go +to the thin muscular layer developing under the skin; and partly sensory nerves +that proceed to the sense-cells of the ciliated epiderm ( <i>f</i>). Many of +the Turbellaria have also special sense-organs; a couple of ciliated smell pits +( <i>na</i>), rudimentary eyes ( <i>au</i>), and, less frequently, auditory +vesicles. +</p> + +<p> +On these principles I assume that the oldest and simplest Turbellaria arose +from Platodaria, and these directly from bilateral Gastræads. The chief +advances were the formation of gonads and nephridia, and of the rudimentary +brain. On this hypothesis, which I advanced in 1872 in the first sketch of the +gastræa-theory ( <i>Monograph on the Sponges</i>), there is no direct affinity +between the Platodes and the Cnidaria. +</p> + +<p> +Next to the ancient stem-group of the Turbellaria come a number of more recent +chordonia ancestors, which we class with the <i>Vermalia</i> or +<i>Helminthes,</i> the unarticulated worms. These true worms ( <i>Vermes,</i> +lately also called <i>Scolecida</i>) are the difficulty or the lumber-room of +the zoological classifier, because the various classes have very complicated +relations to the lower Platodes on the one hand and the more advanced animals +on the other. But if we exclude the Platodes and the Annelids from this stem, +we find a fairly satisfactory unity of organisation +<span class='pagenum'><a name="Page_224" id="Page_224"></a></span> +in the remaining classes. Among these worms we find some important forms that +show considerable advance in organisation from the platode to the chordonia +stage. Three of these phenomena are particularly instructive: (1) The formation +of a true (secondary) body-cavity (cœloma); (2) the formation of a second +aperture of the gut, the anus; and (3) the formation of a vascular system. The +great majority of the Vermalia have these three features, and they are all +wanting in the Platodes; in the rest of the worms at least one or two of them +are developed. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus242"></a> +<img src="images/fig242.gif" width="330" height="374" alt="Figs. 242 and 243. +Chaetonotus, a rudimentary vermalian, of the group of Gastrotricha." /> +<p class="caption">Figs. 242 and 243—<b>Chætonotus, a rudimentary +vermalian,</b> of the group of Gastrotricha. <i>m</i> mouth, <i>s</i> gullet, +<i>d</i> gut, <i>a</i> anus, <i>g</i> brain, <i>n</i> nerves, <i>ss</i> sensory +hairs, <i>au</i> eye, <i>ms</i> muscular cells, <i>h</i> skin, <i>f</i> +ciliated bands of the ventral surface, <i>nc</i> nephridia, <i>nm</i> their +aperture, <i>e</i> ovaries.</p> +</div> + +<p> +Next and very close to the Platodes we have the Ichthydina ( +<i>Gastrotricha</i>), little marine and fresh-water worms, about 1/250 to +1/1000 inch long. Zoologists differ as to their position in classification. In +my opinion, they approach very close to the Rhabdocœla (Figs. 240, 241), and +differ from them chiefly in the possession of an anus at the posterior end +(Fig. 242 <i>a</i>). Further, the cilia that cover the whole surface of the +Turbellaria are confined in the Gastrotricha to two ciliated bands ( <i>f</i>) +on the ventral surface of the oval body, the dorsal surface having bristles. +Otherwise the organisation of the two classes is the same. In both the gut +consists of a muscular gullet ( <i>s</i>) and a glandular primitive gut ( +<i>d</i>). Over the gullet is a double brain (acroganglion, <i>g</i>). At the +side of the gut are two serpentine prorenal canals (water-vessels or +pronephridia, <i>nc</i>), which open on the ventral side ( <i>nm</i>). Behind +are a pair of simple sexual glands or gonads (Fig. 243 <i>e</i>). +</p> + +<p> +While the Ichthydina are thus closely related to the Platodes, we have to go +farther away for the two classes of Vermalia which we unite in the group of the +“snout-worms” ( <i>Frontonia</i>). These are the <i>Nemertina</i> +and the <i>Enteropneusta.</i> +<span class='pagenum'><a name="Page_225" id="Page_225"></a></span> +Both classes have a complete ciliary coat on the epidermis, a heritage from the +Turbellaria and the Gastræads; also, both have two openings of the gut, the +mouth and anus, like the Gastrotricha. But we find also an important organ that +is wanting in the preceding forms—the vascular system. In their more +advanced mesoderm we find a few contractile longitudinal canals which force the +blood through the body by their contractions; these are the first +blood-vessels. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus244"></a> +<img src="images/fig244.gif" width="202" height="356" alt="Fig.244. A simple Nemertine." /> +<p class="caption">Fig. 244—<b>A simple Nemertine.</b> <i>m</i> +mouth, <i>d</i> gut, <i>a</i> anus, <i>g</i> brain, <i>n</i> nerves, <i>h</i> +ciliary coat, <i>ss</i> sensory pits (head-clefts), <i>au</i> eyes, <i>r</i> +dorsal vessel, <i>l</i> lateral vessels. (Diagram.)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus245"></a> +<img src="images/fig245.gif" width="154" height="523" alt="Fig. 245. A young +Enteropneust." /> +<p class="caption">Fig. 245—<b>A young Enteropneust</b> ( <i>Balanaglossus</i>). (From +<i>Alexander Agassiz.</i>) <i>r</i> acorn-shaped snout, <i>h</i> neck, <i>k</i> +gill-clefts and gill-arches of the fore-gut, in long rows on each side, +<i>d</i> digestive hind-gut, filling the greater part of the body-cavity, +<i>v</i> intestinal vein or ventral vessel, lying between the parallel folds of +the skin, <i>a</i> anus.</p> +</div> + +<p> +<span class='pagenum'><a name="Page_226" id="Page_226"></a></span> +The Nemertina were formerly classed with the much less advanced Turbellaria. +But they differ essentially from them in having an anus and blood-vessels, and +several other marks of higher organisation. They have generally long and narrow +bodies, like a more or less flattened cord; there are, besides several small +species, giant-forms with a width of 1/5 to 2/5 inch and a length of several +yards (even ten to fifteen). Most of them live in the sea, but some in fresh +water and moist earth. In their internal structure they approach the +Turbellaria on the one hand and the higher Vermalia (especially the +Enteropneusta) on the other. They have a good deal of interest as the lowest +and oldest of all animals with blood. In them we find blood-vessels for the +first time, distributing real blood through the body. The blood is red, and the +red colouring-matter is hæmoglobin, connected with elliptic discoid +blood-cells, as in the Vertebrates. Most of them have two or three parallel +blood-canals, which run the whole length of the body, and are connected in +front and behind by loops, and often by a number of ring-shaped pieces. The +chief of these primitive blood-vessels is the one that lies above the gut in +the middle line of the back (Fig. 244 <i>r</i>); it may be compared to either +the dorsal vessel of the Articulates or the aorta of the Vertebrates. To the +right and left are the two serpentine lateral vessels (Fig. 244 <i>l</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus246"></a> +<img src="images/fig246.gif" width="256" height="143" alt="Fig.246. Transverse +section of the branchial gut." /> +<p class="caption">Fig. 246—<b>Transverse section of the branchial +gut.</b> <i>A</i> of Balanoglossus, <i>B</i> of Ascidia. <i>r</i> branchial +gut, <i>n</i> pharyngeal groove, * ventral folds between the two. Diagrammatic +illustration from <i>Gegenbaur,</i> to show the relation of the dorsal +branchial-gut cavity ( <i>r</i>) to the pharyngeal or hypobranchial groove ( +<i>n</i>).</p> +</div> + +<p> +After the Nemertina, I take (as distant relatives) the <i>Enteropneusta</i> ; +they may be classed together with them as <i>Frontonia</i> or <i>Rhyncocœla</i> +(snout-worms). There is now only one genus of this class, with several species +( <i>Balanoglossus</i>); but it is very remarkable, and may be regarded as the +last survivor of an ancient and long-extinct class of Vermalia. They are +related, on the one hand, to the Nemertina and their immediate ancestors, the +Platodes, and to the lowest and oldest forms of the Chordonia on the other. +</p> + +<p> +The Enteropneusta (Fig. 245) live in the sea sand, and are long worms of very +simple shape, like the Nemertina. From the latter they have inherited: (1) The +bilateral type, with incomplete segmentation; (2) the ciliary coat of the soft +epidermis; (3) the double rows of gastric pouches, alternating with a single or +double row of gonads; (4) separation of the sexes (the Platode ancestors were +hermaphroditic); (5) the ventral mouth, underneath a protruding snout; (6) the +anus terminating the simple gut-tube; and (7) several parallel blood-canals, +running the length of the body, a dorsal and a ventral principal stem. +</p> + +<p> +On the other hand, the Enteropneusta differ from their Nemertine ancestors in +several features, some of which are important, that we may attribute to +adaptation. The chief of these is the branchial gut (Fig. 245 <i>k</i>). The +anterior section of the gut is converted into a respiratory organ, and pierced +by two rows of gill-clefts; between these there is a branchial (gill) skeleton, +formed of rods and plates of chitine. The water that enters at the mouth makes +its exit by these clefts. They lie in the dorsal half of the fore-gut, and this +is completely separated from the ventral half by two longitudinal folds (Fig. +246 <i>A*</i>). This ventral half, the glandular walls of which are clothed +with ciliary epithelium and secrete mucus, corresponds to the pharyngeal or +hypo-branchial groove of the Chordonia ( <i>Bn</i>), the important organ from +which the later thyroid gland is developed in the Craniota (cf. p. 184). The +agreement in the structure of the branchial gut of the Enteropneusts, +Tunicates, and Vertebrates was first recognised by Gegenbaur (1878); it is the +more significant as at first we find only a couple of gill-clefts in the young +animals of all three groups; the number gradually increases. We can infer from +this the common descent of the three groups with all the more +<span class='pagenum'><a name="Page_227" id="Page_227"></a></span> +confidence when we find the <i>Balanoglossus</i> approaching the Chordonia in +other respects. Thus, for instance, the chief part of the central nervous +system is a long dorsal neural string that runs above the gut and corresponds +to the medullary tube of the Chordonia. Bateson believes he has detected a +rudimentary chorda between the two. +</p> + +<p> +Of all extant invertebrate animals the Enteropneusts come nearest to the +Chordonia in virtue of these peculiar characters; hence we may regard them as +the survivors of the ancient gut-breathing Vermalia from which the Chordonia +also have descended. Again, of all the chorda-animals the Copelata (Fig. 225) +and the tailed larvæ of the ascidia approach nearest to the young +<i>Balanoglossus.</i> Both are, on the other hand, very closely related to the +<i>Amphioxus,</i> the Primitive Vertebrate of which we have considered the +importance (Chapters XVI and XVII). As we saw there, the unarticulated +Tunicates and the articulated Vertebrates must be regarded as two independent +stems, that have developed in divergent directions. But the common root of the +two stems, the extinct group of the Prochordonia, must be sought in the +vermalia stem; and of all the living Vermalia those we have considered give us +the safest clue to their origin. It is true that the actual representatives of +the important groups of the Copelata, Balanoglossi, Nemertina, Icthydina, etc., +have more or less departed from the primitive model owing to adaptation to +special environment. But we may just as confidently affirm that the main +features of their organisation have been preserved by heredity. +</p> + +<p> +We must grant, however, that in the whole stem-history of the Vertebrates the +long stretch from the Gastræads and Platodes up to the oldest Chordonia remains +by far the most obscure section. We might frame another hypothesis to raise the +difficulty—namely, that there was a long series of very different and +totally extinct forms between the Gastræa and the Chordæa. Even in this +modified chordæa-theory the six fundamental organs of the chordula would retain +their great value. The medullary tube would be originally a chemical sensory +organ, a dorsal olfactory tube, taking in respiratory-water and food by the +neuroporus in front and conveying them by the neurenteric canal into the +primitive gut. This olfactory tube would afterwards become the nervous centre, +while the expanding gonads (lying to right and left of the primitive mouth) +would form the cœloma. The chorda may have been originally a digestive +glandular groove in the dorsal middle line of the primitive gut. The two +secondary gut-openings, mouth and anus, may have arisen in various ways by +change of functions. In any case, we should ascribe the same high value to the +chordula as we did before to the gastrula. +</p> + +<p> +In order to explain more fully the chief stages in the advance of our race, I +add the hypothetical sketch of man’s ancestry that I published in my +<i>Last Link</i> [a translation by Dr. Gadow of the paper read at the +International Zoological Congress at Cambridge in 1898]:— +<span class='pagenum'><a name="Page_228" id="Page_228"></a></span> +</p> + +<p class="center"> +<b>A.—Man’s Genealogical Tree, First Half:</b><br/> +EARLIER SERIES OF ANCESTORS, <br/> WITHOUT FOSSIL EVIDENCE.<br/><br/> +</p> + +<table border="1" cellspacing="0" +cellpadding="4" summary="Chief Stages, Ancestral Stem-groups, Living Relatives +of Ancestors."> +<tr> +<td align="center" valign="bottom"> <b>Chief Stages</b> </td> <td +align="center" valign="bottom"> <b>Ancestral Stem-groups</b> </td> <td +align="center" valign="bottom"> <b> Living Relatives of <br/> Ancestors </b> +</td> </tr> + +<tr> +<td align="center" valign="middle" rowspan="2"> Stages 1–5: <br/> <b> +Protist <br/> ancestors </b> <br/> Unicellular <br/> organisms. <br/> +1–2: <br/> + +Prototypes <br/> 3–5: <br/> Protozoa </td> <td align="center"> 1. +<b>Monera</b> <br/> + +Without nucleus <br/> <br/> 2. <b>Algaria</b> <br/> Unicellular algæ +<br/> </td> <td align="center"> 1. <b>Chromacea</b> <br/> <i> +(Chroococcus) <br/> Phycochromacea </i> <br/> 2. <b>Paulotomea</b> <br/> <i> +Palmellacea <br/> Eremosphæra </i> </td> </tr> + +<tr> +<td align="center"> 3. <b>Lobosa</b> <br/> Unicellular (amœbina) <br/> +rhizopods <br/> 4. <b>Infusoria</b> <br/> Unicellular <br/> <br/> 5. +<b>Blastæades</b> <br/> Multicellular hollow spheres <br/> </td> <td +align="center"> 3. <b>Amœbina</b> <br/> <i>Amœba Leucocyta</i> <br/> +<br/> + +4. <b>Flagellata</b> <br/> <i> Euflagellata <br/> Zoomonades </i> <br/> 5. +<b>Catallacta</b> <br/> <i> Magosphæra, Volvocina, <br/> Blastula </i> </td> +</tr> + +<tr> +<td align="center" valign="middle" rowspan="2"> Stages 6–11: <br/> <b> +Invertebrate <br/> metazoa <br/> ancestors </b> <br/> 6–8: <br/> +Cœlenteria <br/> without anus and <br/> body-cavity <br/> 9–11: <br/> +Vermalia, with <br/> anus and <br/> body-cavity </td> <td align="center"> 6. +<b>Gastræades</b> <br/> With two germ-layers <br/> <br/> 7 <b>Platodes +I</b> <br/> <i>Platodaria</i> <br/> (without nephridia) <br/> 8. <b>Platodes +II</b> <br/> <i>Platodinia</i> (with nephridia) </td> <td align="center"> 6. +<b>Gastrula</b> <br/> <i> Hydra, Olynthus, <br/> Gastremaria </i> <br/> 7. +<b>Cryptocœla</b> <br/> <i>Convoluta, Porporus</i> <br/> <br/> 8. +<b>Rhabdocœla</b> <br/> <i>Vortex, Monolus</i> </td> </tr> + +<tr> +<td align="center"> 9. <b>Provermalia</b> <br/> (Primitive worms) <br/> +<i>Rotatoria</i> <br/> 10. <b>Frontonia</b> <br/> <i>(Rhynchelminthes)</i> +<br/> Snout-worms <br/> 11. <b>Prochordonia</b> <br/> Chorda-worms <br/> +</td> <td align="center"> 9. <b>Gastrotricha</b> <br/> <i>Trochozoa, +Trochophora</i> <br/> <br/> 10. <b>Enteropneusta</b> <br/> <i> +Balanglossus <br/> Cephalodiscus </i> <br/> 11. <b>Copelata</b> <br/> +<i>Appendicaria</i> <br/> Chordula-larvæ </td> </tr> + +<tr> +<td align="center" valign="middle"> Stages 12–15: <br/> <b> Monorhina +<br/> ancestors </b> <br/> Oldest vertebrates <br/> without jaws or <br/> pairs +of limbs, <br/> single nose </td> <td align="center"> 12. <b>Acrania I</b> +<br/> (Prospondylia) <br/> 13. <b>Acrania II</b> <br/> More recent <br/> 14. +<b>Cyclostoma I</b> <br/> (Archicrania) <br/> 15. <b>Cyclostoma II</b> <br/> +More recent </td> + +<td align="center"> 12. <b>Amphioxus larvæ</b> <br/> <br/> 13. +<b>Leptocardia</b> <br/> Amphioxus <br/> 14. <b>Petromyzonta larvæ</b> <br/> + <br/> 15. <b>Marsipobranchia</b> <br/> Petromyzonta </td> </tr> + +</table> + +<p> +<br/><br/> +</p> + +<p class="center"> +<b>B.—Man’s Genealogical Tree, Second Half:</b><br/> +LATER ANCESTORS, WITH FOSSIL EVIDENCE.<br/><br/> +</p> + +<table border="1" cellspacing="0" +cellpadding="4" summary="Geological Periods, Ancestral Stem-groups, Living +Relatives of Ancestors."> +<tr> +<td align="center" valign="bottom"> <b> Geological <br/> Periods </b> </td> <td +align="center" valign="bottom"> <b>Ancestral Stem-groups</b> </td> <td +align="center" valign="bottom"> <b> Living Relatives of <br/> Ancestors </b> +</td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Silurian</b> </td> <td align="center"> +16. <b>Selachii</b> <br/> Primitive fishes <br/> <i>Proselachii</i> </td> <td +align="center"> 16. <b>Natidanides</b> <br/> Chlamydoselachius <br/> Heptanchus +</td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Silurian</b> </td> <td align="center"> +17. <b>Ganoids</b> <br/> Plated-fishes <br/> <i>Proganoids</i> </td> <td +align="center"> 17. <b>Accipenserides</b> <br/> (Sturgeons) <br/> Polypterus +</td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Devonian</b> </td> <td align="center" +valign="middle"> 18. <b>Dipneusta</b> <br/> <i>Paladipneusta</i> </td> <td +align="center"> 18. <b>Neodipneusta</b> <br/> Ceratodus <br/> Proptopterus +</td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Carboniferous</b> </td> <td +align="center" valign="middle"> 19. <b>Amphibia</b> <br/> <i>Stegocephala</i> +</td> <td align="center"> 19. <b>Phanerobranchia</b> <br/> Salamandrina <br/> +(Proteus, triton) </td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Permian</b> </td> <td align="center" +valign="middle"> 20. <b>Reptilia</b> <br/> <i>Proreptilia</i> </td> <td +align="center"> 20. <b>Rhynchocephalia</b> <br/> Primitive lizards <br/> +Hatteria </td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Triassic</b> </td> <td align="center" +valign="middle"> 21. <b>Monotrema</b> <br/> <i>Promammalia</i> </td> <td +align="center"> 21. <b>Ornithodelphia</b> <br/> <i> Echidna <br/> +Ornithorhyncus </i> </td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Jurassic</b> </td> <td align="center" +valign="middle"> 22. <b>Marsupialia</b> <br/> <i>Prodidelphia</i> </td> <td +align="center"> 22. <b>Didelphia</b> <br/> <i> Didelphys <br/> Perameles </i> +</td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Cretaceous</b> </td> <td align="center" +valign="middle"> 23. <b>Mallotheria</b> <br/> <i>Prochoriata</i> </td> <td +align="center"> 23. <b>Insectivora</b> <br/> Erinaceida <br/> (Ictopsia +) +</td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Older Eocene</b> </td> <td +align="center"> 24. <b>Lemuravida</b> <br/> Older lemurs <br/> Dentition 3. 1. +4. 3. </td> <td align="center"> 24. <b>Pachylemures</b> <br/> <i> (Hyopsodus +) +<br/> (Adapis +) </i> </td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Neo-Eocene</b> </td> <td align="center"> +25. <b>Lemurogona</b> <br/> Later lemurs <br/> Dentition 2. 1. 4. 3. </td> <td +align="center"> 25. <b>Autolemures</b> <br/> <i> Eulemur <br/> Stenops </i> +</td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Oligocene</b> </td> <td align="center"> +26. <b>Dysmopitheca</b> <br/> Western apes <br/> Dentition 2. 1. 3. 3. </td> +<td align="center"> 26. <b>Platyrrhinæ</b> <br/> <i> (Anthropops +) <br/> +(Homunculus +) </i> </td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Older Miocene</b> </td> <td +align="center"> 27. <b>Cynopitheca</b> <br/> Dog-faced apes (tailed) </td> <td +align="center"> 27. <b>Papiomorpha</b> <br/> <i>Cynocephalus</i> </td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Neo-Miocene</b> </td> <td align="center" +valign="middle"> 28. <b>Anthropoides</b> <br/> Man-like apes (tail-less) </td> +<td align="center"> 28. <b>Hylobatida</b> <br/> Hylobates <br/> Satyrus </td> +</tr> + +<tr> +<td align="center" valign="middle"> <b>Pliocene</b> </td> <td align="center" +valign="middle"> 29. <b>Pithecanthropi</b> <br/> Ape-men (alali, speechless) +</td> <td align="center"> 29. <b>Anthropitheca</b> <br/> Chimpanzee <br/> +Gorilla </td> </tr> + +<tr> +<td align="center" valign="middle"> <b>Pleistocene</b> </td> <td +align="center"> 30. <b>Homines</b> <br/> Men with speech </td> <td +align="center"> 30. <b>Weddahs</b> <br/> Australian negroes </td> </tr> +</table> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap21"></a> +<span class='pagenum'><a name="Page_229" id="Page_229"></a></span> +Chapter XXI.<br/> +OUR FISH-LIKE ANCESTORS</h2> + +<p> +Our task of detecting the extinct ancestors of our race among the vast numbers +of animals known to us encounters very different difficulties in the various +sections of man’s stem-history. These were very great in the series of +our invertebrate ancestors; they are much slighter in the subsequent series of +our vertebrate ancestors. Within the vertebrate stem there is, as we have +already seen, so complete an agreement in structure and embryology that it is +impossible to doubt their phylogenetic unity. In this case the evidence is much +clearer and more abundant. +</p> + +<p> +The characteristics that distinguish the Vertebrates as a whole from the +Invertebrates have already been discussed in our description of the +hypothetical Primitive Vertebrate (Chapter XI, Figs. 98–102). The chief +of these are: (1) The evolution of the primitive brain into a dorsal medullary +tube; (2) the formation of the chorda between the medullary tube and the gut; +(3) the division of the gut into branchial (gill) and hepatic (liver) gut; and +(4) the internal articulation or metamerism. The first three features are +shared by the Vertebrates with the ascidia-larvæ and the Prochordonia; the +fourth is peculiar to them. Thus the chief advantage in organisation by which +the earliest Vertebrates took precedence of the unsegmented Chordonia consisted +in the development of internal segmentation. +</p> + +<p> +The whole vertebrate stem divides first into the two chief sections of Acrania +and Craniota. The Amphioxus is the only surviving representative of the older +and lower section, the Acrania (“skull-less”). All the other +vertebrates belong to the second division, the Craniota +(“skull-animals”). The Craniota descend directly from the Acrania, +and these from the primitive Chordonia. The exhaustive study that we made of +the comparative anatomy and ontogeny of the Ascidia and the Amphioxus has +proved these relations for us. (See Chapters XVI and XVII.) The Amphioxus, the +lowest Vertebrate, and the Ascidia, the nearest related Invertebrate, descend +from a common extinct stem-form, the Chordæa; and this must have had, +substantially, the organisation of the chordula. +</p> + +<p> +However, the Amphioxus is important not merely because it fills the deep gulf +between the Invertebrates and Vertebrates, but also because it shows us to-day +the typical vertebrate in all its simplicity. We owe to it the most important +data that we proceed on in reconstructing the gradual historical development of +the whole stem. All the Craniota descend from a common stem-form, and this was +substantially identical in structure with the Amphioxus. This stem-form, the +Primitive Vertebrate (<i>Prospondylus,</i> Figs. 98–102), had the +characteristics of the vertebrate as such, but not the important features that +distinguish the Craniota from the Acrania. Though the Amphioxus has many +peculiarities of structure and has much degenerated, and though it cannot be +regarded as an unchanged descendant of the Primitive Vertebrate, it must have +inherited from it the specific characters we enumerated above. We may not say +that “Amphioxus is the ancestor of the Vertebrates”; but we can +say: “Amphioxus is the nearest relation to the ancestor of all the +animals we know.” Both belong to the same small family, or lowest class +of the Vertebrates, that we call the Acrania. In our genealogical tree this +group forms the twelfth stage, or the first stage among the vertebrate +ancestors (p. 228). From this group of Acrania both the Amphioxus and the +Craniota were evolved. +</p> + +<p> +The vast division of the Craniota embraces all the Vertebrates known to us, +with the exception of the Amphioxus. All of them have a head clearly +differentiated from the trunk, and a skull enclosing a brain. The head has also +three pairs of higher sense-organs (nose, eyes, and ears). The brain is very +rudimentary at first, a mere bulbous enlargement of the +<span class='pagenum'><a name="Page_230" id="Page_230"></a></span> +fore end of the medullary tube. But it is soon divided by a number of +transverse constrictions into, first three, then five successive cerebral +vesicles. In this formation of the head, skull, and brain, with further +development of the higher sense-organs, we have the advance that the Craniota +made beyond their skull-less ancestors. Other organs also attained a higher +development; they acquired a compact centralised heart with valves and a more +advanced liver and kidneys, and made progress in other important respects. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus247"></a> +<img src="images/fig247.gif" width="165" height="664" alt="Fig.247. The large +marine lamprey (Petromyzon marinus)." /> +<p class="caption">Fig. 247—<b>The large marine lamprey</b> +<i>(Petromyzon marinus),</i> much reduced. Behind the eye there is a row of +seven gill-clefts visible on the left, in front the round suctorial +mouth.</p> +</div> + +<p> +We may divide the Craniota generally into <i>Cyclostoma</i> +(“round-mouthed”) and <i>Gnathostoma</i> +(“jaw-mouthed”). There are only a few groups of the former in +existence now, but they are very interesting, because in their whole structure +they stand midway between the Acrania and the Gnathostoma. They are much more +advanced than the Acrania, much less so than the fishes, and thus form a very +welcome connecting-link between the two groups. We may therefore consider them +a special intermediate group, the fourteenth and fifteenth stages in the series +of our ancestors. +</p> + +<p> +The few surviving species of the Cyclostoma are divided into two +orders—the <i>Myxinoides</i> and the <i>Petromyzontes.</i> The former, +the hag-fishes, have a long, cylindrical, worm-like body. They were classed by +Linné with the worms, and by later zoologists, with the fishes, or the +amphibia, or the molluscs. They live in the sea, usually as parasites of +fishes, into the skin of which they bore with their round suctorial mouths and +their tongues, armed with horny teeth. They are sometimes found alive in the +body cavity of fishes (such as the torsk or sturgeon); in these cases they have +passed through the skin into the interior. The second order consists of the +Petromyzontes or lampreys; the small river lamprey (<i>Petromyzon +fluviatilis</i>) and the large marine lamprey (<i>Petromyzon marinus,</i> Fig. +247). They also have a round suctorial mouth, with horny teeth inside it; by +means of this they attach themselves by sucking to fishes, stones, and other +objects (hence the name <i>Petromyzon</i> = stone-sucker). It seems that this +habit was very widespread among the earlier Vertebrates; the larvæ of many of +the Ganoids and frogs have suctorial disks near the mouth. +</p> + +<p> +The class that is formed of the Myxinoides and Petromyzontes is called the +Cyclostoma (round-mouthed), because their mouth has a circular or semi-circular +aperture. The jaws (upper and lower) that we find in all the higher Vertebrates +are completely wanting in the Cyclostoma, as in the Amphioxus. Hence the other +Vertebrates are collectively opposed to them as Gnathostoma (jaw-mouthed). The +Cyclostoma might also be called <i>Monorhina</i> (single-nosed), because they +have only a single nasal passage, while all the Gnathostoma have two nostrils +(<i>Amphirhina</i> = double-nosed). But apart from these peculiarities the +Cyclostoma differ more widely from the fishes in other special features of +their structure than the fishes do from man. Hence they are obviously the last +survivors of a very ancient class of Vertebrates, that was far from attaining +the advanced organisation of the true fish. To mention only the chief points, +the Cyclostoma show no trace of pairs of limbs. Their mucous skin is quite +naked and smooth and devoid of scales. There is no bony skeleton. A very +rudimentary skull is developed at the foremost end of their chorda. At this +point a soft membranous (partly turning into cartilage), small skull-capsule is +formed, and encloses the brain. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus248"></a> +<img src="images/fig248.gif" width="212" height="649" alt="Fig.248. Fossil Permian +primitive fish (Pleuracanthus Dechenii), from the red sandstone of +Saarbrücken." /> +<p class="caption">Fig. 248—<b>Fossil Permian primitive fish</b> +<i>(Pleuracanthus Dechenii),</i> from the red sandstone of Saarbrücken. (From +<i>Döderlein.</i>) <i>I</i> Skull and branchial skeleton: <i>o</i> eye-region, +<i>pq</i> palatoquadratum, <i>nd</i> lower jaw, <i>hm</i> hyomandibular, +<i>hy</i> tongue-bone, <i>k</i> gill-radii, <i>kb</i> gill-arches, <i>z</i> +jaw-teeth, <i>sz</i> gullet-teeth, <i>st</i> neck-spine. <i>II</i> Vertebral +column: <i>ob</i> upper arches, <i>ub</i> lower arches, <i>hc</i> intercentra, +<i>r</i> ribs. <i>III</i> Single fins: <i>d</i> dorsal fin, <i>c</i> tail-fin +(tail-end wanting), <i>an</i> anus-fin, <i>ft</i> supporter of fin-rays. +<i>IV</i> Breast-fin: <i>sg</i> shoulder-zone, <i>ax</i> fin-axis, <i>ss</i> +double lines of fin-rays, <i>bs</i> additional rays, <i>sch</i> plates. +<i>V</i> Ventral fin: <i>p</i> pelvis, <i>ax</i> fin-axis, <i>ss</i> single row +of fin-rays, <i>bs</i> additional rays, <i>sch</i> scales, <i>cop</i> +penis.</p> +</div> + +<p> +<span class='pagenum'><a name="Page_231" id="Page_231"></a></span> +The brain of the Cyclostoma is merely a very small and comparatively +insignificant swelling of the spinal marrow, a simple vesicle at first. It +afterwards divides into five successive cerebral vesicles, like the brain of +the Gnathostoma. These five primitive cerebral vesicles, that are found in the +embryos of all the higher vertebrates from the fishes to man, and grow into +very complex structures, remain at a very rudimentary stage in the Cyclostoma. +The histological structure of the nerves is also less advanced than in the rest +of the vertebrates. In these the auscultory organ always contains three +circular canals, but in the lampreys there are only two, and in the hag-fishes +only one. In most other respects the organisation of the Cyclostoma is much +simpler—for instance, in the structure of the heart, circulation, and +kidneys. We must especially note the absence of a very important organ that we +find in the fishes, the floating-bladder, from which the lungs of the higher +Vertebrates have been developed. +</p> + +<p> +When we consider all these peculiarities in the structure of the Cyclostoma, we +may formulate the following thesis: Two divergent lines proceeded from the +earliest Craniota, or the primitive Craniota (<i>Archicrania</i>). One of these +lines is preserved in a greatly modified condition: these are the Cyclostoma, a +very backward and partly degenerate side-line. The other, the chief line of the +Vertebrate stem, advanced straight to the fishes, and by fresh adaptations +acquired a number of important improvements. +</p> + +<p> +The Cyclostoma are almost always classified by zoologists among the fishes; but +the incorrectness of this may be judged from the fact that in all the chief and +distinctive features of organisation they are further removed from the fishes +than the fishes are from the Mammals, and even man. With the fishes we enter +upon the vast division of the jaw-mouthed +<span class='pagenum'><a name="Page_232" id="Page_232"></a></span> +or double-nosed Vertebrates (<i>Gnathostoma</i> or <i>Amphirhina</i>). We have +to consider the fishes carefully as the class which, on the evidence of +palæontology, comparative anatomy, and ontogeny, may be regarded with absolute +certainty as the stem-class of all the higher Vertebrates or Gnathostomes. +Naturally, none of the actual fishes can be considered the direct ancestor of +the higher Vertebrates. But it is certain that all the Vertebrates or +Gnathostomes, from the fishes to man, descend from a common, extinct, fish-like +ancestor. If we had this ancient stem-form before us, we would undoubtedly +class it as a true fish. Fortunately the comparative anatomy and classification +of the fishes are now so far advanced that we can get a very clear idea of +these interesting and instructive features. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus249"></a> +<img src="images/fig249.gif" width="189" height="546" alt="Fig.249. Embryo of a +shark (Scymnus lichia), seen from the ventral side." /> +<p class="caption">Fig. 249—<b>Embryo of a shark</b> (<i>Scymnus +lichia</i>), seen from the ventral side. <i>v</i> breast-fins (in front five +pairs of gill-clefts), <i>h</i> belly-fins, <i>a</i> anus, <i>s</i> tail-fin, +<i>k</i> external gill-tuft, <i>d</i> yelk-sac (removed for most part), +<i>g</i> eye, <i>n</i> nose, <i>m</i> mouth-cleft.</p> +</div> + +<p> +In order to understand properly the genealogical tree of our race within the +vertebrate stem, it is important to bear in mind the characteristics that +separate the whole of the Gnathostomes from the Cyclostomes and Craniota. In +these respects the fishes agree entirely with all the other Gnathostomes up to +man, and it is on this that we base our claim of relationship to the fishes. +The following characteristics of the Gnathostomes are anatomic features of this +kind: (1) The internal gill-arch apparatus with the jaw arches; (2) the pair of +nostrils; (3) the floating bladder or lungs; and (4) the two pairs of limbs. +</p> + +<p> +The peculiar formation of the frame work of the branchial (gill) arches and the +connected maxillary (jaw) apparatus is of importance in the whole group of the +Gnathostomes. It is inherited in rudimentary form by all of them, from the +earliest fishes to man. It is true that the primitive transformation (which we +find even in the Ascidia) of the fore gut into the branchial gut can be traced +in all the Vertebrates to the same simple type; in this respect the +gill-clefts, which pierce the walls of the branchial gut in all the Vertebrates +and in the Ascidia, are very characteristic. But the <i>external,</i> +superficial branchial skeleton that supports the gill-crate in the Cyclostoma +is replaced in the Gnathostomes by an <i>internal</i> branchial skeleton. It +consists of a number of successive cartilaginous arches, which lie in the wall +of the gullet between the gill-clefts, and run round the gullet from both +sides. The foremost pair of gill-arches become the maxillary arches, from which +we get our upper and lower jaws. +</p> + +<p> +The olfactory organs are at first found in the same form in all the +Gnathostomes, as a pair of depressions in the fore part of the skin of the +head, above the mouth; hence, they are also called the Amphirhina +<span class='pagenum'><a name="Page_233" id="Page_233"></a></span> +(“double-nosed”). The Cyclostoma are “one-nosed” +(<i>Monorhina</i>); their nose is a single passage in the middle of the frontal +surface. But as the olfactory nerve is double in both cases, it is possible +that the peculiar form of the nose in the actual Cyclostomes is a secondary +acquisition (by adaptation to suctorial habits). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus250"></a> +<img src="images/fig250.gif" width="256" height="649" alt="Fig.250. +Fully-developed man-eating shark (Carcharias melanopterus), left view." /> +<p class="caption">Fig. 250—<b>Fully developed man-eating shark</b> +(<i>Carcharias melanopterus</i>), left view. <i>r<sub>1</sub></i> first, +<i>r<sub>2</sub></i> second dorsal fin, <i>s</i> tail-fin, <i>a</i> anus-fin, +<i>v</i> breast-fins, <i>h</i> belly-fins.)</p> +</div> + +<p> +A third essential character of the Gnathostomes, that distinguishes them very +conspicuously from the lower vertebrates we have dealt with, is the formation +of a blind sac by invagination from the fore part of the gut, which becomes in +the fishes the air-filled floating-bladder. This organ acts as a hydrostatic +apparatus, increasing or reducing the specific gravity of the fish by +compressing or altering the quantity of air in it. The fish can rise or sink in +the water by means of it. This is the organ from which the lungs of the higher +vertebrates are developed. +</p> + +<p> +Finally, the fourth character of the Gnathostomes in their simple embryonic +form is the two pairs of extremities or limbs—a pair of fore legs +(breast-fins in the fish, Fig. 250 <i>v</i>) and a pair of hind legs (ventral +fins in the fish, Fig. 250 <i>h</i>). The comparative anatomy of these fins is +very interesting, because they contain the rudiments of all the skeletal parts +that form the framework of the fore and hind legs in all the higher vertebrates +right up to man. There is no trace of these pairs of limbs in the Acrania and +Cyclostomes. +</p> + +<p> +Turning, now, to a closer inspection of the fish class, we may first divide it +into three groups or sub-classes, the genealogy of which is well known to us. +The first and oldest group is the sub-class of the <i>Selachii</i> or primitive +fishes; the best-known representatives of which to-day are the orders of the +sharks and rays (Figs. 248–252). Next to this is the more advanced +sub-class of the plated fishes or <i>Ganoids</i> (Figs. 253–5). It has +been long extinct for the most part, and has very few living representatives, +such as the sturgeon and the bony pike; but we can form some idea of the +earlier extent of this interesting group from the large numbers of fossils. +From these plated fishes the sub-class of the bony fishes +<span class='pagenum'><a name="Page_234" id="Page_234"></a></span> +or <i>Teleostei</i> was developed, to which the great majority of living fishes +belong (especially nearly all our river fishes). Comparative anatomy and +ontogeny show clearly that the Ganoids descended from the Selachii, and the +Teleostei from the Ganoids. On the other hand, a collateral line, or rather the +advancing chief line of the vertebrate stem, was developed from the earlier +Ganoids, and this leads us through the group of the Dipneusta to the important +division of the Amphibia. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus251"></a> +<img src="images/fig251.gif" width="228" height="424" alt="Fig.251. Fossil +angel-shark (Squatina alifera) from the upper Jurassic at Eichstätt." /> +<p class="caption">Fig. 251—<b>Fossil angel-shark</b> (<i>Squatina +alifera</i>), from the upper Jurassic at Eichstätt. (From <i>Zittel.</i>) The +cartilaginous skull is clearly seen in the broad head, and the gill-arches +behind. The wide breast-fin and the narrower belly-fin have a number of radii; +between these and the vertebral column are a number of ribs.</p> +</div> + +<p> +The earliest fossil remains of Vertebrates that we know were found in the Upper +Silurian (p. 201), and belong to two groups—the Selachii and the Ganoids. +The most primitive of all known representatives of the earliest fishes are +probably the remarkable <i>Pleuracanthida,</i> the genera <i>Pleuracanthus, +Xenacanthus, Orthocanthus,</i> etc. (Fig. 248). These ancient cartilaginous +fishes agree in most points of structure with the real sharks (Figs. 249, 250); +but in other respects they seem to be so much simpler in organisation that many +palæontologists separate them altogether, and regard them as +<i>Proselachii</i>; they are probably closely related to the extinct ancestors +of the Gnathostomes. We find well-preserved remains of them in the Permian +period. Well-preserved impressions of other sharks are found in the Jurassic +schist, such as of the angel-fish (<i>Squatina,</i> Fig. 251). Among the +extinct earlier sharks of the Tertiary period there were some twice as large as +the biggest living fishes; <i>Carcharodon</i> was more than 100 feet long. The +sole surviving species of this genus (<i>C. Rondeleti</i>) is eleven yards +long, and has teeth two inches long; but among the fossil species we find teeth +six inches long (Fig. 252). +</p> + +<p> +From the primitive fishes or Selachii, the earliest Gnathostomes, was developed +the legion of the Ganoids. There are very few genera now of this interesting +and varied group—the ancient sturgeons (<i>Accipenser</i>), the eggs of +which are eaten as caviare, and the stratified pikes (<i>Polypterus,</i> Fig. +255) in African rivers, and bony pikes (<i>Lepidosteus</i>) in the rivers of +North America. On the other hand, we have a great variety of specimens of this +group in the fossil state, from the Upper Silurian onward. Some of these fossil +Ganoids approach closely to the Selachii; others are nearer to the Dipneusts; +others again represent a transition to the Teleostei. For our genealogical +purposes the most interesting are the intermediate forms between the Selachii +and the Dipneusts. Huxley, to whom we owe particularly important works on the +fossil Ganoids, classed them in the order of the <i>Crossopterygii.</i> Many +genera and species of this order are found in the Devonian and Carboniferous +strata (Fig. 253); a single, greatly modified survivor of the group is still +found in the large rivers of Africa (<i>Polypterus,</i> Fig. 255, and the +closely related <i>Calamichthys</i>). In many impressions of the Crossopterygii +the floating bladder seems to be ossified, +<span class='pagenum'><a name="Page_235" id="Page_235"></a></span> +and therefore well preserved—for instance, in the <i>Undina</i> (Fig. +254, immediately behind the head). +</p> + +<p> +Part of these Crossopterygii approach very closely in their chief anatomic +features to the Dipneusts, and thus represent phylogenetically the transition +from the Devonian Ganoids to the earliest air-breathing vertebrates. This +important advance was made in the Devonian period. The numerous fossils that we +have from the first two geological sections, the Laurentian and Cambrian +periods, belong exclusively to aquatic plants and animals. From this +paleontological fact, in conjunction with important geological and biological +indications, we may infer with some confidence that there were no terrestrial +animals at that time. During the whole of the vast archeozoic period—many +millions of years—the living population of our planet consisted almost +exclusively of aquatic organisms; this is a very remarkable fact, when we +remember that this period embraces the larger half of the whole history of +life. The lower animal-stems are wholly (or with very few exceptions) aquatic. +But the higher stems also remained in the water during the primordial epoch. It +was only towards its close that some of them came to live on land. We find +isolated fossil remains of terrestrial animals first in the Upper Silurian, and +in larger numbers in the Devonian strata, which were deposited at the beginning +of the second chief section of geology (the paleozoic age). The number +increases considerably in the Carboniferous and Permian deposits. We find many +species both of the articulate and the vertebrate stem that lived on land and +breathed the atmosphere; their aquatic ancestors of the Silurian period only +breathed water. This important change in respiration is the chief modification +that the animal organism underwent in passing from the water to the solid land. +The first consequence was the formation of lungs for breathing air; up to that +time the gills alone had served for respiration. But there was at the same time +a great change in the circulation and its organs; these are always very closely +correlated to the respiratory organs. Moreover, the limbs and other organs were +also more or less modified, either in consequence of remote correlation to the +preceding or owing to new adaptations. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus252"></a> +<img src="images/fig252.gif" width="265" height="262" alt="Fig.252. Tooth of a +gigantic shark (Carcharodon megalodon), from the Pliocene at Malta." /> +<p class="caption">Fig. 252—<b>Tooth of a gigantic shark</b> +(<i>Carcharodon megalodon</i>), from the Pliocene at Malta. (From +<i>Zittel.</i>)</p> +</div> + +<p> +In the vertebrate stem it was unquestionably a branch of the fishes—in +fact, of the Ganoids—that made the first fortunate experiment during the +Devonian period of adapting themselves to terrestrial life and breathing the +atmosphere. This led to a modification of the heart and the nose. The true +fishes have merely a pair of blind olfactory pits on the surface of the head; +but a connection of these with the cavity of the mouth was now formed. A canal +made its appearance on each side, and led directly from the nasal depression +into the mouth-cavity, thus conveying atmospheric air to the lungs even when +the mouth was closed. Further, in all true fishes the heart has only two +sections—an atrium that receives the venous blood from the veins, and a +ventricle that propels it through a conical artery to the gills; the atrium was +now divided into two halves, or right and left auricles, by an incomplete +partition. The right auricle alone now received the venous blood from the body, +while the left auricle received the venous blood that flowed from the lungs and +gills to the heart. Thus the double circulation of the higher vertebrates was +evolved from the simple +<span class='pagenum'><a name="Page_236" id="Page_236"></a></span> +circulation of the true fishes, and, in accordance with the laws of +correlation, this advance led to others in the structure of other organs. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus253"></a> +<img src="images/fig253.gif" width="492" height="507" alt="Fig.253. A Devonian +Crossopterygius (Holoptychius nobilissimus), from the Scotch old red sandstone. +Fig. 254. A Jurassic Crossopterygius (Undina penicillata), from the upper +Jurassic at Eichstätt. Fig. 255. A living Crossopterygius, from the Upper +Nile." /> +<p class="caption">Fig. 253—<b>A Devonian Crossopterygius</b> +(<i>Holoptychius nobilissimus</i>), from the Scotch old red sandstone. (From +<i>Huxley.</i>)<br/> Fig. 254.—<b>A Jurassic Crossopterygius</b> +(<i>Undina penicillata</i>), from the upper Jurassic at Eichstätt. (From +<i>Zittel.</i>) <i>j</i> jugular plates, <i>b</i> three ribbed scales.<br/> +Fig. 255—<b>A living Crossopterygius,</b> from the Upper Nile +((Polypterus bichir).</p> +</div> + +<p> +The vertebrate class, that thus adapted itself to breathing the atmosphere, and +was developed from a branch of the Ganoids, takes the name of the +<i>Dipneusts</i> or <i>Dipnoa</i> (“double-breathers”), because +they retained the earlier gill-respiration along with the new pulmonary (lung) +respiration, like the lowest amphibia. This class was represented during the +paleozoic age (or the Devonian, Carboniferous, and Permian periods) by a number +of different genera. There are only three genera of the class living to-day: +<i>Protopterus annectens</i> in the rivers +<span class='pagenum'><a name="Page_237" id="Page_237"></a></span> +of tropical Africa (the White Nile, the Niger, Quelliman, etc.), <i>Lepidosiren +paradoxa</i> in tropical South America (in the tributaries of the Amazon), and +<i>Ceratodus Forsteri</i> in the rivers of East Australia. This wide +distribution of the three isolated survivors proves that they represent a group +that was formerly very large. In their whole structure they form a transition +from the fishes to the amphibia. The transitional formation between the two +classes is so pronounced in the whole organisation of these remarkable animals +that zoologists had a lively controversy over the question whether they were +really fishes or amphibia. Several distinguished zoologists classed them with +the amphibia, though most now associate them with the fishes. As a matter of +fact, the characters of the two classes are so far united in the Dipneusts that +the answer to the question depends entirely on the definition we give of +“fish” and “amphibian.” In habits they are true +amphibia. During the tropical winter, in the rainy season, they swim in the +water like the fishes, and breathe water by gills. During the dry season they +bury themselves in the dry mud, and breathe the atmosphere through lungs, like +the amphibia and the higher vertebrates. In this double respiration they +resemble the lower amphibia, and have the same characteristic formation of the +heart; in this they are much superior to the fishes. But in most other features +<span class='pagenum'><a name="Page_238" id="Page_238"></a></span> +they approach nearer to the fishes, and are inferior to the amphibia. +Externally they are entirely fish-like. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus256"></a> +<a name="illus257"></a> +<img src="images/fig256.gif" width="457" height="372" alt="Fig.256. Fossil +Dipneust (Dipterus Valenciennesi), from the old red sandstone (Devon). Fig. +257. The Australian Dipneust (Ceratodus Forsteri)." /> +<p class="caption">Fig. 256—<b>Fossil Dipneust</b> (<i>Dipterus +Valenciennesi</i>), from the old red sandstone (Devon). (From +<i>Pander.</i>)<br/> Fig. 257—<b>The Australian Dipneust</b> +(<i>Ceratodus Forsteri</i>). <i>B</i> view from the right, <i>A</i> lower side +of the skull, <i>C</i> lower jaw. (From <i>Gunther.</i>) <i>Qu</i> quadrate +bone, <i>Psph</i> parasphenoid, <i>Pt P</i> pterygopalatinum, <i>Vo</i> vomer, +<i>d</i> teeth, <i>na</i> nostrils, <i>Br</i> branchial cavity, <i>C</i> first +rib. <i>D</i> lower-jaw teeth of the fossil <i>Ceratodus Kaupi</i> (from the +Triassic).</p> +</div> + +<p> +In the Dipneusts the head is not marked off from the trunk. The skin is covered +with large scales. The skeleton is soft, cartilaginous, and at a low stage of +development, as in the lower Selachii and the earliest Ganoids. The chorda is +completely retained, and surrounded by an unsegmented sheath. The two pairs of +limbs are very simple fins of a primitive type, like those of the lowest +Selachii. The formation of the brain, the gut, and the sexual organs is also +the same as in the Selachii. Thus the Dipneusts have preserved by heredity many +of the less advanced features of our primitive fish-like ancestors, and at the +same time have made a great step forward in adaptation to air-breathing by +means of lungs and the correlative improvement of the heart. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus258"></a> +<img src="images/fig258.gif" width="423" height="281" alt="Fig.258. Young +ceratodus, shortly after issuing from the egg. Fig. 259. Young ceratodus six +weeks after issuing from the egg." /> +<p class="caption">Fig. 258—<b>Young ceratodus,</b> shortly after +issuing from the egg, magnified. <i>k</i> gill-cover,<br/><i>l</i> liver. (From +<i>Richard Semon.</i>)<br/> Fig. 259—<b>Young ceratodus</b> six weeks +after issuing from the egg. <i>s</i> spiral fold of gut,<br/><i>b</i> +rudimentary belly-fin. (From <i>Richard Semon.</i>)</p> +</div> + +<p> +Ceratodus is particularly interesting on account of the primitive build of its +skeleton; the cartilaginous skeleton of its two pairs of fins, for instance, +has still the original form of a bi-serial or feathered leaf, and was on that +account described by Gegenbaur as a “primitive fin-skeleton.” On +the other hand, the skeleton of the pairs of fins is greatly reduced in the +African dipneust (<i>Protopterus</i>) and the American (<i>Lepidosiren</i>). +Further, the lungs are double in these modern dipneusts, as in all the other +air-breathing vertebrates; they have on that account been called +“double-lunged” (<i>Dipneumones</i>) in contrast to the Ceratodus; +the latter has only a single lung (<i>Monopneumones</i>). At the same time the +gills also are developed as water-breathing organs in all these lung-fishes. +Protopterus has external as well as internal gills. +</p> + +<p> +The paleozoic Dipneusts that are in the direct line of our ancestry, and form +the connecting-bridge between the Ganoids and the Amphibia, differ in many +respects +<span class='pagenum'><a name="Page_239" id="Page_239"></a></span> +from their living descendants, but agree with them in the above essential +features. This is confirmed by a number of interesting facts that have lately +come to our knowledge in connection with the embryonic development of the +Ceratodus and Lepidosiren; they give us important information as to the +stem-history of the lower Vertebrates, and therefore of our early ancestors of +the paleozoic age. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap22"></a>Chapter XXII.<br/> +OUR FIVE-TOED ANCESTORS</h2> + +<p> +With the phylogenetic study of the four higher classes of Vertebrates, which +must now engage our attention, we reach much firmer ground and more light in +the construction of our genealogy than we have, perhaps, enjoyed up to the +present. In the first place, we owe a number of very valuable data to the very +interesting class of Vertebrates that come next to the Dipneusts and have been +developed from them—the Amphibia. To this group belong the salamander, +the frog, and the toad. In earlier days all the reptiles were, on the example +of Linne, classed with the Amphibia (lizards, serpents, crocodiles, and +tortoises). But the reptiles are much more advanced than the Amphibia, and are +nearer to the birds in the chief points of their structure. The true Amphibia +are nearer to the Dipneusta and the fishes; they are also much older than the +reptiles. There were plenty of highly-developed (and sometimes large) Amphibia +during the Carboniferous period; but the earliest reptiles are only found in +the Permian period. It is probable that the Amphibia were evolved even +earlier—during the Devonian period—from the Dipneusta. The extinct +Amphibia of which we have fossil remains from that remote period (very numerous +especially in the Triassic strata) were distinguished for a graceful scaly coat +or a powerful bony armour on the skin (like the crocodile), whereas the living +amphibia have usually a smooth and slippery skin. +</p> + +<p> +The earliest of these armoured Amphibia (<i>Phractamphibia</i>) form the order +of <i>Stegocephala</i> (“roof-headed”) (Fig. 260). It is among +these, and not among the actual Amphibia, that we must look for the forms that +are directly related to the genealogy of our race, and are the ancestors of the +three higher classes of Vertebrates. But even the existing Amphibia have such +important relations to us in their anatomic structure, and especially their +embryonic development, that we may say: Between the Dipneusts and the Amniotes +there was a series of extinct intermediate forms which we should certainly +class with the Amphibia if we had them before us. In their whole organisation +even the actual Amphibia seem to be an instructive transitional group. In the +important respects of respiration and circulation they approach very closely to +the Dipneusta, though in other respects they are far superior to them. +</p> + +<p> +This is particularly true of the development of their limbs or extremities. In +them we find these for the first time as five-toed feet. The thorough +investigations of Gegenbaur have shown that the fish’s fins, of which +very erroneous opinions were formerly held, are many-toed feet. The various +cartilaginous or bony radii that are found in large numbers in each fin +correspond to the fingers or toes of the higher Vertebrates. The several joints +of each fin-radius correspond to the various parts of the toe. Even in the +Dipneusta the fin is of the same construction as in the fishes; it was +afterwards gradually evolved into the five-toed form, which we first encounter +in the Amphibia. This reduction of the number of the toes to six, and then to +five, probably took place in the second half of the Devonian period—at +the latest, in the subsequent Carboniferous period—in those Dipneusta +which we regard as the ancestors of the Amphibia. We have several fossil +remains of five-toed Amphibia from this period. There are numbers of fossil +impressions of them in the Triassic of Thuringia (<i>Chirotherium</i>). +</p> + +<p> +<span class='pagenum'><a name="Page_240" id="Page_240"></a></span> +The fact that the toes number five is of great importance, because they have +clearly been transmitted from the Amphibia to all the higher Vertebrates. Man +entirely resembles his amphibian ancestors in this respect, and indeed in the +whole structure of the bony skeleton of his five-toed extremities. A careful +comparison of the skeleton of the frog with our own is enough to show this. It +is well known that this hereditary number of the toes has assumed a very great +practical importance from remote times; on it our whole system of enumeration +(the decimal system applied to measurement of time, mass, weight, etc.) is +based. There is absolutely no reason why there should be five toes in the fore +and hind feet in the lowest Amphibia, the reptiles, and the higher Vertebrates, +unless we +<span class='pagenum'><a name="Page_241" id="Page_241"></a></span> +ascribe it to inheritance from a common stem-form. Heredity alone can explain +it. It is true that we find less than five toes in many of the Amphibia and of +the higher Vertebrates. But in all these cases we can prove that some of the +toes atrophied, and were in time lost altogether. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus260"></a> +<img src="images/fig260.gif" width="456" height="493" alt="Fig.260. Fossil +amphibian from the Permian, found in the Plauen terrain near Dresden +(Branchiosaurus amblystomus)." /> +<p class="caption">Fig. 260—<b>Fossil amphibian from the Permian,</b> +found in the Plauen terrain near Dresden (<i>Branchiosaurus amblystomus</i>). +(From <i>Credner.</i>) <i>A</i> skeleton of a young larva. <i>B</i> larva, +restored, with gills. <i>C</i> the adult form.)</p> +</div> + +<p> +The causes of this evolution of the five-toed foot from the many-toed fin in +the amphibian ancestor must be sought in adaptation to the entire change of +function that the limbs experienced in passing from an exclusively aquatic to a +partly terrestrial life. The many-toed fin had been used almost solely for +motion in the water; it had now also to support the body in creeping on the +solid ground. This led to a modification both of the skeleton and the muscles +of the limbs. The number of the fin-radii was gradually reduced, and sank +finally to five. But these five remaining radii became much stronger. The soft +cartilaginous radii became bony rods. The rest of the skeleton was similarly +strengthened. Thus from the one-armed lever of the many-toed fish-fin arose the +improved many-armed lever system of the five-toed amphibian limbs. The +movements of the body gained in variety as well as in strength. The various +parts of the skeletal system and correlated muscular system began to +differentiate more and more. In view of the close correlation of the muscular +and nervous systems, this also made great advance in structure and function. +Hence we find, as a matter of fact, that the brain is much more developed in +the higher Amphibia than in the fishes, the Dipneusta, and the lower Amphibia. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus261"></a> +<img src="images/fig261.gif" width="141" height="298" alt="Fig.261. Larva of the +Spotted Salamander (Salamandra maculata), seen from the ventral side." /> +<p class="caption">Fig. 261—<b>Larva of the Spotted Salamander</b> +(<i>Salamandra maculata</i>), seen from the ventral side. In the centre a +yelk-sac still hangs from the gut. The external gills are gracefully ramified. +The two pairs of legs are still very small.</p> +</div> + +<p> +The first advance in organisation that was occasioned by the adoption of life +on land was naturally the construction of an organ for breathing air—a +lung. This was formed directly from the floating-bladder inherited from the +fishes. At first its function was insignificant beside that of the gills, the +older organ for water-respiration. Hence we find in the lowest Amphibia, the +gilled Amphibia, that, like the Dipneusta, they pass the greater part of their +life in the water, and breathe water through gills. They only come to the +surface at brief intervals, or creep on to the land, and then breathe air by +their lungs. But some of the tailed Amphibia—the salamanders—remain +entirely in the water when they are young, and afterwards spend most of their +time on land. In the adult state they only breathe air through lungs. The same +applies to the most advanced of the Amphibia, the Batrachia (frogs and toads); +some of them have entirely lost the gill-bearing larva form.<a href="#linknote-30" name="linknoteref-30" id="linknoteref-30"><sup>[30]</sup></a> This +is also the case with certain small, serpentine Amphibia, the Cæcilia (which +live in the ground like earth-worms). +</p> + +<p class="footnote"> +<a name="linknote-30" id="linknote-30"></a> <a href="#linknoteref-30">[30]</a> +The tree-frog of Martinique (<i>Hylades martinicensis</i>) loses the gills +on the seventh, and the tail and yelk-sac on the eighth, day of fœtal life. On +the ninth or tenth day after fecundation the frog emerges from the egg. +</p> + +<p> +The great interest of the natural history of the Amphibia consists especially +in their intermediate position between the lower and higher Vertebrates. The +lower Amphibia approach very closely to the Dipneusta in their whole +organisation, live mainly in the water, and breathe by gills; but the higher +Amphibia are just as close to the Amniotes, live mainly on land, and breathe by +lungs. But in their younger state the latter resemble the former, and only +reach the higher stage by a complete metamorphosis. The embryonic development +of most of the +<span class='pagenum'><a name="Page_242" id="Page_242"></a></span> +higher Amphibia still faithfully reproduces the stem-history of the whole +class, and the various stages of the advance that was made by the lower +Vertebrates in passing from aquatic to terrestrial life during the Devonian or +the Carboniferous period are repeated in the spring by every frog that develops +from an egg in our ponds. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus262"></a> +<img src="images/fig262.gif" width="144" height="321" alt="Fig.262. Larva of the +common grass-frog (Rana temporaria), or “tadpole.”" /> +<p class="caption">Fig. 262—<b>Larva of the common grass-frog</b> +(<i>Rana temporaria</i>), or “tadpole.” <i>m</i> mouth, <i>n</i> a +pair of suckers for fastening on to stones, <i>d</i> skin-fold from which the +gill-cover develops; behind it the gill-clefts, from which the branching gills +(<i>k</i>) protrude, <i>s</i> tail-muscles, <i>f</i> cutaneous fin-fringe of +the tail.</p> +</div> + +<p> +The common frog leaves the egg in the shape of a larva, like the tailed +salamander (Fig. 261), and this is altogether different from the mature frog +(Fig. 262). The short trunk ends in a long tail, with the form and structure of +a fish’s tail (<i>s</i>). There are no limbs at first. The respiration is +exclusively branchial, first through external (<i>k</i>) and then internal +gills. In harmony with this the heart has the same structure as in the fish, +and consists of two sections—an atrium that receives the venous blood +from the body, and a ventricle that forces it through the arteries into the +gills. +</p> + +<p> +We find the larvæ of the frog (or tadpoles, <i>Gyrini</i>) in great numbers in +our ponds every spring in this fish-form, using their muscular tails in +swimming, just like the fishes and young Ascidia. When they have reached a +certain size, the remarkable metamorphosis from the fish-form to the frog +begins. A blind sac grows out of the gullet, and expands into a couple of +spacious sacs: these are the lungs. The simple chamber of the heart is divided +into two sections by the development of a partition, and there are at the same +time considerable changes in the structure of the chief arteries. Previously +all the blood went from the auricle through the aortic arches into the gills, +but now only part of it goes to the gills, the other part passing to the lungs +through the new-formed pulmonary artery. From this point arterial blood returns +to the left auricle of the heart, while the venous blood gathers in the right +auricle. As both auricles open into a single ventricle, this contains mixed +blood. The dipneust form has now succeeded to the fish-form. In the further +course of the metamorphosis the gills and the branchial vessels entirely +disappear, and the respiration becomes exclusively pulmonary. Later, the long +swimming tail is lost, and the frog now hops to the land with the legs that +have grown meantime. +</p> + +<p> +This remarkable metamorphosis of the Amphibia is very instructive in connection +with our human genealogy, and is particularly interesting from the fact that +the various groups of actual Amphibia have remained at different stages of +their stem-history, in harmony with the biogenetic law. We have first of all a +very low order of Amphibia—the <i>Sozobranchia</i> +(“gilled-amphibia”), which retain their gills throughout life, like +the fishes. In a second order of the salamanders the gills are lost in the +metamorphosis, and when fully grown they have only pulmonary respiration. Some +of the tailed Amphibia still retain the gill-clefts in the side of the neck, +though they have lost the gills themselves (<i>Menopoma</i>). If we force the +larvæ of our salamanders (Fig. 261) and tritons to remain in the water, and +prevent them from reaching the land, we can in favourable circumstances make +them retain their gills. In this fish-like condition they reach sexual +maturity, and remain throughout life at the lower stage of the gilled Amphibia. +</p> + +<p> +<span class='pagenum'><a name="Page_243" id="Page_243"></a></span> +fish-like axolotl (<i>Siredon pisciformis</i>). It was formerly regarded as a +permanent gilled amphibian persisting throughout life at the fish-stage. But +some of the hundreds of these animals that are kept in the Botanical Garden at +Paris got on to the land for some reason or other, lost their gills, and +changed into a form closely resembling the salamander (<i>Amblystoma</i>). +Other species of the genus became sexually mature for the first time in this +condition. This has been regarded as an astounding phenomenon, although every +common frog and salamander repeats the metamorphosis in the spring. The whole +change from the aquatic and gill-breathing animal to the terrestrial +lung-breathing form may be followed step by step in this case. But what we see +here in the development of the individual has happened to the whole class in +the course of its stem-history. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus263"></a> +<img src="images/fig263.gif" width="457" height="142" alt="Fig.263. Fossil mailed +amphibian, from the Bohemian Carboniferous (Seeleya)." /> +<p class="caption">Fig. 263—<b>Fossil mailed amphibian,</b> from the +Bohemian Carboniferous (<i>Seeleya</i>). (From <i>Fritsch.</i>) The scaly coat +is retained on the left.</p> +</div> + +<p> +The metamorphosis goes farther in a third order of Amphibia, the +<i>Batrachia</i> or <i>Anura,</i> than in the salamander. To this belong the +various kinds of toads, ringed snakes, water-frogs, tree-frogs, etc. These +lose, not only the gills, but also (sooner or later) the tail, during +metamorphosis. +</p> + +<p> +The ontogenetic loss of the gills and the tail in the frog and toad can only be +explained on the assumption that they are descended from long-tailed Amphibia +of the salamander type. This is also clear from the comparative anatomy of the +two groups. This remarkable metamorphosis is, however, also interesting because +it throws a certain light on the phylogeny of the tail-less apes and man. Their +ancestors also had long tails and gills like the gilled Amphibia, as the tail +and the gill-arches of the human embryo clearly show. +</p> + +<p> +For comparative anatomical and ontogenetic reasons, we must not seek these +amphibian ancestors of ours—as one would be inclined to do, +perhaps—among the tail-less Batrachia, but among the tailed lower +Amphibia. +</p> + +<p> +The vertebrate form that comes next to the Amphibia in the series of our +ancestors is a lizard-like animal, the earlier existence of which can be +confidently deduced from the facts of comparative anatomy and ontogeny. The +living <i>Hatteria</i> of New Zealand (Fig. 264) and the extinct +<i>Rhyncocephala</i> of the Permian period (Fig. 265) are closely related to +this important stem-form; we may call them the <i>Protamniotes,</i> or +Primitive Amniotes. All the Vertebrates above the Amphibia—or the three +classes of reptiles, birds, and mammals—differ so much in their whole +organisation from all the lower Vertebrates we have yet considered, and have so +great a resemblance to each other, that we put them all together in a single +group with the title of <i>Amniotes.</i> In these three classes alone we find +the remarkable embryonic membrane, already mentioned, which we called the +<i>amnion</i>; a cenogenetic adaptation that we may regard as a result of the +sinking of the growing embryo into the yelk-sac. +</p> + +<p> +All the Amniotes known to us—all reptiles, birds, and mammals (including +man)—agree in so many important points of internal structure and +development that their descent from a common ancestor can be affirmed with +tolerable certainty. If the evidence of comparative +<span class='pagenum'><a name="Page_244" id="Page_244"></a></span> +anatomy and ontogeny is ever entirely beyond suspicion, it is certainly the +case here. All the peculiarities that accompany and follow the formation of the +amnion, and that we have learned in our consideration of human embryology; all +the peculiarities in the development of the organs which we will presently +follow in detail; finally, all the principal special features of the internal +structure of the full-grown Amniotes—prove so clearly the common origin +of all the Amniotes from single extinct stem-form that it is difficult to +entertain the idea of their evolution from several independent stems. This +unknown common stem-form is our primitive Amniote (<i>Protamnion</i>). In +outward appearance it was probably something between the salamander and the +lizard. +</p> + +<p> +It is very probable that some part of the Permian period was the age of the +origin of the Protamniotes. This follows from the fact that the Amphibia are +not fully developed until the Carboniferous period, and that the first fossil +reptiles (<i>Palæhatteria, Homœosaurus, Proterosaurus</i>) are found towards +the close of the Permian period. Among the important changes of the vertebrate +organisation that marked the rise of the first Amniotes from salamandrine +Amphibia during this period the following three are especially noteworthy: the +entire disappearance of the water-breathing gills and the conversion of the +gill-arches into other organs, the formation of the allantois or primitive +urinary sac, and the development of the amnion. +</p> + +<p> +One of the most salient characteristics of the Amniotes is the complete loss of +the gills. All Amniotes, even if living in water (such as sea-serpents and +whales), breathe air through lungs, never water through gills. All the Amphibia +(with very rare exceptions) retain their gills for some time when young, and +have for a time (if not permanently) branchial respiration; but after these +there is no question of branchial respiration. The Protamniote itself must have +entirely abandoned water-breathing. Nevertheless, the gill-arches are preserved +by heredity, and develop into totally different (in part rudimentary) +organs—various parts of the bone of the tongue, the frame of the jaws, +the organ of hearing, etc. But we do not find in the embryos of the Amniotes +any trace of gill-leaves, or of real respiratory organs on the gill-arches. +</p> + +<p> +With this complete abandonment of the gills is probably connected the formation +of another organ, to which we have already referred in embryology—namely, +the allantois or primitive urinary sac (cf. p. 166). It is very probable that +the urinary bladder of the Dipneusts is the first structure of the allantois. +We find in these a urinary bladder that proceeds from the lower wall of the +hind end of the gut, and serves as receptacle for the renal secretions. This +organ has been transmitted to the Amphibia, as we can see in the frog. +</p> + +<p> +The formation of the amnion and the allantois and the complete disappearance of +the gills are the chief characteristics that distinguish the Amniotes from the +lower Vertebrates we have hitherto considered. To these we may add several +subordinate features that are transmitted to all the Amniotes, and are found in +these only. One striking embryonic character of the Amniotes is the great curve +of the head and neck in the embryo. We also find an advance in the structure of +several of the internal organs of the Amniotes which raises them above the +highest of the anamnia. In particular, a partition is formed in the simple +ventricle of the heart, dividing into right and left chambers. In connection +with the complete metamorphosis of the gill-arches we find a further +development of the auscultory organs. Also, there is a great advance in the +structure of the brain, skeleton, muscular system, and other parts. Finally, +one of the most important changes is the reconstruction of the kidneys. In all +the earlier Vertebrates we have found the primitive kidneys as excretory +organs, and these appear at an early stage in the embryos of all the higher +Vertebrates up to man. But in the Amniotes these primitive kidneys cease to act +at an early stage of embryonic life, and their function is taken up by the +permanent or secondary kidneys, which develop from the terminal section of the +prorenal ducts. +</p> + +<p> +Taking all these peculiarities of the Amniotes together, it is impossible to +doubt that all the animals of this group—all reptiles, birds, and +mammals—have a common origin, and form a single blood-related stem. Our +own race belongs to this stem. Man is, in every feature of his organisation and +embryonic development, a true Amniote, and has descended from the Protamniote +with all the other +<span class='pagenum'><a name="Page_245" id="Page_245"></a></span> +Amniotes. Though they appeared at the end (possibly even in the middle) of the +Paleozoic age, the Amniotes only reached their full development during the +Mesozoic age. The birds and mammals made their first appearance during this +period. Even the reptiles show their greatest growth at this time, so that it +is called “the reptile age.” The extinct Protamniote, the ancestor +of the whole group, belongs in its whole organisation to the reptile class. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus264"></a> +<img src="images/fig264.gif" width="493" height="379" alt="Fig.264. The lizard +(Hatteria punctata = Sphenodon punctatus) of New Zealand." /> +<p class="caption">Fig. 264—<b>The lizard</b> (<i>Hatteria punctata = +Sphenodon punctatus</i>) of New Zealand. The sole surviving proreptile. (From +<i>Brehm.</i>)</p> +</div> + +<p> +The genealogical tree of the amniote group is clearly indicated in its chief +lines by their paleontology, comparative anatomy, and ontogeny. The group +succeeding the Protamniote divided into two branches. The branch that will +claim our whole interest is the class of the Mammals. The other branch, which +developed in a totally different direction, and only comes in contact with the +Mammals at its root, is the combined group of the reptiles and birds; these two +classes may, with Huxley, be conveniently grouped together as the +<i>Sauropsida.</i> Their common stem-form is an extinct lizard-like reptile of +the order of the Rhyncocephalia. From this have been developed in various +directions the serpents, crocodiles, tortoises, etc.—in a word, all the +members of the reptile class. But the remarkable class of the birds has also +been evolved directly from a branch of the reptile group, as is now established +beyond question. The embryos of the reptiles and birds are identical until a +very late stage, and have an astonishing resemblance even later. Their whole +structure agrees so much that no anatomist now questions the descent of the +birds from the reptiles. On the other +<span class='pagenum'><a name="Page_246" id="Page_246"></a></span> +hand, the mammal line has descended from the group of the Sauromammalia, a +different branch of the Proreptilia. It is connected at its deepest roots with +the reptile line, but it then diverges completely from it and follows a +distinctive development. Man is the highest outcome of this class, the +“crown of creation.” The hypothesis that the three higher +Vertebrate classes represent a single Amniote-stem, and that the common root of +this stem is to be found in the amphibian class, is now generally admitted. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus265"></a> +<img src="images/fig265.gif" width="447" height="149" alt="Fig.265. Homoeosaurus +pulchellus, a Jurassic proreptile from Kehlheim." /> +<p class="caption">Fig. 265—<b>Homœosaurus pulchellus,</b> a Jurassic +proreptile from Kehlheim. (From <i>Zittel.</i>)</p> +</div> + +<p> +The instructive group of the Permian Tocosauria, the common root from which the +divergent stems of the Sauropsids and mammals have issued, merits our +particular attention as the stem-group of all the Amniotes. Fortunately a +living representative of this extinct ancestral group has been preserved to our +day; this is the remarkable lizard of New Zealand, <i>Hatteria punctata</i> +(Fig. 264). Externally it differs little from the ordinary lizard; but in many +important points of internal structure, especially in the primitive +construction of the vertebral column, the skull, and the limbs, it occupies a +much lower position, and approaches its amphibian ancestors, the Stegocephala. +Hence Hatteria is the phylogenetically oldest of all living reptiles, an +isolated survivor from the Permian period, closely resembling the common +ancestor of the Amniotes. It must differ so little from this extinct form, our +hypothetical Protamniote, that we put it next to the Proreptilia. The +remarkable Permian <i>Palæhatteria,</i> that Credner discovered in the Plauen +terrain at Dresden in 1888, belongs to the same group (Fig. 266). The Jurassic +genus <i>Homœosaurus</i> (Fig. 265), of which well-preserved skeletons are +found in the Solenhofen schists, is perhaps still more closely related to them. +</p> + +<p> +Unfortunately, the numerous fossil remains of Permian and Triassic Tocosauria +that we have found in the last two decades are, for the most part, very +imperfectly preserved. Very often we can make only precarious inferences from +these skeletal fragments as to the anatomic characters of the soft parts that +went with the bony skeleton of the extinct Tocosauria. Hence it has not yet +been possible to arrange these important fossils with any confidence in the +ancestral series that descend from the Protamniotes to the Sauropsids on the +one side and the Mammals on the other. Opinions are particularly divided as to +the place in classification and the phylogenetic significance of the remarkable +<i>Theromorpha.</i> Cope gives this name to a very interesting and extensive +group of extinct terrestrial reptiles, of which we have only fossil remains +from the Permian and Triassic strata. Forty years ago some of these Therosauria +(fresh-water animals) were described by Owen as <i>Anomodontia.</i> But during +the last twenty years the distinguished American paleontologists, Cope and +Osborn, have greatly increased our knowledge of them, and have claimed that the +stem-forms of the Mammals must be sought in this order. As a matter of fact, +the Theromorpha are nearer to the Mammals in the chief points of structure than +any other reptiles. This is especially true of the Thereodontia, to which the +<i>Pureosauria</i> and <i>Pelycosauria</i> belong (Fig. 267). The whole +structure of their pelvis and hind-feet has attained the same form as in the +Monotremes, the lowest Mammals. The formation of the +<span class='pagenum'><a name="Page_247" id="Page_247"></a></span> +scapula and the quadrate bone shows an approach to the Mammals such as we find +in no other group of reptiles. The teeth also are already divided into +incisors, canines, and molars. Nevertheless, it is very doubtful whether the +Theromorpha really are in the ancestral line of the Sauromammals, or lead +direct from the Tocosauria to the earliest Mammals. Other experts on this group +believe that it is an independent legion of the reptiles, connected, perhaps, +at its lowest root, with the Sauromammals, but developed quite independently of +the Mammals—though parallel to them in many ways. +</p> + +<p> +One of the most important of the zoological facts that we rely on in our +investigation of the genealogy of the human race is the position of man in the +Mammal class. However different the views of zoologists may have been as to +this position in detail, and as to his relations to the apes, no scientist has +ever doubted that man is a true mammal in his whole organisation and +development. Linné drew attention to this fact in the first edition of his +famous <i>Systema Naturæ</i> (1735). As will be seen in any museum of anatomy +or any manual of comparative anatomy; the human frame has all the +characteristics that are common to the Mammals and distinguish them +conspicuously from all other animals. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus266"></a> +<img src="images/fig266.gif" width="258" height="211" alt="Fig.266. Skull of a +Permian lizard (Palaehatteria longicaudata)." /> +<p class="caption">Fig. 266—<b>Skull of a Permian lizard</b> +(<i>Palæhatteria longicaudata</i>). (From <i>Credner.</i>) <i>n</i> nasal bone, +<i>pf</i> frontal bone, <i>l</i> lachrymal bone, <i>po</i> postorbital bone, +<i>sq</i> covering bone, <i>i</i> cheek-bone, <i>vo</i> vomer, <i>im</i> +inter-maxillary.</p> +</div> + +<p> +If we examine this undoubted fact from the point of view of phylogeny, in the +light of the theory of descent, it follows at once that man is of a common stem +with all the other Mammals, and comes from the same root as they. But the +various features in which the Mammals agree and by which they are distinguished +are of such a character as to make a polyphyletic hypothesis quite +inadmissible. It is impossible to entertain the idea that all the living and +extinct Mammals come from a number of separate roots. If we accept the general +theory of evolution, we are bound to admit the monophyletic hypothesis of the +descent of all the Mammals (including man) from a single mammalian stem-form. +We may call this long-extinct root-form and its earliest descendants (a few +genera of one family) “primitive mammals” or +“stem-mammals” (<i>Promammalia</i>). As we have already seen, this +root-form developed from the primitive Proreptile stem in a totally different +direction from the birds, and soon separated from the main stem of the +reptiles. The differences between the Mammals and the reptiles and birds are so +important and characteristic that we can assume with complete confidence this +division of the vertebrate stem at the commencement of the development of the +Amniotes. The reptiles and birds, which we group together as the +<i>Sauropsids,</i> generally agree in the characteristic structure of the skull +and brain, and this is notably different from that of the Mammals. In most of +the reptiles and birds the skull is connected with the first cervical vertebra +(the <i>atlas</i>) by a single, and in the Mammals (and Amphibia) by a double, +condyle at the back of the head. In the former the lower jaw is composed of +several pieces, and connected with the skull so that it can move by a special +maxillary bone (the <i>quadratum</i>); in the Mammals the lower jaw consists of +one pair of bony pieces, which articulate directly with the temporal bone. +Further, in the Sauropsids the skin is clothed with scales or feathers; in the +Mammals with hair. The red blood-cells of the former have a nucleus; those of +the latter have not. In fine, two quite characteristic features of the Mammals, +which distinguish them not only from the birds and reptiles, but from all other +animals, are the possession of a +<span class='pagenum'><a name="Page_248" id="Page_248"></a></span> +complete diaphragm and of mammary glands that produce the milk for the +nutrition of the young. It is only in the Mammals that the diaphragm forms a +transverse partition of the body-cavity, completely separating the pectoral +from the abdominal cavity. It is only in the mammals that the mother suckles +its young, and this rightly gives the name to the whole class (<i>mamma</i> = +breast). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus267"></a> +<img src="images/fig267.gif" width="251" height="271" alt="Fig.267. Skull of a +Triassic theromorphum (Galesaurus planiceps), from the Karoo formation in South +Africa." /> +<p class="caption">Fig. 267—<b>Skull of a Triassic theromorphum</b> +(<i>Galesaurus planiceps</i>), from the Karoo formation in South Africa. (From +<i>Owen.</i>) a from the right, <i>b</i> from below, <i>c</i> from above, +<i>d</i> tricuspid tooth. <i>N</i> nostrils, <i>Na</i> nasal bone, <i>Mx</i> +upper jaw, <i>Prf</i> prefrontal, <i>Fr</i> frontal bone, <i>A</i> eye-pits, +<i>S</i> temple-pits. <i>Pa</i> Parietal eye, <i>Bo</i> joint at back of head, +<i>Pt</i> pterygoid-bone, <i>Md</i> lower jaw.</p> +</div> + +<p> +From these pregnant facts of comparative anatomy and ontogeny it follows +absolutely that the whole of the Mammals belong to a single natural stem, which +branched off at an early date from the reptile-root. It follows further with +the same absolute certainty that the human race is also a branch of this stem. +Man shares all the characteristics I have described with all the Mammals, and +differs in them from all other animals. Finally, from these facts we deduce +with the same confidence those advances in the vertebrate organisation by which +one branch of the Sauromammals was converted into the stem-form of the Mammals. +Of these advances the chief were: (1) The characteristic modification of the +skull and the brain; (2) the development of a hairy coat; (3) the complete +formation of the diaphragm; and (4) the construction of the mammary glands and +adaptation to suckling. Other important changes of structure proceeded step by +step with these. +</p> + +<p> +The epoch at which these important advances were made, and the foundation of +the Mammal class was laid, may be put with great probability in the first +section of the Mesozoic or secondary age—the Triassic period. The oldest +fossil remains of mammals that we know were found in strata that belong to the +earliest Triassic period—the upper Kueper. One of the earliest forms is +the genus <i>Dromatherium,</i> from the North American Triassic (Fig. 268). +Their teeth still strikingly recall those of the Pelycosauria. Hence we may +assume that this small and probably insectivorous mammal belonged to the +stem-group of the Promammals. We do not find any positive trace of the third +and most advanced division of the Mammals—the Placentals. These +(including man) are much younger, and we do not find indisputable fossil +remains of them until the Cenozoic age, or the Tertiary period. This +paleontological fact is very important, because it fully harmonises with the +evolutionary succession of the Mammal orders that is deduced from their +comparative anatomy and ontogeny. +</p> + +<p> +The latter science teaches us that the whole Mammal class divides into three +main groups or sub-classes, which correspond to three successive phylogenetic +stages. These three stages, which also represent three important stages in our +human genealogy, were first distinguished in 1816 by the eminent French +zoologist, Blainville, and received the names of <i>Ornithodelphia, +Didelphia,</i> and <i>Monodelphia,</i> according to the construction of the +female organs (<i>delphys</i> = uterus or womb). Huxley afterwards gave them +the names of <i>Prototheria, Metatheria,</i> and <i>Epitheria.</i> But the +three sub-classes differ so widely from each other, not only in the +construction of the sexual organs, but in many other respects also, that we may +confidently draw up the following +<span class='pagenum'><a name="Page_249" id="Page_249"></a></span> +important phylogenetic thesis: The Monodelphia or Placentals descend from the +Didelphia or Marsupials; and the latter, in turn, are descended from the +Monotremes or Ornithodelphia. +</p> + +<p> +Thus we must regard as the twenty-first stage in our genealogical tree the +earliest and lowest chief group of the Mammals—the sub-class of the +Monotremes (“cloaca-animals,” Ornithodelphia, or Prototheria, Figs. +269 and 270). They take their name from the cloaca which they share with all +the lower Vertebrates. This cloaca is the common outlet for the passage of the +excrements, the urine, and the sexual products. The urinary ducts and sexual +canals open into the hindmost part of the gut, while in all the other Mammals +they are separated from the rectum and anus. The latter have a special +uro-genital outlet (<i>porus urogenitalis</i>). The bladder also opens into the +cloaca in the Monotremes, and, indeed, apart from the two urinary ducts; in all +the other Mammals the latter open directly into the bladder. It was proved by +Haacke and Caldwell in 1884 that the Monotremes lay large eggs like the +reptiles, while all the other Mammals are viviparous. In 1894 Richard Semon +further proved that these large eggs, rich in food-yelk, have a partial +segmentation and discoid gastrulation, as I had hypothetically assumed in 1879; +here again they resemble their reptilian ancestors. The construction of the +mammary gland is also peculiar in the Monotremes. In them the glands have no +teats for the young animal to suck, but there is a special part of the breast +pierced with holes like a sieve, from which the milk issues, and the young +Monotreme must lick it off. Further, the brain of the Monotremes is very little +advanced. It is feebler than that of any of the other Mammals. The fore-brain +or cerebrum, in particular, is so small that it does not cover the cerebellum. +In the skeleton (Fig. 270) the formation of the scapula among other parts is +curious; it is quite different from that of the other Mammals, and rather +agrees with that of the reptiles and Amphibia. Like these, the Monotremes have +a strongly developed <i>caracoideum.</i> From these and other less prominent +characteristics it follows absolutely that the Monotremes occupy the lowest +place among the Mammals, and represent a transitional group between the +Tocosauria and the rest of the Mammals. All these remarkable reptilian +characters must have been possessed by the stem-form of the whole mammal class, +the Promammal of the Triassic period, and have been inherited from the +Proreptiles. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus268"></a> +<img src="images/fig268.gif" width="208" height="87" alt="Fig.268. Lower jaw of a +Primitive Mammal or Promammal (Dromatherium silvestre) from the North American +Triassic." /> +<p class="caption">Fig. 268—<b>Lower jaw of a Primitive Mammal or +Promammal</b> (<i>Dromatherium silvestre</i>) from the North American Triassic. +<i>i</i> incisors, <i>c</i> canine, <i>p</i> premolars, <i>m</i> molars. (From +<i>Döderlein.</i>)</p> +</div> + +<p> +During the Triassic and Jurassic periods the sub-class of the Monotremes was +represented by a number of different stem-mammals. Numerous fossil remains of +them have lately been discovered in the Mesozoic strata of Europe, Africa, and +America. To-day there are only two surviving specimens of the group, which we +place together in the family of the duck-bills, <i>Ornithostoma.</i> They are +confined to Australia and the neighbouring island of Van Diemen’s Land +(or Tasmania); they become scarcer every year, and will soon, like their +blood-relatives, be counted among the extinct animals. One form lives in the +rivers, and builds subterraneous dwellings on the banks; this is the +<i>Ornithorhyncus paradoxus,</i> with webbed feet, a thick soft fur, and broad +flat jaws, which look very much like the bill of a duck (Figs. 269, 270). The +other form, the land duck-bill, or spiny ant-eater (<i>Echidna hystrix</i>), is +very much like the anteaters in its habits and the peculiar construction of its +thin snout and very long tongue; it is covered with needles, and can roll +itself up like a hedgehog. A cognate form (<i>Parechidna Bruyni</i>) has lately +been found in New Guinea. +</p> + +<p> +These modern Ornithostoma are the scattered survivors of the vast Mesozoic +group of Monotremes; hence they have the same interest in connection with the +stem history of the Mammals as the living stem-reptiles (<i>Hatteria</i>) for +that of the reptiles, and the isolated Acrania (<i>Amphioxus</i>) for the +phylogeny of the Vertebrate stem. +</p> + +<p> +The Australian duck-bills are distinguished externally by a toothless bird-like +<span class='pagenum'><a name="Page_250" id="Page_250"></a></span> +beak or snout. This absence of real bony teeth is a late result of adaptation, +as in the toothless Placentals (<i>Edentata,</i> armadillos and ant-eaters). +The extinct Monotremes, to which the Promammalia belonged, must have had +developed teeth, inherited from the reptiles. Lately small rudiments of real +molars have been discovered in the young of the Ornithorhyncus, which has horny +plates in the jaws instead of real teeth. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus269"></a> +<img src="images/fig269.gif" width="421" height="176" alt="Fig. 269. The +Ornithorhyncus or Duck-mole. (Ornithorhyncus paradoxus)." /> +<p class="caption">Fig. 269—<b>The Ornithorhyncus or Duck-mole.</b> +(<i>Ornithorhyncus paradoxus</i>).</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus270"></a> +<img src="images/fig270.gif" width="421" height="143" alt="Fig. 270. Skeleton of the Ornithorhyncus." /> +<p class="caption">Fig. 270—<b>Skeleton of the +Ornithorhyncus.</b></p> +</div> + +<p> +The living Ornithostoma and the stem-forms of the Marsupials (or +<i>Didelphia</i>) must be regarded as two widely diverging lines from the +Promammals. This second sub-class of the Mammals is very interesting as a +perfect intermediate stage between the other two. While the Marsupials retain a +great part of the characteristics of the Monotremes, they have also acquired +some of the chief features of the Placentals. Some features +<span class='pagenum'><a name="Page_251" id="Page_251"></a></span> +are also peculiar to the Marsupials, such as the construction of the male and +female sexual organs and the form of the lower jaw. The Marsupials are +distinguished by a peculiar hook-like bony process that bends from the corner +of the lower jaw and points inwards. As most of the Placentals have not this +process, we can, with some probability, recognise the Marsupial from this +feature alone. Most of the mammal remains that we have from the Jurassic and +Cretaceous deposits are merely lower jaws, and most of the jaws found in the +Jurassic deposits at Stonesfield and Purbeck have the peculiar hook-like +process that characterises the lower jaw of the Marsupial. On the strength of +this paleontological fact, we may suppose that they belonged to Marsupials. +Placentals do not seem to have existed at the middle of the Mesozoic +age—not until towards its close (in the Cretaceous period). At all +events, we have no fossil remains of indubitable Placentals from that period. +</p> + +<p> +The existing Marsupials, of which the plant-eating kangaroo and the carnivorous +opossum (Fig. 272) are the best known, differ a good deal in structure, shape, +and size, and correspond in many respects to the various orders of Placentals. +Most of them live in Australia, and a small part of the Australian and East +Malayan islands. There is now not a single living Marsupial on the mainland of +Europe, Asia, or Africa. It was very different during the Mesozoic and even +during the Cenozoic age. The sedimentary deposits of these periods contain a +great number and variety of marsupial remains, sometimes of a colossal size, in +various parts of the earth, and even in Europe. We may infer from this that the +existing Marsupials are the remnant of an extensive earlier group that was +distributed all over the earth. It had to give way in the struggle for life to +the more powerful Placentals during the Tertiary period. The survivors of the +group were able to keep alive in Australia and South America because the one +was completely separated from the other parts of the earth during the whole of +the Tertiary period, and the other during the greater part of it. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus271"></a> +<img src="images/fig271.gif" width="225" height="93" alt="Fig.271. Lower jaw of a +Promammal (Dryolestes priscus), from the Jurassic of the Felsen strata." /> +<p class="caption">Fig. 271—<b>Lower jaw of a Promammal</b> +(<i>Dryolestes priscus</i>), from the Jurassic of the Felsen strata. (From +<i>Marsh.</i>)</p> +</div> + +<p> +From the comparative anatomy and ontogeny of the existing Marsupials we may +draw very interesting conclusions as to their intermediate position between the +earlier Monotremes and the later Placentals. The defective development of the +brain (especially the cerebrum), the possession of marsupial bones, and the +simple construction of the allantois (without any placenta as yet) were +inherited by the Marsupials, with many other features, from the Monotremes, and +preserved. On the other hand, they have lost the independent bone +(<i>caracoideum</i>) at the shoulder-blade. But we have a more important +advance in the disappearance of the cloaca; the rectum and anus are separated +by a partition from the uro-genital opening (<i>sinus urogenitalis</i>). +Moreover, all the Marsupials have teats on the mammary glands, at which the +new-born animal sucks. The teats pass into the cavity of a pouch or pocket on +the ventral side of the mother, and this is supported by a couple of marsupial +bones. The young are born in a very imperfect condition, and carried by the +mother for some time longer in her pouch, until they are fully developed (Fig. +272). In the giant kangaroo, which is as tall as a man, the embryo only +develops for a month in the uterus, is then born in a very imperfect state, and +finishes its growth in the mother’s pouch (<i>marsupium</i>); it remains +in this about nine months, and at first hangs continually on to the teat of the +mammary gland. +</p> + +<p> +From these and other characteristics (especially the peculiar construction of +the internal and external sexual organs in male and female) it is clear that we +must conceive the whole sub-class of the Marsupials as one stem group, which +has been developed from the Promammalia. From one branch of these Marsupials +(possibly from more than one) the stem-forms of the higher Mammals, the +Placentals, were afterwards evolved. Of the existing forms of the Marsupials, +<span class='pagenum'><a name="Page_252" id="Page_252"></a></span> +which have undergone various modifications through adaptation to different +environments, the family of the opossums (<i>Didelphida</i> or <i>Pedimana</i>) +seems to be the oldest and nearest to the common stem-form of the whole class. +To this family belong the crab-eating opossum of Brazil (Fig. 272) and the +opossum of Virginia, on the embryology of which Selenka has given us a valuable +work (cf. Figs. 63–67 and 131–135). These Didelphida climb trees +like the apes, grasping the branches with their hand-shaped hind feet. We may +conclude from this that the stem-forms of the Primates, which we must regard as +the earliest Lemurs, were evolved directly from the opossum. We must not +forget, however, that the conversion of the five-toed foot into a prehensile +hand is polyphyletic. By the same adaptation to climbing trees the habit of +grasping their branches with the feet has in many different cases brought about +that opposition of the thumb or great toe to the other toes which makes the +hand prehensile. We see this in the climbing lizards (chameleon), the birds, +and the tree-dwelling mammals of various orders. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus272"></a> +<img src="images/fig272.gif" width="316" height="373" alt="Fig.272. The +crab-eating Opossum (Philander cancrivorus). The female has three young in the +pouch." /> +<p class="caption">Fig. 272—<b>The crab-eating Opossum</b> +(<i>Philander cancrivorus</i>). The female has three young in the pouch. (From +<i>Brehm.</i>)</p> +</div> + +<p> +Some zoologists have lately advanced the opposite opinion, that the Marsupials +represent a completely independent +<span class='pagenum'><a name="Page_253" id="Page_253"></a></span> +sub-class of the Mammals, with no direct relation to the Placentals, and +developing independently of them from the Monotremes. But this opinion is +untenable if we examine carefully the whole organisation of the three +sub-classes, and do not lay the chief stress on incidental features and +secondary adaptations (such as the formation of the marsupium). It is then +clear that the Marsupials—viviparous Mammals without placenta—are a +necessary transition from the oviparous Monotremes to the higher Placentals +with chorion-villi. In this sense the Marsupial class certainly contains some +of man’s ancestors. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap23"></a>Chapter XXIII.<br/> +OUR APE ANCESTORS</h2> + +<p> +The long series of animal forms which we must regard as the ancestors of our +race has been confined within narrower and narrower circles as our phylogenetic +inquiry has progressed. The great majority of known animals do not fall in the +line of our ancestry, and even within the vertebrate stem only a small number +are found to do so. In the most advanced class of the stem, the mammals, there +are only a few families that belong directly to our genealogical tree. The most +important of these are the apes and their predecessors, the half-apes, and the +earliest Placentals (<i>Prochoriata</i>). +</p> + +<p> +The Placentals (also called <i>Choriata, Monodelphia, Eutheria</i> or +<i>Epitheria</i>) are distinguished from the lower mammals we have just +considered, the Monotremes and Marsupials, by a number of striking +peculiarities. Man has all these distinctive features; that is a very +significant fact. We may, on the ground of the most careful +comparative-anatomical and ontogenetic research, formulate the thesis: +“Man is in every respect a true Placental.” He has all the +characteristics of structure and development that distinguish the Placentals +from the two lower divisions of the mammals, and, in fact, from all other +animals. Among these characteristics we must especially notice the more +advanced development of the brain. The fore-brain or cerebrum especially is +much more developed in them than in the lower animals. The <i>corpus +callosum,</i> which forms a sort of wide bridge connecting the two hemispheres +of the cerebrum, is only fully formed in the Placentals; it is very rudimentary +in the Marsupials and Monotremes. It is true that the lowest Placentals are not +far removed from the Marsupials in cerebral development; but within the +placental group we can trace an unbroken gradation of progressive development +of the brain, rising gradually from this lowest stage up to the elaborate +psychic organ of the apes and man. The human soul—a physiological +function of the brain—is in reality only a more advanced ape-soul. +</p> + +<p> +The mammary glands of the Placentals are provided with teats like those of the +Marsupials; but we never find in the Placentals the pouch in which the latter +carry and suckle their young. Nor have they the marsupial bones in the ventral +wall at the anterior border of the pelvis, which the Marsupials have in common +with the Monotremes, and which are formed by a partial ossification of the +sinews of the inner oblique abdominal muscle. There are merely a few +insignificant remnants of them in some of the Carnivora. The Placentals are +also generally without the hook-shaped process at the angle of the lower jaw +which is found in the Marsupials. +</p> + +<p> +However, the feature that characterises the Placentals above all others, and +that has given its name to the whole sub-class, is the formation of the +placenta. We have already considered the formation and significance of this +remarkable embryonic organ when we traced the development of the chorion and +the allantois in the human embryo (pp.165–9) The urinary sac or the +allantois, the +<span class='pagenum'><a name="Page_254" id="Page_254"></a></span> +curious vesicle that grows out of the hind part of the gut, has essentially the +same structure and function in the human embryo as in that of all the other +Amniotes (cf. Figs. 194–6). There is a quite secondary difference, on +which great stress has wrongly been laid, in the fact that in man and the +higher apes the original cavity of the allantois quickly degenerates, and the +rudiment of it sticks out as a solid projection from the primitive gut. The +thin wall of the allantois consists of the same two layers or membranes as the +wall of the gut—the gut-gland layer within and the gut-fibre layer +without. In the gut-fibre layer of the allantois there are large blood-vessels, +which serve for the nutrition, and especially the respiration, of the +embryo—the umbilical vessels (p. 170).In the reptiles and birds the +allantois enlarges into a spacious sac, which encloses the embryo with the +amnion, and does not combine with the outer fœtal membrane (the chorion). This +is the case also with the lowest mammals, the oviparous Monotremes and most of +the Marsupials. It is only in some of the later Marsupials (<i>Peramelida</i>) +and all the Placentals that the allantois develops into the distinctive and +remarkable structure that we call the <i>placenta.</i> +</p> + +<div class="fig" style="width:100%;"> +<a name="illus273"></a> +<img src="images/fig273.gif" width="199" height="219" alt="Fig.273. Foetal +membranes of the human embryo (diagrammatic)." /> +<p class="caption">Fig. 273—<b>Fœtal membranes of the human embryo</b> +(diagrammatic). <i>m</i> the thick muscular wall of the womb. <i>plu</i> +placenta [the inner layer (<i>plu</i>′) of which penetrates into the +chorion-villi (<i>chz</i>) with its processes]. <i>chf</i> tufted, <i>chl</i> +smooth chorion. <i>a</i> amnion, <i>ah</i> amniotic cavity, <i>as</i> amniotic +sheath of the umbilical cord (which passes under into the navel of the +embryo—not given here), <i>dg</i> vitelline duct, <i>ds</i> yelk sac, +<i>dv, dr</i> decidua (vera and reflexa). The uterine cavity (<i>uh</i>) opens +below into the vagina and above on the right into an oviduct (<i>t</i>). (From +<i>Kölliker.</i>)</p> +</div> + +<p> +The placenta is formed by the branches of the blood-vessels in the wall of the +allantois growing into the hollow ectodermic tufts (villi) of the chorion, +which run into corresponding depressions in the mucous membrane of the womb. +The latter also is richly permeated with blood-vessels which bring the +mother’s blood to the embryo. As the partition in the villi between the +maternal blood-vessels and those of the fœtus is extremely thin, there is a +direct exchange of fluid between the two, and this is of the greatest +importance in the nutrition of the young mammal. It is true that the maternal +vessels do not entirely pass into the fœtal vessels, so that the two kinds of +blood are simply mixed. But the partition between them is so thin that the +nutritive fluid easily transudes through it. By means of this transudation or +diosmosis the exchange of fluids takes place without difficulty. The larger the +embryo is in the placentals, and the longer it remains in the womb, the more +necessary it is to have special structures to meet its great consumption of +food. +</p> + +<p> +In this respect there is a very conspicuous difference between the lower and +higher mammals. In the Marsupials, in which the embryo is only a comparatively +short time in the womb and is born in a very immature condition, the vascular +arrangements in the yelk-sac and the allantois suffice for its nutrition, as we +find them in the Monotremes, birds, and reptiles. But in the Placentals, where +gestation lasts a long time, and the embryo reaches its full development under +the protection of its enveloping membranes, there has to be a new mechanism for +the direct supply of a large quantity of food, and this is admirably met by the +formation of the placenta. +</p> + +<p> +Branches of the blood-vessels penetrate into the chorion-villi from within, +starting from the gut-fibre layer of the allantois, and bringing the blood of +the fœtus through the umbilical vessels (Fig. 273 <i>chz</i>). On the other +hand, a thick network of blood-vessels develops in the mucous membrane that +clothes the inner surface of the womb, especially in the region of the +depressions into which the chorion-villi penetrate (<i>plu</i>). This network +of arteries contains maternal blood, brought by the uterine vessels. As the +connective tissue between the enlarged capillaries of +<span class='pagenum'><a name="Page_255" id="Page_255"></a></span> +the uterus disappears, wide cavities filled with maternal blood appear, and +into these the chorion-villi of the embryo penetrate. The sum of these vessels +of both kinds, that are so intimately correlated at this point, together with +the connective and enveloping tissue, is the <i>placenta.</i> The placenta +consists, therefore, properly speaking, of two different though intimately +connected parts—the fœtal placenta (Fig. 273 <i>chz</i>) within and the +maternal or uterine placenta (<i>plu</i>) without. The latter is made up of the +mucous coat of the uterus and its blood-vessels, the former of the tufted +chorion and the umbilical vessels of the embryo (cf. Fig. 196). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus274"></a> +<img src="images/fig274.gif" width="321" height="109" alt="Fig.274. Skull of a +fossil lemur (Adapis parisiensis,), from the Miocene at Quercy." /> +<p class="caption">Fig. 274—<b>Skull of a fossil lemur</b> (<i>Adapis +parisiensis</i>), from the Miocene at Quercy. <i>A</i> lateral view from the +right. <i>B</i> lower jaw, <i>C</i> lower molar, <i>i</i> incisors, <i>c</i> +canines, <i>p</i> premolars, <i>m</i> molars.</p> +</div> + +<p> +The manner in which these two kinds of vessels combine in the placenta, and the +structure, form, and size of it, differ a good deal in the various Placentals; +to some extent they give us valuable data for the natural classification, and +therefore the phylogeny, of the whole of this sub-class. On the ground of these +differences we divide it into two principal sections; the lower Placentals or +<i>Indecidua,</i> and the higher Placentals or <i> Deciduata.</i> +</p> + +<p> +To the Indecidua belong three important groups of mammals: the Lemurs +(<i>Prosimiæ</i>), the Ungulates (tapirs, horses, pigs, ruminants, etc.), and +the Cetacea (dolphins and whales). In these Indecidua the villi are distributed +over the whole surface of the chorion (or its greater part) either singly or in +groups. They are only loosely connected with the mucous coat of the uterus, so +that the whole fœtal membrane with its villi can be easily withdrawn from the +uterine depressions like a hand from a glove. There is no real coalescence of +the two placentas at any part of the surface of contact. Hence at birth the +fœtal placenta alone comes away; the uterine placenta is not torn away with it. +</p> + +<p> +The formation of the placenta is very different in the second and higher +section of the Placentals, the <i>Deciduata.</i> Here again the whole surface +of the chorion is thickly covered with the villi in the beginning. But they +afterwards disappear from one part of the surface, and grow proportionately +thicker on the other part. We thus get a differentiation between the smooth +chorion (<i>chorion laeve,</i> Fig. 273 <i>chl</i>) and the thickly-tufted +chorion (<i>chorion frondosum,</i> Fig. 273 <i>chf</i>). The former has only a +few small villi or none at all; the latter is thickly covered with large and +well-developed villi; this alone now constitutes the placenta. In the great +majority of the Deciduata the placenta has the same shape as in man >(Figs. 197 +and 200)—namely a thick, circular disk like a cake; so we find in the +Insectivora, Chiroptera, Rodents, and Apes. This <i>discoplacenta</i> lies on +one side of the chorion. But in the Sarcotheria (both the Carnivora and the +seals, <i>Pinnipedia</i>) and in the elephant and several other Deciduates we +find a <i>zonoplacenta</i>; in these the rich mass of villi runs like a girdle +round the middle of the ellipsoid chorion, the two poles of it being free from +them. +</p> + +<p> +Still more characteristic of the Deciduates is the peculiar and very intimate +connection between the <i>chorion frondosum</i> and the corresponding part of +the mucous coat of the womb, which we must regard as a real coalescence of the +two. The villi of the chorion push their branches into the blood-filled tissues +of the coat of the uterus, and the vessels of each loop together so intimately +that it is no longer possible to separate the fœtal +<span class='pagenum'><a name="Page_256" id="Page_256"></a></span> +from the maternal placenta; they form henceforth a compact and apparently +simple placenta. In consequence of this coalescence, a whole piece of the +lining of the womb comes away at birth with the fœtal membrane that is +interlaced with it. This piece is called the “falling-away” +membrane (<i>decidua</i>). It is also called the serous (spongy) membrane, +because it is pierced like a sieve or sponge. All the higher Placentals that +have this decidua are classed together as the “Deciduates.” The +tearing away of the decidua at birth naturally causes the mother to lose a +quantity of blood, which does not happen in the Indecidua. The last part of the +uterine coat has to be repaired by a new growth after birth in the Deciduates. +(Cf. Figs. 199, 200, pp. 168–70.) +</p> + +<div class="fig" style="width:100%;"> +<a name="illus275"></a> +<img src="images/fig275.gif" width="228" height="352" alt="Fig.275. The Slender +Lori (Stenops gracilis) of Ceylon, a tail-less lemur." /> +<p class="caption">Fig. 275—<b>The Slender Lori</b> (<i>Stenops +gracilis</i>) of Ceylon, a tail-less lemur.</p> +</div> + +<p> +In the various orders of the Deciduates, the placenta differs considerably both +in outer form and internal structure. The extensive investigations of the last +ten years have shown that there is more variation in these respects among the +higher mammals than was formerly supposed. The physiological work of this +important embryonic organ, the nutrition of the fœtus during its long sojourn +in the womb, is accomplished in the various groups of the Placentals by very +different and sometimes very elaborate structures. They have lately been fully +described by Hans Strahl. +</p> + +<p> +The phylogeny of the placenta has become more intelligible from the fact that +we have found a number of transitional forms of it. Some of the Marsupials +(<i>Perameles</i>) have the beginning of a placenta. In some of the Lemurs +(<i>Tarsius</i>) a discoid placenta with decidua is developed. +</p> + +<p> +While these important results of comparative embryology have been throwing +further light on the close blood-relationship of man and the anthropoid apes in +the last few years (p. 172), the great advance of paleontology has at the same +time been affording us a deeper insight into the stem-history of the Placental +group. In the seventh chapter of my <i>Systematic Phylogeny of the +Vertebrates</i> I advanced the hypothesis that the Placentals form a single +stem with many branches, which has been evolved from an older group of the +Marsupials (<i>Prodidelphia</i>). The four great legions of the +Placentals—Rodents, Ungulates, Carnassia, and Primates—are sharply +separated to-day by important features of organisation. But if we consider +their extinct ancestors of the Tertiary period, the differences gradually +disappear, the deeper we go in the Cenozoic deposits; in the end we find that +they vanish altogether. +<span class='pagenum'><a name="Page_257" id="Page_257"></a></span> +The primitive stem-forms of the Rodents (<i>Esthonychida</i>), the Ungulates +(<i>Chondylarthra</i>), the Carnassia (<i>Ictopsida</i>), and the Primates +(<i>Lemuravida</i>) are so closely related at the beginning of the Tertiary +period that we might group them together as different families of one order, +the Proplacentals (<i>Mallotheria</i> or <i>Prochoriata</i>). +</p> + +<p> +Hence the great majority of the Placentals have no direct and close +relationship to man, but only the legion of the <i> Primates.</i> This is now +generally divided into three orders—the half-apes (<i>Prosimiæ</i>), apes +(<i>Simiæ</i>), and man (<i>Anthropi</i>). The lemurs or half-apes are the +stem-group, descending from the older <i> Mallotheria</i> of the Cretaceous +period. From them the apes were evolved in the Tertiary period, and man was +formed from these towards its close. +</p> + +<p> +The Lemurs (<i>Prosimiæ</i>) have few living representatives. But they are very +interesting, and are the last survivors of a once extensive group. We find many +fossil remains of them in the older Tertiary deposits of Europe and North +America, in the Eocene and Miocene. We distinguish two sub-orders, the fossil +<i>Lemuravida</i> and the modern <i>Lemurogona.</i> The earliest and most +primitive forms of the Lemuravida are the Pachylemurs (<i>Hypopsodina</i>); +they come next to the earliest Placentals (<i>Prochoriata</i>), and have the +typical full dentition, with forty-four teeth (3.1.4.3. over 3.1.4.3.). The +Necrolemurs (<i>Adapida,</i> Fig. 274) have only forty teeth, and have lost an +incisor in each jaw (2.1.4.3. over 2.1.4.3.). The dentition is still further +reduced in the Lemurogona (<i>Autolemures</i>), which usually have only +thirty-six teeth (2.1.3.3. over 2.1.3.3.). These living survivors are scattered +far over the southern part of the Old World. Most of the species live in +Madagascar, some in the Sunda Islands, others on the mainland of Asia and +Africa. They are gloomy and melancholic animals; they live a quiet life, +climbing trees, and eating fruit and insects. They are of different kinds. Some +are closely related to the Marsupials (especially the opossum). Others +(<i>Macrotarsi</i>) are nearer to the Insectivora, others again +(<i>Chiromys</i>) to the Rodents. Some of the lemurs (<i>Brachytarsi</i>) +approach closely to the true apes. The numerous fossil remains of half-apes and +apes that have been recently found in the Tertiary deposits justify us in +thinking that man’s ancestors were represented by several different +species during this long period. Some of these were almost as big as men, such +as the diluvial lemurogonon <i>Megaladapis</i> of Madagascar. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus276"></a> +<img src="images/fig276.gif" width="220" height="231" alt="Fig.276. The +white-nosed ape (Cercopithecus petaurista)." /> +<p class="caption">Fig. 276—<b>The white-nosed ape</b> +(<i>Cercopithecus petaurista</i>).</p> +</div> + +<p> +Next to the lemurs come the true apes (<i>Simiæ</i>), the twenty-sixth stage in +our ancestry. It has been beyond question for some time now that the apes +approach nearest to man in every respect of all the animals. Just as the lowest +apes come close to the lemurs, so the highest come next to man. When we +carefully study the comparative anatomy of the apes and man, we can trace a +gradual and uninterrupted advance in the organisation of the ape up to the +purely human frame, and, after impartial examination of the “ape +problem” that has been discussed of late years with such passionate +interest, we come infallibly to the important conclusion, first formulated by +Huxley in 1863: “Whatever systems of organs we take, the comparison of +their modifications in the series of apes leads to the same result: that the +anatomic differences that separate man from the gorilla and chimpanzee are not +as great as those that separate the gorilla from the lower apes.” +Translated into phylogenetic language, this “pithecometra-law,” +formulated in such masterly fashion by Huxley, is quite equivalent to the +popular saying: “Man is descended from the apes.” +</p> + +<p> +In the very first exposition of his profound natural classification (1735) +Linné +<span class='pagenum'><a name="Page_258" id="Page_258"></a></span> +placed the anthropoid mammals at the head of the animal kingdom, with three +genera: man, the ape, and the sloth. He afterwards called them the +“Primates”—the “lords” of the animal world; he +then also separated the lemur from the true ape, and rejected the sloth. Later +zoologists divided the order of Primates. First the Gottingen anatomist, +Blumenbach, founded a special order for man, which he called <i> Bimana</i> +(“two-handed”); in a second order he united the apes and lemurs +under the name of <i>Quadrumana</i> (“four-handed”); and a third +order was formed of the distantly-related <i>Chiroptera</i> (bats, etc.). The +separation of the Bimana and Quadrumana was retained by Cuvier and most of the +subsequent zoologists. It seems to be extremely important, but, as a matter of +fact, it is totally wrong. This was first shown in 1863 by Huxley, in his +famous <i>Man’s Place in Nature.</i> On the strength of careful +comparative anatomical research he proved that the apes are just as truly +“two-handed” as man; or, if we prefer to reverse it, that man is as +truly four-handed as the ape. He showed convincingly that the ideas of hand and +foot had been wrongly defined, and had been improperly based on physiological +instead of morphological grounds. The circumstance that we oppose the +<span class='pagenum'><a name="Page_259" id="Page_259"></a></span> +thumb to the other four fingers in our hand, and so can grasp things, seemed to +be a special distinction of the hand in contrast to the foot, in which the +corresponding great toe cannot be opposed in this way to the others. But the +apes can grasp with the hind-foot as well as the fore, and so were regarded as +quadrumanous. However, the inability to grasp that we find in the foot of +civilised man is a consequence of the habit of clothing it with tight coverings +for thousands of years. Many of the bare-footed lower races of men, especially +among the negroes, use the foot very freely in the same way as the hand. As a +result of early habit and continued practice, they can grasp with the foot (in +climbing trees, for instance) just as well as with the hand. Even new-born +infants of our own race can grasp very strongly with the great toe, and hold a +spoon with it as firmly as with the hand. Hence the physiological distinction +between hand and foot can neither be pressed very far, nor has it a scientific +basis. We must look to morphological characters. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus277"></a> +<img src="images/fig277.gif" width="319" height="344" alt="Fig.277. The +drill-baboon (Cynocephalus leucophaeus) (From Brehm.)" /> +<p class="caption">Fig. 277—<b>The drill-baboon</b> (<i>Cynocephalus +leucophæus</i>).<br/> (From <i>Brehm.</i>)</p> +</div> + +<p> +As a matter of fact, it is possible to draw such a sharp morphological +distinction—a distinction based on anatomic structure—between the +fore and hind extremity. In the formation both of the bony skeleton and of the +muscles that are connected with the hand and foot before and behind there are +material and constant differences; and these are found both in man and the ape. +For instance, the number and arrangement of the smaller bones of the hand and +foot are quite different. There are similar constant differences in the +muscles. The hind extremity always has three muscles (a short flexor muscle, a +short extensor muscle, and a long calf-muscle) that are not found in the fore +extremity. The arrangement of the muscles also is different before and behind. +These characteristic differences between the fore and hind extremities are +found in man as well as the ape. There can be no doubt, therefore, that the +ape’s foot deserves that name just as much as the human foot does, and +that all true apes are just as “bimanous” as man. The common +distinction of the apes as “quadrumanous” is altogether wrong +morphologically. +</p> + +<p> +But it may be asked whether, quite apart from this, we can find any other +features that distinguish man more sharply from the ape than the various +species of apes are distinguished from each other. Huxley gave so complete and +demonstrative a reply to this question that the opposition still raised on many +sides is absolutely without foundation. On the ground of careful comparative +anatomical research, Huxley proved that in all morphological respects the +differences between the highest and lowest apes are greater than the +corresponding differences between the highest apes and man. He thus restored +Linné’s order of the Primates (excluding the bats), and divided it into +three sub-orders, the first composed of the half-apes (<i>Lemuridæ</i>), the +second of the true apes (<i>Simiadæ</i>), the third of men (<i>Anthropidæ</i>). +</p> + +<p> +But, as we wish to proceed quite consistently and impartially on the laws of +systematic logic, we may, on the strength of Huxley’s own law, go a good +deal farther in this division. We are justified in going at least one important +step farther, and assigning man his natural place within one of the sections of +the order of apes. All the features that characterise this group of apes are +found in man, and not found in the other apes. We do not seem to be justified, +therefore, in founding for man a special order distinct from the apes. +</p> + +<p> +The order of the true apes (<i>Simiæ</i> or <i> Pitheca</i>)—excluding +the lemurs—has long been divided into two principal groups, which also +differ in their geographical distribution. One group (<i>Hesperopitheca,</i> or +western apes) live in America. The other group, to which man belongs, are the +<i> Eopitheca</i> or eastern apes; they are found in Asia and Africa, and were +formerly in Europe. All the eastern apes agree with man in the features that +are chiefly used in zoological classification to distinguish between the two +simian groups, especially in the dentition. The objection might be raised that +the teeth are too subordinate an organ physiologically for us to lay stress on +them in so important a question. But there is a good reason for it; it is with +perfect justice that zoologists have for more than a century paid particular +attention to the teeth in the systematic division and arrangement of the orders +of mammals. The number, form, and arrangement of the teeth are much more +faithfully inherited in the various orders than most other characters. +</p> + +<p> +Hence the form of dentition in man is very important. In the fully developed +<span class='pagenum'><a name="Page_260" id="Page_260"></a></span> +condition we have thirty-two teeth; of these eight are incisors, four canine, +and twenty molars. The eight incisors, in the middle of the jaws, have certain +characteristic differences above and below. In the upper jaw the inner incisors +are larger than the outer; in the lower jaw the inner are the smaller. Next to +these, at each side of both jaws, is a canine (or “eye tooth”), +which is larger than the incisors. Sometimes it is very prominent in man, as it +is in most apes and many of the other mammals, and forms a sort of tusk. Next +to this there are five molars above and below on each side, the first two of +which (the +<span class='pagenum'><a name="Page_261" id="Page_261"></a></span> +“pre-molars”) are small, have only one root, and are included in +the change of teeth; the three back ones are much larger, have two roots, and +only come with the second teeth. The apes of the Old World, or all the living +or fossil apes of Asia, Africa, and Europe, have the same dentition as man. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus278"></a> +<img src="images/fig278.gif" width="480" height="294" alt="Figs. 278 to 282. +Skeletons of a man and the four anthropoid apes. Fig. 278. Gibbon (Hylobates). +Fig. 279. Orang (Satyrus). Fig. 280. Chimpanzee (Anthropithecus). Fig. 281. +Gorilla (Gorilla). Fig. 282. Man (Homo)." /> +<p class="caption">Fig. 278–282—<b>Skeletons of a man and the four +anthropoid apes.</b><br/> (Fig. 278, Gibbon; Fig. 279, Orang; Fig. 280, +Chimpanzee; Fig. 281, Gorilla; Fig. 282, Man.<br/> (From <i>Huxley.</i>) Cf. +Figs. 203–209.</p> +</div> + +<p> +On the other hand, all the American apes have an additional pre-molar in each +half of the jaw. They have six molars above and below on each side, or +thirty-six teeth altogether. This characteristic difference between the eastern +and western apes has been so faithfully inherited that it is very instructive +for us. It is true that there seems to be an exception in the case of a small +family of South American apes. The small silky apes (<i>Arctopitheca</i> or +<i>Hapalidæ</i>), which include the tamarin (<i>Midas</i>) and the brush-monkey +(<i>Jacchus</i>), have only five molars in each half of the jaw (instead of +six), and so seem to be nearer to the eastern apes. But it is found, on closer +examination, that they have three premolars, like all the western apes, and +that only the last molar has been lost. Hence the apparent exception really +confirms the above distinction. +</p> + +<p> +Of the other features in which the two groups of apes differ, the structure of +the nose is particularly instructive and conspicuous. All the eastern apes have +the same type of nose as man—a comparatively narrow partition between the +two halves, so that the nostrils run downwards. In some of them the nose +protrudes as far as in man, and has the same characteristic structure. We have +already alluded to the curious long-nosed apes, which have a long, +finely-curved nose. Most of the eastern apes have, it is true, rather flat +noses, like, for instance, the white-nosed monkey (Fig. 276); but the nasal +partition is thin and narrow in them all. The American apes have a different +type of nose. The partition is very broad and thick at the bottom, and the +wings of the nostrils are not developed, so that they point outwards instead of +downwards. This difference in the form of the nose is so constantly inherited +in both groups that the apes of the New World are called +“flat-nosed” (<i>Platyrrhinæ</i>), and those of the Old World +“narrow-nosed” (<i>Catarrhinæ</i>). The bony passage of the ear (at +the bottom of which is the tympanum) is short and wide in all the Platyrrhines, +but long and narrow in all the Catarrhines; and in man this difference also is +significant. +</p> + +<p> +This division of the apes into Platyrrhines and Catarrhines, on the ground of +the above hereditary features, is now generally admitted in zoology, and +receives strong support from the geographical distribution of the two groups in +the east and west. It follows at once, as regards the phylogeny of the apes, +that two divergent lines proceeded from the common stem-form of the ape-order +in the early Tertiary period, one of which spread over the Old, the other over +the New, World. It is certain that all the Platyrrhines come of one stock, and +also all the Catarrhines; but the former are phylogenetically older, and must +be regarded as the stem-group of the latter. +</p> + +<p> +What can we deduce from this with regard to our own genealogy? Man has just the +same characters, the same form of dentition, auditory passage, and nose, as all +the Catarrhines; in this he radically differs from the Platyrrhines. We are +thus forced to assign him a position among the eastern apes in the order of +Primates, or at least place him alongside of them. But it follows that man is a +direct blood relative of the apes of the Old World, and can be traced to a +common stem-form together with all the Catarrhines. In his whole organisation +and in his origin man is a true Catarrhine; he originated in the Old World from +an unknown, extinct group of the eastern apes. The apes of the New World, or +the Platyrrhines, form a divergent branch of our genealogical tree, and this is +only distantly related at its root to the human race. We must assume, of +course, that the earliest Eocene apes had the full dentition of the +Platyrrhines; hence we may regard this stem-group as a special stage (the +twenty-sixth) in our ancestry, and deduce from it (as the twenty-seventh stage) +the earliest Catarrhines. +</p> + +<p> +We have now reduced the circle of our nearest relatives to the small and +comparatively scanty group that is represented by the sub-order of the +Catarrhines; and we are in a position to answer the question of man’s +place in this sub-order, and say whether we can deduce anything further from +this position as to our immediate ancestors. In answering this question the +comprehensive and able studies that Huxley gives of +<span class='pagenum'><a name="Page_262" id="Page_262"></a></span> +the comparative anatomy of man and the various Catarrhines in his +<i>Man’s Place in Nature</i> are of great assistance to us. It is quite +clear from these that the differences between man and the highest Catarrhines +(gorilla, chimpanzee, and orang) are in every respect slighter than the +corresponding differences between the highest and the lowest Catarrhines +(white-nosed monkey, macaco, baboon, etc.). In fact, within the small group of +the tail-less anthropoid apes the differences between the various genera are +not less than the differences between them and man. This is seen by a glance at +the skeletons that Huxley has put together (Figs. 278–282). Whether we +take the skull or the vertebral column or the ribs or the fore or hind limbs, +or whether we extend the comparison to the muscles, blood-vessels, brain, +placenta, etc., we always reach the same result on impartial +examination—that man is not more different from the other Catarrhines +than the extreme forms of them (for instance, the gorilla and baboon) differ +from each other. We may now, therefore, complete the Huxleian law we have +already quoted with the following thesis: “Whatever system of organs we +take, a comparison of their modifications in the series of Catarrhines always +leads to the same conclusion; the anatomic differences that separate man from +the most advanced Catarrhines (orang, gorilla, chimpanzee) are not as great as +those that separate the latter from the lowest Catarrhines (white-nosed monkey, +macaco, baboon).” +</p> + +<p> +We must, therefore, consider the descent of man from other Catarrhines to be +fully proved. Whatever further information on the comparative anatomy and +ontogeny of the living Catarrhines we may obtain in the future, it cannot +possibly disturb this conclusion. Naturally, our Catarrhine ancestors must have +passed through a long series of different forms before the human type was +produced. The chief advances that effected this “creation of man,” +or his differentiation from the nearest related Catarrhines, were: the adoption +of the erect posture and the consequent greater differentiation of the fore and +hind limbs, the evolution of articulate speech and its organ, the larynx, and +the further development of the brain and its function, the soul; sexual +selection had a great influence in this, as Darwin showed in his famous work. +</p> + +<p> +With an eye to these advances we can distinguish at least four important stages +in our simian ancestry, which represent prominent points in the historical +process of the making of man. We may take, after the Lemurs, the earliest and +lowest Platyrrhines of South America, with thirty-six teeth, as the +twenty-sixth stage of our genealogy; they were developed from the Lemurs by a +peculiar modification of the brain, teeth, nose, and fingers. From these Eocene +stem-apes were formed the earliest Catarrhines or eastern apes, with the human +dentition (thirty-two teeth), by modification of the nose, lengthening of the +bony channel of the ear, and the loss of four pre-molars. These oldest +stem-forms of the whole Catarrhine group were still thickly coated with hair, +and had long tails—baboons (<i>Cynopitheca</i>) or tailed apes +(<i>Menocerca,</i> Fig. 276). They lived during the Tertiary period, and are +found fossilised in the Miocene. Of the actual tailed apes perhaps the nearest +to them are the <i> Semnopitheci.</i> +</p> + +<p> +If we take these Semnopitheci as the twenty-seventh stage in our ancestry, we +may put next to them, as the twenty-eighth, the tail-less anthropoid apes. This +name is given to the most advanced and man-like of the existing Catarrhines. +They were developed from the other Catarrhines by losing the tail and part of +the hair, and by a higher development of the brain, which found expression in +the enormous growth of the skull. Of this remarkable family there are only a +few genera to-day, and we have already dealt with them (Chapter XV)—the +gibbon (<i>Hylobates,</i> Fig. 203) and orang (<i>Satyrus,</i> Figs. 204, 205) +in South-Eastern Asia and the Archipelago; and the chimpanzee +(<i>Anthropithecus,</i> Figs. 206, 207) and gorilla (<i>Gorilla,</i> Fig. 208) +in Equatorial Africa. +</p> + +<p> +The great interest that every thoughtful man takes in these nearest relatives +of ours has found expression recently in a fairly large literature. The most +distinguished of these works for impartial treatment of the question of +affinity is Robert Hartmann’s little work on <i>The Anthropoid Apes.</i> +Hartmann divides the primate order into two families: (1) <i> Primarii</i> (man +and the anthropoid apes); and (2) <i> Simianæ</i> (true apes, Catarrhines and +Platyrrhines). Professor Klaatsch, of Heidelberg, has advanced a different view +in his interesting and richly illustrated work on <i>The Origin and Development +of the Human</i> +<span class='pagenum'><a name="Page_263" id="Page_263"></a></span> +<i>Race.</i> This is a substantial supplement to my <i>Anthropogeny,</i> in so +far as it gives the chief results of modern research on the early history of +man and civilisation. But when Klaatsch declares the descent of man from the +apes to be “irrational, narrow-minded, and false,” in the belief +that we are thinking of some living species of ape, we must remind him that no +competent scientist has ever held so narrow a view. All of us look +merely—in the sense of Lamarck and Darwin—to the original unity +(admitted by Klaatsch) of the primate stem. This common descent of all the +Primates (men, apes, and lemurs) from one primitive stem-form, from which the +most far-reaching conclusions follow for the whole of anthropology and +philosophy, is admitted by Klaatsch as well as by myself and all other +competent zoologists who accept the theory of evolution in general. He says +explicitly (p. 172): “The three anthropoid apes—gorilla, +chimpanzee, and orang—seem to be branches from a common root, and this +was not far from that of the gibbon and man.” That is in the main the +opinion that I have maintained (especially against Virchow) in a number of +works ever since 1866. The hypothetical common ancestor of all the Primates, +which must have lived in the earliest Tertiary period (more probably in the +Cretaceous), was called by me <i>Archiprimus</i>; Klaatsch now calls it +<i>Primatoid.</i> Dubois has proposed the appropriate name of +<i>Prothylobates</i> for the common and much younger stem-form of the +anthropomorpha (man and the anthropoid apes). The actual <i> Hylobates</i> is +nearer to it than the other three existing anthropoids. None of these can be +said to be absolutely the most man-like. The gorilla comes next to man in the +structure of the hand and foot, the chimpanzee in the chief features of the +skull, the orang in brain development, and the gibbon in the formation of the +chest. None of these existing anthropoid apes is among the direct ancestors of +our race; they are scattered survivors of an ancient branch of the Catarrhines, +from which the human race developed in a particular direction. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus283"></a> +<img src="images/fig283.gif" width="289" height="202" alt="Fig.283. Skull of the +fossil ape-man of Java (Pithecanthropus erectus), restored by Eugen Dubois." /> +<p class="caption">Fig. 283—<b>Skull of the fossil ape-man of +Java</b> (<i>Pithecanthropus erectus</i>), restored by <i>Eugen +Dubois.</i></p> +</div> + +<p> +Although man is directly connected with this anthropoid family and originates +from it, we may assign an important intermediate form between the +<i>Prothylobates</i> and him (the twenty-ninth stage in our ancestry), the +ape-men (<i>Pithecanthropi</i>). I gave this name in the <i>History of +Creation</i> to the “speechless primitive men” (<i>Alali</i>), +which were men in the ordinary sense as far as the general structure is +concerned (especially in the differentiation of the limbs), but lacked one of +the chief human characteristics, articulate speech and the higher intelligence +that goes with it, and so had a less developed brain. The phylogenetic +hypothesis of the organisation of this “ape-man” which I then +advanced was brilliantly confirmed twenty-four years afterwards by the famous +discovery of the fossil <i>Pithecanthropus erectus</i> by Eugen Dubois (then +military surgeon in Java, afterwards professor at Amsterdam). In 1892 he found +at Trinil, in the residency of Madiun in Java, in Pliocene deposits, certain +remains of a large and very man-like ape (roof of the skull, femur, and teeth), +which he described as “an erect ape-man” and a survivor of a +“stem-form of man” (Fig. 283). Naturally, the Pithecanthropus +excited the liveliest interest, as the long-sought transitional form between +man and the ape: we seemed to have found “the missing link.” There +were very interesting scientific discussions of it at the last three +International Congresses of Zoology (Leyden, 1895, Cambridge, 1898, and Berlin, +1901). I took an active part in the discussion at +<span class='pagenum'><a name="Page_264" id="Page_264"></a></span> +Cambridge, and may refer the reader to the paper I read there on “The +Present Position of Our Knowledge of the Origin of Man” (translated by +Dr. Gadow with the title of <i> The Last Link</i>). +</p> + +<p> +An extensive and valuable literature has grown up in the last ten years on the +Pithecanthropus and the pithecoid theory connected with it. A number of +distinguished anthropologists, anatomists, paleontologists, and phylogenists +have taken part in the controversy, and made use of the important data +furnished by the new science of pre-historic research. Hermann Klaatsch has +given a good summary of them, with many fine illustrations, in the +above-mentioned work. I refer the reader to it as a valuable supplement to the +present work, especially as I cannot go any further here into these +anthropological and pre-historic questions. I will only repeat that I think he +is wrong in the attitude of hostility that he affects to take up with regard to +my own views on the descent of man from the apes. +</p> + +<p> +The most powerful opponent of the pithecoid theory—and the theory of +evolution in general—during the last thirty years (until his death in +September, 1902) was the famous Berlin anatomist, Rudolf Virchow. In the +speeches which he delivered every year at various congresses and meetings on +this question, he was never tired of attacking the hated “ape +theory.” His constant categorical position was: “It is quite +certain that man does not descend from the ape or any other animal.” This +has been repeated incessantly by opponents of the theory, especially +theologians and philosophers. In the inaugural speech that he delivered in 1894 +at the Anthropological Congress at Vienna, he said that “man might just +as well have descended from a sheep or an elephant as from an ape.” +Absurd expressions like this only show that the famous pathological anatomist, +who did so much for medicine in the establishment of cellular pathology, had +not the requisite attainments in comparative anatomy and ontogeny, systematic +zoology and paleontology, for sound judgment in the province of anthropology. +The Strassburg anatomist, Gustav Schwalbe, deserved great praise for having the +moral courage to oppose this dogmatic and ungrounded teaching of Virchow, and +showing its untenability. The recent admirable works of Schwalbe on the +Pithecanthropus, the earliest races of men, and the Neanderthal skull +(1897–1901) will supply any candid and judicious reader with the +empirical material with which he can convince himself of the baselessness of +the erroneous dogmas of Virchow and his clerical friends (J. Ranke, J. +Bumüller, etc.). +</p> + +<p> +As the Pithecanthropus walked erect, and his brain (judging from the capacity +of his skull, Fig. 283) was midway between the lowest men and the anthropoid +apes, we must assume that the next great step in the advance from the +Pithecanthropus to man was the further development of human speech and reason. +</p> + +<p> +Comparative philology has recently shown that human speech is polyphyletic in +origin; that we must distinguish several (probably many) different primitive +tongues that were developed independently. The evolution of language also +teaches us (both from its ontogeny in the child and its phylogeny in the race) +that human speech proper was only gradually developed after the rest of the +body had attained its characteristic form. It is probable that language was not +evolved until after the dispersal of the various species and races of men, and +this probably took place at the commencement of the Quaternary or Diluvial +period. The speechless ape-men or <i>Alali</i> certainly existed towards the +end of the Tertiary period, during the Pliocene, possibly even the Miocene, +period. +</p> + +<p> +The third, and last, stage of our animal ancestry is the true or speaking man +(<i>Homo</i>), who was gradually evolved from the preceding stage by the +advance of animal language into articulate human speech. As to the time and +place of this real “creation of man” we can only express tentative +opinions. It was probably during the Diluvial period in the hotter zone of the +Old World, either on the mainland in tropical Africa or Asia or on an earlier +continent (Lemuria—now sunk below the waves of the Indian Ocean), which +stretched from East Africa (Madagascar, Abyssinia) to East Asia (Sunda Islands, +Further India). I have given fully in my <i>History of Creation,</i> (chapter +xxviii) the weighty reasons for claiming this descent of man from the +anthropoid eastern apes, and shown how we may conceive the spread of the +various races from this “Paradise” over the whole earth. I have +also dealt fully with the relations of the various races and species of men to +each other. +<span class='pagenum'><a name="Page_265" id="Page_265"></a></span> +</p> + +<h4>SYNOPSIS OF THE CHIEF SECTIONS OF OUR STEM-HISTORY</h4> + +<p class="center"> +First Stage: <b>The Protists</b> +</p> + +<p> +Man’s ancestors are unicellular protozoa, originally unnucleated Monera +like the Chromacea, structureless green particles of plasm; afterwards real +nucleated cells (first plasmodomous <i>Protophyta,</i> like the Palmella; then +plasmophagous <i>Protozoa,</i> like the Amœba). +</p> + +<p class="center"> +Second Stage: <b>The Blastæads</b> +</p> + +<p> +Man’s ancestors are round cœnobia or colonies of Protozoa; they consist +of a close association of many homogeneous cells, and thus are individuals of +the second order. They resemble the round cell-communities of the Magospheræ +and Volvocina, equivalent to the ontogenetic blastula: hollow globules, the +wall of which consists of a single layer of ciliated cells (blastoderm). +</p> + +<p class="center"> +Third Stage: <b>The Gastræads</b> +</p> + +<p +>Man’s ancestors are Gastræads, like the simplest of the +actual Metazoa (Prophysema, Olynthus, Hydra, Pemmatodiscus). Their body +consists merely of a primitive gut, the wall of which is made up of the two +primary germinal layers. +</p> + +<p class="center"> +Fourth Stage: <b>The Platodes</b> +</p> + +<p> +Man’s ancestors have substantially the organisation of simple Platodes +(at first like the cryptocœlic Platodaria, later like the rhabdocœlic +Turbellaria). The leaf-shaped bilateral-symmetrical body has only one +gut-opening, and develops the first trace of a nervous centre from the ectoderm +in the middle line of the back (Figs. 239, 240). +</p> + +<p class="center"> +Fifth Stage: <b>The Vermalia</b> +</p> + +<p> +Man’s ancestors have substantially the organisation of unarticulated +Vermalia, at first Gastrotricha (Ichthydina), afterwards Frontonia (Nemertina, +Enteropneusta). Four secondary germinal layers develop, two middle layers +arising between the limiting layers (cœloma). The dorsal ectoderm forms the +vertical plate, acroganglion (Fig. 243). +</p> + +<p class="center"> +Sixth Stage: <b>The Prochordonia</b> +</p> + +<p> +Man’s ancestors have substantially the organisation of a simple +unarticulated Chordonium (Copelata and Ascidia-larvæ). The unsegmented chorda +develops between the dorsal medullary tube and the ventral gut-tube. The simple +cœlom-pouches divide by a frontal septum into two on each side; the dorsal +pouch (episomite) forms a muscle-plate; the ventral pouch (hyposomite) forms a +gonad. Head-gut with gill-clefts. +</p> + +<p class="center"> +Seventh Stage: <b>The Acrania</b> +</p> + +<p> +Man’s ancestors are skull-less Vertebrates, like the Amphioxus. The body +is a series of metamera, as several of the primitive segments are developed. +The head contains in the ventral half the branchial gut, the trunk the hepatic +gut. The medullary tube is still simple. No skull, jaws, or limbs. +</p> + +<p class="center"> +Eighth Stage: <b>The Cyclostoma</b> +</p> + +<p> +Man’s ancestors are jaw-less Craniotes (like the Myxinoida and +Petromyzonta). The number of metamera increases. The fore-end of the medullary +tube expands into a vesicle and forms the brain, which soon divides into five +cerebral vesicles. In the sides of it appear the three higher sense-organs: +nose, eyes, and auditory vesicles. No jaws, limbs, or floating bladder. +</p> + +<p class="center"> +Ninth Stage: <b>The Ichthyoda</b> +</p> + +<p> +Man’s ancestors are fish-like Craniotes: (1) Primitive fishes (Selachii); +(2) plated fishes (Ganoida); (3) amphibian fishes (Dipneusta); (4) mailed +amphibia (Stegocephala). The ancestors of this series develop two pairs of +limbs: a pair of fore (breast-fins) and of hind (belly-fins) legs. The +gill-arches are formed between the gill-clefts: the first pair form the +maxillary arches (the upper and lower jaws). The floating bladder (lung) and +pancreas grow out of the gut. +</p> + +<p class="center"> +Tenth Stage: <b>The Amniotes</b> +</p> + +<p> +Man’s ancestors are Amniotes or gill-less Vertebrates: (1) Primitive +Amniotes (Proreptilia); (2) Sauromammals; (3) Primitive Mammals (Monotremes); +(4) Marsupials; (5) Lemurs (Prosimiæ); (6) Western apes (Platyrrhinæ); (7) +Eastern apes (Catarrhinæ): at first tailed Cynopitheca; then tail-less +anthropoids; later speechless ape-men (Alali); finally speaking man. The +ancestors of these Amniotes develop an amnion and allantois, and gradually +assume the mammal, and finally the specifically human, form. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap24"></a> +<span class='pagenum'><a name="Page_266" id="Page_266"></a></span> +Chapter XXIV.<br/> +EVOLUTION OF THE NERVOUS SYSTEM</h2> + +<p> +The previous chapters have taught us how the human body as a whole develops +from the first simple rudiment, a single layer of cells. The whole human race +owes its origin, like the individual man, to a simple cell. The unicellular +stem-form of the race is reproduced daily in the unicellular embryonic stage of +the individual. We have now to consider in detail the evolution of the various +parts that make up the human frame. I must, naturally, confine myself to the +most general and principal outlines; to make a special study of the evolution +of each organ and tissue is both beyond the scope of this work, and probably +beyond the anatomic capacity of most of my readers to appreciate. In tracing +the evolution of the various organs we shall follow the method that has +hitherto guided us, except that we shall now have to consider the ontogeny and +phylogeny of the organs together. We have seen, in studying the evolution of +the body as a whole, that phylogeny casts a light over the darker paths of +ontogeny, and that we should be almost unable to find our way in it without the +aid of the former. We shall have the same experience in the study of the organs +in detail, and I shall be compelled to give simultaneously their ontogenetic +and phylogenetic origin. The more we go into the details of organic +development, and the more closely we follow the rise of the various parts, the +more we see the inseparable connection of embryology and stem-history. The +ontogeny of the organs can only be understood in the light of their phylogeny, +just as we found of the embryology of the whole body. Each embryonic form is +determined by a corresponding stem-form. This is true of details as well as of +the whole. +</p> + +<p> +We will consider first the animal and then the vegetal systems of organs of the +body. The first group consists of the psychic and the motor apparatus. To the +former belong the skin, the nervous system, and the sense-organs. The motor +apparatus is composed of the passive and the active organs of movement (the +skeleton and the muscles). The second or vegetal group consists of the +nutritive and the reproductive apparatus. To the nutritive apparatus belong the +alimentary canal with all its appendages, the vascular system, and the renal +(kidney) system. The reproductive apparatus comprises the different organs of +sex (embryonic glands, sexual ducts, and copulative organs). +</p> + +<p> +As we know from previous chapters (XI–XIII), the animal systems of organs +(the organs of sensation and presentation) develop for the most part out of the +<i>outer</i> primary germ-layer, or the cutaneous (skin) layer. On the other +hand, the vegetal systems of organs arise for the most part from the +<i>inner</i> primary germ-layer, the visceral layer. It is true that this +antithesis of the animal and vegetal spheres of the body in man and all the +higher animals is by no means rigid; several parts of the animal apparatus (for +instance, the greater part of the muscles) are formed from cells that come +originally from the entoderm; and a great part of the vegetative apparatus (for +instance, the mouth-cavity and the gonoducts) are composed of cells that come +from the ectoderm. +</p> + +<p> +In the more advanced animal body there is so much interlacing and displacement +of the various parts that it is often very difficult to indicate the sources of +them. But, broadly speaking, we may take it as a positive and important fact +that in man and the higher animals the chief part of the animal organs comes +from the ectoderm, and the greater part of the vegetative organs from the +entoderm. It was for this reason that Carl Ernst von Baer called the one the +animal and the other the vegetative layer (see p. 16). +</p> + +<p> +The solid foundation of this important thesis is the <i>gastrula,</i> the most +instructive embryonic form in the animal world, which we still find in the same +shape in the most diverse classes of animals. This form points demonstrably to +a +<span class='pagenum'><a name="Page_267" id="Page_267"></a></span> +common stem-form of all the Metazoa, the <i>Gastræa;</i> in this long-extinct +stem-form the whole body consisted throughout life of the two primary germinal +layers, as is now the case temporarily in the gastrula; in the Gastræa the +simple cutaneous (skin) layer <i>actually</i> represented all the animal organs +and functions, and the simple visceral (gut) layer all the vegetal organs and +functions. This is the case with the modern Gastræads (Fig. 233); and it is +also the case potentially with the gastrula. +</p> + +<p> +We shall easily see that the gastræa theory is thus able to throw a good deal +of light, both morphologically and physiologically, on some of the chief +features of embryonic development, if we take up first the consideration of the +chief element in the animal sphere, the psychic apparatus or sensorium and its +evolution. This apparatus consists of two very different parts, which seem at +first to have very little connection with each other—the outer skin, with +all its hairs, nails, sweat-glands, etc., and the nervous system. The latter +comprises the central nervous system (brain and spinal cord), the peripheral, +cerebral, and spinal nerves, and the sense-organs. In the fully-formed +vertebrate body these two chief elements of the sensorium lie far apart, the +skin being external to, and the central nervous system in the very centre of, +the body. The one is only connected with the other by a section of the +peripheral nervous system and the sense-organs. Nevertheless, as we know from +human embryology, the medullary tube is formed from the cutaneous layer. The +organs that discharge the most advanced functions of the animal body—the +organs of the soul, or of psychic life—develop from the external skin. +This is a perfectly natural and necessary process. If we reflect on the +historical evolution of the psychic and sensory functions, we are forced to +conclude that the cells which accomplish them must originally have been located +on the outer surface of the body. Only elementary organs in this superficial +position could directly receive the influences of the environment. Afterwards, +under the influence of natural selection, the cellular group in the skin which +was specifically “sensitive” withdrew into the inner and more +protected part of the body, and formed there the foundation of a central +nervous organ. As a result of increased differentiation, the skin and the +central nervous system became further and further separated, and in the end the +two were only permanently connected by the afferent peripheral sensory nerves. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus284"></a> +<img src="images/fig284.gif" width="244" height="270" alt="Fig.244. The human skin +in vertical section." /> +<p class="caption">Fig. 284—<b>The human skin in vertical section</b> +(from <i>Ecker</i>), highly magnified, <i>a</i> horny layer of the epidermis, +<i>b</i> mucous layer of the epidermis, <i>c</i> papillæ of the corium, +<i>d</i> blood-vessels of same, <i>ef</i> ducts of the sweat-glands (<i>g</i>), +<i>h</i> fat-glands in the corium, <i>i</i> nerve, passing into a tactile +corpuscle above.</p> +</div> + +<p> +The observations of the comparative anatomist are in complete accord with this +view. He tells us that large numbers of the lower animals have no nervous +system, though they exercise the functions of sensation and will like the +higher animals. In the unicellular Protozoa, which do not form germinal layers, +there is, of course, neither nervous system nor skin. But in the second +division of the animal kingdom also, the Metazoa, there is at first no nervous +system. Its functions are represented by the simple cell-layer of the ectoderm, +which the lower Metazoa have inherited from the Gastræa (Fig. 30 <i>e</i>). We +find this in the lowest Zoophytes—the Gastræads, Physemaria, and Sponges +(Figs. 233–238). The lowest Cnidaria (the hydroid polyps) also are little +superior to the Gastræads in structure. Their vegetative functions are +accomplished by the simple visceral layer, and their animal functions by the +simple cutaneous layer. In these +<span class='pagenum'><a name="Page_268" id="Page_268"></a></span> +cases the simple cell-layer of the ectoderm is at once skin, locomotive +apparatus, and nervous system. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus285"></a> +<img src="images/fig285.gif" width="136" height="110" alt="Fig.285. Epidermic cells +of a human embryo of two months." /> +<p class="caption">Fig. 285—<b>Epidermic cells</b> of a human embryo +of two months. (From <i>Kölliker.</i>)</p> +</div> + +<p> +When we come to the higher Metazoa, in which the sensory functions and their +organs are more advanced, we find a division of labour among the ectodermic +cells. Groups of sensitive nerve cells separate from the ordinary epidermic +cells; they retire into the more protected tissue of the mesodermic under-skin, +and form special neural ganglia there. Even in the Platodes, especially the +<i>Turbellaria,</i> we find an independent nervous system, which has separated +from the outer skin. This is the “upper pharyngeal ganglion,” or +<i>acroganglion,</i> situated above the gullet (Fig. 241 <i>g</i>).From this +rudimentary structure has been developed the elaborate central nervous system +of the higher animals. In some of the higher worms, such as the earth-worm, the +first rudiment of the central nervous system (Fig. 74 <i>n</i>) is a local +thickening of the skin-sense layer (<i>hs</i>), which afterwards separates +altogether from the horny plate. In the earliest Platodes (<i>Cryptocœla</i>) +and Vermalia (<i>Gastrotricha</i>) the acroganglion remains in the epidermis. +But the medullary tube of the Vertebrates originates in the same way. Our +embryology has taught us that this first structure of the central nervous +system also develops originally from the outer germinal layer. +</p> + +<p> +Let us now examine more closely the evolution of the human skin, with its +various appendages, the hairs and glands. This external covering has, +physiologically, a double and important part to play. It is, in the first +place, the common integument that covers the whole surface of the body, and +forms a protective envelope for the other organs. As such it also effects a +certain exchange of matter between the body and the surrounding atmosphere +(exhalation, perspiration). In the second place, it is the earliest and +original sense organ, the common organ of feeling that experiences the +sensation of the temperature of the environment and the pressure or resistance +of bodies that come into contact. +</p> + +<p> +The human skin (like that of all the higher animals) is composed of two layers, +the outer and the inner or underlying skin. The outer skin or <i>epidermis,</i> +consists of simple ectodermic cells, and contains no blood-vessels (Fig. 284 +<i>a, b</i>). It develops from the outer germinal layer, or skin-sense layer. +The underlying skin (<i>corium</i> or <i>hypodermis</i>) consists chiefly of +connective tissue, contains numerous blood-vessels and nerves, and has a +totally different origin. It comes from the outermost parietal stratum of the +middle germinal layer, or the skin-fibre layer. The corium is much thicker than +the epidermis. In its deeper strata (the <i>subcutis</i>) there are clusters of +fat-cells (Fig. 284 <i>h</i>). Its uppermost stratum (the cutis proper, or the +papillary stratum) forms, over almost the whole surface of the body, a number +of conical microscopic papillæ (something like warts), which push into the +overlying epidermis (<i>c</i>). These tactile or sensory particles contain the +finest sensory organs of the skin, the touch corpuscles. Others contain merely +end-loops of the blood-vessels that nourish the skin (<i>c, d</i>). The various +parts of the corium arise by division of labour from the originally homogeneous +cells of the cutis-plate, the outermost lamina of the mesodermic skin-fibre +layer (Fig. 145 <i>hpr,</i> and Figs. 161, 162 <i>cp</i>). +</p> + +<p> +In the same way, all the parts and appendages of the epidermis develop by +differentiation from the homogeneous cells of this horny plate (Fig. 285). At +an early stage the simple cellular layer of this horny plate divides into two. +The inner and softer stratum (Fig. 284 <i>b</i>) is known as the mucous +stratum, the outer and harder (<i>a</i>) as the horny (corneous) stratum. This +horny layer is being constantly used up and rubbed away at the surface; new +layers of cells grow up in their place out of the underlying mucous stratum. At +first the epidermis is a simple covering of the surface of the body. Afterwards +various appendages develop from it, some internally, others externally. The +internal appendages are the cutaneous glands—sweat, fat, etc. +<span class='pagenum'><a name="Page_269" id="Page_269"></a></span> +The external appendages are the hairs and nails. +</p> + +<p> +The cutaneous glands are originally merely solid cone-shaped growths of the +epidermis, which sink into the underlying corium (Fig. 286 <i>1</i>). +Afterwards a canal (<i>2, 3</i>) is formed inside them, either by the softening +and dissolution of the central cells or by the secretion of fluid internally. +Some of the glands, such as the sudoriferous, do not ramify (Fig. 284 +<i>efg</i>). These glands, which secrete the perspiration, are very long, and +have a spiral coil at the end, but they never ramify; so also the wax-glands of +the ears. Most of the other cutaneous glands give out buds and ramify; thus, +for instance, the lachrymal glands of the upper eye-lid that secrete tears +(Fig. 286), and the sebaceous glands which secrete the fat in the skin and +generally open into the hair-follicles. Sudoriferous and sebaceous glands are +found only in mammals. But we find lachrymal glands in all the three classes of +Amniotes—reptiles, birds, and mammals. They are wanting in the lower +aquatic vertebrates. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus286"></a> +<img src="images/fig286.gif" width="155" height="232" alt="Fig.286. Rudimentary +lachrymal glands from a human embryo of four months." /> +<p class="caption">Fig. 286—<b>Rudimentary lachrymal glands</b> from +a human embryo of four months. (From <i>Kölliker.</i>) <i>1</i> earliest +structure, in the shape of a simple solid cone, <i>2</i> and <i>3</i> more +advanced structures, ramifying and hollowing out. <i>a</i> solid buds, <i>e</i> +cellular coat of the hollow buds, <i>f</i> structure of the fibrous envelope, +which afterwards forms the corium about the glands.</p> +</div> + +<p> +The mammary glands (Figs. 287, 288) are very remarkable; they are found in all +mammals, and in these alone. They secrete the milk for the feeding of the +new-born mammal. In spite of their unusual size these structures are nothing +more than large sebaceous glands in the skin. The milk is formed by the +liquefaction of the fatty milk-cells inside the branching mammary-gland tubes +(Fig. 287 <i>c</i>), in the same way as the skin-grease or hair-fat, by the +solution of fatty cells inside the sebaceous glands. The outlets of the mammary +glands enlarge and form sac-like mammary ducts (<i>b</i>); these narrow again +(<i>a</i>), and open in the teats or nipples of the breast by sixteen to +twenty-four fine apertures. The first structure of this large and elaborate +gland is a very simple cone in the epidermis, which penetrates into the corium +and ramifies. In the new-born infant it consists of twelve to eighteen +radiating lobes (Fig. 288). These gradually ramify, their ducts become hollow +and larger, and rich masses of fat accumulate between the lobes. Thus is formed +the prominent female breast (<i>mamma</i>), on the top of which rises the teat +or nipple (<i>mammilla</i>). The latter is only developed later on, when the +mammary gland is fully-formed; and this ontogenetic phenomenon is extremely +interesting, because the earlier mammals (the stem-forms of the whole class) +have no teats. In them the milk comes out through a flat portion of the ventral +skin that is pierced like a sieve, as we still find in the lowest living +mammals, the oviparous Monotremes of Australia. The young animal licks the milk +from the mother instead of sucking it. In many of the lower mammals we find a +number of milk-glands at different parts of the ventral surface. In the human +female there is usually only one pair of glands, at the breast; and it is the +same with the apes, bats, elephants, and several other mammals. Sometimes, +however, we find two successive pairs of glands (or even more) in the human +female. Some women have four or five pairs of breasts, like pigs and hedgehogs +(Fig. 103). This polymastism points back to an older stem-form. We often find +these accessory breasts in the male also (Fig. 103 <i>D</i>). Sometimes, +moreover, the normal mammary glands are fully developed and can suckle in the +male; but as a rule they are merely rudimentary organs without functions in the +male. We have already (Chapter XI) dealt with this remarkable and interesting +instance of atavism. +</p> + +<p> +While the cutaneous glands are inner growths of the epidermis, the appendages +<span class='pagenum'><a name="Page_270" id="Page_270"></a></span> +which we call hairs and nails are external local growths in it. The nails +(<i>Ungues</i>) which form important protective structures on the back of the +most sensitive parts of our limbs, the tips of the fingers and toes, are horny +growths of the epidermis, which we share with the apes. The lower mammals +usually have claws instead of them; the ungulates, hoofs. The stem-form of the +mammals certainly had claws; we find them in a rudimentary form even in the +salamander. The horny claws are highly developed in most of the reptiles (Fig. +264), and the mammals have inherited them from the earliest representatives of +this class, the stem-reptiles (<i>Tocosauria</i>). Like the hoofs +(<i>ungulæ</i>) of the Ungulates, the nails of apes and men have been evolved +from the claws of the older mammals. In the human embryo the first rudiment of +the nails is found (between the horny and the mucous stratum of the epidermis) +in the fourth month. But their edges do not penetrate through until the end of +the sixth month. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus287"></a> +<img src="images/fig287.gif" width="167" height="243" alt="Fig.287. The female +breast (mamma) in vertical section." /> +<p class="caption">Fig. 287—<b>The female breast</b> (<i>mamma</i>) +in vertical section. <i>c</i> racemose glandular lobes, <i>b</i> enlarged +milk-ducts, a narrower outlets, which open into the nipple. (From <i>H. +Meyer.</i>)</p> +</div> + +<p> +The most interesting and important appendages of the epidermis are the hairs; +on account of their peculiar composition and origin we must regard them as +highly characteristic of the whole mammalian class. It is true that we also +find hairs in many of the lower animals, such as insects and worms. But these +hairs, like the hairs of plants, are thread-like appendages of the surface, and +differ entirely from the hairs of the mammals in the details of their structure +and development. +</p> + +<p> +The embryology of the hairs is known in all its details, but there are two +different views as to their phylogeny. On the older view the hairs of the +mammals are equivalent or homologous to the feathers of the bird or the horny +scales of the reptile. As we deduce all three classes of Amniotes from a common +stem-group, we must assume that these Permian stem-reptiles had a complete +scaly coat, inherited from their Carboniferous ancestors, the mailed amphibia +(<i>Stegocephala</i>); the bony scales of their corium were covered with horny +scales. In passing from aquatic to terrestrial life the horny scales were +further developed, and the bony scales degenerated in most of the reptiles. As +regards the bird’s feathers, it is certain that they are modifications of +the horny scales of their reptilian ancestors. But it is otherwise with the +hairs of the mammals. In their case the hypothesis has lately been advanced on +the strength of very extensive research, especially by Friedrich Maurer, that +they have been evolved from the cutaneous sense-organs of amphibian ancestors +by modification of functions; the epidermic structure is very similar in both +in its embryonic rudiments. This modern view, which had the support of the +greatest expert on the vertebrates, Carl Gegenbaur, can be harmonised with the +older theory to an extent, in the sense that both formations, scales and hairs, +were very closely connected originally. Probably the conical budding of the +skin-sense layer grew up <i>under the protection of the horny scale,</i> and +became an organ of touch subsequently by the cornification of the hairs; many +hairs are still sensory organs (tactile hairs on the muzzle and cheeks of many +mammals: pubic hairs). +</p> + +<p> +This middle position of the genetic connection of scales and hairs was advanced +in my <i>Systematic Phylogeny of the Vertebrates</i> (p. 433). It is confirmed +by the similar arrangement of the two cutaneous formations. As Maurer pointed +out, the hairs, as well as the cutaneous sense-organs and the scales, are at +first arranged in regular longitudinal series, and they afterwards break into +alternate groups. In the embryo of a bear two +<span class='pagenum'><a name="Page_271" id="Page_271"></a></span> +inches long, which I owe to the kindness of Herr von Schmertzing (of Arva +Varallia, Hungary), the back is covered with sixteen to twenty alternating +longitudinal rows of scaly protuberances (Fig. 289). They are at the same time +arranged in regular transverse rows, which converge at an acute angle from both +sides towards the middle of the back. The tip of the scale-like wart is turned +inwards. Between these larger hard scales (or groups of hairs) we find numbers +of rudimentary smaller hairs. +</p> + +<p> +The human embryo is, as a rule, entirely clothed with a thick coat of fine wool +during the last three or four weeks of gestation. This embryonic woollen coat +(<i>Lanugo</i>) generally disappears in part during the last weeks of fœtal +life but in any case, as a rule, it is lost immediately after birth, and is +replaced by the thinner coat of the permanent hair. These permanent hairs grow +out of hair-follicles, which are formed from the root-sheaths of the +disappearing wool-fibres. The embryonic wool-coat usually, in the case of the +human embryo, covers the whole body, with the exception of the palms of the +hands and soles of the feet. These parts are always bare, as in the case of +apes and of most other mammals. Sometimes the wool-coat of the embryo has a +striking effect, by its colour, on the later permanent hair-coat. Hence it +happens occasionally, for instance, among our Indo-Germanic races, that +children of blond parents seem—to the dismay of the latter—to be +covered at birth with a dark brown or even a black woolly coat. Not until this +has disappeared do we see the permanent blond hair which the child has +inherited. Sometimes the darker coat remains for weeks, and even months, after +birth. This remarkable woolly coat of the human embryo is a legacy from the +apes, our ancient long-haired ancestors. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus288"></a> +<img src="images/fig288.gif" width="163" height="180" alt="Fig.288. Mammary gland +of a new-born infant." /> +<p class="caption">Fig. 288—<b>Mammary gland of a new-born +infant,</b> <i>a</i> original central gland, <i>b</i> small and <i>c</i> large +buds of same. (From <i>Langer.</i>)</p> +</div> + +<p> +It is not less noteworthy that many of the higher apes approach man in the +thinness of the hair on various parts of the body. With most of the apes, +especially the higher Catarrhines (or narrow-nosed apes), the face is mostly, +or entirely, bare, or at least it has hair no longer or thicker than that of +man. In their case, too, the back of the head is usually provided with a +thicker growth of hair; this is lacking, however, in the case of the +bald-headed chimpanzee (<i>Anthropithecus calvus</i>). The males of many +species of apes have a considerable beard on the cheeks and chin; this sign of +the masculine sex has been acquired by sexual selection. Many species of apes +have a very thin covering of hair on the breast and the upper side of the +limbs—much thinner than on the back or the under side of the limbs. On +the other hand, we are often astonished to find tufts of hair on the shoulders, +back, and extremities of members of our Indo-Germanic and of the Semitic races. +Exceptional hair on the face, as on the whole body, is hereditary in certain +families of hairy men. The quantity and the quality of the hair on head and +chin are also conspicuously transmitted in families. These extraordinary +variations in the total and partial hairy coat of the body, which are so +noticeable, not only in comparing different races of men, but also in comparing +different families of the same race, can only be explained on the assumption +that in man the hairy coat is, on the whole, a rudimentary organ, a useless +inheritance from the more thickly-coated apes. In this man resembles the +elephant, rhinoceros, hippopotamus, whale, and other mammals of various orders, +which have also, almost entirely or for the most part, lost their hairy coats +by adaptation. +</p> + +<p> +The particular process of adaptation by which man lost the growth of hair on +most parts of his body, and retained or augmented it at some points, was most +probably sexual selection. As Darwin luminously showed in his <i>Descent of +Man,</i> sexual selection has been very active +<span class='pagenum'><a name="Page_272" id="Page_272"></a></span> +in this respect. As the male anthropoid apes chose the females with the least +hair, and the females favoured the males with the finest growths on chin and +head, the general coating of the body gradually degenerated, and the hair of +the beard and head was more strongly developed. The growth of hair at other +parts of the body (arm-pit, pubic region) was also probably due to sexual +selection. Moreover, changes of climate, or habits, and other adaptations +unknown to us, may have assisted the disappearance of the hairy coat. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus289"></a> +<img src="images/fig289.gif" width="348" height="299" alt="Fig.289. Embryo of a +bear (Ursus arctos)." /> +<p class="caption">Fig. 289—<b>Embryo of a bear</b> (<i>Ursus +arctos</i>). <i>A</i> seen from ventral side, <i>B</i> from the left.</p> +</div> + +<p> +The fact that our coat of hair is inherited directly from the anthropoid apes +is proved in an interesting way, according to Darwin, by the direction of the +rudimentary hairs on our arms, which cannot be explained in any other way. Both +on the upper and the lower part of the arm they point towards the elbow. Here +they meet at an obtuse angle. This curious arrangement is found only in the +anthropoid apes—gorilla, chimpanzee, orang, and several species of +gibbons—besides man (Figs. 203, 207). In other species of gibbon the +hairs are pointed towards the hand both in the upper and lower arm, as in the +rest of the mammals. We can easily explain this remarkable peculiarity of the +anthropoids and man on the theory that our common ancestors were accustomed (as +the anthropoid apes are to-day) to place their hands over their heads, or +across a branch above their heads, during rain. In this position, the fact that +the hairs point downwards helps the rain to run off. Thus the direction of the +hair on the lower part of our arm reminds us to-day of that useful custom of +our anthropoid ancestors. +</p> + +<p> +The nervous system in man and all the other Vertebrates is, when fully formed, +an extremely complex apparatus, that we may compare, in anatomic structure and +physiological function, with an extensive telegraphic system. The chief station +of +<span class='pagenum'><a name="Page_273" id="Page_273"></a></span> +the system is the central marrow or central nervous system, the innumerable +ganglionic cells or <i>neurona</i> (Fig. 9)of which are connected by branching +processes with each other and with numbers of very fine conducting wires. The +latter are the peripheral and ubiquitous nerve-fibres; with their terminal +apparatus, the sense-organs, etc., they constitute the conducting marrow or +peripheral nervous system. Some of them—the sensory +nerve-fibres—conduct the impressions from the skin and other sense-organs +to the central marrow; others—the motor nerve-fibres—convey the +commands of the will to the muscles. +</p> + +<p> +The central nervous system or central marrow (<i>medulla centralis</i>) is the +real organ of psychic action in the narrower sense. However we conceive the +intimate connection of this organ and its functions, it is certain that its +characteristic actions, which we call sensation, will, and thought, are +inseparably dependent on the normal development of the material organ in man +and all the higher animals. We must, therefore, pay particular attention to the +evolution of the latter. As it can give us most important information regarding +the nature of the “soul,” it should be full of interest. If the +central marrow develops in just the same way in the human embryo as in the +embryo of the other mammals, the evolution of the human psychic organ from the +central organ of the other mammals, and through them from the lower +vertebrates, must be beyond question. No one can doubt the momentous bearing of +these embryonic phenomena. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus290"></a> +<a name="illus291"></a> +<img src="images/fig290.gif" width="193" height="301" alt="CaFig.290. Human embryo, +three months old, from the dorsal side: brain and spinal cord exposed. Fig. +291. Central marrow of a human embryo, four months old, from the back.nyon" /> +<p class="caption">Fig. 290—<b>Human embryo,</b> three months old, +from the dorsal side: brain and spinal cord exposed. (From <i>Kölliker.</i>) +<i>h</i> cerebral hemispheres (fore brain), <i>m</i> corpora quadrigemina +(middle brain), <i>c</i> cerebellum (hind brain): under the latter is the +triangular medulla oblongata (after brain).<br/> Fig. 291—<b>Central +marrow of a human embryo,</b> four months old, from the back. (From +<i>Kölliker.</i>) <i>h</i> large hemispheres, <i>v</i> quadrigemina, <i>c</i> +cerebellum, <i>mo</i> medulla oblongata: underneath it the spinal +cord.</p> +</div> + +<p> +In order to understand them fully we must first say a word or two of the +general form and the anatomic composition of the mature human central marrow. +Like the central nervous system of all the other Craniotes, it consists of two +parts, the head-marrow or brain (<i>medulla capitis</i> or <i>encephalon</i>) +and the spinal-marrow (<i>medulla spinalis</i> or <i>notomyelon</i>). The one +is enclosed in the bony skull, the other in the bony vertebral column. Twelve +pairs of cerebral nerves proceed from the brain, and thirty-one pairs of spinal +nerves from the spinal cord, to the rest of the body (Fig. 171). On general +anatomic investigation the spinal marrow is found to be a cylindrical cord, +with a spindle-shaped bulb both in the region of the neck above (at the last +cervical vertebra) and the region of the loins (at the first lumbar vertebra) +below (Fig. 291). At the cervical bulb the strong nerves of the upper limbs, +and at the lumbar bulb those of the lower limbs, proceed from the spinal cord. +Above, the latter passes into the brain through the medulla oblongata (Fig. 291 +<i>mo</i>). The spinal cord seems to be a thick mass of nervous matter, but it +has a narrow canal at its axis, which passes into the further cerebral +ventricles above, and is filled, like these, with a clear fluid. +</p> + +<p> +The brain is a large nerve-mass, occupying the greater part of the skull, of +most elaborate structure. On general examination it divides into two parts, the +cerebrum and cerebellum. The cerebrum lies in front and above, and has the +familiar characteristic convolutions and furrows on its surface (Figs. 292, +293). On the upper side it is divided by a deep longitudinal fissure into two +halves, the +<span class='pagenum'><a name="Page_274" id="Page_274"></a></span> +cerebral hemispheres; these are connected by the <i>corpus callosum.</i> The +large cerebrum is separated from the small cerebellum by a deep transverse +furrow. The latter lies behind and below, and has also numbers of furrows, but +much finer and more regular, with convolutions between, at its surface. The +cerebellum also is divided by a longitudinal fissure into two halves, the +“small hemispheres”; these are connected by a worm-shaped piece, +the <i>vermis cerebelli,</i> above, and by the broad <i>pons Varolii</i> below +(Fig. 292 <i>VI</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus292"></a> +<img src="images/fig292.gif" width="199" height="205" alt="Fig.292. The human +brain, seen from below." /> +<p class="caption">Fig. 292—<b>The human brain,</b> seen from below. +(From <i>H. Meyer.</i>) Above (in front) is the cerebrum with its extensive +branching furrows; below (behind) the cerebellum with its narrow parallel +furrows. The Roman numbers indicate the roots of the twelve pairs of cerebral +nerves in a series towards the rear.</p> +</div> + +<p> +But comparative anatomy and ontogeny teach us that in man and all the other +Craniotes the brain is at first composed, not of these two, but of three, and +afterwards five, consecutive parts. These are found in just the same +form—as five consecutive vesicles—in the embryo of all the +Craniotes, from the Cyclostoma and fishes to man. But, however much they agree +in their rudimentary condition, they differ considerably afterwards. In man and +the higher mammals the first of these ventricles, the cerebrum, grows so much +that in its mature condition it is by far the largest and heaviest part of the +brain. To it belong not only the large hemispheres, but also the corpus +callosum that unites them, the olfactory lobes, from which the olfactory nerves +start, and most of the structures that are found at the roof and bottom of the +large lateral ventricles inside the two hemispheres, such as the <i>corpora +striata.</i> On the other hand, the <i>optic thalami,</i> which lie between the +latter, belong to the second division, which develops from the +“intermediate brain ”; to the same section belong the single third +cerebral ventricle and the structures that are known as the corpora geniculata, +the infundibulum, and the pineal gland. Behind these parts we find, between the +cerebrum and cerebellum, a small ganglion composed of two prominences, which is +called the <i>corpus quadrigeminum</i> on account of a superficial transverse +fissure cutting across (Figs. 290 <i>m</i> and 291 <i>v</i>). Although this +quadrigeminum is very insignificant in man and the higher mammals, it forms a +special third section, greatly developed in the lower vertebrates, the +“middle brain.” The fourth section is the “hind-brain” +or little brain (cerebellum) in the narrower sense, with the single median +part, the vermis, and the pair of lateral parts, the “small +hemispheres” (Fig. 291 <i>c</i>). Finally, we have the fifth and last +section, the medulla oblongata (Fig. 291 <i>mo</i>), which contains the single +fourth cerebral cavity and the contiguous parts (pyramids, olivary bodies, +corpora restiformia). The medulla oblongata passes straight into the medulla +spinalis (spinal cord). The narrow central canal of the spinal cord continues +above into the quadrangular fourth cerebral cavity of the medulla oblongata, +the floor of which is the quadrangular depression. From here a narrow duct, +called “the aqueduct of Sylvius,” passes through the corpus +quadrigeminum to the third cerebral ventricle, which lies between the two optic +thalami; and this in turn is connected with the pairs of lateral ventricles +which lie to the right and left in the large hemispheres. Thus all the cavities +of the central marrow are directly interconnected. All these parts of the brain +have an infinitely complex structure in detail, but we cannot go into this. +Although it is much more elaborate in man and the higher Vertebrates than in +the lower classes, it develops in them all from the same rudimentary structure, +the five simple cerebral vesicles of the embryonic brain. +</p> + +<p> +But before we consider the development of the complicated structure of the +brain from this simple series of vesicles, let +<span class='pagenum'><a name="Page_275" id="Page_275"></a></span> +us glance for a moment at the lower animals, which have no brain. Even in the +skull-less vertebrate, the Amphioxus, we find no independent brain, as we have +seen. The whole central marrow is merely a simple cylindrical cord which runs +the length of the body, and ends equally simply at both extremities—a +plain medullary tube. All that we can discover is a small vesicular bulb at the +foremost part of the tube, a degenerate rudiment of a primitive brain. We meet +the same simple medullary tube in the first structure of the ascidia larva, in +the same characteristic position, above the chorda. On closer examination we +find here also a small vesicular swelling at the fore end of the tube, the +first trace of a differentiation of it into brain and spinal cord. It is +probable that this differentiation was more advanced in the extinct +Provertebrates, and the brain-bulb more pronounced (Figs. 98–102). The +brain is phylogenetically older than the spinal cord, as the trunk was not +developed until after the head. If we consider the undeniable affinity of the +Ascidiæ to the Vermalia, and remember that we can trace all the Chordonia to +lower Vermalia, it seems probable that the simple central marrow of the former +is equivalent to the simple nervous ganglion, which lies above the gullet in +the lower worms, and has long been known as the “upper pharyngeal +ganglion” (<i>ganglion pharyngeum superius</i>); it would be better to +call it the primitive or vertical brain (acroganglion). +</p> + +<p> +Probably this upper pharyngeal ganglion of the lower worms is the structure +from which the complex central marrow of the higher animals has been evolved. +The medullary tube of the Chordonia has been formed by the lengthening of the +vertical brain on the dorsal side. In all the other animals the central nervous +system has been developed in a totally different way from the upper pharyngeal +ganglion; in the Articulates, especially, a pharyngeal ring, with ventral +marrow, has been added. The Molluscs also have a pharyngeal ring, but it is not +found in the Vertebrates. In these the central marrow has been prolonged down +the dorsal side; in the Articulates down the ventral side. This fact proves of +itself that there is no direct relationship between the Vertebrates and the +Articulates. The unfortunate attempts to derive the dorsal marrow of the former +from the ventral marrow of the latter have totally failed (cf. p. 219). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus293"></a> +<img src="images/fig293.gif" width="258" height="189" alt="Fig.293. The human +brain, seen from the left." /> +<p class="caption">Fig. 293—<b>The human brain,</b> seen from the +left. (From <i>H. Meyer.</i>) The furrows of the cerebrum are indicated by +thick, and those of the cerebellum by finer lines. Under the latter we can see +the medulla oblongata. <i>f1–f2</i> frontal convolutions, <i>C</i> +central convolutions, <i>S</i> fissure of Sylvius, <i>T</i> temporal furrow, +<i>Pa</i> parietal lobes, <i>An</i> angular gyrus, <i>Po</i> parieto-occipital +fissure.</p> +</div> + +<p> +When we examine the embryology of the human nervous system, we must start from +the important fact, which we have already seen, that the first structure of it +in man and all the higher Vertebrates is the simple medullary tube, and that +this separates from the outer germinal layer in the middle line of the +sole-shaped embryonic shield. As the reader will remember, the straight +medullary furrow first appears in the middle of the sandal-shaped embryonic +shield. At each side of it the parallel borders curve over in the form of +dorsal or medullary swellings. These bend together with their free borders, and +thus form the closed medullary tube (Figs. 133–137). At first this tube +lies directly underneath the horny plate; but it afterwards travels inwards, +the upper edges of the provertebral plates growing together between the horny +plate and the tube, joining above the latter, and forming a completely closed +canal. As Gegenbaur very properly observes, “this gradual imbedding in +the +<span class='pagenum'><a name="Page_276" id="Page_276"></a></span> +inner part of the body is a process acquired with the progressive +differentiation and the higher potentiality that this secures; by this process +the organ of greater value to the organism is buried within the frame.” +(Cf. Figs. 143–146). +</p> + +<p> +In the Cyclostoma—a stage above the Acrania—the fore end of the +cylindrical medullary tube begins early to expand into a pear-shaped vesicle; +this is the first outline of an independent brain. In this way the central +marrow of the Vertebrates divides clearly into its two chief sections, brain +and spinal cord. The simple vesicular form of the brain, which persists for +some time in the Cyclostoma, is found also at first in all the higher +Vertebrates (Fig. 153 <i>hb</i>). But in these it soon passes away, the one +vesicle being divided into several successive parts by transverse +constrictions. There are first two of these constrictions, dividing the brain +into three consecutive vesicles (fore brain, middle brain, and hind brain, Fig. +154 <i>v, m, h</i>). Then the first and third are sub-divided by fresh +constrictions, and thus we get five successive sections (Fig. 155). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus294"></a> +<img src="images/fig294.gif" width="254" height="138" alt="Fig.294. Central marrow +of the human embryo from the seventh week, 4/5 inch long. Fig. 294. The brain +from above. Fig. 295. The brain with the uppermost part of the cord, from the +left. Fig. 296. Back view of the whole embryo: brain and spinal cord exposed." /> +<p class="caption">Fig. 294–296—<b>Central marrow of the human +embryo</b> from the seventh week, 4/5 inch long. (From <i>Kölliker.</i>) Fig. +294. The brain from above, <i>v</i> fore brain, <i>z</i> intermediate brain, +<i>m</i> middle brain, <i>h</i> hind brain, <i>n</i> after brain. Fig. 2955. +The brain with the uppermost part of the cord, from the left. Fig. 296. Back +view of the whole embryo: brain and spinal cord exposed.</p> +</div> + +<p> +In all the Craniotes, from the Cyclostoma up to man, the same parts develop +from these five original cerebral vesicles, though in very different ways. The +first vesicle, the fore brain (Fig. 155 <i>v</i>), forms by far the largest +part of the cerebrum—namely, the large hemispheres, the olfactory lobes, +the corpora striata, the callosum, and the fornix. From the second vesicle, the +intermediate brain (<i>z</i>), originate especially the optic thalami, the +other parts that surround the third cerebral ventricle, and the infundibulum +and pineal gland. The third vesicle, the middle brain (<i>m</i>), produces the +corpora quadrigemina and the aqueduct of Sylvius. From the fourth vesicle, the +hind brain (<i>h</i>), develops the greater part of the +cerebellum—namely, the vermis and the two small hemispheres. Finally, the +fifth vesicle, the after brain (<i>n</i>), forms the medulla oblongata, with +the quadrangular pit (the floor of the fourth ventricle), the pyramids, olivary +bodies, etc. +</p> + +<p> +We must certainly regard it as a comparative-anatomical and ontogenetic fact of +the greatest significance that in all the Craniotes, from the lowest +Cyclostomes and fishes up to the apes and man, the brain develops in just the +same way in the embryo. The first rudiment of it is always a simple vesicular +enlargement of the fore end of the medullary tube. In every case, first three, +then five, vesicles develop from this bulb, and the permanent brain with all +its complex anatomic structures, of so great a variety in the various classes +of Vertebrates, is formed from the five primitive vesicles. When we compare the +mature brain of a fish, an amphibian, a reptile, a bird, and a mammal, it seems +incredible that we can trace the various parts of these organs, that differ so +much internally and externally, to common types. Yet all these different +Craniote brains have started with the same rudimentary structure. To convince +ourselves of this we have only to compare the corresponding stages of +development of the embryos of these different animals. +</p> + +<p> +This comparison is extremely instructive. If we extend it through the whole +series of the Craniotes, we soon discover this interesting fact: In the +Cyclostomes (the Myxinoida and Petromyzonta), which we have recognised as the +lowest and earliest Craniotes, the whole brain remains throughout life at a +very low stage, which is very brief and passing in the embryos of the higher +Craniotes; they retain the five original sections of the brain unchanged. In +the fishes we find an essential and considerable modification of the five +vesicles; it is clearly the brain of the Selachii in the first place, and +subsequently the brain of the Ganoids, from which the brain of the rest of the +fishes on the one hand and of the Dipneusts and Amphibia, and through these of +the higher Vertebrates, on the other hand, must be derived. In the fishes and +Amphibia (Fig. 300) there is a preponderant development of the middle brain, +and also the after brain, the first, second, and +<span class='pagenum'><a name="Page_277" id="Page_277"></a></span> +fourth sections remaining very primitive. It is just the reverse in the higher +Vertebrates, in which the first and third sections, the cerebrum and +cerebellum, are exceptionally developed; while the middle brain and after brain +remain small. The corpora quadrigemina are mostly covered by the cerebrum, and +the oblongata by the cerebellum. But we find a number of stages of development +within the higher Vertebrates themselves. From the Amphibia upwards the brain +(and with it the psychic life) develops in two different directions; one of +these is followed by the reptiles and birds, and the other by the mammals. The +development of the first section, the fore brain, is particularly +characteristic of the mammals. It is only in them that the cerebrum becomes so +large as to cover all the other parts of the brain (Figs. 293, 301–304). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus297"></a> +<img src="images/fig297.gif" width="158" height="220" alt="Fig.297. Head of a +chick embryo (hatched fifty-eight hours), from the back." /> +<p class="caption">Fig. 297—<b>Head of a chick embryo</b> (hatched +fifty-eight hours), from the back. (From <i>Mihalkovics.</i>) <i>vw</i> +anterior wall of the fore brain. <i>vh</i> its ventricle. <i>au</i> optic +vesicles, <i>mh</i> middle brain, <i>kh</i> hind brain, <i>nh</i> after brain, +<i>hz</i> heart (seen from below), <i>vw</i> vitelline veins, <i>us</i> +primitive segment, <i>rm</i> spinal cord.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus298"></a> +<a name="illus299"></a> +<img src="images/fig298.gif" width="204" height="227" alt="Fig.298. Brain of three craniote embryos in vertical +section. Fig. 299. Brain of a shark (Scyllium), back view." /> +<p class="caption">Fig. 298—<b>Brain of three craniote embryos</b> in +vertical section. <i>A</i> of a shark (<i>Heptarchus</i>), <i>B</i> of a +serpent (<i>Coluber</i>), <i>C</i> of a goat (<i>Capra</i>). <i>a</i> fore +brain, <i>b</i> intermediate brain, <i>c</i> middle brain, <i>d</i> hind brain, +<i>e</i> after brain, <i>s</i> primitive cleft. (From <i>Gegenbaur.</i>)<br/> +Fig. 299—<b>Brain of a shark</b> (<i>Scyllium</i>), back view. <i>g</i> +fore-brain, <i>h</i> olfactory lobes, which send the large olfactory nerves to +the nasal capsule (<i>o</i>), <i>d</i> intermediate brain, <i>b</i> middle +brain; behind this the insignificant structure of the hind brain, <i>a</i> +after brain. (From <i>Gegenbaur.</i>)</p> +</div> + +<p> +There are also notable variations in the relative position of the cerebral +vesicles. In the lower Craniotes they lie originally almost in the same plane. +When we examine the brain laterally, we can cut through all five vesicles with +a straight line. But in the Amniotes there is a considerable curve in the brain +along with the bending of the head and neck; the whole of the upper dorsal +surface of the brain develops much more than the under ventral surface. This +causes a curve, so that the parts come to lie as follows: The fore brain is +right in front and below, the intermediate brain a little higher, and the +middle brain highest of all; the hind brain lies a little lower, and the after +brain lower still. We find this only in the Amniotes—the reptiles, birds, +and mammals. +</p> + +<p> +Thus, while the brain of the mammals agrees a good deal in general growth with +that of the birds and reptiles, there are some striking differences between the +two. In the Sauropsids (birds and reptiles) the middle brain and the middle +part of the hind brain are well developed. In the mammals these parts do not +grow, and the fore-brain develops so much that it overlies the other vesicles. +As it continues to grow towards the rear, it at last covers the whole of the +rest of the brain, and also encloses the middle parts from the sides (Figs. +301–303). This process is of great importance, because the fore brain is +the organ of the higher psychic life, and in it those functions of the +nerve-cells are discharged which we sum up in +<span class='pagenum'><a name="Page_278" id="Page_278"></a></span> +the word “soul.” The highest achievements of the animal +body—the wonderful manifestations of consciousness and the complex +molecular processes of thought—have their seat in the fore brain. We can +remove the large hemispheres, piece by piece, from the mammal without killing +it, and we then see how the higher functions of consciousness, thought, will, +and sensation, are gradually destroyed, and in the end completely extinguished. +If the animal is fed artificially, it may be kept alive for a long time, as the +destruction of the psychic organs by no means involves the extinction of the +faculties of digestion, respiration, circulation, urination—in a word, +the vegetative functions. It is only conscious sensation, voluntary movement, +thought, and the combination of various higher psychic functions that are +affected. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus300"></a> +<img src="images/fig300.gif" width="146" height="268" alt="Fig.300. Brain and +spinal cord of the frog." /> +<p class="caption">Fig. 300—<b>Brain and spinal cord of the frog.</b> +<i>A</i> from the dorsal, <i>B</i> from the ventral side. <i>a</i> olfactory +lobes before the (<i>b</i>) fore brain, <i>i</i> infundibulum at the base of +the intermediate brain, <i>c</i> middle brain, <i>d</i> hind brain, <i>s</i> +quadrangular pit in the after brain, <i>m</i> spinal cord (very short in the +frog), <i>m</i>′ roots of the spinal nerves, <i>t</i> terminal fibres of +the spinal cord. (From <i>Gegenbaur.</i>)</p> +</div> + +<p> +The fore brain, the organ of these functions, only attains this high level of +development in the more advanced Placentals, and thus we have the simple +explanation of the intellectual superiority of the higher mammals. The soul of +most of the lower Placentals is not much above that of the reptiles, but among +the higher Placentals we find an uninterrupted gradation of mental power up to +the apes and man. In harmony with this we find an astonishing variation in the +degree of development of their fore brain, not only qualitatively, but also +quantitatively. The mass and weight of the brain are much greater in modern +mammals, and the differentiation of its various parts more important, than in +their extinct Tertiary ancestors. This can be shown paleontologically in any +particular order. The brains of the living ungulates are (relatively to the +size of the body) four to six times (in the highest groups even eight times) as +large as those of their earlier Tertiary ancestors, the well-preserved skulls +of which enable us to determine the size and weight of the brain. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus301"></a> +<img src="images/fig301.gif" width="211" height="104" alt="Fig.301. Brain of an +ox-embryo, two inches in length." /> +<p class="caption"> +Fig. 301—<b>Brain of an ox-embryo,</b> two inches +in length. (From <i>Mihalkovics.</i>) Left view; the lateral wall of the left +hemisphere has been removed, <i>st</i> corpora striata, <i>ml</i> +Monro-foramen, <i>ag</i> arterial plexus, <i>ah</i> Ammon’s horn, +<i>mh</i> middle brain, <i>kh</i> cerebellum, <i>dv</i> roof of the fourth +ventricle, <i>bb</i> pons Varolii, <i>na</i> medulla oblongata.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus302"></a> +<img src="images/fig302.gif" width="136" height="121" alt="Fig. 302. Brain of a human embryo, twelve +weeks old." /> +<p class="caption">Fig. 302—<b>Brain of a human embryo,</b> twelve +weeks old. (From <i>Mihalkovics.</i>) Seen from behind and above. <i>ms</i> +mantle-furrow, <i>mh</i> corpora quadrigemina (middle brain), <i>vs</i> +anterior medullary ala, <i>kh</i> cerebellum, <i>vv</i> fourth ventricle, +<i>na</i> medulla oblongata.</p> +</div> + +<p> +In the lower mammals the surface of the cerebral hemispheres is quite smooth +and level, as in the rabbit (Fig. 304). Moreover, the fore brain remains so +small that it does not cover the middle brain. At a stage higher the middle +<span class='pagenum'><a name="Page_279" id="Page_279"></a></span> +brain is covered, but the hind brain remains free. Finally, in the apes and +man, the latter also is covered by the fore brain. We can trace a similar +gradual development in the fissures and convolutions that are found on the +surface of the cerebrum of the higher mammals (Figs. 292, 293). If we compare +different groups of mammals in regard to these fissures and convolutions, we +find that their development proceeds step by step with the advance of mental +life. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus303"></a> +<img src="images/fig303.gif" width="241" height="152" alt="Fig.303. Brain of a human embryo, twenty-four +weeks old, halved in the median plane: right hemisphere seen from inside." /> +<p class="caption">Fig. 303—<b>Brain of a human embryo,</b> +twenty-four weeks old, halved in the median plane: right hemisphere seen from +inside. (From <i>Mihalkovics.</i>) <i>rn</i> olfactory nerve, <i>tr</i> funnel +of the intermediate brain, <i>vc</i> anterior commissure, <i>ml</i> +Monro-foramen, <i>gw</i> fornix, <i>ds</i> transparent sheath, <i>bl</i> corpus +callosum, <i>br</i> fissure at its border, <i>hs</i> occipital fissure, +<i>zh</i> cuneus, <i>sf</i> occipital transverse fissure, <i>zb</i> pineal +gland, <i>mh</i> corpora quadrigemina, <i>kh</i> cerebellum.</p> +</div> + +<p> +Of late years great attention has been paid to this special branch of cerebral +anatomy, and very striking individual differences have been detected within the +limits of the human race. In all human beings of special gifts and high +intelligence the convolutions and fissures are much more developed than in the +average man; and they are more developed in the latter than in idiots and +others of low mental capacity. There is a similar gradation among the mammals +in the internal structure of the fore brain. In particular the corpus callosum, +that unites the two cerebral hemispheres, is only developed in the Placentals. +Other structures—for instance, in the lateral ventricles—that seem +at first to be peculiar to man, are also found in the higher apes, and these +alone. It was long thought that man had certain distinctive organs in his +cerebrum which were not found in any other animal. But careful examination has +discovered that this is not the case, but that the characteristic features of +the human brain are found in a rudimentary form in the lower apes, and are more +or less fully developed in the higher apes. Huxley has convincingly shown, in +his <i>Man’s Place in Nature</i> (1863), that the differences in the +formation of the brain within the ape-group constitute a deeper gulf between +the lower and higher apes than between the higher apes and man. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus304"></a> +<img src="images/fig304.gif" width="253" height="169" alt="Fig.304. Brain of the rabbit." /> +<p class="caption">Fig. 304—<b>Brain of the rabbit.</b> <i>A</i> from +the dorsal, <i>B</i> from the ventral side, <i>lo</i> olfactory lobes, <i>I</i> +fore brain, <i>h</i> hypophysis at the base of the intermediate brain, +<i>III</i> middle brain, <i>IV</i> hind brain, <i>V</i> after brain, <i>2</i> +optic nerve, <i>3</i> oculo-motor nerve, <i>5–8</i> cerebral nerves. In +<i>A</i> the roof of the right hemisphere (<i>I</i>) is removed, so that we can +see the corpora striata in the lateral ventricle. (From +<i>Gegenbaur.</i>)</p> +</div> + +<p> +The comparative anatomy and physiology of the brain of the higher and lower +mammals are very instructive, and give important information in connection with +the chief questions of psychology. +</p> + +<p> +The central marrow (brain and spinal cord) develops from the medullary tube in +man just as in all the other mammals, and the same applies to the conducting +marrow or “peripheral nervous system.” It consists of the +<i>sensory</i> nerves, which conduct centripetally the impressions from the +skin and the sense-organs to the central marrow, and of the <i>motor</i> +nerves, which convey centrifugally the movements of the will from the central +marrow to the muscles. All these +<span class='pagenum'><a name="Page_280" id="Page_280"></a></span> +peripheral nerves grow out of the medullary tube (Fig. 171), and are, like it, +products of the skin-sense layer. +</p> + +<p> +The complete agreement in the structure and development of the psychic organs +which we find between man and the highest mammals, and which can only be +explained by their common origin, is of profound importance in the monistic +psychology. This is only seen in its full light when we compare these +morphological facts with the corresponding physiological phenomena, and +remember that every psychic action requires the complete and normal condition +of the correlative brain structure for its full and normal exercise. The very +complex molecular movements inside the neural cells, which we describe +comprehensively as “the life of the soul,” can no more exist in the +vertebrate, and therefore in man, without their organs than the circulation +without the heart and blood. And as the central marrow develops in man from the +same medullary tube as that of the other vertebrates, and as man shares the +characteristic structure of his cerebrum (the organ of thought) with the +anthropoid apes, his psychic life also must have the same origin as theirs. +</p> + +<p> +If we appreciate the full weight of these morphological and physiological +facts, and put a proper phylogenetic interpretation on the observations of +embryology, we see that the older idea of the personal immortality of the human +soul is scientifically untenable. Death puts an end, in man as in any other +vertebrate, to the physiological function of the cerebral neurona, the +countless microscopic ganglionic cells, the collective activity of which is +known as “the soul.” I have shown this fully in the eleventh +chapter of my <i>Riddle of the Universe.</i> +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap25"></a>Chapter XXV.<br/> +EVOLUTION OF THE SENSE-ORGANS</h2> + +<p> +The sense-organs are indubitably among the most important and interesting parts +of the human body; they are the organs by means of which we obtain our +knowledge of objects in the surrounding world. <i>Nihil est in intellectu quod +non prius fuerit in sensu.</i> They are the first sources of the life of the +soul. There is no other part of the body in which we discover such elaborate +anatomical structures, co-operating with a definite purpose; and there is no +other organ in which the wonderful and purposive structure seems so clearly to +compel us to admit a Creator and a preconceived plan. Hence we find special +efforts made by dualists to draw our attention here to the “wisdom of the +Creator” and the design visible in his works. As a matter of fact, you +will discover, on mature reflection, that on this theory the Creator is at +bottom only playing the part of a clever mechanic or watch-maker; all these +familiar teleological ideas of Creator and creation are based, in the long run, +on a similar childlike anthropomorphism. +</p> + +<p> +However, we must grant that at the first glance the teleological theory seems +to give the simplest and most satisfactory explanation of these purposive +structures. If we merely examine the structure and functions of the most +advanced sense-organs, it seems impossible to explain them without postulating +a creative act. Yet evolution shows us quite clearly that this popular idea is +totally wrong. With its assistance we discover that the purposive and +remarkable sense-organs were developed, like all other organs, without any +preconceived design—developed by the same mechanical process of natural +selection, the same constant correlation of adaptation and heredity, by which +the other purposive structures in the animal frame were slowly and gradually +brought forth in the struggle for life. +</p> + +<p> +Like most other Vertebrates, man has six sensory organs, which serve for eight +<span class='pagenum'><a name="Page_281" id="Page_281"></a></span> +different classes of sensations. The skin serves for sensations of pressure and +temperature. This is the oldest, lowest, and vaguest of the sense-organs; it is +distributed over the surface of the body. The other sensory activities are +localised. The sexual sense is bound up with the skin of the external sexual +organs, the sense of taste with the mucous lining of the mouth (tongue and +palate), and the sense of smell with the mucous lining of the nasal cavity. For +the two most advanced and most highly differentiated sensory functions there +are special and very elaborate mechanical structures—the eye for the +sense of sight, and the ear for the sense of hearing and space (equilibrium). +</p> + +<p> +Comparative anatomy and physiology teach us that there are no differentiated +sense-organs in the lower animals; all their sensations are received by the +surface of the skin. The undifferentiated skin-layer or ectoderm of the Gastræa +is the simple stratum of cells from which the differentiated sense-organs of +all the Metazoa (including the Vertebrates) have been evolved. Starting from +the assumption that necessarily only the superficial parts of the body, which +are in direct touch with the outer world, could be concerned in the origin of +sensations, we can see at once that the sense-organs also must have arisen +there. This is really the case. The chief part of all the sense-organs +originates from the skin-sense layer, partly directly from the horny plate, +partly from the brain, the foremost part, of the medullary tube, after it has +separated from the horny plate. If we compare the embryonic development of the +various sense-organs, we see that they all make their appearance in the +simplest conceivable form; the wonderful contrivances that make the higher +sense-organs among the most remarkable and elaborate structures in the body +develop only gradually. In the phylogenetic explanation of them comparative +anatomy and ontogeny achieve their greatest triumphs. But at first all the +sense-organs are merely parts of the skin in which sensory nerves expand. These +nerves themselves were originally of a homogeneous character. The different +functions or specific energies of the differentiated sense-nerves were only +gradually developed by division of labour. At the same time, their simple +terminal expansions in the skin were converted into extremely complex organs. +</p> + +<p> +The great instructiveness of these historical facts in connection with the life +of the soul is not difficult to see. The whole philosophy of the future will be +transformed as soon as psychology takes cognisance of these genetic phenomena +and makes them the basis of its speculations. When we examine impartially the +manuals of psychology that have been published by the most distinguished +speculative philosophers and are still widely distributed, we are astonished at +the naivete with which the authors raise their airy metaphysical speculations, +regardless of the momentous embryological facts that completely refute them. +Yet the science of evolution, in conjunction with the great advance of the +comparative anatomy and physiology of the sense-organs, provides the one sound +empirical basis of a natural psychology. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus305"></a> +<img src="images/fig305.gif" width="177" height="134" alt="Fig.305. Head of a shark +(Scyllium), from the ventral side." /> +<p class="caption">Fig. 305—<b>Head of a shark</b> (<i>Scyllium</i>), +from the ventral side. <i>m</i> mouth, <i>o</i> olfactory pits, <i>r</i> nasal +groove, <i>n</i> nasal fold in natural position, <i>n′</i> nasal fold +drawn up. (The dots are openings of the mucous canals.) (From +<i>Gegenbaur.</i>)</p> +</div> + +<p> +In respect of the terminal expansions of the sensory nerves, we can distribute +the human sense-organs in three groups, which correspond to three stages of +development. The first group comprises those organs the nerves of which spread +out quite simply in the free surface of the skin itself (organs of the sense of +pressure, warmth, and sex). In the second group the nerves spread out in the +mucous coat of cavities which are at first depressions in or invaginations of +the skin (organs of the sense of smell and taste). The third group is formed of +the very elaborate organs, the nerves of which spread out in an internal +vesicle, separated from the skin (organs of the sense of sight, hearing, and +space). +</p> + +<p> +There is little to be said of the development of the lower sense-organs. We +<span class='pagenum'><a name="Page_282" id="Page_282"></a></span> +have already considered (p. 268) the organ of touch and temperature in the +skin. I need only add that in the corium of man and all the higher Vertebrates +countless microscopic sense-organs develop, but the precise relation of these +to the sensations of pressure or resistance, of warmth and cold, has not yet +been explained. Organs of this kind, in or on which sensory cutaneous nerves +terminate, are the “tactile corpuscles” (or the Pacinian +corpuscles) and end-bulbs. We find similar corpuscles in the organs of the +sexual sense, the male penis and the female clitoris; they are processes of the +skin, the development of which we will consider later (together with the rest +of the sexual parts, Chapter XXIX). The evolution of the organ of taste, the +tongue and palate, will also be treated later, together with that of the +alimentary canal to which these parts belong (Chapter XXVII). I will only point +out for the present that the mucous coat of the tongue and palate, in which the +gustatory nerve ends, originates from a part of the outer skin. As we have +seen, the whole of the mouth-cavity is formed, not as a part of the gut-tube +proper, but as a pit-like fold in the outer skin (p. 139). Its mucous lining is +therefore formed, not from the visceral, but from the cutaneous layer, and the +taste-cells at the surface of the tongue and palate are not products of the +gut-fibre layer, but of the skin-sense layer. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus306"></a> +<img src="images/fig306.gif" width="357" height="324" alt="Figs. 306 and 307. Head +of a chick embryo, three days old. Fig. 308. Head of a chick embryo, four days +old, from below. Figs. 309 and 310. Heads of chick embryos: 309 from the end of +the fourth, 310 from the beginning of the fifth week." /> +<p class="caption">Fig. 306 and 307—<b>Head of a chick embryo,</b> +three days old: 2.306 front view, 2.307 from the right. <i>n</i> rudimentary +nose (olfactory pits), <i>l</i> rudimentary eyes (optic pits), <i>g</i> +rudimentary ear (auscultory pit), <i>v</i> fore brain, <i>gl</i> eye-cleft, +<i>o</i> process of upper jaw, <i>u</i> process of lower jaw of the first +gill-arch.</p> +</div> + +<p class="caption"> +Fig. 308—<b>Head of a chick embryo,</b> four days old, from below. +<i>n</i> nasal pit, <i>o</i> upper-jaw process of the first gill-arch, <i>u</i> +lower-jaw process of same, <i>k″</i> second gill-arch, <i>sp</i> choroid +fissure of eye, <i>s</i> gullet.<br/> + +Fig. 309 and 310—<b>Heads of chick embryos:</b> 309 from the end of the +fourth, 310 from the beginning of the fifth week. Letters as in Fig. 308, +except: in inner, an outer, nasal process, <i>nf</i> nasal furrow, <i>st</i> +frontal process, <i>m</i> mouth. (From <i>Kölliker.</i>). +</p> + +<p> +This applies also to the mucous lining of the olfactory organ, the nose. +However, the development of this organ is much more interesting. Although the +nose seems superficially to be simple and single, it really consists, in man +and all +<span class='pagenum'><a name="Page_283" id="Page_283"></a></span> +other Gnathostomes, of two completely separated halves, the right and left +cavities. They are divided by a vertical partition, so that the right nostril +leads into the right cavity alone and the left nostril into the left cavity. +They open internally (and separately) by the posterior nasal apertures into the +pharynx, so that we can get direct into the gullet through the nasal passages +without touching the mouth. This is the way the air usually passes in +respiration; the mouth being closed, it goes through the nose into the gullet, +and through the larynx and bronchial tubes into the lungs. The nasal cavities +are separated from the mouth by the horizontal bony palate, to which is +attached behind (as a dependent process) the soft palate with the uvula. In the +upper and hinder parts of the nasal cavities the olfactory nerve, the first +pair of cerebral nerves, expands in the mucous coat which clothes them. The +terminal branches of it spread partly over the septum (partition), partly on +the side walls of the internal cavities, to which are attached the turbinated +bones. These bones are much more developed in many of the higher mammals than +in man, but there are three of them in all mammals. The sensation of smell +arises by the passage of a current of air containing odorous matter over the +mucous lining of the cavities, and stimulating the olfactory cells of the +nerve-endings. +</p> + +<p> +Man has all the features which distinguish the olfactory organ of the mammals +from that of the lower Vertebrates. In all essential points the human nose +entirely resembles that of the Catarrhine apes, some of which have quite a +human external nose (compare the face of the long-nosed apes). However, the +first structure of the olfactory organ in the human embryo gives no indication +of the future ample proportions of our catarrhine nose. It has the form in +which we find it permanently in the fishes—a couple of simple depressions +in the skin at the outer surface of the head. We find these blind olfactory +pits in all the fishes; sometimes they lie near the eyes, sometimes more +forward at the point of the muzzle, sometimes lower down, near the mouth (Fig. +249). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus311"></a> +<img src="images/fig311.gif" width="222" height="270" alt="Fig.311. Frontal section +of the mouth and throat of a human embryo, neck half-inch long." /> +<p class="caption">Fig. 311—<b>Frontal section of the mouth and +throat of a human embryo,</b> neck half-inch long. “Invented” by +<i>Wilhelm His.</i> The vertical section (in the frontal plane, from left to +right) is so constructed that we see the nasal pits in the upper third of the +figure and the eyes at the sides: in the middle third the primitive gullet with +the gill-clefts (gill-arches in section); in the lower third the pectoral +cavity with the bronchial tubes and the rudimentary lungs.</p> +</div> + +<p> +This first rudimentary structure of the double nose is the same in all the +Gnathostomes; it has no connection with the primitive mouth. But even in a +section of the fishes a connection of this kind begins to make its appearance, +a furrow in the surface of the skin running from each side of the nasal pit to +the nearest corner of the mouth. This furrow, the nasal groove or furrow (Fig. +305 <i>r</i>), is very important. In many of the sharks, such as the +<i>Scyllium,</i> a special process of the frontal skin, the nasal fold or +internal nasal process, is formed internally over the groove (<i>n, +n″</i>). In contrast to this the outer edge of the furrow rises in an +“external nasal process.” As the two processes meet and coalesce +over the nasal groove in the Dipneusts and Amphibia, it is converted into a +canal, the nasal canal. Henceforth we can penetrate from the external pits +through the nasal canals direct into the mouth, which has been formed quite +independently. In the Dipneusts and the lower Amphibia the internal aperture of +the nasal canals lies in front (behind the lips); in the higher Amphibia it is +right behind. Finally, in the three higher classes of Vertebrates the primary +mouth-cavity is divided by the formation of the horizontal +<span class='pagenum'><a name="Page_284" id="Page_284"></a></span> +palate-roof into two distinct cavities—the upper (secondary) nasal cavity +and the lower (secondary) mouth-cavity. The nasal cavity in turn is divided by +the construction of the vertical septum into two halves—right and left. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus312"></a> +<img src="images/fig312.gif" width="124" height="113" alt="Fig.312. Diagrammatic +section of the mouth-nose cavity." /> +<p class="caption">Fig. 312—<b>Diagrammatic section of the mouth-nose +cavity.</b> While the palate-plates (<i>p</i>) divide the original mouth-cavity +into the lower secondary mouth (<i>m</i>) and the upper nasal cavity, the +latter in turn is divided by the vertical partition (<i>e</i>) into two halves +(<i>n, n</i>). (From <i>Gegenbaur.</i>)</p> +</div> + +<p> +Comparative anatomy shows us to-day, in the series of the double-nosed +Vertebrates, from the fishes up to man, all the different stages in the +development of the nose, which the advanced olfactory organ of the higher +mammals has passed through at various periods in the course of its phylogeny. +It first appears in the embryo of man and the higher Vertebrates, in which the +double fish-nose persists throughout life. At an early stage, before there is +any trace of the characteristic human face, a pair of small pits are formed in +the head over the original mouth-cavity; these were first discovered by Baer, +and rightly called the “olfactory pits” (Figs. 306 <i>n</i>, 307 +<i>n</i>). These primitive nasal pits are quite separate from the rudimentary +mouth, which also originates as a pit-like depression in the skin, in front of +the blind fore end of the gut. Both the pair of nasal pits and the single +mouth-pit (Fig. 310 <i>m</i>) are clothed with the horny plate. The original +separation of the former from the latter is, however, presently abolished, a +process forming above the mouth-pit—the “frontal process” +(Fig. 309 <i>st</i>). Its outer edge rises to the right and left in the shape +of two lateral processes; these are the inner nasal processes or folds +(<i>in</i>). Opposite to these a parallel ridge is formed on either side +between the eye and the nasal pit; these are the outer nasal processes +(<i>an</i>). Thus between the inner and outer nasal processes a groove-like +depression is formed on either side, which leads from the nasal pit towards the +mouth-pit (<i>m</i>); this groove is, as the reader will guess, the same nasal +furrow or groove that we have already seen in the shark (Fig. 305 <i>r</i>). As +the parallel edges of the inner and outer nasal processes bend towards each +other and join above the nasal groove, this is converted into a tube, the +primitive nasal canal. Hence the nose of man and all the other Amniotes +consists at this embryonic stage of a couple of narrow tubes, the nasal canals, +which lead from the outer surface of the forehead into the rudimentary mouth. +This transitory condition resembles that in which we find the nose permanently +in the Dipneusts and Amphibia. +</p> + +<p> +A cone-shaped structure, which grows from below towards the lower ends of the +two nasal processes and joins with them, plays an important part in the +conversion of the open nasal groove into the closed canal. This is the +upper-jaw process (Figs. 306–310 <i>o</i>). Below the mouth-pit are the +gill-arches, which are separated by the gill-clefts. The first of these +gill-arches, and the most important for our purpose, which we may call the +maxillary (jaw) arch, forms the skeleton of the jaws. Above at the basis a +small process grows out of this first gill-arch; this is the upper-jaw process. +The first gill-arch itself develops a cartilage at one of its inner sides, the +“Meckel cartilage” (named after its discoverer), on the outer +surface of which the lower jaw is formed (Figs. 306–310 <i>u</i>). The +upper-jaw process forms the chief part of the skeleton of that jaw, the palate +bone, and the pterygoid bone. On its outer side is afterwards formed the +upper-jaw bone, in the narrower sense, while the middle part of the skeleton of +the upper jaw, the intermaxillary, develops from the foremost part of the +frontal process. +</p> + +<p> +The two upper-jaw processes are of great importance in the further development +of the face. From them is formed, growing into the primitive mouth-cavity, the +important horizontal partition (the palate) that divides the former into two +distinct cavities. The upper cavity, into which the nasal canals open, now +develops into the nasal cavity, the air-passage and the organ of smell. The +lower cavity forms the permanent secondary mouth (Fig. 312 <i>m</i>), the +food-passage and the organ of taste. Both the upper and lower cavities open +behind into the gullet (pharynx). The hard +<span class='pagenum'><a name="Page_285" id="Page_285"></a></span> +palate that separates them is formed by the joining of two lateral halves, the +horizontal plates of the two upper-jaw processes, or the palate-plates +(<i>p</i>). When these do not, sometimes, completely join in the middle, a +longitudinal cleft remains, through which we can penetrate from the mouth +straight into the nasal cavity. This is the malformation known as +“wolf’s throat.” “Hare-lip” is the lesser form of +the same defect. At the same time as the horizontal partition of the hard +palate a vertical partition is formed by which the single nasal cavity is +divided into two sections—a right and left half (Fig. 312 <i>n, n</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus313"></a> +<img src="images/fig313.gif" width="391" height="241" alt="Figs. 313 and 314. +Upper part of the body of a human embryo, two-thirds of an inch long, of the +sixth week; Fig. 313 from the left, Fig. 314 from the front." /> +<p class="caption">Figs. 313 and 314—<b>Upper part of the body of a +human embryo,</b> two-thirds of an inch long, of the sixth week; Fig. 313 from +the left, Fig. 314 from the front. The origin of the nose and the upper lip +from two lateral and originally separate halves can be clearly seen. Nose and +upper lip are large in proportion to the rest of the face, and especially to +the lower lip. (From <i>Kollmann.</i>)</p> +</div> + +<p> +The double nose has now acquired the characteristic form that man shares with +the other mammals. Its further development is easy to follow; it consists of +the formation of the inner and outer processes of the walls of the two +cavities. The external nose is not formed until long after all these essential +parts of the internal organ of smell. The first traces of it in the human +embryo are found about the middle of the second month (Figs. 313–316). As +can be seen in any human embryo during the first month, there is at first no +trace of the external nose. It only develops afterwards from the foremost nasal +part of the primitive skull, growing forwards from behind. The characteristic +human nose is formed very late. Much stress is at times laid on this organ as +an exclusive privilege of man. But there are apes that have similar noses, such +as the long-nosed ape. +</p> + +<p> +The evolution of the eye is not less interesting and instructive than that of +the nose. Although this noblest of the sensory organs is one of the most +elaborate and purposive on account of its optic perfection and remarkable +structure, it nevertheless develops, without preconceived design, from a simple +process of the outer germinal layer. The fully-formed human eye is a round +capsule, the eye-ball (Fig. 317). This lies in the bony cavity of the skull, +surrounded by protective fat and motor muscles. The greater part of it is taken +up with a semi-fluid, transparent gelatinous substance, the <i>corpus +vitreum.</i> The crystalline lens is fitted into the anterior surface of the +ball (Fig. 317 <i>l</i>). It is a lenticular, bi-convex, transparent body, the +most important of the refractive media in the eye. Of this group we have, +besides the corpus vitreum and the lens, the watery fluid (<i>humor aqueus</i>) +that is found in front of the lens (at the letter <i>m</i> in Fig. 317). These +three transparent refractive media, by which the rays of light that +<span class='pagenum'><a name="Page_286" id="Page_286"></a></span> +enter the eye are broken up and re-focussed, are enclosed in a solid round +capsule, composed of several different coats, something like the concentric +layers of an onion. The outermost and thickest of these envelopes is the white +sclerotic coat of the eye. It consists of tough white connective tissue. In +front of the lens a circular, strongly-curved, transparent plate is fitted into +the sclerotic, like the glass of a watch—the <i>cornea</i> (<i>b</i>). At +its outer surface the cornea is covered with a very thin layer of the +epidermis; this is known as the <i>conjunctiva.</i> It goes from the cornea +over the inner surface of the eye-lids, the upper and lower folds which we draw +over the eye in closing it. At the inner corner of the eye we have a +rudimentary organ in the shape of the relic of a third (inner) eye-lid, which +is greatly developed, as “nictitating (winking) membrane,” in the +lower Vertebrates (p. 5). Underneath the upper eye-lid are the lachrymal +glands, the product of which, the lachrymal fluid, keeps the outer surface of +the eye smooth and clean. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus315"></a> +<img src="images/fig315.gif" width="188" height="220" alt="Fig.315. Face of a human +embryo, seven weeks old." /> +<p class="caption">Fig. 315—<b>Face of a human embryo,</b> seven +weeks old. (From <i>Kollmann.</i>) Joining of the nasal processes (<i>e</i> +outer, <i>i</i> inner) with the upper-jaw process (<i>o</i>), <i>n</i> nasal +wall, <i>a</i> ear-opening.</p> +</div> + +<p> +Immediately under the sclerotic we find a very delicate, dark-red membrane, +very rich in blood-vessels—the <i>choroid coat</i>—and inside this +the retina (<i>o</i>), the expansion of the optic nerve (<i>i</i>). The latter +is the second cerebral nerve. It proceeds from the optic thalami (the second +cerebral vesicle) to the eye; penetrates its outer envelopes, and then spreads +out like a net between the choroid and the corpus vitreum. Between the retina +and the choroid there is a very delicate membrane, which is usually (but +wrongly) associated with the latter. This is the black pigment-membrane +(<i>n</i>). It consists of a single stratum of graceful, hexagonal, +regularly-joined cells, full of granules of black colouring matter. This +pigment membrane clothes, not only the inner surface of the choroid proper, but +also the hind surface of its anterior muscular continuation, which covers the +edge of the lens in front as a circular membrane, and arrests the rays of light +at the sides. This is the well-known <i>iris</i> of the eye (<i>h</i>), +coloured differently in different individuals (blue, grey, brown, etc.); it +forms the anterior border of the choroid. The circular opening that is left in +the middle is the <i>pupil,</i> through which the rays of light penetrate into +the eye. At the point where the iris leaves the anterior border of the choroid +proper the latter is very thick, and forms a delicate crown of folds +(<i>g</i>), which surrounds the edge of the lens with about seventy large and +many smaller rays (<i>corona ciliaris.</i>) +</p> + +<div class="fig" style="width:100%;"> +<a name="illus316"></a> +<img src="images/fig316.gif" width="88" height="124" alt="Fig.316. Face of a human +embryo, eight weeks old." /> +<p class="caption">Fig. 316—<b>Face of a human embryo,</b> eight +weeks old. (From <i>Ecker.</i>)</p> +</div> + +<p> +At a very early stage a couple of pear-shaped vesicles develop from the +foremost part of the first cerebral vesicle in the embryo of man and the other +Craniotes (Figs. 155 <i>a</i>, 297 <i>au</i>). These growths are the primary +optic vesicles. They are at first directed outwards and forwards, but presently +grow downward, so that, after the complete separation of the five cerebral +vesicles, they lie at the base of the intermediate brain. The inner cavities of +these pear-shaped vesicles, which soon attain a considerable size, are openly +connected with the ventricle of the intermediate brain by their hollow stems. +They are covered externally by the epidermis. +</p> + +<p> +At the point where this comes into direct contact with the most curved part of +the primary optic vesicle there is a thickening (<i>l</i>) and also a +depression (<i>o</i>) of the horny plate (Fig. 318, <i>I</i>). This pit, which +we may call the lens-pit, is converted into a closed sac, the thick- +<span class='pagenum'><a name="Page_287" id="Page_287"></a></span> +walled lens-vesicle (<i>2, l</i>), the thick edges of the pit joining together +above it. In the same way in which the medullary tube separates from the outer +germinal layer, we now see this lens-sac sever itself entirely from the horny +plate (<i>h</i>), its source of origin. The hollow of the sac is afterwards +filled with the cells of its thick walls, and thus we get the solid crystalline +lens. This is, therefore, a purely epidermic structure. Together with the lens +the small underlying piece of corium-plate also separates from the skin. +</p> + +<p> +As the lens separates from the corneous plate and grows inwards, it necessarily +hollows out the contiguous primary optic vesicle (Fig. 318, <i>1–3</i>). +This is done in just the same way as the invagination of the blastula, which +gives rise to the gastrula in the amphioxus (Fig. 38 <i>C–F</i>). In both +cases the hollowing of the closed vesicle on one side goes so far that at last +the inner, folded part touches the outer, not folded part, and the cavity +disappears. As in the gastrula the first part is converted into the entoderm +and the latter into the ectoderm, so in the invagination of the primary optic +vesicle the retina (<i>r</i>) is formed from the first (inner) part, and the +black pigment membrane (<i>u</i>) from the latter (outer, non-invaginated) +part. The hollow stem of the primary optic vesicle is converted into the optic +nerve. The lens (<i>l</i>), which has so important a part in this process, lies +at first directly on the invaginated part, or the retina (<i>r</i>). But they +soon separate, a new structure, the corpus vitreum (<i>gl</i>), growing between +them. While the lenticular sac is being detached and is causing the +invagination of the primary optic vesicle, another invagination is taking place +from below; this proceeds from the superficial part of the skin-fibre +layer—the corium of the head. Behind and under the lens a last-shaped +process rises from the cutis-plate (Fig. 319 <i>g</i>), hollows out the +cup-shaped optic vesicle from below, and presses between the lens (<i>l</i>) +and the retina (<i>i</i>). In this way the optic vesicle acquires the form of a +hood. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus317"></a> +<img src="images/fig317.gif" width="253" height="265" alt="Fig.317. The human eye +in section." /> +<p class="caption">Fig. 317—<b>The human eye in section.</b> <i>a</i> +sclerotic coat, <i>b</i> cornea, <i>c</i> conjunctiva, <i>d</i> circular veins +of the iris, <i>e</i> choroid coat, <i>f</i> ciliary muscle, <i>g</i> corona +ciliaris, <i>h</i> iris, <i>i</i> optic nerve, <i>k</i> anterior border of the +retina, <i>l</i> crystalline lens, <i>m</i> inner covering of the cornea +(aqueous membrane), <i>n</i> pigment membrane, <i>o</i> retina, <i>p</i> +Petit’s canal, <i>q</i> yellow spot of the retina. (From +<i>Helmholtz.</i>)</p> +</div> + +<p> +Finally, a complete fibrous envelope, the fibrous capsule of the eye-ball, is +formed about the secondary optic vesicle and its stem (the secondary optic +nerve). It originates from the part of the head-plates which immediately +encloses the eye. This fibrous envelope takes the form of a closed round +vesicle, surrounding the whole of the ball and pushing between the lens and the +horny plate at its outer side. The round wall of the capsule soon divides into +two different membranes by surface-cleavage. The inner membrane becomes the +choroid or vascular coat, and in front the ciliary corona and iris. The outer +membrane is converted into the white protective or sclerotic coat—in +front, the transparent cornea. The eye is now formed in all its essential +parts. The further development—the complicated differentiation and +composition of the various parts—is a matter of detail. +</p> + +<p> +The chief point in this remarkable evolution of the eye is the circumstance +that the optic nerve, the retina, and the pigment membrane originate really +from a part of the brain—an outgrowth of the intermediate +brain—while the lens, the chief refractive body, develops from the outer +skin. From the skin—the horny +<span class='pagenum'><a name="Page_288" id="Page_288"></a></span> +plate—also arises the delicate conjunctiva, which afterwards covers the +outer surface of the eyeball. The lachrymal glands are ramified growths from +the conjunctiva (Fig. 286). All these important parts of the eye are products +of the outer germinal layer. The remaining parts—the corpus vitreum (with +the vascular capsule of the lens), the choroid (with the iris), and the +sclerotic (with the cornea)—are formed from the middle germinal layer. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus318"></a> +<img src="images/fig318.gif" width="218" height="111" alt="Fig.318. Eye of the +chick embryo in longitudinal section (1. from an embryo sixty-five hours old; +2. from a somewhat older embryo; 3. from an embryo four days old)." /> +<p class="caption">Fig. 318—<b>Eye of the chick embryo</b> in +longitudinal section (<i>1.</i> from an embryo sixty-five hours old; <i>2.</i> +from a somewhat older embryo; <i>3.</i> from an embryo four days old). <i>h</i> +horny plate, <i>o</i> lens-pit, <i>l</i> lens (in <i>1.</i> still part of the +epidermis, in <i>2.</i> and <i>3.</i> separated from it), <i>x</i> thickening +of the horny plate at the point where the lens has severed itself, <i>gl</i> +corpus vitreum, <i>r</i> retina, <i>u</i> pigment membrane. (From +<i>Remak.</i>)</p> +</div> + +<p> +The outer protection of the eye, the eye-lids, are merely folds of the skin, +which are formed in the third month of human embryonic life. In the fourth +month the upper eye-lid reaches the lower, and the eye remains covered with +them until birth. As a rule, they open wide shortly before birth (sometimes +only after birth). Our craniote ancestors had a third eye-lid, the nictitating +membrane, which was drawn over the eye from its inner angle. It is still found +in many of the Selachii and Amniotes. In the apes and man it has degenerated, +and there is now only a small relic of it at the inner corner of the eye, the +semi-lunar fold, a useless rudimentary organ (cf. p. 32). The apes and man have +also lost the Harderian gland that opened under the nictitating membrane; we +find this in the rest of the mammals, and the birds, reptiles, and amphibia. +</p> + +<p> +The peculiar embryonic development of the vertebrate eye does not enable us to +draw any definite conclusions as to its obscure phylogeny; it is clearly +cenogenetic to a great extent, or obscured by the reduction and curtailment of +its original features. It is probable that many of the earlier stages of its +phylogeny have disappeared without leaving a trace. It can only be said +positively that the peculiar ontogeny of the complicated optic apparatus in man +follows just the same laws as in all the other Vertebrates. Their eye is a part +of the fore brain, which has grown forward towards the skin, not an original +cutaneous sense-organ, as in the Invertebrates. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus319"></a> +<img src="images/fig319.gif" width="169" height="151" alt="Fig.319. Horizontal +transverse section of the eye of a human embryo, four weeks old." /> +<p class="caption">Fig. 319—<b>Horizontal transverse section of the +eye of a human embryo,</b> four weeks old. (From <i>Kölliker.</i>) <i>t</i> +lens (the dark wall of which is as thick as the diameter of the central +cavity), <i>g</i> corpus vitreum (connected by a stem, <i>g,</i> with the +corium), <i>v</i> vascular loop (pressing behind the lens inside the corpus +vitreum by means of this stem <i>g</i>), <i>i</i> retina (inner thicker, +invaginated layer of the primary optic vesicle), <i>a</i> pigment membrane +(outer, thin, non-invaginated layer of same), <i>h</i> space between retina and +pigment membrane (remainder of the cavity of the primary optic +vesicle).</p> +</div> + +<p> +The vertebrate ear resembles the eye and nose in many important respects, but +is different in others, in its development. The auscultory organ in the +fully-developed man is like that of the other mammals, and especially the apes, +in the main features. As in them, it consists of two chief parts—an +apparatus for conducting sound (external and middle ear) and an apparatus for +the sensation of sound (internal ear). The external ear opens in the shell at +the side of the head (Fig. 320 <i>a</i>). From this point the external passage +(<i>b</i>), about an inch in length, leads into the head. The inner end of it +is closed by the tympanum, a vertical, but not quite upright, thin membrane of +an oval shape (<i>c</i>). This tympanum separates the external passage from the +tympanic cavity (<i>d</i>). This is a small cavity, filled with air, in the +temporal bone; it is connected with the mouth by a special tube. This tube is +rather longer, but much narrower, than the outer passage, leads inwards +obliquely from the anterior wall of the tympanic cavity, and opens in the +throat below, behind the nasal +<span class='pagenum'><a name="Page_289" id="Page_289"></a></span> +openings. It is called the Eustachian tube (<i>e</i>); it serves to equalise +the pressure of the air within the tympanic cavity and the outer atmosphere +that enters by the external passage. Both the Eustachian tube and the tympanic +cavity are lined with a thin mucous coat, which is a direct continuation of the +mucous lining of the throat. Inside the tympanic cavity there are three small +bones which are known (from their shape) as the hammer, anvil, and stirrup +(Fig. 320, <i>f, g, h</i>). The hammer (<i>f</i>) is the outermost, next to the +tympanum. The anvil (<i>g</i>) fits between the other two, above and inside the +hammer. The stirrup (<i>h</i>) lies inside the anvil, and touches with its base +the outer wall of the internal ear, or auscultory vesicle. All these parts of +the external and middle ear belong to the apparatus for conducting sound. Their +chief task is to convey the waves of sound through the thick wall of the head +to the inner-lying auscultory vesicle. They are not found at all in the fishes. +In these the waves of sound are conveyed directly by the wall of the head to +the auscultory vesicle. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus320"></a> +<img src="images/fig320.gif" width="206" height="252" alt="Fig.320. The human ear +(left ear, seen from the front)." /> +<p class="caption">Fig. 320—<b>The human ear</b> (left ear, seen from +the front), <i>a</i> shell of ear, <i>b</i> external passage, <i>c</i> +tympanum, <i>d</i> tympanic cavity, <i>e</i> Eustachian tube, <i>f, g, h</i> +the three bones of the ear (<i>f</i> hammer, <i>g</i> anvil, <i>h</i> stirrup), +<i>i</i> utricle, <i>k</i> the three semi-circular canals, <i>l</i> the +sacculus, <i>m</i> cochlea, <i>n</i> auscultory nerve.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus321"></a> +<img src="images/fig321.gif" width="155" height="72" alt="Fig. 321. The bony +labyrinth of the human ear (left side)." /> +<p class="caption">Fig. 321—<b>The bony labyrinth of the human +ear</b> (left side). <i>a</i> vestibulum, <i>b</i> cochlea, <i>c</i> upper +canal, <i>d</i> posterior canal, <i>e</i> outer canal, <i>f</i> oval fenestra, +<i>g</i> round fenestra. (From <i>Meyer.</i>)</p> +</div> + +<p> +The internal apparatus for the sensation of sound, which receives the waves of +sound from the conducting apparatus, consists in man and all other mammals of a +closed auscultory vesicle filled with fluid and an auditory nerve, the ends of +which expand over the wall of this vesicle. The vibrations of the sound-waves +are conveyed by these media to the nerve-endings. In the labyrinthic water that +fills the auscultory vesicle there are small stones at the points of entry of +the acoustic nerves, which are composed of groups of microscopic calcareous +crystals (otoliths). The auscultory organ of most of the Invertebrates has +substantially the same composition. It usually consists of a closed vesicle, +filled with fluid, and containing otoliths, with the acoustic nerve expanding +on its wall. But, while the auditory vesicle is usually of a simple round or +oval shape in the Invertebrates, it has in the Vertebrates a special and +curious structure, the labyrinth. This thin-membraned labyrinth is enclosed in +a bony capsule of the same shape, the osseous labyrinth (Fig. 321), and this +lies in the middle of the petrous bone of the skull. The labyrinth is divided +into two vesicles in all the Gnathostomes. The larger one is called the +<i>utriculus,</i> and has three arched appendages, called the +“semi-circular canals” (<i>c, d, e</i>). The smaller vesicle is +called the sacculus, and is connected with a peculiar appendage, with (in man +and the higher mammals) a spiral form something like a snail’s shell, and +therefore called the <i>cochlea</i> (= snail, <i>b</i>). On the thin wall of +this delicate labyrinth the acoustic nerve, which comes from the after-brain, +spreads out in most elaborate fashion. It divides into two main +branches—a cochlear nerve (for the cochlea) and a vestibular nerve (for +the rest of the labyrinth). The former seems to have more to do with the +quality, the latter with the quantity, of the acoustic sensations. Through the +cochlear nerves we learn the height and timbre, through the vestibular nerves +the intensity, of tones. +</p> + +<p> +The first structure of this highly elaborate organ is very simple in the embryo +of man and all the other Craniotes; it is a +<span class='pagenum'><a name="Page_290" id="Page_290"></a></span> +pit-like depression in the skin. At the back part of the head at both sides, +near the after brain, a small thickening of the horny plate is formed at the +upper end of the second gill-cleft (Fig. 322 <i>A fl</i>). This sinks into a +sort of pit, and severs from the epidermis, just as the lens of the eye does. +In this way is formed at each side, directly under the horny plate of the back +part of the head, a small vesicle filled with fluid, the primitive auscultory +vesicle, or the primary labyrinth. As it separates from its source, the horny +plate, and presses inwards and backwards into the skull, it changes from round +to pear-shaped (Figs. 322 <i>B lv</i>, 323 <i>o</i>). The outer part of it is +lengthened into a thin stem, which at first still opens outwards by a narrow +canal. This is the labyrinthic appendage (Fig. 322 <i>lr</i>). In the lower +Vertebrates it develops into a special cavity filled with calcareous crystals, +which remains open permanently in some of the primitive fishes, and opens +outwards in the upper part of the skull. But in the mammals the labyrinthic +appendage degenerates. In these it has only a phylogenetic interest as a +rudimentary organ, with no actual physiological significance. The useless relic +of it passes through the wall of the petrous bone in the shape of a narrow +canal, and is called the vestibular aqueduct. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus322"></a> +<img src="images/fig322.gif" width="310" height="113" alt="Fig.322. Development of the auscultory labyrinth +of the chick, in five successive stages (A to E)." /> +<p class="caption">Fig. 322—<b>Development of the auscultory +labyrinth</b> of the chick, in five successive stages (<i>A–E</i>). +(Vertical transverse sections of the skull.) <i>fl</i> auscultory pits, +<i>lv</i> auscultory vesicles, <i>lr</i> labyrinthic appendage, <i>c</i> +rudimentary cochlea, <i>csp</i> posterior canal, <i>cse</i> external canal, +<i>jv</i> jugular vein. (From <i>Reissner.</i>)</p> +</div> + +<p> +It is only the inner and lower bulbous part of the separated auscultory vesicle +that develops into the highly complex and differentiated structure that is +afterwards known as the secondary labyrinth. This vesicle divides at an early +stage into an upper and larger and a lower and smaller section. From the one we +get the <i>utriculus</i> with the semi-circular canals; from the other the +<i>sacculus</i> and the cochlea (Fig. 320 <i>c</i>). The canals are formed in +the shape of simple pouch-like involutions of the utricle (<i>cse</i> and +<i>csp</i>). The edges join together in the middle part of each fold, and +separate from the utricle, the two ends remaining in open connection with its +cavity. All the Gnathostomes have these three canals like man, whereas among +the Cyclostomes the lampreys have only two and the hag-fishes only one. The +very complex structure of the cochlea, one of the most elaborate and wonderful +outcomes of adaptation in the mammal body, develops originally in very simple +fashion as a flask-like projection from the sacculus. As Hasse and Retzius have +pointed out, we find the successive ontogenetic stages of its growth +represented permanently in the series of the higher Vertebrates. The cochlea is +wanting even in the Monotremes, and is restricted to the rest of the mammals +and man. +</p> + +<p> +The auditory nerve, or eighth cerebral nerve, expands with one branch in the +cochlea, and with the other in the remaining parts of the labyrinth. This nerve +is, as Gegenbaur has shown, the sensory dorsal branch of a cerebro-spinal +nerve, the motor ventral branch of which acts for the muscles of the face +(<i>nervus facialis</i>). It has therefore originated phylogenetically from an +ordinary cutaneous nerve, and so is of quite different origin from the optic +and olfactory nerves, which both represent direct outgrowths of the brain. In +this respect the auscultory organ is essentially different from the organs of +sight and smell. The acoustic nerve is formed from ectodermic cells of the hind +brain, and develops from the nervous structure that appears at its dorsal +limit. On the other hand, all the membranous, cartilaginous, and osseous +coverings of the labyrinth are formed from the mesodermic head-plates. +</p> + +<p> +The apparatus for conducting sound which we find in the external and middle ear +of mammals develops quite separately from the apparatus for the sensation of +sound. It is both phylogenetically and ontogenetically an independent secondary +formation, a later accession +<span class='pagenum'><a name="Page_291" id="Page_291"></a></span> +to the primary internal ear. Nevertheless, its development is not less +interesting, and is explained with the same ease by comparative anatomy. In all +the fishes and in the lowest Vertebrates there is no special apparatus for +conducting sound, no external or middle ear; they have only a labyrinth, an +internal ear, which lies within the skull. They are without the tympanum and +tympanic cavity, and all its appendages. From many observations made in the +last few decades it seems that many of the fishes (if not all) cannot +distinguish tones; their labyrinth seems to be chiefly (if not exclusively) an +organ for the sense of space (or equilibrium). If it is destroyed, the fishes +lose their balance and fall. In the opinion of recent physiologists this +applies also to many of the Invertebrates (including the nearer ancestors of +the Vertebrates). The round vesicles which are considered to be their +auscultory vesicles, and which contain an otolith, are supposed to be merely +organs of the sense of space (“static vesicles or statocysts”). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus323"></a> +<img src="images/fig323.gif" width="173" height="124" alt="Fig.323. Primitive skull +of the human embryo, four weeks old, vertical section, left half seen +internally." /> +<p class="caption">Fig. 323—<b>Primitive skull of the human +embryo,</b> four weeks old, vertical section, left half seen internally. <i>v, +z, m, h, n</i> the five pits of the cranial cavity, in which the five cerebral +vesicles lie (fore, intermediate, middle, hind, and after brains), <i>o</i> +pear-shaped primary auscultory vesicle (appearing through), <i>a</i> eye +(appearing through), <i>no</i> optic nerve, <i>p</i> canal of the hypophysis, +<i>t</i> central prominence of the skull. (From <i>Kölliker.</i>)</p> +</div> + +<p> +The middle ear makes its first appearance in the amphibian class, where we find +a tympanum, tympanic cavity, and Eustachian tube; these animals, and all +terrestrial Vertebrates, certainly have the faculty of hearing. All these +essential parts of the middle ear originate from the first gill-cleft and its +surrounding part; in the Selachii this remains throughout life an open +squirting-hole, and lies between the first and second gill-arch. In the embryo +of the higher Vertebrates it closes up in the centre, and thus forms the +tympanic membrane. The outlying remainder of the first gill-cleft is the +rudiment of the external meatus. From its inner part we get the tympanic +cavity, and, further inward still, the Eustachian tube. Connected with this is +the development of the three bones of the mammal ear from the first two +gill-arches; the hammer and anvil are formed from the first, the stirrup from +the upper end of the second, gill-arch. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus324"></a> +<img src="images/fig324.gif" width="199" height="187" alt="Fig.324. The +rudimentary muscles of the ear in the human skull." /> +<p class="caption">Fig. 324—<b>The rudimentary muscles of the ear</b> +in the human skull. <i>a</i> raising muscle (<i>M. attollens</i>), <i>b</i> +drawing muscle (<i>M. attrahens</i>), <i>c</i> withdrawing muscle (<i>M. +retrahens</i>), <i>d</i> large muscle of the helix (<i>M. helicis major</i>), +<i>e</i> small muscle of the helix (<i>M. helicis minor</i>), <i>f</i> muscle +of the angle of the ear (<i>M. tragicus</i>), <i>g</i> anti-angular muscle +(<i>M. antitragicus</i>). (From <i>H. Meyer.</i>)</p> +</div> + +<p> +Finally, the shell (pinna or concha) and external meatus (passage to the +tympanum) of the outer ear are developed in a very simple fashion from the skin +that borders the external aperture of the first gill-cleft. The shell rises in +the shape of a circular fold of the skin, in which cartilage and muscles are +afterwards formed (Figs. 313, 315). This organ is only found in the mammalian +class. It is very rudimentary in the lowest section, the Monotremes. In the +others it is found at very different stages of development, and sometimes of +degeneration. It is degenerate in most of the aquatic mammals. The majority of +them have lost it altogether—for instance, the walruses and whales and +most of the seals. On the other hand, the pinna is well developed in the great +majority of the Marsupials and Placentals; it receives and collects the waves +of sound, and is equipped with a very elaborate muscular apparatus, by means of +which the pinna +<span class='pagenum'><a name="Page_292" id="Page_292"></a></span> +can be turned freely in any direction and its shape be altered. It is well +known how readily domestic animals—horses, cows, dogs, hares, +etc.—point their ears and move them in different directions. Most of the +apes do the same, and our earlier ape ancestors were also able to do it. But +our later simian ancestors, which we have in common with the anthropoid apes, +abandoned the use of these muscles, and they gradually became rudimentary and +useless. However, we possess them still (Fig. 324). In fact, some men can still +move their ears a little backward and forward by means of the drawing and +withdrawing muscles (<i>b</i> and <i>c</i>); with practice this faculty can be +much improved. But no man can now lift up his ears by the raising muscle +(<i>a</i>), or change the shape of them by the small inner muscles (<i>d, e, f, +g</i>). These muscles were very useful to our ancestors, but are of no +consequence to us. This applies to most of the anthropoid apes as well. +</p> + +<p> +We also share with the higher anthropoid apes (gorilla, chimpanzee, and orang) +the characteristic form of the human outer ear, especially the folded border, +the helix and the lobe. The lower apes have pointed ears, without folded border +or lobe, like the other mammals. But Darwin has shown that at the upper part of +the folded border there is in many men a small pointed process, which most of +us do not possess. In some individuals this process is well developed. It can +only be explained as the relic of the original point of the ear, which has been +turned inwards in consequence of the curving of the edge. If we compare the +pinna of man and the various apes in this respect, we find that they present a +connected series of degenerate structures. In the common catarrhine ancestors +of the anthropoids and man the degeneration set in with the folding together of +the pinna. This brought about the helix of the ear, in which we find the +significant angle which represents the relic of the salient point of the ear in +our earlier simian ancestors. Here again, therefore, comparative anatomy +enables us to trace with certainty the human ear to the similar, but more +developed, organ of the lower mammals. At the same time, comparative physiology +shows that it was a more or less useful implement in the latter, but it is +quite useless in the anthropoids and man. The conducting of the sound has +scarcely been affected by the loss of the pinna. We have also in this the +explanation of the extraordinary variety in the shape and size of the shell of +the ear in different men; in this it resembles other rudimentary organs. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap26"></a>Chapter XXVI.<br/> +EVOLUTION OF THE ORGANS OF MOVEMENT</h2> + +<p> +The peculiar structure of the locomotive apparatus is one of the features that +are most distinctive of the vertebrate stem. The chief part of this apparatus +is formed, as in all the higher animals, by the active organs of movement, the +muscles; in consequence of their contractility they have the power to draw up +and shorten themselves. This effects the movement of the various parts of the +body, and thus the whole body is conveyed from place to place. But the +arrangement of these muscles and their relation to the solid skeleton are +different in the Vertebrates from the Invertebrates. +</p> + +<p> +In most of the lower animals, especially the Platodes and Vermalia, we find +that the muscles form a simple, thin layer of flesh immediately underneath the +skin. This muscular layer is very closely connected with the skin itself; it is +the same in the Mollusc stem. Even in the large division of the Articulates, +the classes of crabs, spiders, myriapods, and +<span class='pagenum'><a name="Page_293" id="Page_293"></a></span> +<span class='pagenum'><a name="Page_294" id="Page_294"></a></span> +insects, we find a similar feature, with the difference that in this case the +skin forms a solid armour—a rigid cutaneous skeleton made of chitine (and +often also of carbonate of lime). This external chitine coat undergoes a very +elaborate articulation both on the trunk and the limbs of the Articulates, and +in consequence the muscular system also, the contractile fibres of which are +attached inside the chitine tubes, is highly articulated. The Vertebrates form +a direct contrast to this. In these alone a solid internal skeleton is +developed, of cartilage or bone, to which the muscles are attached. This bony +skeleton is a complex lever apparatus, or <i>passive</i> apparatus of movement. +Its rigid parts, the arms of the levers, or the bones, are brought together by +the actively mobile muscles, as if by drawing-ropes. This admirable +locomotorium, especially its solid central axis, the vertebral column, is a +special feature of the Vertebrates, and has given the name to the group. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus325"></a> +<img src="images/fig325.gif" width="317" height="599" alt="Fig.325. The human +skeleton from the right. Fig. 326. The human skeleton. Front." /> +<p class="caption">Fig. 325—<b>The human skeleton.</b> From the +right.<br/> Fig. 326—<b>The human skeleton.</b> Front.</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus327"></a> +<img src="images/fig327.gif" width="89" height="324" alt="Fig.327. The human +vertebral column (standing upright, from the right side)." /> +<p class="caption">Fig. 327—<b>The human vertebral column</b> +(standing upright, from the right side). (From <i>H. Meyer.</i>)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus328"></a> +<img src="images/fig328.gif" width="88" height="145" alt="Fig.328. A +piece of the axial rod (chorda dorsalis), from a sheep embryo." /> +<p class="caption">Fig. 328—<b>A piece of the axial rod</b> +(<i>chorda dorsalis</i>), from a sheep embryo. <i>a</i> cuticular sheath, +<i>b</i> cells. (From <i>Kölliker.</i>)</p> +</div> + +<p> +In order to get a clear idea of the chief features of the development of the +human skeleton, we must first examine its composition in the adult frame (Fig. +325, the human skeleton seen from the right; Fig. 326, front view of the whole +skeleton). As in other mammals, we distinguish first between the axial or +dorsal skeleton and the skeleton of the limbs. The axial skeleton consists of +the vertebral column (the skeleton of the trunk) and the skull (skeleton of the +head); the latter is a peculiarly modified part of the former. As appendages of +the vertebral column we have the ribs, and of the skull we have the hyoid bone, +the lower jaw, and the other products of the gill-arches. +</p> + +<p> +The skeleton of the limbs or extremities is composed of two groups of +parts—the skeleton of the extremities proper and the zone-skeleton, which +connects these with the vertebral column. The zone-skeleton of the arms (or +fore legs) is the shoulder-zone; the zone-skeleton of the legs (or hind legs) +is the pelvic zone. +</p> + +<p> +The vertebral column (Fig. 327) in man is composed of thirty-three to +thirty-five ring-shaped bones in a continuous series (above each other, in +man’s upright position). These <i>vertebræ</i> are separated from each +other by elastic ligaments, and at the same time connected by joints, so that +the whole column forms a firm and solid, but flexible and elastic, axial +skeleton, moving freely in all directions. The vertebræ differ in shape and +connection at the various parts of the trunk, and we distinguish the following +groups in the series, beginning at the top: Seven cervical vertebræ, twelve +dorsal vertebræ, five lumbar vertebræ, five sacral vertebræ, and four to six +caudal vertebræ. The uppermost, or those next to the skull, are the cervical +vertebræ (Fig. 327); they have a hole in each of the lateral processes. There +are seven of these vertebræ in man and almost all the other mammals, even if +the neck is as long as that of the camel or giraffe, or as short as that of the +mole or hedgehog. This constant number, which has few exceptions (due to +adaptation), is a strong proof of the common descent of the mammals; it can +only be explained by faithful heredity from a common stem-form, a primitive +mammal with seven cervical vertebræ. If each species had been created +separately, it would have been better to have given the long-necked mammals +more, and the short-necked animals less, cervical vertebræ. Next to these come +the dorsal (or pectoral) +<span class='pagenum'><a name="Page_295" id="Page_295"></a></span> +vertebræ, which number twelve to thirteen (usually twelve) in man and most of +the other mammals. Each dorsal vertebra (Fig. 165) has at the side, connected +by joints, a couple of ribs, long bony arches that lie in and protect the wall +of the chest. The twelve pairs of ribs, together with the connecting +intercostal muscles and the sternum, which joins the ends of the right and left +ribs in front, form the chest (<i>thorax</i>). In this strong and elastic frame +are the lungs, and between them the heart. Next to the dorsal vertebræ comes a +short but stronger section of the column, formed of five large vertebræ. These +are the lumbar vertebræ (Fig. 166); they have no ribs and no holes in the +transverse processes. To these succeeds the sacral bone, which is fitted +between the two halves of the pelvic zone. The sacrum is formed of five +vertebræ, completely blended together. Finally, we have at the end a small +rudimentary caudal column, the <i>coccyx.</i> This consists of a varying number +(usually four, more rarely three, or five or six) of small degenerated +vertebræ, and is a useless rudimentary organ with no actual physiological +significance. Morphologically, however, it is of great interest as an +irrefragable proof of the descent of man and the anthropoids from long-tailed +apes. On no other theory can we explain the existence of this rudimentary tail. +In the earlier stages of development the tail of the human embryo protrudes +considerably. It afterwards atrophies; but the relic of the atrophied caudal +vertebræ and of the rudimentary muscles that once moved it remains permanently. +Sometimes, in fact, the external tail is preserved. The older anatomists say +that the tail is usually one vertebra longer in the human female than in the +male (or four against five); Steinbach says it is the reverse. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus329"></a> +<img src="images/fig329.gif" width="98" height="147" alt="Fig.329. Three dorsal vertebræ, from a human +embryo, eight weeks old, in lateral longitudinal section." /> +<p class="caption">Fig. 329—<b>Three dorsal vertebræ,</b> from a +human embryo, eight weeks old, in lateral longitudinal section. <i>v</i> +cartilaginous vertebral body, <i>li</i> inter-vertebral disks, <i>ch</i> +chorda. (From <i>Kölliker.</i>)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus330"></a> +<img src="images/fig330.gif" width="180" height="113" alt="Fig.330. A dorsal vertebra of the same +embryo, in lateral transverse section." /> +<p class="caption">Fig. 330—<b>A dorsal vertebra of the same +embryo,</b> in lateral transverse section. <i>cv</i> cartilaginous vertebral +body, <i>ch</i> chorda, <i>pr</i> transverse process, <i>a</i> vertebral arch +(upper arch), <i>c</i> upper end of the rib (lower arch). (From +<i>Kölliker.</i>)</p> +</div> + +<p> +In the human vertebral column there are usually thirty-three vertebræ. It is +interesting to find, however, that the number often changes, one or two +vertebræ dropping out or an additional one appearing. Often, also, a mobile rib +is formed at the last cervical or the first lumbar vertebra, so that there are +then thirteen dorsal vertebræ, besides six cervical and four lumbar. In this +way the contiguous vertebræ of the various sections of the column may take each +other’s places. +</p> + +<p> +In order to understand the embryology of the human vertebral column we must +first carefully consider the shape and connection of the vertebræ. Each +vertebra has, in general, the shape of a seal-ring (Figs. 164–166). The +thicker portion, which is turned towards the ventral side, is called the body +of the vertebra, and forms a short osseous disk; the thinner part forms a +semi-circular arch, the <i>vertebral arch,</i> and is turned towards the back. +The arches of the successive vertebræ are connected by thin intercrural +ligaments in such a way that the cavity they collectively enclose represents a +long canal. In this vertebral canal we find the trunk part of the central +nervous system, the spinal cord. Its head part, the brain, is enclosed by the +skull, and the skull itself is merely the uppermost part of the vertebral +column, distinctively modified. The base or ventral side of the vesicular +cranial capsule corresponds originally to a number of developed vertebral +bodies; its vault or dorsal side to their combined upper vertebral arches. +</p> + +<p> +While the solid, massive bodies of the vertebræ represent the real central axis +of the skeleton, the dorsal arches serve to protect the central marrow they +enclose. But similar arches develop on the ventral side for the protection of +the viscera in the breast and belly. These lower or +<span class='pagenum'><a name="Page_296" id="Page_296"></a></span> +ventral vertebral arches, proceeding from the ventral side of the vertebral +bodies, form, in many of the lower Vertebrates, a canal in which the large +blood-vessels are enclosed on the lower surface of the vertebral column (aorta +and caudal vein). In the higher Vertebrates the majority of these vertebral +arches are lost or become rudimentary. But at the thoracic section of the +column they develop into independent strong osseous arches, the ribs +(<i>costæ</i>). In reality the ribs are merely large and independent lower +vertebral arches, which have lost their original connection with the vertebral +bodies. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus331"></a> +<img src="images/fig331.gif" width="203" height="162" alt="Fig.331. Intervertebral +disk of a new-born infant, transverse section." /> +<p class="caption">Fig. 331—<b>Intervertebral disk</b> of a new-born +infant, transverse section. <i>a</i> rest of the chorda. (From +<i>Kölliker.</i>)</p> +</div> + +<p> +If we turn from this anatomic survey of the composition of the column to the +question of its development, I may refer the reader to earlier pages with +regard to the first and most important points (pp. 145–148). It will be +remembered that in the human embryo and that of the other vertebrates we find +at first, instead of the segmented column, only a simple unarticulated +cartilaginous rod. This solid but flexible and elastic rod is the axial rod (or +the <i>chorda dorsalis</i>). In the lowest Vertebrate, the Amphioxus, it +retains this simple form throughout life, and permanently represents the whole +internal skeleton (Fig. 210 <i>i</i>). In the Tunicates, also, the nearest +Invertebrate relatives of the Vertebrates, we meet the same +chorda—transitorily in the passing larva tail of the Ascidia, permanently +in the Copelata (Fig. 225 <i>c</i>). Undoubtedly both the Tunicates and Acrania +have inherited the chorda from a common unsegmented stem-form; and these +ancient, long-extinct ancestors of all the chordonia are our hypothetical +Prochordonia. +</p> + +<p> +Long before there is any trace of the skull, limbs, etc., in the embryo of man +or any of the higher Vertebrates—at the early stage in which the whole +body is merely a sole-shaped embryonic shield—there appears in the middle +line of the shield, directly under the medullary furrow, the simple chorda. +(Cf. Figs. 131–135 <i>ch</i>). It follows the long axis of the body in +the shape of a cylindrical axial rod of elastic but firm composition, equally +pointed at both ends. In every case the chorda originates from the dorsal wall +of the primitive gut; the cells that compose it (Fig. 328 <i>b</i>) belong to +the entoderm (Figs. 216–221). At an early stage the chorda develops a +transparent structureless sheath, which is secreted from its cells (Fig. 328 +<i>a</i>). This <i>chordalemma</i> is often called the “inner +chorda-sheath,” and must not be confused with the real external sheath, +the mesoblastic perichorda. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus332"></a> +<img src="images/fig332.gif" width="150" height="128" alt="Fig. 332. Human skull." /> +<p class="caption">Fig. 332—<b>Human skull.</b></p> +</div> + +<p> +But this unsegmented primary axial skeleton is soon replaced by the segmented +secondary axial skeleton, which we know as the vertebral column. The +provertebral plates (Fig. 124 <i>s</i>) differentiate from the innermost, +median part of the visceral layer of the cœlom-pouches at each side of the +chorda. As they grow round the chorda and enclose it they form the skeleton +plate or skeletogenetic layer—that is to say, the skeleton-forming +stratum of cells, which provides the mobile foundation of the permanent +vertebral column and skull (scleroblast). In the head-half of the embryo the +skeletal plate remains a continuous, simple, undivided layer of tissue, and +presently enlarges into a thin-walled capsule enclosing the brain, the +primordial skull. In the trunk-half the provertebral +<span class='pagenum'><a name="Page_297" id="Page_297"></a></span> +plate divides into a number of homogeneous, cubical, successive pieces; these +are the several primitive vertebræ. They are not numerous at first, but soon +increase as the embryo grows longer (Figs. 153–155). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus333"></a> +<img src="images/fig133.gif" width="230" height="191" alt="Fig. 333. Skull of a new-born child." /> +<p class="caption">Fig. 333—<b>Skull of a new-born child.</b> (From +<i>Kollmann.</i>) Above, in the three bones of the roof of the skull, we see +the lines that radiate from the central points of ossification; in front, the +frontal bone; behind, the occipital bone; between the two the large parietal +bone, <i>p. s</i> the scurf bone, <i>w</i> mastoid fontanelle, <i>f</i> petrous +bone, <i>t</i> tympanic bone, <i>l</i> lateral part, <i>b</i> bulla, <i>j</i> +cheek-bone, <i>a</i> large wing of cuneiform bone, <i>k</i> fontanelle of +cuneiform bone.</p> +</div> + +<p> +In all the Craniotes the soft, indifferent cells of the mesoderm, which +originally compose the skeletal plate, are afterwards converted for the most +part into cartilaginous cells, and these secrete a firm and elastic +intercellular substance between them, and form cartilaginous tissue. Like most +of the other parts of the skeleton, the membranous rudiments of the vertebræ +soon pass into a cartilaginous state, and in the higher Vertebrates this is +afterwards replaced by the hard osseous tissue with its characteristic stellate +cells (Fig. 6). The primary axial skeleton remains a simple chorda throughout +life in the Acrania, the Cyclostomes, and the lowest fishes. In most of the +other Vertebrates the chorda is more or less replaced by the cartilaginous +tissue of the secondary perichorda that grows round it. In the lower Craniotes +(especially the fishes) a more or less considerable part of the chorda is +preserved in the bodies of the vertebræ. In the mammals it disappears for the +most part. By the end of the second month in the human embryo the chorda is +merely a slender thread, running through the axis of the thick, cartilaginous +vertebral column (Figs. 182 <i>ch,</i> 329 <i>ch</i>). In the cartilaginous +vertebral bodies themselves, which afterwards ossify, the slender remnant of +the chorda presently disappears (Fig. 330 <i>ch</i>). But in the elastic +inter-vertebral disks, which develop from the skeletal plate between each pair +of vertebral bodies (Fig. 329 <i>li</i>), a relic of the chorda remains +permanently. In the new-born child there is a large pear-shaped cavity in each +intervertebral disk, filled with a gelatinous mass of cells (Fig. 331 +<i>a</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus334"></a> +<img src="images/fig334.gif" width="278" height="153" alt="Fig.334. Head-skeleton of a primitive fish." /> +<p class="caption">Fig. 334—<b>Head-skeleton of a primitive fish.</b> +<i>n</i> nasal pit, <i>eth</i> cribriform bone region, <i>orb</i> orbit of eye, +<i>la</i> wall of auscultory labyrinth, <i>occ</i> occipital region of +primitive skull, <i>cv</i> vertebral column, <i>a</i> fore, <i>bc</i> hind-lip +cartilage, <i>o</i> primitive upper jaw (<i>palato-quadratum</i>), <i>u</i> +primitive lower jaw, <i>II</i> hyaloid bone, <i>III–VIII</i> first to +sixth branchial arches. (From <i>Gegenbaur.</i>)</p> +</div> + +<p> +Though less sharply defined, this gelatinous nucleus of the elastic +cartilaginous disks persists throughout life in the mammals, but in the birds +and most reptiles the last trace of the chorda disappears. In the subsequent +ossification of the cartilaginous vertebra the first deposit of bony matter +(“first osseous nucleus”) takes place in the vertebral body +immediately round the remainder of the chorda, and soon displaces it +altogether. Then there is a special osseous nucleus formed in each half of the +vertebral arch. The ossification does not reach the point at which the three +nuclei are joined until after birth. In the first year the two osseous halves +of the arches unite; but it is much later—in the second to the eighth +year— +<span class='pagenum'><a name="Page_298" id="Page_298"></a></span> +<span class='pagenum'><a name="Page_299" id="Page_299"></a></span> +that they connect with the osseous vertebral bodies. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus335"></a> +<img src="images/fig335.gif" width="345" height="484" alt="Fig.345. Roofs of the +skulls of nine Primates (Cattarrhines), seen from above and reduced to a common +size." /> +<p class="caption">Fig. 335—<b>Roofs of the skulls of nine +Primates</b> (<i>Cattarrhines</i>), seen from above and reduced to a common +size. <i>1</i> European, <i>2</i> Brazilian, <i>3</i> Pithecanthropus, <i>4</i> +Gorilla, <i>5</i> Chimpanzee, <i>6</i> Orang, <i>7</i> Gibbon, <i>8</i> Tailed +ape, <i>9</i> Baboon.</p> +</div> + +<p> +The bony skull (<i>cranium</i>), the head-part of the secondary axial skeleton, +develops in just the same way as the vertebral column. The skull forms a bony +envelope for the brain, just as the vertebral canal does for the spinal cord; +and as the brain is only a peculiarly differentiated part of the head, while +the spinal cord represents the longer trunk-section of the originally +homogeneous medullary tube, we shall expect to find that the osseous coat of +the one is a special modification of the osseous envelope of the other. When we +examine the adult human skull in itself (Fig. 332), it is difficult to conceive +how it can be merely the modified fore part of the vertebral column. It is an +elaborate and extensive bony structure, composed of no less than twenty bones +of different shapes and sizes. Seven of them form the spacious shell that +surrounds the brain, in which we distinguish the solid ventral base below and +the curved dorsal vault above. The other thirteen bones form the facial skull, +which is especially the bony envelope of the higher sense-organs, and at the +same time encloses the entrance of the alimentary canal. The lower jaw is +articulated at the base of the skull (usually regarded as the XXI cranial +bone). Behind the lower jaw we find the hyoid bone at the root of the tongue, +also formed from the gill-arches, and a part of the lower arches that have +developed as “head-ribs” from the ventral side of the base of the +cranium. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus336"></a> +<img src="images/fig336.gif" width="357" height="244" alt="Fig.336. Skeleton of +the breast-fin of Ceratodus (biserial feathered skeleton). Fig. 337. Skeleton +of the breast-fin of an early Selachius (Acanthias). Fig. 338. Skeleton of the +breast-fin of a young Selachius." /> +<p class="caption">Fig. 336—<b>Skeleton of the breast-fin of +Ceratodus</b> (biserial feathered skeleton). <i>A, B,</i> cartilaginous series +of the fin-stem. <i>rr</i> cartilaginous fin-radii. (From <i>Gunther.</i>)<br/> +Fig. 337—<b>Skeleton of the breast-fin of an early Selachius</b> +(<i>Acanthias</i>). The radii of the median fin-border (<i>B</i>) have +disappeared for the most part; a few only (<i>R</i>) are left. <i>R, R,</i> +radii of the lateral fin-border, <i>mt</i> metapterygium, <i>ms</i> +mesopterygium, <i>p</i> propterygium. (From <i>Gegenbaur.</i>)<br/> Fig. +338—<b>Skeleton of the breast-fin of a young Selachius.</b> The radii of +the median fin-border have wholly disappeared. The shaded part on the right is +the section that persists in the five-fingered hand of the higher Vertebrates. +(<i>b</i> the three basal pieces of the fin: <i>mt</i> metapterygium, rudiment +of the humerus, <i>ms</i> mesopterygium, <i>p</i> propterygium.) (From +<i>Gegenbaur.</i>)</p> +</div> + +<p> +Although the fully-developed skull of the higher Vertebrates, with its peculiar +shape, its enormous size, and its complex composition, seems to have nothing in +common with the ordinary vertebræ, nevertheless even the older comparative +anatomists came to recognise at the end of the eighteenth century that it is +really nothing else originally than a series of modified vertebræ. When Goethe +in 1790 “picked up the skull of a slain victim from the sand of the +Jewish cemetery at Venice, he noticed at once +<span class='pagenum'><a name="Page_300" id="Page_300"></a></span> +that the bones of the face also could be traced to vertebræ (like the three +hind-most cranial vertebræ).” And when Oken (without knowing anything of +Goethe’s discovery) found at Ilenstein, “a fine bleached skull of a +hind, the thought flashed across him like lightning: ‘It is a vertebral +column.’” +</p> + +<p> +This famous vertebral theory of the skull has interested the most distinguished +zoologists for more than a century: the chief representatives of comparative +anatomy have devoted their highest powers to the solution of the problem, and +the interest has spread far beyond their circle. But it was not until 1872 that +it was happily solved, after seven years’ labour, by the comparative +anatomist who surpassed all other experts of this science in the second half of +the nineteenth century by the richness of his empirical knowledge and the +acuteness and depth of his philosophic speculations. Carl Gegenbaur has shown, +in his classic <i>Studies of the Comparative Anatomy of the Vertebrates</i> +(third section), that we find the most solid foundation for the vertebral +theory of the skull in the head-skeleton of the Selachii. Earlier anatomists +had wrongly started from the mammal skull, and had compared the several bones +that compose it with the several parts of the vertebra (Fig. 333) they thought +they could prove in this way that the fully-formed mammal skull was made of +from three to six vertebræ. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus339"></a> +<a name="illus340"></a> +<a name="illus341"></a> +<img src="images/fig339.gif" width="420" height="250" alt="Fig.339. Skeleton of +the fore leg of an amphibian. Fig. 340. Skeleton of gorilla’s hand. Fig. +341. Skeleton of human hand, back." /> +<p class="caption">Fig. 339—<b>Skeleton of the fore leg of an +amphibian.</b> <i>h</i> upper-arm (humerus), <i>ru</i> lower arm (<i>r</i> +radius, <i>u</i> ulna), <i>rcicu′,</i> wrist-bones of first series +(<i>r</i> radiale, <i>i</i> intermedium, <i>c</i> centrale, <i>u′</i> +ulnare). <i>1, 2, 3, 4, 5</i> wrist-bones of the second series. (From +<i>Gegenbaur.</i>)<br/> +Fig. 340—<b>Skeleton of gorilla’s hand.</b> (From +<i>Huxley.</i>)<br/> +Fig. 341—<b>Skeleton of human hand,</b> back. (From +<i>Meyer.</i>)</p> +</div> + +<p> +The older theory was refuted by simple and obvious facts, which were first +pointed out by Huxley. Nevertheless, the fundamental idea of it—the +belief that the skull is formed from the head-part of the perichordal axial +skeleton, just as the brain is from the simple medullary tube, by +differentiation and modification—remained. The work now was to discover +the proper way of supplying this philosophic theory with an empirical +foundation, and it was reserved for Gegenbaur to achieve this. He first opened +out the phylogenetic path which here, as in all morphological questions, leads +most confidently to the goal. He showed that the primitive fishes (Figs. +249–251), the ancestors of all the Gnathostomes, still preserve +permanently in the form of their skull the structure out of which the +transformed skull of the higher Vertebrates, including man, has been evolved. +He further showed that +<span class='pagenum'><a name="Page_301" id="Page_301"></a></span> +the branchial arches of the Selachii prove that their skull originally +consisted of a large number of (at least nine or ten) provertebræ, and that the +cerebral nerves that proceed from the base of the brain entirely confirm this. +These cerebral nerves are (with the exception of the first and second pair, the +olfactory and optic nerves) merely modifications of spinal nerves, and are +essentially similar to them in their peripheral expansion. The comparative +anatomy of these cerebral nerves, their origin and their expansion, furnishes +one of the strongest arguments for the new vertebral theory of the skull. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus342"></a> +<img src="images/fig342.gif" width="418" height="228" alt="Fig.342. Skeleton of +the hand or fore foot of six mammals. I man, II dog, III pig, IV ox, V tapir, +VI horse." /> +<p class="caption">Fig. 342—<b>Skeleton of the hand or fore foot of +six mammals.</b> <i>I</i> man, <i>II</i> dog, <i>III</i> pig, <i>IV</i> ox, +<i>V</i> tapir, <i>VI</i> horse. <i>r</i> radius, <i>u</i> ulna, <i>a</i> +scaphoideum, <i>b</i> lunare, <i>a</i> triquetrum, <i>d</i> trapezium, <i>e</i> +trapezoid, <i>f</i> capitatum, <i>g</i> hamatum, <i>p</i> pisiforme. <i>1</i> +thumb, <i>2</i> index finger, <i>3</i> middle finger, <i>4</i> ring finger, +<i>5</i> little finger. (From <i>Gegenbaur.</i>)</p> +</div> + +<p> +We have not space here to go into the details of Gegenbaur’s theory of +the skull. I must be content to refer the reader to the great work I have +mentioned, in which it is thoroughly established from the +empirico-philosophical point of view. He has also given a comprehensive and +up-to-date treatment of the subject in his <i>Comparative Anatomy of the +Vertebrates</i> (1898). Gegenbaur indicates as original “cranial +ribs,” or “lower arches of the cranial vertebræ,” at each +side of the head of the Selachii (Fig. 334), the following pairs of arches: +<i>I</i> and <i>II,</i> two lip-cartilages, the anterior (<i>a</i>) of which is +composed of an upper piece only, the posterior (<i>bc</i>) from an upper and +lower piece; <i>III,</i> the maxillary arches, also consisting of two pieces on +each side—the primitive upper jaw (<i>os palato-quadratum, o</i>) and the +primitive lower jaw (<i>u</i>); <i>IV,</i> the hyaloid bone (<i>II</i>); +finally, <i>V–X,</i> six branchial arches in the narrower sense +(<i>III–VIII</i>). From the anatomic features of these nine to ten +cranial ribs or “lower vertebral arches” and the cranial nerves +that spread over them, it is clear that the apparently simple cartilaginous +primitive skull of the Selachii was originally formed from so many (at least +nine) somites or provertebræ. The blending of these primitive segments into a +single capsule is, however, so ancient that, in virtue of the law of curtailed +heredity, the original division seems to have disappeared; in the embryonic +development it is very difficult to detect it in isolated traces, and in some +respects quite impossible. It is claimed that several (three to six) traces of +provertebræ have been discovered in the anterior (pre-chordal) part of the +Selachii-skull; this would bring up the number of cranial somites to twelve or +sixteen, or even more. +</p> + +<p> +In the primitive skull of man (Fig. 323) and the higher Vertebrates, which has +been evolved from that of the Selachii, five consecutive sections are +discoverable at a certain early period of development, and one might be induced +to trace these to five primitive vertebræ; but these sections are due entirely +to adaptation to +<span class='pagenum'><a name="Page_302" id="Page_302"></a></span> +<span class='pagenum'><a name="Page_303" id="Page_303"></a></span> +the five primitive cerebral vesicles, and correspond, like these, to a large +number of metamera. That we have in the primitive skull of the mammals a +greatly modified and transformed organ, and not at all a primitive formation, +is clear from the circumstance that its original soft membranous form only +assumes the cartilaginous character for the most part at the base and the +sides, and remains membranous at the roof. At this part the bones of the +subsequent osseous skull develop as external coverings over the membranous +structure, without an intermediate cartilaginous stage, as there is at the base +of the skull. Thus a large part of the cranial bones develop originally as +covering bones from the corium, and only secondarily come into close touch with +the primitive skull (Fig. 333). We have previously seen how this very +rudimentary beginning of the skull in man is formed ontogenetically from the +“head-plates,” and thus the fore end of the chorda is enclosed in +the base of the skull. (Cf. Fig. 145 and pp. 138, 144, and 149.) +</p> + +<div class="fig" style="width:100%;"> +<a name="illus343"></a> +<img src="images/fig343.gif" width="305" height="573" alt="Fig.343-345. Arm and +hand of three anthropoids. Fig. 343. Chimpanzee (Anthropithecus niger). Fig. +344. Veddah of Ceylon (Homo veddalis). Fig. 345. European (Homo +mediterraneus)." /> +<p class="caption">Figs. 343–345—<b>Arm and hand of three +anthropoids.</b> Fig. 343—Chimpanzee (<i>Anthropithecus niger</i>). Fig. +344—Veddah of Ceylon (<i>Homo veddalis</i>). Fig. 345—European +(<i>Homo mediterraneus</i>). (From <i>Paul</i> and <i>Fritz +Sarasin.</i>)</p> +</div> + +<p> +The phylogeny of the skull has made great progress during the last three +decades through the joint attainments of comparative anatomy, ontogeny, and +paleontology. By the judicious and comprehensive application of the +phylogenetic method (in the sense of Gegenbaur) we have found the key to the +great and important problems that arise from the thorough comparative study of +the skull. Another school of research, the school of what is called +“exact craniology” (in the sense of Virchow), has, meantime, made +fruitless efforts to obtain this result. We may gratefully acknowledge all that +this descriptive school has done in the way of accurately describing the +various forms and measurements of the human skull, as compared with those of +other mammals. But the vast empirical material that it has accumulated in its +extensive literature is mere dead and sterile erudition until it is vivified +and illumined by phylogenetic speculation. +</p> + +<p> +Virchow confined himself to the most careful analysis of large numbers of human +skulls and those of anthropoid mammals. He saw only the differences between +them, and sought to express these in figures. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus346"></a> +<img src="images/fig346.gif" width="190" height="157" alt="Fig.346. Transverse +section of a fish’s tail (from the tunny)." /> +<p class="caption">Fig. 346—<b>Transverse section of a fish’s +tail</b> (from the tunny). (From <i>Johannes Müller.</i>) <i>a</i> upper +(dorsal) lateral muscles, <i>a′, b′</i> lower (ventral) lateral +muscles, <i>d</i> vertebral bodies, <i>b</i> sections of incomplete conical +mantle, <i>B</i> attachment lines of the inter-muscular ligaments (from the +side).</p> +</div> + +<p> +Without adducing a single solid reason, or offering any alternative +explanation, he rejected evolution as an unproved hypothesis. He played a most +unfortunate part in the controversy as to the significance of the fossil human +skulls of Spy and Neanderthal, and the comparison of them with the skull of the +Pithecanthropus (Fig. 283). All the interesting features of these skulls that +clearly indicated the transition from the anthropoid to the man were declared +by Virchow to be chance pathological variations. He said that the roof of the +skull of Pithecanthropus (Fig. 335, <i>3</i>) must have belonged to an ape, +because so pronounced an <i>orbital stricture</i> (the horizontal constriction +between the outer edge of the eye-orbit and the temples) is not found in any +human being. Immediately afterwards Nehring showed in the skull of a Brazilian +Indian (Fig. 335, <i>2</i>), found in the Sambaquis of Santos, that this +stricture can be even deeper in man than in many of the apes. It is very +instructive in this connection to compare the roofs of the skulls (seen from +above) of different primates. I have, therefore, arranged nine such skulls in +Fig. 335, and reduced them to a common size. +</p> + +<p> +We turn now to the branchial arches, which were regarded even by the earlier +natural philosophers as “head-ribs.” (Cf. Figs. 167–170). Of +the four original gill-arches of the mammals the first lies between the +primitive mouth and the first gill-cleft. From the base of this arch is formed +the upper-jaw process, which joins with the inner and outer nasal processes on +each side, in the manner we have previously explained, and forms the chief +parts of the skeleton of the upper jaw (palate bone, pterygoid bone, etc.) (Cf. +p. 284.) The remainder of the first branchial arch, which is now called, by +<span class='pagenum'><a name="Page_304" id="Page_304"></a></span> +way of contrast, the “upper-jaw process,” forms from its base two +of the ear-ossicles (hammer and anvil), and as to the rest is converted into a +long strip of cartilage that is known, after its discoverer, as +“Meckel’s cartilage,” or the <i>promandibula.</i> At the +outer surface of the latter is formed from the cellular matter of the corium, +as covering or accessory bone, the permanent bony lower jaw. From the first +part or base of the second branchial arch we get, in the mammals, the third +ossicle of the ear, the stirrup; and from the succeeding parts we get (in this +order) the muscle of the stirrup, the styloid process of the temporal bone, the +styloid-hyoid ligament, and the little horn of the hyoid bone. The third +branchial arch is only cartilaginous at the foremost part, and here the body of +the hyoid bone and its larger horn are formed at each side by the junction of +its two halves. The fourth branchial arch is only found transitorily in the +mammal embryo as a rudimentary organ, and does not develop special parts; and +there is no trace in the embryo of the higher Vertebrates of the posterior +branchial arches (fifth and sixth pair), which are permanent in the Selachii. +They have been lost long ago. Moreover, the four gill-clefts of the human +embryo are only interesting as rudimentary organs, and they soon close up and +disappear. The first alone (between the first and second branchial arches) has +any permanent significance; from it are developed the tympanic cavity and the +Eustachian tube. (Cf. Figs. 169, 320.) +</p> + +<p> +It was Carl Gegenbaur again who solved the difficult problem of tracing the +skeleton of the limbs of the Vertebrates to a common type. Few parts of the +vertebrate body have undergone such infinitely varied modifications in regard +to size, shape, and adaptation of structure as the limbs or extremities; yet we +are in a position to reduce them all to the same hereditary standard. We may +generally distinguish three groups among the Vertebrates in relation to the +formation of their limbs. The lowest and earliest Vertebrates, the Acrania and +Cyclostomes, had, like their invertebrate ancestors, no pairs of limbs, as we +see in the Amphioxus and the Cyclostomes to-day (Figs. 210, 247). The second +group is formed of the two classes of the true fishes and the Dipneusts; here +there are always two pairs of limbs at first, in the shape of many-toed +fins—one pair of breast-fins or fore legs, and one pair of belly-fins or +hind legs (Figs. 248–259). The third group comprises the four higher +classes of Vertebrates—the amphibia, reptiles, birds, and mammals; in +these quadrupeds there are at first the same two pairs of limbs, but in the +shape of five-toed feet. Frequently we find less than five toes, and sometimes +the feet are wholly atrophied (as in the serpents). But the original stem-form +of the group had five toes or fingers before and behind (Figs. 263–265). +</p> + +<p> +The true primitive form of the pairs of limbs, such as they were found in the +primitive fishes of the Silurian period, is preserved for us in the Australian +dipneust, the remarkable <i>Ceratodus</i> (Fig. 257). Both the breast-fin and +the belly-fin are flat oval paddles, in which we find a biserial cartilaginous +skeleton (Fig. 336). This consists, firstly, of a much segmented fin-rod or +“stem” (<i>A, B</i>), which runs through the fin from base to tip; +and secondly of a double row of thin articulated fin-radii (<i>r, r</i>), which +are attached to both sides of the fin-rod, like the feathers of a feathered +leaf. This primitive fin, which Gegenbaur first recognised, is attached to the +vertebral column by a simple zone in the shape of a cartilaginous arch. It has +probably originated from the branchial arches.<a href="#linknote-31" name="linknoteref-31" id="linknoteref-31"><sup>[31]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-31" id="linknote-31"></a> <a href="#linknoteref-31">[31]</a> +While Gegenbaur derives the fins from two pairs of posterior separated +branchial arches, Balfour holds that they have been developed from segments of +a pair of originally continuous lateral fins or folds of the skin.) +</p> + +<p> +We find the same biserial primitive fin more or less preserved in the +fossilised remains of the earliest Selachii (Fig. 248), Ganoids (Fig. 253), and +Dipneusts (Fig. 256). It is also found in modified form in some of the actual +sharks and pikes. But in the majority of the Selachii it has already +degenerated to the extent that the radii on one side of the fin-rod have been +partly or entirely lost, and are retained only on the other (Fig. 337). We thus +get the uniserial fin, which has been transmitted from the Selachii to the rest +of the fishes (Fig. 338). +</p> + +<p> +Gegenbaur has shown how the five-toed leg of the Amphibia, that has been +inherited by the three classes of Amniotes, was evolved from the uniserial +fish-fin.<a href="#linknote-32" name="linknoteref-32" id="linknoteref-32"><sup>[32]</sup></a> +<span class='pagenum'><a name="Page_305" id="Page_305"></a></span> +<span class='pagenum'><a name="Page_306" id="Page_306"></a></span> +</p> + +<p class="footnote"> +<a name="linknote-32" id="linknote-32"></a> <a href="#linknoteref-32">[32]</a> +The limb of the four higher classes of Vertebrates is now explained in the +sense that the original fin-rod passes along its outer (ulnar or fibular) side, +and ends in the fifth toe. It was formerly believed to go along the inner +(radial or tibial) side, and end in the first toe, as Fig. 339 shows.) In the +dipneust ancestors of the Amphibia the radii gradually atrophy, and are lost, +for the most part, on the other side of the fin-rod as well (the lighter +cartilages in Fig. 338). Only the four lowest radii (shaded in the +illustration) are preserved; and these are the four inner toes of the foot +(first to fourth). The little or fifth toe is developed from the lower end of +the fin-rod. From the middle and upper part of the fin-rod was developed the +long stem of the limb—the important radius and ulna (Fig. 339 <i>r</i> +and <i>u</i>) and humerus (<i>h</i>) of the higher Vertebrates. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus347"></a> +<a name="illus348"></a> +<img src="images/fig347.gif" width="334" height="527" alt="Fig.347. Human +skeleton. Fig. 348. Skeleton of the giant gorilla." /> +<p class="caption">Fig. 347—<b>Human skeleton.</b> (Cf. Figure +326.)<br/> Fig. 348—<b>Skeleton of the giant gorilla.</b> (Cf. Figure +209.)</p> +</div> + +<p> +In this way the five-toed foot of the Amphibia, which we first meet in the +Carboniferous Stegocephala (Fig. 260), and which was inherited from them by the +reptiles on one side and the mammals on the other, was formed by gradual +degeneration and differentiation from the many-toed fish-fin (Fig. 341). The +reduction of the radii to four was accompanied by a further differentiation of +the fin-rod, its transverse segmentation into upper and lower halves, and the +formation of the zone of the limb, which is composed originally of three limbs +before and behind in the higher Vertebrates. The simple arch of the original +shoulder-zone divides on each side into an upper (dorsal) piece, the +shoulder-blade (<i>scapula</i>), and a lower (ventral) piece; the anterior part +of the latter forms the primitive clavicle (<i>procoracoideum</i>), and the +posterior part the <i>coracoideum.</i> In the same way the simple arch of the +pelvic zone breaks up into an upper (dorsal) piece, the iliac-bone (<i>os +ilium</i>), and a lower (ventral) piece; the anterior part of the latter forms +the pubic bone (<i>os pubis</i>), and the posterior the ischial bone (<i>os +ischii</i>). +</p> + +<p> +There is also a complete agreement between the fore and hind limb in the stem +or shaft. The first section of the stem is supported by a single strong +bone—the humerus in the fore, the femur in the hind limb. The second +section contains two bones: in front the radius (<i>r</i>) and ulna (<i>u</i>), +behind the tibia and fibula. (Cf. the skeletons in Figs. 260, 265, 270, +278–282, and 348.) The succeeding numerous small bones of the wrist +(<i>carpus</i>) and ankle (<i>tarsus</i>) are also similarly arranged in the +fore and hind extremities, and so are the five bones of the middle-hand +(<i>metacarpus</i>) and middle-foot (<i>metatarsus</i>). Finally, it is the +same with the toes themselves, which have a similar characteristic composition +from a series of bony pieces before and behind. We find a complete parallel in +all the parts of the fore leg and the hind leg. +</p> + +<p> +When we thus learn from comparative anatomy that the skeleton of the human +limbs is composed of just the same bones, put together in the same way, as the +skeleton in the four higher classes of Vertebrates, we may at once infer a +common descent of them from a single stem-form. This stem-form was the earliest +amphibian that had five toes on each foot. It is particularly the outer parts +of the limbs that have been modified by adaptation to different conditions. We +need only recall the immense variations they offer within the mammal class. We +have the slender legs of the deer and the strong springing legs of the +kangaroo, the climbing feet of the sloth and the digging feet of the mole, the +fins of the whale and the wings of the bat. It will readily be granted that +these organs of locomotion differ as much in regard to size, shape, and special +function as can be conceived. Nevertheless, the bony skeleton is substantially +the same in every case. In the different limbs we always find the same +characteristic bones in essentially the same rigidly hereditary connection; +this is as splendid a proof of the theory of evolution as comparative anatomy +can discover in any organ of the body. It is true that the skeleton of the +limbs of the various mammals undergoes many distortions and degenerations +besides the special adaptations (Fig. 342). Thus we find the first finger or +the thumb atrophied in the fore-foot (or hand) of the dog (<i>II</i>). It has +entirely disappeared in the pig (<i>III</i>) and tapir (<i>V</i>). In the +ruminants (such as the ox, <i>IV</i>) the second and fifth toes are also +atrophied, and only the third and fourth are well developed (<i>VI, 3</i>). +Nevertheless, all these different fore-feet, as well as the hand of the ape +(Fig. 340) and of man (Fig. 341), were originally developed from a common +pentadactyle stem-form. This is proved by the rudiments of the degenerated +toes, and by the similarity of the arrangement of the wrist-bones in all the +pentanomes (Fig. 342 <i>a–p</i>). +</p> + +<p> +If we candidly compare the bony skeleton of the human arm and hand with that of +the nearest anthropoid apes, we find an almost perfect identity. This is +especially true of the chimpanzee. In regard to the proportions of the various +<span class='pagenum'><a name="Page_307" id="Page_307"></a></span> +parts, the lowest living races of men (the Veddahs of Ceylon, Fig. 344) are +midway between the chimpanzee (Fig. 343) and the European (Fig. 345). More +considerable are the differences in structure and the proportions of the +various parts between the different genera of anthropoid apes (Figs. +278–282); and still greater is the morphological distance between these +and the lowest apes (the <i>Cynopitheca</i>). Here, again, impartial and +thorough anatomic comparison confirms the accuracy of Huxley’s +pithecometra principle p. 171. +</p> + +<p> +The complete unity of structure which is thus revealed by the comparative +anatomy of the limbs is fully confirmed by their embryology. However different +the extremities of the four-footed Craniotes may be in their adult state, they +all develop from the same rudimentary structure. In every case the first trace +of the limb in the embryo is a very simple protuberance that grows out of the +side of the hyposoma. These simple structures develop directly into fins in the +fishes and Dipneusts by differentiation of their cells. In the higher classes +of Vertebrates each of the four takes the shape in its further growth of a leaf +with a stalk, the inner half becoming narrower and thicker and the outer half +broader and thinner. The inner half (the stalk of the leaf) then divides into +two sections—the upper and lower parts of the limb. Afterwards four +shallow indentations are formed at the free edge of the leaf, and gradually +deepen; these are the intervals between the five toes (Fig. 174). The toes soon +make their appearance. But at first all five toes, both of fore and hind feet, +are connected by a thin membrane like a swimming-web; they remind us of the +original shaping of the foot as a paddling fin. The further development of the +limbs from this rudimentary structure takes place in the same way in all the +Vertebrates according to the laws of heredity. +</p> + +<p> +The embryonic development of the muscles, or <i>active</i> organs of +locomotion, is not less interesting than that of the skeleton, or +<i>passive</i> organs. But the comparative anatomy and ontogeny of the muscular +system are much more difficult and inaccessible, and consequently have hitherto +been less studied. We can therefore only draw some general phylogenetic +conclusions therefrom. +</p> + +<p> +It is incontestable that the musculature of the Vertebrates has been evolved +from that of lower Invertebrates; and among these we have to consider +especially the unarticulated Vermalia. They have a simple cutaneous muscular +layer, developing from the mesoderm. This was afterwards replaced by a pair of +internal lateral muscles, that developed from the middle wall of the +cœlom-pouches; we still find the first rudiments of the muscles arising from +the muscle-plate of these in the embryos of all the Vertebrates (cf. Figs. 124, +158–160, 222–224 <i>mp</i>). In the unarticulated stem-forms of the +Chordonia, which we have called the Prochordonia, the two cœlom-pouches, and +therefore also the muscle-plates of their walls, were not yet segmented. A +great advance was made in the articulation of them, as we have followed it step +by step in the Amphioxus (Figs. 124, 158). This segmentation of the muscles was +the momentous historical process with which vertebration, and the development +of the vertebrate stem, began. The articulation of the skeleton came after this +segmentation of the muscular system, and the two entered into very close +correlation. +</p> + +<p> +The episomites or dorsal cœlom-pouches of the Acrania, Cyclostomes, and +Selachii (Fig. 161 <i>h</i>) first develop from their inner or median wall +(from the cell-layer that lies directly on the skeletal plate [<i>sk</i>] and +the medullary tube [<i>nr</i>]) a strong muscle-plate (<i>mp</i>). By dorsal +growth (<i>w</i>) it also reaches the external wall of the cœlom-pouches, and +proceeds from the dorsal to the ventral wall. From these segmental +muscle-plates, which are chiefly concerned in the segmentation of the +Vertebrates, proceed the lateral muscles of the stem, as we find in the +simplest form in the Amphioxus (Fig. 210). By the formation of a horizontal +frontal septum they divide on each side into an upper and lower series of +myotomes, dorsal and ventral lateral muscles. This is seen with typical +regularity in the transverse section of the tail of a fish (Fig. 346). From +these earlier lateral muscles of the trunk develop the greater part of the +subsequent muscles of the trunk, and also the much later “muscular +buds” of the limbs.<a href="#linknote-33" name="linknoteref-33" id="linknoteref-33"><sup>[33]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-33" id="linknote-33"></a> <a href="#linknoteref-33">[33]</a> +The ontogeny of the muscles is mostly cenogenetic. The greater part of the +muscles of the head (or the visceral muscles) belong originally to the hyposoma +of the vertebrate organism, and develop from the wall of the hyposomites or +ventral cœlom-pouches. This also applies originally to the primary muscles of +the limbs, as these too belong phylogenetically to the hyposoma. (Cf. Chapter +XIV.) +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap27"></a> +<span class='pagenum'><a name="Page_308" id="Page_308"></a></span> +Chapter XXVII.<br/> +THE EVOLUTION OF THE ALIMENTARY SYSTEM</h2> + +<p> +The chief of the vegetal organs of the human frame, to the evolution of which +we now turn our attention, is the alimentary canal. The gut is the oldest of +all the organs of the metazoic body, and it leads us back to the earliest age +of the formation of organs—to the first section of the Laurentian period. +As we have already seen, the result of the first division of labour among the +homogeneous cells of the earliest multicellular animal body was the formation +of an alimentary cavity. The first duty and first need of every organism is +self-preservation. This is met by the functions of the nutrition and the +covering of the body. When, therefore, in the primitive globular <i>Blastæa</i> +the homogeneous cells began to effect a division of labour, they had first to +meet this twofold need. One half were converted into alimentary cells and +enclosed a digestive cavity, the gut. The other half became covering cells, and +formed an envelope round the alimentary tube and the whole body. Thus arose the +primary germinal layers—the inner, alimentary, or vegetal layer, and the +outer, covering, or animal layer. (Cf. pp. 214–17.) +</p> + +<p> +When we try to construct an animal frame of the simplest conceivable type, that +has some such primitive alimentary canal and the two primary layers +constituting its wall, we inevitably come to the very remarkable embryonic form +of the gastrula, which we have found with extraordinary persistence throughout +the whole range of animals, with the exception of the unicellulars—in the +Sponges, Cnidaria, Platodes, Vermalia, Molluscs, Articulates, Echinoderms, +Tunicates, and Vertebrates. In all these stems the gastrula recurs in the same +very simple form. It is certainly a remarkable fact that the gastrula is found +in various animals as a larva-stage in their individual development, and that +this gastrula, though much disguised by cenogenetic modifications, has +everywhere essentially the same palingenetic structure (Figs. 30–35). The +elaborate alimentary canal of the higher animals develops ontogenetically from +the same simple primitive gut of the <i>gastrula.</i> +</p> + +<p> +This gastræa theory is now accepted by nearly all zoologists. It was first +supported and partly modified by Professor Ray-Lankester; he proposed three +years afterwards (in his essay on the development of the Molluscs, 1875) to +give the name of <i>archenteron</i> to the primitive gut and <i>blastoporus</i> +to the primitive mouth. +</p> + +<p> +Before we follow the development of the human alimentary canal in detail, it is +necessary to say a word about the general features of its composition in the +fully-developed man. The mature alimentary canal in man is constructed in all +its main features like that of all the higher mammals, and particularly +resembles that of the Catarrhines, the narrow-nosed apes of the Old World. The +entrance into it, the mouth, is armed with thirty-two teeth, fixed in rows in +the upper and lower jaws. As we have seen, our dentition is exactly the same as +that of the Catarrhines, and differs from that of all other animals p. 257. +Above the mouth-cavity is the double nasal cavity; they are separated by the +palate-wall. But we saw that this separation is not there from the first, and +that originally there is a common mouth-nasal cavity in the embryo; and this is +only divided afterwards by the hard palate into two—the nasal cavity +above and that of the mouth below (Fig. 311). +</p> + +<p> +At the back the cavity of the mouth is half closed by the vertical curtain that +we call the soft palate, in the middle of which is the uvula. A glance into a +mirror with the mouth wide open will show its shape. The uvula is interesting +because, besides man, it is only found in the ape. At each side of the soft +palate are the tonsils. Through the curved opening that we find +<span class='pagenum'><a name="Page_309" id="Page_309"></a></span> +underneath the soft palate we penetrate into the gullet or pharynx behind the +mouth-cavity. Into this opens on either side a narrow canal (the Eustachian +tube), through which there is direct communication with the tympanic cavity of +the ear (Fig. 320 <i>e</i>). The pharynx is continued in a long, narrow tube, +the œsophagus ( <i>sr</i>). By this the food passes into the stomach when +masticated and swallowed. Into the gullet also opens, right above, the trachea +( <i>lr</i>), that leads to the lungs. The entrance to it is covered by the +epiglottis, over which the food slides. The cartilaginous epiglottis is found +only in the mammals, and has developed from the fourth branchial arch of the +fishes and amphibia. The lungs are found, in man and all the mammals, to the +right and left in the pectoral cavity, with the heart between them. At the +upper end of the trachea there is, under the epiglottis, a specially +differentiated part, strengthened by a cartilaginous skeleton, the larynx. This +important organ of human speech also develops from a part of the alimentary +canal. In front of the larynx is the thyroid gland, which sometimes enlarges +and forms goitre. +</p> + +<p> +The œsophagus descends into the pectoral cavity along the vertebral column, +behind the lungs and the heart, pierces the diaphragm, and enters the visceral +cavity. The diaphragm is a membrano-muscular partition that completely +separates the thoracic from the abdominal cavity in all the mammals (and these +alone). This separation is not found in the beginning; there is at first a +common breast-belly cavity, the cœloma or pleuro-peritoneal cavity. The +diaphragm is formed later on as a muscular horizontal partition between the +thoracic and abdominal cavities. It then completely separates the two cavities, +and is only pierced by several organs that pass from the one to the other. One +of the chief of these organs is the œsophagus. After this has passed through +the diaphragm, it expands into the gastric sac in which digestion chiefly takes +place. The stomach of the adult man (Fig. 349) is a long, somewhat oblique sac, +expanding on the left into a blind sac, the fundus of the stomach ( +<i>b′</i>), but narrowing on the right, and passing at the pylorus ( +<i>e</i>) into the small intestine. At this point there is a valve, the pyloric +valve ( <i>d</i>), between the two sections of the canal; it opens only when +the pulpy food passes from the stomach into the intestine. In man and the +higher Vertebrates the stomach itself is the chief organ of digestion, and is +especially occupied with the solution of the food; this is not the case in many +of the lower Vertebrates, which have no stomach, and discharge its function by +a part of the gut farther on. The muscular wall of the stomach is comparatively +thick; it has externally strong muscles that accomplish the digestive +movements, and internally a large quantity of small glands, the peptic glands, +which secrete the gastric juice. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus349"></a> +<img src="images/fig349.gif" width="231" height="158" alt="Fig.349. Human stomach +and duodenum, longitudinal section." /> +<p class="caption">Fig. 349—<b>Human stomach and duodenum,</b> +longitudinal section. <i>a</i> cardiac (end of œsophagus), <i>b</i> fundus +(blind sac of the left side), <i>c</i> pylorus-fold, <i>d</i> pylorus-valves, +<i>e</i> pylorus-cavity, <i>fgh</i> duodenum, <i>i</i> entrance of the +gall-duct and the pancreatic duct. (From <i>Meyer.</i>)</p> +</div> + +<p> +Next to the stomach comes the longest section of the alimentary canal, the +middle gut or small intestine. Its chief function is to absorb the peptonised +fluid mass of food, or the chyle, and it is subdivided into several sections, +of which the first (next to the stomach) is called the duodenum (Fig. 349 +<i>fgh</i>). It is a short, horseshoe-shaped loop of the gut. The largest +glands of the alimentary canal open into it—the liver, the chief +digestive gland, that secretes the gall, and the pancreas, which secretes the +pancreatic juice. The two glands pour their secretions, the bile and pancreatic +juice, close together into the duodenum ( <i>i</i>). The opening of the +gall-duct is of particular phylogenetic importance, as it is the same in all +the Vertebrates, and indicates the principal point of the hepatic or trunk-gut +(Gegenbaur). The liver, phylogenetically older than the stomach, is a large +gland, rich in blood, in the adult man, immediately under the diaphragm on the +left +<span class='pagenum'><a name="Page_310" id="Page_310"></a></span> +side, and separated by it from the lungs. The pancreas lies a little further +back and more to the left. The remaining part of the small intestine is so long +that it has to coil itself in many folds in order to find room in the narrow +space of the abdominal cavity. It is divided into the jejunum above and the +ileum below. In the last section of it is the part of the small intestine at +which in the embryo the yelk-sac opens into the gut. This long and thin +intestine then passes into the large intestine, from which it is cut off by a +special valve. Immediately behind this “Bauhin-valve” the first +part of the large intestine forms a wide, pouch-like structure, the cæcum. The +atrophied end of the cæcum is the famous rudimentary organ, the vermiform +appendix. The large intestine ( <i>colon</i>) consists of three parts—an +ascending part on the right, a transverse middle part, and a descending part on +the left. The latter finally passes through an S-shaped bend into the last +section of the alimentary canal, the rectum, which opens behind by the anus. +Both the large and small intestines are equipped with numbers of small glands, +which secrete mucous and other fluids. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus350"></a> +<img src="images/fig350.gif" width="206" height="179" alt="Fig.350. Median section +of the head of a hare-embryo, one-fourth of an inch in length." /> +<p class="caption">Fig. 350—<b>Median section of the head of a +hare-embryo,</b> one-fourth of an inch in length. (From <i>Mihalcovics.</i>) +The deep mouth-cleft ( <i>hp</i>) is separated by the membrane of the throat ( +<i>rh</i>) from the blind cavity of the head-gut ( <i>kd</i>). <i>hz</i> heart, +<i>ch</i> chorda, <i>hp</i> the point at which the hypophysis develops from the +mouth-cleft, <i>vh</i> ventricle of the cerebrum, <i>v3</i> , third ventricle +(intermediate brain), <i>v4</i> fourth ventricle (hind brain), <i>ck</i> spinal +canal.</p> +</div> + +<p> +For the greater part of its length the alimentary canal is attached to the +inner dorsal surface of the abdominal cavity, or to the lower surface of the +vertebral column. The fixing is accomplished by means of the thin membranous +plate that we call the mesentery. +</p> + +<p> +Although the fully-formed alimentary canal is thus a very elaborate organ, and +although in detail it has a quantity of complex structural features into which +we cannot enter here, nevertheless the whole complicated structure has been +historically evolved from the very simple form of the primitive gut that we +find in our gastræad-ancestors, and that every gastrula brings before us +to-day. We have already pointed out (Chapter IX) how the epigastrula of the +mammals (Fig. 67) can be reduced to the original type of the bell-gastrula, +which is now preserved by the amphioxus alone (Fig. 35). Like the latter, the +human gastrula and that of all other mammals must be regarded as the +ontogenetic reproduction of the phylogenetic form that we call the Gastræa, in +which the whole body is nothing but a double-walled gastric sac. +</p> + +<p> +We already know from embryology the manner in which the gut develops in the +embryo of man and the other mammals. From the gastrula is first formed the +spherical embryonic vesicle filled with fluid ( <i>gastrocystis,</i> Fig. 106). +In the dorsal wall of this the sole-shaped embryonic shield is developed, and +on the under-side of this a shallow groove appears in the middle line, the +first trace of the later, secondary alimentary tube. The gut-groove becomes +deeper and deeper, and its edges bend towards each other, and finally form a +tube. +</p> + +<p> +As we have seen, this simple cylindrical gut-tube is at first completely closed +before and behind in man and in the Vertebrates generally (Fig. 148); the +permanent openings of the alimentary canal, the mouth and anus, are only formed +later on, and from the outer skin. A mouth-pit appears in the skin in front +(Fig. 350 <i>hp</i>), and this grows towards the blind fore-end of the cavity +of the head-gut ( <i>kd</i>), and at length breaks into it. In the same way a +shallow anus-pit is formed in the skin behind, which grows deeper and deeper, +advances towards the blind hinder end of the pelvic gut, and at last connects +with it. There is at first, both before and behind, a thin partition between +the external cutaneous pit and the blind end of the gut—the +throat-membrane in front and the anus-membrane behind; these disappear when the +connection takes place. +</p> + +<p> +Directly in front of the anus-opening the allantois develops from the hind gut; +this is the important embryonic structure +<span class='pagenum'><a name="Page_311" id="Page_311"></a></span> +that forms into the placenta in the Placentals (including man). In this more +advanced form the human alimentary canal (and that of all the other mammals) is +a slightly bent, cylindrical tube, with an opening at each end, and two +appendages growing from its lower wall: the anterior one is the umbilical +vesicle or yelk-sac, and the posterior the allantois or urinary sac (Fig. 195). +</p> + +<p> +The thin wall of this simple alimentary tube and its ventral appendages is +found, on microscopic examination, to consist of two strata of cells. The inner +stratum, lining the entire cavity, consists of larger and darker cells, and is +the gut-gland layer. The outer stratum consists of smaller and lighter cells, +and is the gut-fibre layer. The only exception is in the cavities of the mouth +and anus, because these originate from the skin. The inner coat of the +mouth-cavity is not provided by the gut-gland layer, but by the skin-sense +layer; and its muscular substratum is provided, not by the gut-fibre, but the +skin-fibre, layer. It is the same with the wall of the small anus-cavity. +</p> + +<p> +If it is asked how these constituent layers of the primitive gut-wall are +related to the various tissues and organs that we find afterwards in the +fully-developed system, the answer is very simple. It can be put in a single +sentence. The epithelium of the gut—that is to say, the internal soft +stratum of cells that lines the cavity of the alimentary canal and all its +appendages, and is immediately occupied with the processes of +nutrition—is formed solely from the gut-gland layer; all other tissues +and organs that belong to the alimentary canal and its appendages originate +from the gut-fibre layer. From the latter is also developed the whole of the +outer envelope of the gut and its appendages; the fibrous connective tissue and +the smooth muscles that compose its muscular layer, the cartilages that support +it (such as the cartilages of the larynx and the trachea), the blood-vessels +and lymph-vessels that absorb the nutritive fluid from the intestines—in +a word, all that there is in the alimentary system besides the epithelium of +the gut. From the same layer we also get the whole of the mesentery, with all +the organs embedded in it—the heart, the large blood-vessels of the body, +etc. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus351"></a> +<img src="images/fig351.gif" width="144" height="266" alt="Fig.351. Scales or +cutaneous teeth of a shark (Centrophorus calceus)." /> +<p class="caption">Fig. 351—<b>Scales or cutaneous teeth of a +shark</b> ( <i>Centrophorus calceus</i>). A three-pointed tooth rises obliquely +on each of the quadrangular bony plates that lie in the corium. (From +<i>Gegenbaur.</i>)</p> +</div> + +<p> +Let us now leave this original structure of the mammal gut for a moment, in +order to compare it with the alimentary canal of the lower Vertebrates, and of +those Invertebrates that we have recognised as man’s ancestors. We find, +first of all, in the lowest Metazoa, the Gastræads, that the gut remains +permanently in the very simple form in which we find it transitorily in the +palingenetic gastrula of the other animals; it is thus in the Gastremaria ( +<i>Pemmatodiscus</i>), the Physemaria ( <i>Prophysema</i>), the simplest +Sponges ( <i>Olynthus</i>), the freshwater Polyps ( <i>Hydra</i>), and the +ascula-embryos of many other Cœlenteria (Figs. 233–238). Even in the +simplest forms of the Platodes, the Rhabdocœla (Fig. 240), the gut is still a +simple straight tube, lined with the entoderm; but with the important +difference that in this case its single opening, the primitive mouth ( +<i>m</i>), has formed a muscular gullet ( <i>sd</i>) by invagination of the +skin. +</p> + +<p> +We have the same simple form in the gut of the lowest Vermalia (Gastrotricha, +Fig. 242, Nematodes, Sagitta, etc.). But in these a second important opening of +the gut has been formed at the opposite end to the mouth, the anus (Fig. 242 +<i>a</i>). +</p> + +<p> +<span class='pagenum'><a name="Page_312" id="Page_312"></a></span> +We see a great advance in the structure of the vermalian gut in the remarkable +<i>Balanoglossus</i> (Fig. 245), the sole survivor of the Enteropneust class. +Here we have the first appearance of the division of the alimentary tube into +two sections that characterises the Chordonia. The fore half, the head-gut ( +<i>cephalogaster</i>), becomes the organ of respiration (branchial gut, Fig. +245 <i>k</i>); the hind half, the trunk-gut ( <i>truncogaster</i>), alone acts +as digestive organ (hepatic gut, <i>d</i>). The differentiation of these two +parts of the gut in the Enteropneust is just the same as in all the Tunicates +and Vertebrates. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus352"></a> +<img src="images/fig352.gif" width="368" height="322" alt="Fig.352. Gut of a +human embryo, one-sixth of an inch long." /> +<p class="caption">Fig. 352—<b>Gut of a human embryo,</b> one-sixth +of an inch long. (From <i>His.</i>)</p> +</div> + +<p> +It is particularly interesting and instructive in this connection to compare +the Enteropneusts with the Ascidia and the Amphioxus (Figs. 220, 210)—the +remarkable animals that form the connecting link between the Invertebrates and +the Vertebrates. In both forms the gut is of substantially the same +construction; the anterior section forms the respiratory branchial gut, the +posterior the digestive hepatic gut. In both it develops palingenetically from +the primitive gut of the gastrula, and in both the hinder end of the medullary +tube covers the primitive mouth to such an extent that the remarkable medullary +intestinal duct is formed, the passing communication between the neural and +intestinal tubes ( <i>canalis neurentericus,</i> Figs. 83, 85 <i>ne</i>). In +the vicinity of the closed primitive mouth, possibly in its place, the later +anus is developed. In the same way the mouth is a fresh formation in the +Amphioxus and the Ascidia. It is the same with the human mouth and that of the +Craniotes generally. The secondary formation of the mouth in the Chordonia is +probably connected with the development of the gill-clefts which are formed in +the gut-wall immediately behind the mouth. In this way the anterior section of +the gut is converted into a respiratory organ. I have already pointed out that +this modification is distinctive of the +<span class='pagenum'><a name="Page_313" id="Page_313"></a></span> +Vertebrates and Tunicates. The phylogenetic appearance of the gill-clefts +indicates the commencement of a new epoch in the stem-history of the +Vertebrates. +</p> + +<p> +In the further ontogenetic development of the alimentary canal in the human +embryo the appearance of the gill-clefts is the most important process. At a +very early stage the gullet-wall joins with the external body-wall in the head +of the human embryo, and this is followed by the formation of four clefts, +which lead directly into the gullet from without, on the right and left sides +of the neck, behind the mouth. These are the gill or gullet clefts, and the +partitions that separate them are the gill or gullet-arches (Fig. 171). These +are most interesting embryonic structures. They show us that all the higher +Vertebrates reproduce in their earlier stages, in harmony with the biogenetic +law, the process that had so important a part in the rise of the whole +Chordonia-stem. This process was the differentiation of the gut into two +sections—an anterior respiratory section, the branchial gut, that was +restricted to breathing, and a posterior digestive section, the hepatic gut. As +we find this highly characteristic differentiation of the gut into two +different sections in all the Vertebrates and all the Tunicates, we may +conclude that it was also found in their common ancestors, the +Prochordonia—especially as even the Enteropneusts have it. (Cf. pp. 119, +151, 227, Figs. 210, 220, 245.) It is entirely wanting in all the other +Invertebrates. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus353"></a> +<img src="images/fig353.gif" width="208" height="226" alt="Fig.353. Gut of a +dog-embryo (shown in Fig. 202, from Bischoff), seen from the ventral side. +Fig. 354. The same gut seen from the right." /> +<p class="caption">Fig. 353—<b>Gut of a dog-embryo</b> (shown in +Fig. 202, from <i>Bischoff</i>), seen from the ventral side. <i>a</i> +gill-arches (four pairs), <i>b</i> rudiments of pharynx and larynx, <i>c</i> +lungs, <i>d</i> stomach, <i>f</i> liver, <i>g</i> walls of the open yelk-sac +(into which the middle gut opens with a wide aperture), <i>h</i> rectum. <br/> + +Fig. 354—<b>The same gut</b> seen from the right. <i>a</i> lungs, +<i>b</i> stomach, <i>c</i> liver, <i>d</i> yelk-sac, <i>e</i> rectum.)</p> +</div> + +<p> +There is at first only one pair of gill-clefts in the Amphioxus, as in the +Ascidia and Enteropneusts; and the Copelata (Fig. 225) have only one pair +throughout life. But the number presently increases in the former. In the +Craniotes, however, it decreases still further. The Cyclostomes have six to +eight pairs (Fig. 247); some of the Selachii six or seven pairs, most of the +fishes only four or five pairs. In the embryo of man, and the higher +Vertebrates generally, where they make an appearance at an early stage, only +three or four pairs are developed. In the fishes they remain throughout life, +and form an exit for the water taken in at the mouth (Figs. 249–251). But +they are partly lost in the amphibia, and entirely in the higher Vertebrates. +In these nothing is left but a relic of the first gill-cleft. This is formed +into a part of the organ of hearing; from it are developed the external meatus, +the tympanic cavity, and the Eustachian tube. We have already considered these +remarkable structures, and need only point here to the interesting fact that +our middle and external ear is a modified inheritance from the fishes. The +branchial arches also, which separate the clefts, develop into very different +parts. In the fishes they remain gill-arches, supporting the respiratory +gill-leaves. It is the same with the lowest amphibia, but in the higher +amphibia they undergo various modifications; and in the three higher classes of +Vertebrates (including man) the hyoid bone and the ossicles of the ear develop +from them. (Cf. p. 291.) +</p> + +<p> +From the first gill-arch, from the inner surface of which the muscular tongue +proceeds, we get the first structure of the maxillary skeleton—the upper +and lower jaws, which surround the mouth and support the teeth. These important +parts are wholly wanting in the two lowest classes of Vertebrates, the Acrania +and Cyclostoma. They appear first in the earliest Selachii (Figs. +248–251), and have been transmitted from this stem-group of the +Gnathostomes to the higher +<span class='pagenum'><a name="Page_314" id="Page_314"></a></span> +Vertebrates. Hence the original formation of the skeleton of the mouth can be +traced to these primitive fishes, from which we have inherited it. The teeth +are developed from the skin that clothes the jaws. As the whole mouth cavity +originates from the outer integument (Fig. 350), the teeth also must come from +it. As a fact, this is found to be the case on microscopic examination of the +development and finer structure of the teeth. The scales of the fishes, +especially of the shark type (Fig. 351), are in the same position as their +teeth in this respect (Fig. 252). The osseous matter of the tooth (dentine) +develops from the corium; its enamel covering is a secretion of the epidermis +that covers the corium. It is the same with the cutaneous teeth or placoid +scales of the Selachii. At first the whole of the mouth was armed with these +cutaneous teeth in the Selachii and in the earliest amphibia. Afterwards the +formation of them was restricted to the edges of the jaws. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus355"></a> +<img src="images/fig355.gif" width="309" height="145" alt="Fig.355. Median section of the head of a +Petromyzon-larva." /> +<p class="caption">Fig. 355—<b>Median section of the head of a +Petromyzon-larva.</b> (From <i>Gegenbaur.</i>) <i>h</i> hypobranchial groove +(above it in the gullet we see the internal openings of the seven gill-clefts), +<i>v</i> velum, <i>o</i> mouth, <i>c</i> heart, <i>a</i> auditory vesicle, +<i>n</i> neural tube, <i>ch</i> chorda.</p> +</div> + +<p> +Hence our human teeth are, in relation to their original source, modified +fish-scales. For the same reason we must regard the salivary glands, which open +into the mouth, as epidermic glands, as they are formed, not from the glandular +layer of the gut like the rest of the alimentary glands, but from the +epidermis, from the horny plate of the outer germinal layer. Naturally, in +harmony with this evolution of the mouth, the salivary glands belong +genetically to one series with the sudoriferous, sebaceous, and mammary glands. +</p> + +<p> +Thus the human alimentary canal is as simple as the primitive gut of the +gastrula in its original structure. Later it resembles the gut of the earliest +Vermalia (Gastrotricha). It then divides into two sections, a fore or branchial +gut and a hind or hepatic gut, like the alimentary canal of the Balanoglossus, +the Ascidia, and the Amphioxus. The formation of the jaws and the branchial +arches changes it into a real fish-gut ( <i>Selachii</i>). But the branchial +gut, the one reminiscence of our fish-ancestors, is afterwards atrophied as +such. The parts of it that remain are converted into entirely different +structures. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus356"></a> +<img src="images/fig356.gif" width="186" height="113" alt="Fig.356. Transverse +section of the head of a Petromyzon-larva." /> +<p class="caption">Fig. 356—<b>Transverse section of the head of a +Petromyzon-larva.</b> (From <i>Gegenbaur.</i>) Beneath the pharynx ( <i>d</i>) +we see the hypobranchial groove; above it the chorda and neural tube. <i>A, B, +C</i> stages of constriction.</p> +</div> + +<p> +But, although the anterior section of our alimentary canal thus entirely loses +its original character of branchial gut, it retains the physiological character +of respiratory gut. We are now astonished to find that the permanent +respiratory organ of the higher Vertebrates, the air-breathing lung, is +developed from this first part of the alimentary canal. Our lungs, trachea, and +larynx are formed from the ventral wall of the branchial gut. The whole of the +respiratory apparatus, which occupies the greater part of the pectoral cavity +in the adult man, is at first merely a small pair of vesicles or sacs, which +grow out of the floor of the head-gut immediately behind the gills (Figs. 354 +<i>c,</i> 147 <i>l</i>). These vesicles are found in all the Vertebrates except +the two lowest classes, the Acrania and Cyclostomes. In the lower Vertebrates +they do not develop +<span class='pagenum'><a name="Page_315" id="Page_315"></a></span> +into lungs, but into a large air-filled bladder, which occupies a good deal of +the body-cavity and has a quite different purport. It serves, not for +breathing, but to effect swimming movements up and down, and so is a sort of +hydrostatic apparatus—the floating bladder of the fishes ( +<i>nectocystis,</i> p. 233). However, the human lungs, and those of all +air-breathing Vertebrates, develop from the same simple vesicular appendage of +the head-gut that becomes the floating bladder in the fishes. +</p> + +<p> +At first this bladder has no respiratory function, but merely acts as +hydrostatic apparatus for the purpose of increasing or lessening the specific +gravity of the body. The fishes, which have a fully-developed floating bladder, +can press it together, and thus condense the air it contains. The air also +escapes sometimes from the alimentary canal, through an air-duct that connects +the floating bladder with the pharynx, and is ejected by the mouth. This +lessens the size of the bladder, and so the fish becomes heavier and sinks. +When it wishes to rise again, the bladder is expanded by relaxing the pressure. +In many of the Crossopterygii the wall of the bladder is covered with bony +plates, as in the Triassic <i>Undina</i> (Fig. 254). +</p> + +<p> +This hydrostatic apparatus begins in the Dipneusts to change into a respiratory +organ; the blood-vessels in the wall of the bladder now no longer merely +secrete air themselves, but also take in fresh air through the air-duct. This +process reaches its full development in the Amphibia. In these the floating +bladder has turned into lungs, and the air-passage into a trachea. The lungs of +the Amphibia have been transmitted to the three higher classes of Vertebrates. +In the lowest Amphibia the lungs on either side are still very simple +transparent sacs with thin walls, as in the common water-salamander, the +Triton. It still entirely resembles the floating bladder of the fishes. It is +true that the Amphibia have two lungs, right and left. But the floating bladder +is also double in many of the fishes (such as the early Ganoids), and divides +into right and left halves. On the other hand, the lung is single in Ceratodus +(Fig. 257). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus357"></a> +<img src="images/fig357.gif" width="157" height="175" alt="Fig.357. Thoracic and +abdominal viscera of a human embryo of twelve weeks." /> +<p class="caption">Fig. 357—<b>Thoracic and abdominal viscera</b> +of a human embryo of twelve weeks. (From <i>Kölliker.</i>) The head is omitted. +Ventral and pectoral walls are removed. The greater part of the body-cavity is +taken up with the liver, from the middle part of which the cæcum and the +vermiform appendix protrude. Above the diaphragm, in the middle, is the conical +heart; to the right and left of it are the two small lungs.</p> +</div> + +<p> +In the human embryo and that of all the other Amniotes the lungs develop from +the hind part of the ventral wall of the head-gut (Fig. 149). Immediately +behind the single structure of the thyroid gland a median groove, the rudiment +of the trachea, is detached from the gullet. From its hinder end a couple of +vesicles develop—the simple tubular rudiments of the right and left +lungs. They afterwards increase considerably in size, fill the greater part of +the thoracic cavity, and take the heart between them. Even in the frogs we find +that the simple sac has developed into a spongy body of peculiar froth-like +tissue. The originally short connection of the pulmonary sacs with the head-gut +extends into a long, thin tube. This is the wind-pipe (trachea); it opens into +the gullet above, and divides below into two branches which go to the two +lungs. In the wall of the trachea circular cartilages develop, and these keep +it open. At its upper end, underneath its pharyngeal opening, the larynx is +formed—the organ of voice and speech. The larynx is found at various +stages of development in the Amphibia, and comparative anatomists are in a +position to trace the progressive growth of this important organ from the +rudimentary structure of the lower Amphibia up to the elaborate and delicate +vocal apparatus that we have in the larynx of man and of the birds. +</p> + +<p> +We must refer here to an interesting rudimentary organ of the respiratory gut, +the thyroid gland, the large gland in front of the larynx, that lies below the +“Adam’s +<span class='pagenum'><a name="Page_316" id="Page_316"></a></span> +apple,” and is often especially developed in the male sex. It has a +certain function—not yet fully understood—in the nutrition of the +body, and arises in the embryo by constriction from the lower wall of the +pharynx. In many mining districts the thyroid gland is peculiarly liable to +morbid enlargement, and then forms goitre, a growth that hangs at the front of +the neck. But it is much more interesting phylogenetically. As Wilhelm Müller, +of Jena, has shown, this rudimentary organ is the last relic of the +hypobranchial groove, which we considered in a previous chapter, and which runs +in the middle line of the gill-crate in the Ascidia and Amphioxus, and conveys +food to the stomach. (Cf. p. 184,Fig. 246). We still find it in its original +character in the larvæ of the Cyclostomes (Figs. 355, 356). +</p> + +<p> +The second section of the alimentary canal, the trunk or hepatic gut, undergoes +not less important modifications among our vertebrate ancestors than the first +section. In tracing the further development of this digestive part of the gut, +we find that most complex and elaborate organs originate from a very +rudimentary original structure. For clearness we may divide the digestive gut +into three sections: the fore gut (with œsophagus and stomach), the middle gut +(duodenum, with liver, pancreas, jejunum, and ileum, and the hind gut (colon +and rectum). Here again we find vesicular growths or appendages of the +originally simple gut developing into a variety of organs. Two of these +embryonic structures, the yelk-sac and allantois, are already known to us. The +two large glands that open into the duodenum, the liver and pancreas, are +growths from the middle and most important part of the trunk-gut. +</p> + +<p> +Immediately behind the vesicular rudiments of the lungs comes the section of +the alimentary canal that forms the stomach (Figs. 353 <i>d,</i> 354 <i>b</i>). +This sac-shaped organ, which is chiefly responsible for the solution and +digestion of the food, has not in the lower Vertebrates the great physiological +importance and the complex character that it has in the higher. In the Acrania +and Cyclostomes and the earlier fishes we can scarcely distinguish a real +stomach; it is represented merely by the short piece from the branchial to the +hepatic gut. In some of the other fishes also the stomach is only a very simple +spindle-shaped enlargement at the beginning of the digestive section of the +gut, running straight from front to back in the median plane of the body, +underneath the vertebral column. In the mammals its first structure is just as +rudimentary as it is permanently in the preceding. But its various parts soon +begin to develop. As the left side of the spindle-shaped sac grows much more +quickly than the right, and as it turns considerably on its axis at the same +time, it soon comes to lie obliquely. The upper end is more to the left, and +the lower end more to the right. The foremost end draws up into the longer and +narrower canal of the œsophagus. Underneath this on the left the blind sac +(fundus) of the stomach bulges out, and thus the later form gradually develops +(Figs. 349, 184 <i>e</i>). The original longitudinal axis becomes oblique, +sinking below to the left and rising to the right, and approaches nearer and +nearer to a transverse position. In the outer layer of the stomach-wall the +powerful muscles that accomplish the digestive movements develop from the +gut-fibre layer. In the inner layer a number of small glandular tubes are +formed from the gut-gland layer; these are the peptic glands that secrete the +gastric juice. At the lower end of the gastric sac is developed the valve that +separates it from the duodenum (the pylorus, Fig. 349 <i>d</i>). +</p> + +<p> +Underneath the stomach there now develops the disproportionately long stretch +of the small intestine. The development of this section is very simple, and +consists essentially in an extremely rapid and considerable growth lengthways. +It is at first very short, quite straight, and simple. But immediately behind +the stomach we find at an early stage a horseshoe-shaped bend and loop of the +gut, in connection with the severance of the alimentary canal from the yelk-sac +and the development of the first mesentery. The thin delicate membrane that +fastens this loop to the ventral side of the vertebral column, and fills the +inner bend of the horseshoe formation, is the first rudiment of the mesentery +(Fig. 147 <i>g</i>). We find at an early stage a considerable growth of the +small intestine; it is thus forced to coil itself in a number of loops. The +various sections that we have to distinguish in it are differentiated in a very +simple way—the duodenum (next to the stomach), the succeeding long +jejunum, and the last section of the small intestine, the ileum. +</p> + +<p> +<span class='pagenum'><a name="Page_317" id="Page_317"></a></span> +From the duodenum are developed the two large glands that we have already +mentioned—the liver and pancreas. The liver appears first in the shape of +two small sacs, that are found to the right and left immediately behind the +stomach (Figs. 353 <i>f,</i> 354 <i>c</i>). In many of the lower Vertebrates +they remain separate for a long time (in the Myxinoides throughout life), or +are only imperfectly joined. In the higher Vertebrates they soon blend more or +less completely to form a single large organ. The growth of the liver is very +brisk at first. In the human embryo it grows so much in the second month of +development that in the third it occupies by far the greater part of the +body-cavity (Fig. 357). At first the two halves develop equally; afterwards the +left falls far behind the right. In consequence of the unsymmetrical +development and turning of the stomach and other abdominal viscera, the whole +liver is now pushed to the right side. Although the liver does not afterwards +grow so disproportionately, it is comparatively larger in the embryo at the end +of pregnancy than in the adult. Its weight relatively to that of the whole body +is 1 : 36 in the adult, and 1 : 18 in the embryo. Hence it is very important +physiologically during embryonic life; it is chiefly concerned in the formation +of blood, not so much in the secretion of bile. +</p> + +<p> +Immediately behind the liver a second large visceral gland develops from the +duodenum, the pancreas or sweetbread. It is wanting in most of the lowest +classes of Vertebrates, and is first found in the fishes. This organ is also an +outgrowth from the gut. +</p> + +<p> +The last section of the alimentary canal, the large intestine, is at first in +the embryo a very simple, short, and straight tube, which opens behind by the +anus. It remains thus throughout life in the lower Vertebrates. But it grows +considerably in the mammals, coils into various folds, and divides into two +sections, the first and longer of which is the colon, and the second the +rectum. At the beginning of the colon there is a valve (valvula <i>Bauhini</i>) +that separates it from the small intestine. Immediately behind this there is a +sac-like growth, which enlarges into the cæcum (Fig. 357 <i>v</i>). In the +plant-eating mammals this is very large, but it is very small or completely +atrophied in the flesh-eaters. In man, and most of the apes, only the first +portion of the cæcum is wide; the blind end-part of it is very narrow, and +seems later to be merely a useless appendage of the former. This +“vermiform appendage” is very interesting as a rudimentary organ. +The only significance of it in man is that not infrequently a cherry-stone or +some other hard and indigestible matter penetrates into its narrow cavity, and +by setting up inflammation and suppuration causes the death of otherwise sound +men. Teleology has great difficulty in giving a rational explanation of, and +attributing to a beneficent Providence, this dreaded appendicitis. In our +plant-eating ancestors this rudimentary organ was much larger and had a useful +function. +</p> + +<p> +Finally, we have important appendages of the alimentary tube in the bladder and +urethra, which belong to the alimentary system. These urinary organs, acting as +reservoir and duct for the urine excreted by the kidneys, originate from the +innermost part of the allantoic pedicle. In the Dipneusts and Amphibia, in +which the allantoic sac first makes its appearance, it remains within the +body-cavity, and functions entirely as bladder. But in all the Amniotes it +grows far outside of the body-cavity of the embryo, and forms the large +embryonic “primitive bladder,” from which the placenta develops in +the higher mammals. This is lost at birth. But the long stalk or pedicle of the +allantois remains, and forms with its upper part the middle vesico-umbilical +ligament, a rudimentary organ that goes in the shape of a solid string from the +vertex of the bladder to the navel. The lowest part of the allantoic pedicle +(or the “urachus”) remains hollow, and forms the bladder. At first +this opens into the last section of the gut in man as in the lower Vertebrates; +thus there is a real cloaca, which takes off both urine and excrements. But +among the mammals this cloaca is only permanent in the Monotremes, as it is in +all the birds, reptiles, and amphibia. In all the other mammals (marsupials and +placentals) a transverse partition is afterwards formed, and this separates the +urogenital aperture in front from the anus-opening behind. (Cf. p. 249 and +Chapter 29.) +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap28"></a> +<span class='pagenum'><a name="Page_318" id="Page_318"></a></span> +Chapter XXVIII.<br/> +EVOLUTION OF THE VASCULAR SYSTEM</h2> + +<p> +The use that we have hitherto made of our biogenetic law will give the reader +an idea how far we may trust its guidance in phylogenetic investigation. This +differs considerably in the various systems of organs; the reason is that +heredity and variability have a very different range in these systems. While +some of them faithfully preserve the original palingenetic development +inherited from earlier animal ancestors, others show little trace of this rigid +heredity; they are rather disposed to follow new and divergent +<i>cenogenetic</i> lines of development in consequence of adaptation. The +organs of the first kind represent the <i>conservative</i> element in the +multicellular state of the human frame, while the latter represent the +<i>progressive</i> element. The course of historic development is a result of +the correlation of the two tendencies, and they must be carefully +distinguished. +</p> + +<p> +There is perhaps no other system of organs in the human body in which this is +more necessary than in that of which we are now going to consider the obscure +development—the vascular system, or apparatus of circulation. If we were +to draw our conclusions as to the original features in our earlier animal +ancestors solely from the phenomena of the development of this system in the +embryo of man and the other higher Vertebrates, we should be wholly misled. By +a number of important embryonic adaptations, the chief of which is the +formation of an extensive food-yelk, the original course of the development of +the vascular system has been so much falsified and curtailed in the higher +Vertebrates that little or nothing now remains in their embryology of some of +the principal phylogenetic features. We should be quite unable to explain these +if comparative anatomy and ontogeny did not come to our assistance. +</p> + +<p> +The vascular system in man and all the Craniotes is an elaborate apparatus of +cavities filled with juices or cell-containing fluids. These +“vessels” (<i>vascula</i>) play an important part in the nutrition +of the body. They partly conduct the nutritive red blood to the various parts +of the body (blood-vessels); partly absorb from the gut the white chyle formed +in digestion (chyle-vessels); and partly collect the used-up juices and convey +them away from the tissues (lymphatic vessels). With the latter are connected +the large cavities of the body, especially the body-cavity, or cœloma. The +lymphatic vessels conduct both the colourless lymph and the white chyle into +the venous part of the circulation. The lymphatic glands act as producers of +new blood-cells, and with them is associated the spleen. The centre of movement +for the circulation of the fluids is the heart, a strong muscular sac, which +contracts regularly and is equipped with valves like a pump. This constant and +steady circulation of the blood makes possible the complex metabolism of the +higher animals. +</p> + +<p> +But, however important the vascular system may be to the more advanced and +larger and highly-differentiated animals, it is not at all so indispensable an +element of animal life as is commonly supposed. The older science of medicine +regarded the blood as the real source of life. Even in the still prevalent +confused notions of heredity the blood plays the chief part. People speak +generally of full blood, half blood, etc., and imagine that the hereditary +transmission of certain characters “lies in the blood.” The +incorrectness of these ideas is clearly seen from the fact that in the act of +generation the blood of the parents is not directly transmitted to the +offspring, nor does the embryo possess blood in its early stages. We have +already seen that not only the differentiation of the four secondary germinal +layers, but also the first structures of the principal organs in the embryo of +all the Vertebrates, take place long before there is any +<span class='pagenum'><a name="Page_319" id="Page_319"></a></span> +trace of the vascular system—the heart and the blood. In accordance with +this ontogenetic fact, we must regard the vascular system as one of the latest +organs from the phylogenetic point of view; just as we have found the +alimentary canal to be one of the earliest. In any case, the vascular system is +much later than the alimentary. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus358"></a> +<a name="illus359"></a> +<img src="images/fig358.gif" width="376" height="265" alt="Fig.358. Red +blood-cells of various Vertebrates. Fig. 359. Vascular tissues or endothelium +(vasalium). A capillary from the mesentery." /> +<p class="caption">Fig. 358—<b>Red blood-cells of various +Vertebrates</b> (equally magnified). <i>1.</i> of man, <i>2.</i> camel, +<i>3.</i> dove, <i>4.</i> proteus, <i>5.</i> water-salamander (<i>Triton</i>), +<i>6.</i> frog, <i>7.</i> merlin (<i>Cobitis</i>), <i>8.</i> lamprey +(<i>Petromyzon</i>). <i>a</i> surface-view, <i>b</i> edge-view. (From +<i>Wagner.</i>)<br/> Fig. 359—<b>Vascular tissues or endothelium</b> +(<i>vasalium</i>). A capillary from the mesentery. <i>a</i> vascular cells, +<i>b</i> their nuclei.</p> +</div> + +<p> +The important nutritive fluid that circulates as blood and lymph in the +elaborate canals of our vascular system is not a clear, simple fluid, but a +very complex chemical juice with millions of cells floating in it. These +blood-cells are just as important in the complicated life of the higher animal +body as the circulation of money is to the commerce of a civilised community. +Just as the citizens meet their needs most conveniently by means of a financial +circulation, so the various tissue-cells, the microscopic citizens of the +multicellular human body, have their food conveyed to them best by the +circulating cells in the blood. These blood cells (<i>hæmocytes</i>) are of two +kinds in man and all the other Craniotes—red cells (<i>rhodocytes</i> or +<i>erythrocytes</i>) and colourless or lymph cells (<i>leucocytes</i>). The red +colour of the blood is caused by the great accumulation of the former, the +others circulate among them in much smaller quantity. When the colourless cells +increase at the expense of the red we get anæmia (or chlorosis). +</p> + +<p> +The lymph-cells (<i>leucocytes</i>), commonly called the “white +corpuscles” of the blood, are phylogenetically older and more widely +distributed in the animal world than the red. The great majority of the +Invertebrates that have acquired an independent vascular system have only +colourless lymph-cells in the circulating fluid. There is an exception in the +Nemertines (Fig. 358) and some groups of Annelids. When we examine the +colourless blood of a cray-fish or a snail (Fig. 358) under a high power of the +microscope, we find in each drop numbers of mobile leucocytes, which behave +just like independent Amoebæ (Fig. 17). Like these unicellular Protozoa, the +colourless blood-cells creep slowly about, their unshapely plasma-body +constantly changing its form, and stretching out finger-like processes first in +one direction, then another. Like the Amoebæ, they take particles into their +cell-body. On account +<span class='pagenum'><a name="Page_320" id="Page_320"></a></span> +of this feature these amoeboid plastids are called “eating cells” +(<i>phagocytes</i>), and on account of their motions “travelling +cells” (<i>planocytes</i>). It has been shown by the discoveries of the +last few decades that these leucocytes are of the greatest physiological and +pathological consequence to the organism. They can absorb either solid or +dissolved particles from the wall of the gut, and convey them to the blood in +the chyle; they can absorb and remove unusable matter from the tissues. When +they pass in large quantities through the fine pores of the capillaries and +accumulate at irritated spots, they cause inflammation. They can consume and +destroy bacteria, the dreaded vehicles of infectious diseases; but they can +also transport these injurious Monera to fresh regions, and so extend the +sphere of infection. It is probable that the sensitive and travelling +leucocytes of our invertebrate ancestors have powerfully co-operated for +millions of years in the phylogenesis of the advancing animal organisation. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus360"></a> +<img src="images/fig360.gif" width="447" height="137" alt="Fig.360. Transverse +section of the trunk of a chick-embryo, forty-five hours old." /> +<p class="caption">Fig. 360—<b>Transverse section of the trunk of a +chick-embryo,</b> forty-five hours old. (From <i>Balfour.</i>) <i>A</i> +ectoderm (horny-plate), <i>Mc</i> medullary tube, <i>ch</i> chorda, <i>C</i> +entoderm (gut-gland layer), <i>Pv</i> primitive segment (episomite), <i>Wd</i> +prorenal duct, <i>pp</i> cœloma (secondary body-cavity). <i>So</i> skin-fibre +layer, <i>Sp</i> gut-fibre layer, <i>v</i> blood-vessels in latter, <i>ao</i> +primitive aortas, containing red blood-cells.</p> +</div> + +<p> +The red blood-cells have a much more restricted sphere of distribution and +activity. But they also are very important in connection with certain functions +of the craniote-organism, especially the exchange of gases or respiration. The +cells of the dark red, carbonised or venous, blood, which have absorbed +carbonic acid from the animal tissues, give this off in the respiratory organs; +they receive instead of it fresh oxygen, and thus bring about the bright red +colour that distinguishes oxydised or arterial blood. The red colouring matter +of the blood (<i>hæmoglobin</i>) is regularly distributed in the pores of their +protoplasm. The red cells of most of the Vertebrates are elliptical flat disks, +and enclose a nucleus of the same shape; they differ a good deal in size (Fig. +358). The mammals are distinguished from the other Vertebrates by the circular +form of their biconcave red cells and by the absence of a nucleus (Fig. 1); +only a few genera still have the elliptic form inherited from the reptiles +(Fig. 2). In the embryos of the mammals the red cells have a nucleus and the +power of increasing by cleavage (Fig. 10). +</p> + +<p> +The origin of the blood-cells and vessels in the embryo, and their relation to +the germinal layers and tissues, are among the most difficult problems of +ontogeny—those obscure questions on which the most divergent opinions are +still advanced by the most competent scientists. In general, it is certain that +the greater part of the cells that compose the vessels and their contents come +from the mesoderm—in fact, from the gut-fibre layer; it was on this +account that Baer gave the name of “vascular layer” to this +visceral layer of the coeloma. But other important observers say that a part of +these cells come from other germinal layers, especially from the gut-gland +layer. It seems to be true that blood-cells may be formed from the cells of the +entoderm before the development of the mesoderm. If we examine sections of +chickens, the earliest and most familiar subjects of embryology, we find at an +early stage the “primitive-aortas” we have already described (Fig. +360 <i>ao</i>) in the ventral angle between the episoma (<i>Pv</i>) and +hyposoma (<i>Sp</i>). The +<span class='pagenum'><a name="Page_321" id="Page_321"></a></span> +thin wall of these first vessels of the amniote embryo consists of flat cells +(<i>endothelia</i> or <i>vascular epithelia</i>); the fluid within already +contains numbers of red blood-cells; both have been developed from the +gut-fibre layer. It is the same with the vessels of the germinative area (Fig. +361 <i>v</i>), which lie on the entodermic membrane of the yelk-sac (<i>c</i>). +These features are seen still more clearly in the transverse section of the +duck-embryo in Fig. 152.In this we see clearly how a number of stellate cells +proceed from the “vascular layer” and spread in all directions in +the “primary body-cavity”—<i>i.e.</i> in the spaces between +the germinal layers. A part of these travelling cells come together and line +the wall of the larger spaces, and thus form the first vessels; others enter +into the cavity, live in the fluid that fills it, and multiply by +cleavage—the first blood-cells. +</p> + +<p> +But, besides these mesodermic cells of the “vascular layer” proper, +other travelling cells, of which the origin and purport are still obscure, take +part in the formation of blood in the meroblastic Vertebrates (especially +fishes). The chief of these are those that Ruckert has most aptly denominated +“merocytes.” These “eating yelk-cells” are found in +large numbers in the food-yelk of the Selachii, especially in the +yelk-wall—the border zone of the germinal disk in which the embryonic +vascular net is first developed. The nuclei of the merocytes become ten times +as large as the ordinary cell-nucleus, and are distinguished by their strong +capacity for taking colour, or their special richness in chromatin. Their +protoplasmic body resembles the stellate cells of osseous tissue (astrocytes), +and behaves just like a rhizopod (such as Gromia); it sends out numbers of +stellate processes all round, which ramify and stretch into the surrounding +food-yelk. These variable and very mobile processes, the pseudopodia of the +merocytes, serve both for locomotion and for getting food; as in the real +rhizopods, they surround the solid particles of food (granules and plates of +yelk), and accumulate round their nucleus the food they have received and +digested. Hence we may regard them both as eating-cells (<i>phagocytes</i>) and +travelling-cells (<i>planocytes</i>). Their lively nucleus divides quickly and +often repeatedly, so that a number of new nuclei are formed in a short time; as +each fresh nucleus surrounds itself with a mantle of protoplasm, it provides a +new cell for the construction of the embryo. Their origin is still much +disputed. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus361"></a> +<img src="images/fig361.gif" width="277" height="200" alt="Fig.361. Merocytes of a shark-embryo, +rhizopod-like yelk-cells underneath the embryonic cavity (B)." /> +<p class="caption">Fig. 361—<b>Merocytes of a shark-embryo,</b> +rhizopod-like yelk-cells underneath the embryonic cavity (<i>B</i>). (From +<i>Ruckert.</i>) <i>z</i> two embryonic cells, <i>k</i> nuclei of the +merocytes, which wander about in the yelk and eat small yelk-plates (<i>d</i>), +<i>k</i> smaller, more superficial, lighter nuclei, <i>k′</i> a deeper +nucleus, in the act of cleavage, <i>k*</i> chromatin-filled border-nucleus, +freed from the surrounding yelk in order to show the numerous pseudopodia of +the protoplasmic cell-body.</p> +</div> + +<p> +Half of the twelve stems of the animal world have no blood-vessels. They make +their first appearance in the Vermalia. Their earliest source is the primary +body-cavity, the simple space between the two primary germinal layers, which is +either a relic of the segmentation-cavity, or is a subsequent formation. +Amoeboid planocytes, which migrate from the entoderm and reach this +fluid-filled primary cavity, live and multiply there, and form the first +colourless blood-cells. We find the vascular system in this very simple form +to-day in the Bryozoa, Rotatoria, Nematoda, and other lower Vermalia. +</p> + +<p> +The first step in the improvement of this primitive vascular system is the +formation of larger canals or blood-conducting tubes. The spaces filled with +blood, the relics of the primary body-cavity, receive a special wall. +“Blood-vessels” of this kind (in the narrower sense) are found +among the higher worms in various forms, sometimes very simple, at other times +very complex. The form +<span class='pagenum'><a name="Page_322" id="Page_322"></a></span> +that was probably the incipient structure of the elaborate vascular system of +the Vertebrates (and of the Articulates) is found in two primordial principal +vessels—a dorsal vessel in the middle line of the dorsal wall of the gut, +and a ventral vessel that runs from front to rear in the middle line of its +ventral wall. From the dorsal vessel is evolved the aorta (or principal +artery), from the ventral vessel the principal or subintestinal vein. The two +vessels are connected in front and behind by a loop that runs round the gut. +The blood contained in the two tubes is propelled by their peristaltic +contractions. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus362"></a> +<img src="images/fig362.gif" width="117" height="282" alt="Fig.362. Vascular system +of an Annelid (Saenuris), foremost section." /> +<p class="caption">Fig. 362—<b>Vascular system of an Annelid</b> +(<i>Sænuris</i>), foremost section. <i>d</i> dorsal vessel, <i>v</i> ventral +vessel, <i>c</i> transverse connection of two (enlarged in shape of heart). The +arrows indicate the direction of the flow of blood. (From +<i>Gegenbaur.</i></p> +</div> + +<p> +The earliest Vermalia in which we first find this independent vascular system +are the Nemertina (Fig. 244). As a rule, they have three parallel longitudinal +vessels connected by loops, a single dorsal vessel above the gut and a pair of +lateral vessels to the right and left. In some of the Nemertina the blood is +already coloured, and the red colouring matter is real hæmoglobin, connected +with elliptical discoid cells, as in the Vertebrates. The further evolution of +this rudimentary vascular system can be gathered from the class of the Annelids +in which we find it at various stages of development. First, a number of +transverse connections are formed between the dorsal and ventral vessels, which +pass round the gut ring-wise (Fig. 362). Other vessels grow into the body-wall +and ramify in order to convey blood to it. In addition to the two large vessels +of the middle plane there are often two lateral vessels, one to the right and +one to the left; as, for instance, in the leech. There are four of these +parallel longitudinal vessels in the Enteropneusts (<i>Balanoglossus,</i> Fig. +245). In these important Vermalia the foremost section of the gut has already +been converted into a gill-crate, and the vascular arches that rise in the wall +of this from the ventral to the dorsal vessel have become branchial vessels. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus363"></a> +<img src="images/fig363.gif" width="228" height="129" alt="Fig.363. Head of a +fish-embryo, with rudimentary vascular system, from the left." /> +<p class="caption">Fig. 363—<b>Head of a fish-embryo,</b> with +rudimentary vascular system, from the left. <i>dc</i> Cuvier’s duct +(juncture of the anterior and posterior principal veins), <i>sv</i> venous +sinus (enlarged end of Cuvier’s duct), <i>a</i> auricle, <i>v</i> +ventricle, <i>abr</i> trunk of branchial artery, <i>s</i> gill-clefts (arterial +arches between), <i>ad</i> aorta, <i>c</i> carotid artery, <i>n</i> nasal pit. +(From <i>Gegenbaur.</i></p> +</div> + +<p> +We have a further important advance in the Tunicates, which we have recognised +as the nearest blood-relatives of our early vertebrate ancestors. Here we find +for the first time a real heart—<i>i.e.</i> a central organ of +circulation, driving the blood into the vessels by the regular contractions of +its muscular wall, it is of a very rudimentary character, a spindle-shaped +tube, passing at both ends into a principal vessel (Fig. 221). By its original +position behind the gill-crate, on ventral side of the Tunicates (sometimes +more, sometimes less, forward), the head shows clearly that it has been formed +by the local enlargement of a section of the ventral vessel. We have already +noticed the remarkable alternation of the direction of the blood stream, the +heart driving it first from one end, then from the other p. 190. This is very +instructive, because in most of the worms (even the Enteropneust) the blood in +the dorsal vessel travels from back to front, but in the Vertebrates in the +opposite direction. As the Ascidia-heart alternates steadily from one direction +to the other, it shows us permanently, in a sense, the phylogenetic transition +from the earlier forward direction of the dorsal current (in the worms) to the +new backward direction (in the Vertebrates). +</p> + +<p> +As the new direction became permanent in the earlier Prochordonia, which gave +rise to the Vertebrate stem, the two vessels that proceed from either end of +the tubular heart acquired a fixed function. +<span class='pagenum'><a name="Page_323" id="Page_323"></a></span> +The foremost section of the ventral vessel henceforth always conveys blood from +the heart, and so acts as an artery; the hind section of the same vessel brings +the blood from the body to the heart, and so becomes a vein. In view of their +relation to the two sections of the gut, we may call the latter the intestinal +vein and the former the branchial artery. The blood contained in both vessels, +and also in the heart, is venous or carbonised blood—<i>i.e.</i> rich in +carbonic acid; on the other hand, the blood that passes from the gills into the +dorsal vessel is provided with fresh oxygen—arterial or oxydised blood. +The finest branches of the arteries and veins pass into each other in the +tissues by means of a network of very fine, ventral, hair-like vessels, or +capillaries (Fig. 359). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus364"></a> +<img src="images/fig364.gif" width="409" height="213" alt="Fig.364. The five +arterial arches of the Craniotes (1 to 5) in their original disposition. Fig. +365. The five arterial arches of the birds; the lighter parts of the structure +disappear; only the shaded parts remain. Fig. 366. The five arterial arches of +mammals." /> +<p class="caption">Fig. 364—<b>The five arterial arches of the +Craniotes</b> (<i>1–5</i>) in their original disposition. <i>a</i> +arterial cone or bulb, <i>a″</i> aorta-trunk, <i>c</i> carotid artery +(foremost continuation of the roots of the aorta). (From <i>Rathke.</i>)<br/> +Fig. 365—<b>The five arterial arches of the birds;</b> the lighter parts +of the structure disappear; only the shaded parts remain. Letters as in Fig. +364. <i>s</i> subclavian arteries, <i>p</i> pulmonary artery, <i>p′</i> +branches of same, <i>c′</i> outer carotid, <i>c″</i> inner carotid. +(From <i>Rathke.</i>)<br/> Fig. 366—<b>The five arterial arches of +mammals;</b> letters as in Fig. 365. <i>v</i> vertebral artery, <i>b</i> +Botall’s duct (open in the embryo, closed afterwards). (From +<i>Rathke.</i>)</p> +</div> + +<p> +When we turn from the Tunicates to the closely-related Amphioxus we are +astonished at first to find an apparent retrogression in the formation of the +vascular system. As we have seen, the Amphioxus has no real heart; its +colourless blood is driven along in its vascular system by the principal vessel +itself, which contracts regularly in its whole length (cf. Fig. 210). A dorsal +vessel that lies above the gut (aorta) receives the arterial blood from the +gills and drives it into the body. Returning from here, the venous blood +gathers in a ventral vessel under the gut (intestinal vein), and goes back to +the gills. A number of branchial vascular arches, which effect respiration and +rise in the wall of the branchial gut from belly to back, absorb oxygen from +the water and give off carbonic acid; they connect the ventral with the dorsal +vessel. As the same section of the ventral vessel, which also forms the heart +in the Craniotes, has developed in the Ascidia into a simple tubular heart, we +may regard the absence of this in the Amphioxus as a result of degeneration, a +return in this case to the earlier form of the vascular system, as we find it +in many of the worms. We may assume that the Acrania that really belong to our +ancestral series did not share this retrogression, but inherited the +one-chambered heart of the Prochordonia, and transmitted it directly to the +earliest Craniotes (cf. the ideal Primitive Vertebrate, <i>Prospondylus,</i> +Figs. 98–102). +</p> + +<p> +The further phylogenetic evolution of the vascular system is revealed to us by +the comparative anatomy of the Craniotes. At the lowest stage of this group, in +the Cyclostomes, we find for the first time the differentiation of the vasorium +into two sections: a system of blood-vessels proper, which convey the +<i>red</i> blood about the body, and a system of lymphatic vessels, +<span class='pagenum'><a name="Page_324" id="Page_324"></a></span> +which absorb the colourless lymph from the tissues and convey it to the blood. +The lymphatics that absorb from the gut and pour into the blood-stream the +milky food-fluid formed by digestion are distinguished by the special name of +“chyle-vessels.” While the chyle is white on account of its high +proportion of fatty particles, the lymph proper is colourless. Both chyle and +lymph contain the colourless amœboid cells (leucocytes, Fig. 12) that we also +find distributed in the blood as colourless blood-cells (or “white +corpuscles”); but the blood also contains a much larger quantity of red +cells, and these give its characteristic colour to the blood of the Craniotes +(rhodocytes, Fig. 358). The distinction between lymph, chyle, and blood-vessels +which is found in all the Craniotes may be regarded as an outcome of division +of labour between various sections of our originally simple vascular system. In +the Gnathostomes the spleen makes its first appearance, an organ rich in blood, +the chief function of which is the extensive formation of new colourless and +red cells. It is not found in the Acrania and Cyclostomes, or any of the +Invertebrates. It has been transmitted from the earliest fishes to all the +Craniotes. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus367"></a> +<img src="images/fig367.gif" width="437" height="169" alt="Figs. 367-70. +Metamorphosis of the five arterial arches in the human embryo." /> +<p class="caption">Figs. 367–70—<b>Metamorphosis of the five +arterial arches in the human embryo</b> (diagram from <i>Rathke</i>). <i>la</i> +arterial cone, <i>1, 2, 3, 4, 5</i> first to fifth pair of arteries, <i>ad</i> +trunk of aorta, <i>aw</i> roots of aorta. In Fig. 367 only three, in Fig. 368 +all five, of the aortic arches are given (the dotted ones only are developed). +In Fig. 369 the first two pairs have disappeared again. In Fig. 370 the +permanent trunks of the artery are shown; the dotted parts disappear, <i>s</i> +subclavian artery, <i>v</i> vertebral, <i>ax</i> axillary, <i>c</i> carotid +(<i>c′</i> outer, <i>c″</i> inner carotid), <i>p</i> +pulmonary.</p> +</div> + +<p> +The heart also, the central organ of circulation in all the Craniotes, shows an +advance in structure in the Cyclostomes. The simple, spindle-shaped heart-tube, +found in the same form in the embryo of all the Craniotes, is divided into two +sections or chambers in the Cyclostomes, and these are separated by a pair of +valves. The hind section, the auricle, receives the venous blood from the body +and passes it on to the anterior section, the ventricle. From this it is driven +through the trunk of the branchial artery (the foremost section of the ventral +vessel or principal vein) into the gills. +</p> + +<p> +In the Selachii an arterial cone is developed from the foremost end of the +ventricle, as a special division, cut off by valves. It passes into the +enlarged base of the trunk of the branchial artery (Fig. 363 <i>abr</i>). On +each side 5–7 arteries proceed from it. These rise between the +gill-clefts (<i>s</i>) on the gill-arches, surround the gullet, and unite above +into a common trunk-aorta, the continuation of which over the gut corresponds +to the dorsal vessel of the worms. As the curved arteries on the gill-arches +spread into a network of respiratory capillaries, they contain venous blood in +their lower part (as arches of the branchial artery) and arterial blood in the +upper part (as arches of the aorta). The junctures of the various aortic arches +on the right and left are called the roots of the aorta. Of an originally large +number of aortic arches there remain at first six, then (owing to degeneration +of the fifth arch) only five, pairs; and from these five pairs (Fig. 364) the +chief parts of the arterial system develop in all the higher Vertebrates. +</p> + +<p> +The appearance of the lungs and the atmospheric respiration connected +therewith, which we first meet in the Dipneusts, is the next important step in +vascular evolution. In the Dipneusts the auricle of the heart is divided by an +incomplete partition into two halves. Only the right +<span class='pagenum'><a name="Page_325" id="Page_325"></a></span> +auricle now receives the venous blood from the veins of the body. The left +auricle receives the arterial blood from the pulmonary veins. The two auricles +have a common opening into the simple ventricle, where the two kinds of blood +mix, and are driven through the arterial cone or bulb into the arterial arches. +From the last arterial arches the pulmonary arteries arise (Fig. 365 <i>p</i>). +These force a part of the mixed blood into the lungs, the other part of it +going through the aorta into the body. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus371"></a> +<img src="images/fig371.gif" width="191" height="140" alt="Fig.371. Heart of a +rabbit-embryo, from behind. Fig. 372. Heart of the same embryo (Fig. 371), from +the front." /> +<p class="caption">Fig. 371—<b>Heart of a rabbit-embryo,</b> from +behind. <i>a</i> vitelline veins, <i>b</i> auricles of the heart, <i>c</i> +atrium, <i>d</i> ventricle, <i>e</i> arterial bulb, <i>f</i> base of the three +pairs of arterial arches. (From <i>Bischoff.</i>)<br/> Fig. 372—<b>Heart +of the same embryo</b> (Fig. 371), from the front. <i>v</i> vitelline veins, +<i>a</i> auricle, <i>ca</i> auricular canal, <i>l</i> left ventricle, <i>r</i> +right ventricle, <i>ta</i> arterial bulb. (From <i>Bischoff.</i>)</p> +</div> + +<p> +From the Dipneusts upwards we now trace a progressive development of the +vascular system, which ends finally with the loss of branchial respiration and +a complete separation of the two halves of the circulation. In the Amphibia the +partition between the two auricles is complete. In their earlier stages, as +tadpoles (Fig. 262), they have still the branchial respiration and the +circulation of the fishes, and their heart contains venous blood alone. +Afterwards the lungs and pulmonary vessels are developed, and henceforth the +ventricle of the heart contains mixed blood. In the reptiles the ventricle and +its arterial cone begin to divide into two halves by a longitudinal partition, +and this partition becomes complete in the higher reptiles and birds on the one +hand, and the stem-forms of the mammals on the other. Henceforth, the right +half of the heart contains only venous, and the left half only arterial, blood, +as we find in all birds and mammals. The right auricle receives its carbonised +or venous blood from the veins of the body, and the right ventricle drives it +through the pulmonary arteries into the lungs. From here the blood returns, as +oxydised or arterial blood, through the pulmonary veins to the left auricle, +and is forced by the left ventricle into the arteries of the body. Between the +pulmonary arteries and veins is the capillary system of the small or pulmonary +circulation. Between the body-arteries and veins is the capillary system of the +large or body-circulation. It is only in the two highest classes of +Vertebrates—the birds and mammals—that we find a complete division +of the circulations. Moreover, this complete separation has been developed +quite independently in the two classes, as the dissimilar formation of the +aortas shows of itself. In the birds the <i>right</i> half of the fourth +arterial arch has become the permanent arch (Fig. 365). In the mammals this has +been developed from the <i>left</i> half of the same fourth arch (Fig. 366). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus373"></a> +<img src="images/fig373.gif" width="208" height="191" alt="Fig.373. Heart and head of a dog-embryo, from the +front. Fig. 374. Heart of the same dog-embryo, from behind." /> +<p class="caption">Fig. 373—<b>Heart and head of a dog-embryo,</b> +from the front. <i>a</i> fore brain, <i>b</i> eyes, <i>c</i> middle brain, +<i>d</i> primitive lower jaw, <i>e</i> primitive upper jaw, <i>f</i> +gill-arches, <i>g</i> right auricle, <i>h</i> left auricle, <i>i</i> left +ventricle, <i>k</i> right ventricle. (From <i>Bischoff.</i>)<br/> Fig. +374—<b>Heart of the same dog-embryo,</b> from behind. <i>a</i> +inosculation of the vitelline veins, <i>b</i> left auricle, <i>c</i> right +auricle, <i>d</i> auricle, <i>e</i> auricular canal, <i>f</i> left ventricle, +<i>g</i> right ventricle, <i>h</i> arterial bulb. (From +<i>Bischoff.</i>)</p> +</div> + +<p> +If we compare the fully-developed arterial system of the various classes of +Craniotes, it shows a good deal of variety, yet it always proceeds from the +same fundamental type. Its development is just the same in man as in the other +mammals; in particular, the modification of the six pairs of arterial arches is +the same in both (Figs. 367–370). At first there is only a single pair of +arches, which +<span class='pagenum'><a name="Page_326" id="Page_326"></a></span> +lie on the inner surface of the first pair of gill-arches. Behind this there +then develop a second and third pair of arches (lying on the inner side of the +second and third gill-arches, Fig. 367). Finally, we get a fourth, fifth, and +sixth pair. Of the six primitive arterial arches of the Amniotes three soon +pass away (the first, second, and fifth); of the remaining three, the third +gives the carotids, the fourth the aortas, and the sixth (number 5 in Figs. 364 +and 368) the pulmonary arteries. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus375"></a> +<img src="images/fig375.gif" width="200" height="219" alt="Fig.375. Heart of a human embryo, four weeks old. +Fig. 376. Heart of a human embryo, six weeks old, front view. Fig. 377. Heart +of a human embryo, eight weeks old, back view." /> +<p class="caption">Fig. +375—<b>Heart of a human embryo,</b> four weeks old; <i>1.</i> front +view, <i>2.</i> back view, <i>3.</i> opened, and upper half of the atrium +removed. <i>a′</i> left auricle, <i>a″</i> right auricle, +<i>v′</i> left ventricle, <i>v″</i> right ventricle, <i>ao</i> +arterial bulb, <i>c</i> superior vena cava (<i>cd</i> right, <i>cs</i> left), +<i>s</i> rudiment of the interventricular wall. (From <i>Kölliker.</i>)<br/> +Fig. 376—<b>Heart of a human embryo,</b> six weeks old, front view. +<i>r</i> right ventricle, <i>t</i> left ventricle, <i>s</i> furrow between +ventricles, <i>ta</i> arterial bulb, <i>af</i> furrow on its surface; to right +and left are the two large auricles. (From <i>Ecker.</i>)<br/> Fig. +377—<b>Heart of a human embryo,</b> eight weeks old, back view. +<i>a′</i> left auricle, <i>a″</i> right auricle, <i>v′</i> +left ventricle, <i>v″</i> right ventricle, <i>cd</i> right superior vena +cava, <i>ci</i> inferior vena cava. (From <i>Kölliker.</i>)</p> +</div> + +<p> +The human heart also develops in just the same way as that of the other mammals +(Fig. 378). We have already seen the first rudiments of its embryology, which +in the main corresponds to its phylogeny (Figs. 201, 202). We saw that the +palingenetic form of the heart is a spindle-shaped thickening of the gut-fibre +layer in the ventral wall of the head-gut. The structure is then hollowed out, +forms a simple tube, detaches from its place of origin, and henceforth lies +freely in the cardiac cavity. Presently the tube bends into the shape of an S, +and turns spirally on an imaginary axis in such a way that the hind part comes +to lie on the dorsal surface of the fore part. The united vitelline veins open +into the posterior end. From the anterior end spring the aortic arches. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus378"></a> +<img src="images/fig378.gif" width="183" height="232" alt="Fig.378. Heart of the +adult man, fully developed, front view, natural position." /> +<p class="caption">Fig. 378—<b>Heart of the adult man,</b> fully +developed, front view, natural position. <i>a</i> right auricle (underneath it +the right ventricle), <i>b</i> left auricle (under it the left ventricle), +<i>C</i> superior vena cava, <i>V</i> pulmonary veins, <i>P</i> pulmonary +artery, <i>d</i> Botalli’s duct, <i>A</i> aorta. (From +<i>Meyer.</i>)</p> +</div> + +<p> +This first structure of the human heart, enclosing a very simple cavity, +corresponds to the tunicate-heart, and is a reproduction of that of the +Prochordonia, but it now divides into two, and subsequently into three, +compartments; this reminds us for a time of the heart of the Cyclostomes and +fishes. The spiral turning and bending of the heart increases, and at the same +time two transverse constrictions appear, dividing it externally into three +sections (Figs. 371, 372). The foremost section, which is turned towards the +ventral side, and from which the aortic arches rise, reproduces the arterial +bulb of the Selachii. The middle section is a simple ventricle, and the +hindmost, the section turned towards the dorsal side, into which the vitelline +veins inosculate, is a simple auricle (or <i>atrium</i>). The latter forms, +like the simple atrium of the fish-heart, a pair of lateral dilatations, the +auricles (Fig. 371 <i>b</i>); and the constriction between the atrium and +ventricle is called the auricular canal (Fig. 372 <i>ca</i>). The heart of the +human embryo is now a complete fish-heart. +</p> + +<p> +<span class='pagenum'><a name="Page_327" id="Page_327"></a></span> +In perfect harmony with its phylogeny, the embryonic development of the human +heart shows a gradual transition from the fish-heart, through the amphibian and +reptile, to the mammal form, The most important point in the transition is the +formation of a longitudinal partition—incomplete at first, but afterwards +complete—which separates all three divisions of the heart into right +(venous) and left (arterial) halves (cf. Figs. 373–378). The atrium is +separated into a right and left half, each of which absorbs the corresponding +auricle; into the right auricle open the body-veins (upper and lower vena cava, +Figs. 375 <i>c,</i> 377 <i>c</i>); the left auricle receives the pulmonary +veins. In the same way a superficial interventricular furrow is soon seen in +the ventricle (Fig. 376 <i>s</i>). This is the external sign of the internal +partition by which the ventricle is divided into two—a right venous and +left arterial ventricle. Finally a longitudinal partition is formed in the +third section of the primitive fish-like heart, the arterial bulb, externally +indicated by a longitudinal furrow (Fig. 376 <i>af</i>). The cavity of the bulb +is divided into two lateral halves, the pulmonary-artery bulb, that opens into +the right ventricle, and the aorta-bulb, that opens into the left ventricle. +When all the partitions are complete, the small (pulmonary) circulation is +distinguished from the large (body) circulation; the motive centre of the +former is the right half, and that of the latter the left half, of the heart. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus379"></a> +<img src="images/fig379.gif" width="449" height="184" alt="Fig.379. Transverse +section of the back of the head of a chick-embryo, forty hours old." /> +<p class="caption">Fig. 379—<b>Transverse section of the back of the +head of a chick-embryo,</b> forty hours old. (From <i>Kölliker.</i>) <i>m</i> +medulla oblongata, <i>ph</i> pharyngeal cavity (head-gut), <i>h</i> horny +plate, <i>h′</i> thicker part of it, from which the auscultory pits +afterwards develop, <i>hp</i> skin-fibre plate, <i>hh</i> cervical cavity +(head-cœlom or cardiocœl), <i>hzp</i> cardiac plate (the outermost mesodermic +wall of the heart), connected by the ventral mesocardium (<i>uhg</i>) with the +gut-fibre layer or visceral cœlom-layer (<i>dfp*prime;</i>), <i>Ent</i> +entoderm, <i>ihh</i> inner (entodermic?) wall of the heart; the two endothelial +cardiac tubes are still separated by the cenogenetic septum (<i>s</i>) of the +Amniotes, <i>g</i> vessels.</p> +</div> + +<p> +The heart of all the Vertebrates belongs originally to the hyposoma of the +head, and we accordingly find it in the embryo of man and all the other +Amniotes right in front on the under-side of the head; just as in the fishes it +remains permanently in front of the gullet. It afterwards descends into the +trunk, with the advance in the development of the neck and breast, and at last +reaches the breast, between the two lungs. At first it lies symmetrically in +the middle plane of the body, so that its long axis corresponds with that of +the body. In most of the mammals it remains permanently in this position. But +in the apes the axis begins to be oblique, and the apex of the heart to move +towards the left side. The displacement is greatest in the anthropoid +apes—chimpanzee, gorilla, and orang—which resemble man in this. +</p> + +<p> +As the heart of all Vertebrates is originally, in the light of phylogeny, only +a local enlargement of the middle principal vein, it is in perfect accord with +the biogenetic law that its first structure in the embryo is a simple +spindle-shaped tube in the ventral wall of the head-gut. A thin membrane, +standing vertically in the middle plane, the mesocardium, connects the ventral +wall of the head-gut with the lower head-wall. As the cardiac tube extends and +detaches from the gut-wall, it divides the mesocardium into an upper (dorsal) +and lower (ventral) plate (usually called the <i>mesocardium anterius</i> and +<i>posterius</i> in man, Fig. 379 <i>uhg</i>). The +<span class='pagenum'><a name="Page_328" id="Page_328"></a></span> +mesocardium divides two lateral cavities, Remak’s +“neck-cavities” (Fig. 379 <i>hh</i>). These cavities afterwards +join and form the simple pericardial cavity, and are therefore called by +Kölliker the “primitive pericardial cavities.” +</p> + +<p> +The double cervical cavity of the Amniotes is very interesting, both from the +anatomical and the evolutionary point of view; it corresponds to a part of the +hyposomites of the head of the lower Vertebrates—that part of the ventral +cœlom-pouches which comes next to Van Wijhe’s “visceral +cavities” below. Each of the cavities still communicates freely behind +with the two cœlom-pouches of the trunk; and, just as these afterwards coalesce +into a simple body-cavity (the ventral mesentery disappearing), we find the +same thing happening in the head. This simple primary pericardial cavity has +been well called by Gegenbaur the “head-cœloma,” and by Hertwig the +“pericardial breast-cavity.” As it now encloses the heart, it may +also be called <i>cardiocœl.</i> +</p> + +<div class="fig" style="width:100%;"> +<a name="illus380"></a> +<img src="images/fig380.gif" width="183" height="211" alt="Fig.380. Frontal section +of a human embryo, one-twelfth of an inch long in the neck." /> +<p class="caption">Fig. 380—<b>Frontal section of a human embryo,</b> +one-twelfth of an inch long in the neck; “invented” by <i>Wilhelm +His.</i> Seen from ventral side. <i>mb</i> mouth-fissure, surrounded by the +branchial processes, <i>ab</i> bulbus of aorta, <i>hm</i> middle part of +ventricle, <i>hl</i> left lateral part of same, <i>ho</i> auricle, <i>d</i> +diaphragm, <i>vc</i> superior vena cava, <i>vu</i> umbilical vein, <i>vo</i> +vitelline space, <i>lb</i> liver, <i>lg</i> hepatic duct.</p> +</div> + +<p> +The cardiocœl, or head-cœlom, is often disproportionately large in the +Amniotes, the simple cardiac tube growing considerably and lying in several +folds. This causes the ventral wall of the amniote embryo, between the head and +the navel, to be pushed outwards as in rupture (cf. Fig. 180 <i>h</i>). A +transverse fold of the ventral wall, which receives all the vein-trunks that +open into the heart, grows up from below between the pericardium and the +stomach, and forms a transverse partition, which is the first structure of the +primary diaphragm (Fig. 380 <i>d</i>). This important muscular partition, which +completely separates the thoracic and abdominal cavities in the mammals alone, +is still very imperfect here; the two cavities still communicate for a time by +two narrow canals. These canals, which belong to the dorsal part of the +head-cœlom, and which we may call briefly <i>pleural ducts,</i> receive the two +pulmonary sacs, which develop from the hind end of the ventral wall of the +head-gut; they thus become the two pleural cavities. +</p> + +<p> +The diaphragm makes its first appearance in the class of the Amphibia (in the +salamanders) as an insignificant muscular transverse fold of the ventral wall, +which rises from the fore end of the transverse abdominal muscle, and grows +between the pericardium and the liver. In the reptiles (tortoises and +crocodiles) a later dorsal part is joined to this earlier ventral part of the +rudimentary diaphragm, a pair of subvertebral muscles rising from the vertebral +column and being added as “columns” to the transverse partition. +But it was probably in the Permian sauro-mammals that the two originally +separate parts were united, and the diaphragm became a complete partition +between the thoracic and abdominal cavities in the mammals; as it considerably +enlarges the chest-cavity when it contracts, it becomes an important +respiratory muscle. The ontogeny of the diaphragm in man and the other mammals +reproduces this phylogenetic process to-day, in accordance with the biogenetic +law; in all the mammals the diaphragm is formed by the secondary conjunction of +the two originally separate structures, the earlier ventral part and the later +dorsal part. +</p> + +<p> +Sometimes the blending of the two diaphragmatic structures, and consequently +the severance of the one pleural duct from the abdominal cavity, is not +completed in man. This leads to a diaphragmatic rupture (<i>hernia +diaphragmatica</i>). The two cavities then remain in communication by an open +pleural duct, and loops of the intestine may penetrate by this “rupture +opening” into the chest-cavity. This is one of those +<span class='pagenum'><a name="Page_329" id="Page_329"></a></span> +fatal mis-growths that show the great part that blind chance has in organic +development. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus381"></a> +<img src="images/fig381.gif" width="273" height="136" alt="Fig.381. Transverse section of the head of a +chick-embryo, thirty-six hours old." /> +<p class="caption">Fig. 381—<b>Transverse section of the head of a +chick-embryo,</b> thirty-six hours old. Underneath the medullary tube the two +primitive aortas (<i>pa</i>) can be seen in the head-plates (<i>s</i>) at each +side of the chorda. Underneath the gullet (<i>d</i>) we see the aorta-end of +the heart (<i>ae</i>), <i>hh</i> cervical cavity or head cœlom, <i>hk</i> top +of heart, <i>ks</i> head-sheath, amniotic fold, <i>h</i> horny plate. (From +<i>Remak.</i></p> +</div> + +<p> +Thus the thoracic cavity of the mammals, with its important contents, the heart +and lungs, belongs originally to the <i>head-part</i> of the vertebrate body, +and its inclusion in the trunk is secondary. This instructive and very +interesting fact is entirely proved by the concordant evidence of comparative +anatomy and ontogeny. The lungs are outgrowths of the head-gut; the heart +develops from its inner wall. The pleural sacs that enclose the lungs are +dorsal parts of the head-cœlom, originating from the pleuroducts; the +pericardium in which the heart afterwards lies is also double originally, being +formed from ventral halves of the head-cœlom, which only combine at a later +stage. When the lung of the air-breathing Vertebrates issues from the +head-cavity and enters the trunk-cavity, it follows the example of the floating +bladder of the fishes, which also originates from the pharyngeal wall in the +shape of a small pouch-like out-growth, but soon grows so large that, in order +to find room, it has to pass far behind into the trunk-cavity. To put it more +precisely, the lung of the quadrupeds retains this hereditary growth-process of +the fishes; for the hydrostatic floating bladder of the latter is the +air-filled organ from which the air-breathing organ of the former has been +evolved. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus382"></a> +<img src="images/fig382.gif" width="265" height="128" alt="Fig.382. Transverse +section of the cardiac region of the same chick-embryo (behind the +preceding)." /> +<p class="caption">Fig. 382—<b>Transverse section of the cardiac +region of the same chick-embryo</b> (behind the preceding). In the cervical +cavity (<i>hh</i>) the heart (<i>h</i>) is still connected by a mesocard +(<i>hg</i>) with the gut-fibre layer (<i>pf</i>). <i>d</i> gut-gland layer, +<i>up</i> provertebral plates, <i>jb</i> rudimentary auditory vesicle in the +horny plate, <i>hp</i> first rise of the amniotic fold. (From +<i>Remak.</i>)</p> +</div> + +<p> +There is an interesting cenogenetic phenomenon in the formation of the heart of +the higher Vertebrates that deserves special notice. In its earliest form the +heart is <i>double,</i> as recent observation has shown, in all the Amniotes, +and the simple spindle-shaped cardiac tube, which we took as our +starting-point, is only formed at a later stage, when the two lateral tubes +move backwards, touch each other, and at last combine in the middle line. In +man, as in the rabbit, the two embryonic hearts are still far apart at the +stage when there are already eight primitive segments (Fig. 134 <i>h</i>). So +also the two cœlom-pouches of the head in which they lie are still separated by +a broad space. It is not until the permanent body of the embryo develops and +detaches from the embryonic vesicle that the separate lateral structures join +together, and finally combine in the middle line. As the median partition +between the right and left cardiocœl disappears, the two cervical cavities +freely communicate (Fig. 381), and form, on the ventral side of the amniote +head, a horseshoe-shaped arch, the points of which advance backwards into the +pleuro-ducts or pleural cavities, and from there into the two peritoneal sacs +of the trunk. But even after the conjunction of the cervical cavities (Fig. +381) the two cardiac tubes remain separate at first; and even after they have +united a delicate partition in the middle of the simple endothelial tube (Figs. +379 <i>s,</i> 382 <i>h</i>) indicates the original separation. This +<i>cenogenetic</i> “primary cardiac +<span class='pagenum'><a name="Page_330" id="Page_330"></a></span> +septum” presently disappears, and has no relation to the subsequent +permanent partition between the halves of the heart, which, as a heritage from +the reptiles, has a great <i>palingenetic</i> importance. +</p> + +<p> +Thorough opponents of the biogenetic law have laid great stress on these and +similar cenogenetic phenomena, and endeavoured to urge them as striking +disproofs of the law. As in every other instance, careful, discriminating, +comparative-morphological examination converts these supposed disproofs of +evolution into strong arguments in its favour. In his excellent work, <i>On the +structure of the Heart in the Amphibia</i> (1886), Carl Rabl has shown how +easily these curious cenogenetic facts can be explained by the secondary +adaptation of the embryonic structure to the great extension of the food-yelk. +</p> + +<p> +The embryology of all the other parts of the vascular system also gives us +abundant and valuable data for the purposes of phylogeny. But as one needs a +thorough knowledge of the intricate structure of the whole vascular system in +man and the other Vertebrates in order to follow this with profit, we cannot go +into it further here. Moreover, many important features in the ontogeny of the +vascular system are still very obscure and controverted. The characters of the +embryonic circulation of the Amniotes, which we have previously considered +(Chapter XV), are late acquisitions and entirely cenogenetic. (Cf. pp. +170–171; Figs. 198–202.) +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap29"></a>Chapter XXIX.<br/> +EVOLUTION OF THE SEXUAL ORGANS</h2> + +<p> +If we measure the importance of the systems of organs in the animal frame +according to the richness and variety of their phenomena and the physiological +interest that this implies, we must regard as one of the principal and most +interesting systems the one which we are now going to examine—the system +of the reproductive organs. Just as nutrition is the first and most urgent +condition for the self-maintenance of the individual organism, so reproduction +alone secures the maintenance of the species—or, rather, the maintenance +of the long series of generations which the totality of the organic stem +represents in their genealogical connection. No individual organism has the +prerogative of immortality. To each is allotted only a brief span of personal +development, an evanescent moment in the million-year course of the history of +life. +</p> + +<p> +Hence, reproduction and the correlative phenomenon, heredity, have long been +regarded, together with nutrition, as the most important and fundamental +function of living things, and it has been attempted to distinguish them from +“lifeless bodies” on this very score. As a matter of fact, this +division is not so profound and thorough as it seems to be, and is generally +supposed to be. If we examine carefully the nature of the reproductive process, +we soon see that it can be reduced to a general property that is found in +inorganic as well as organic bodies—growth. Reproduction is a nutrition +and growth of the organism beyond the individual limit, which raises a part of +it into the whole. This is most clearly seen when we study it in the simplest +and lowest organisms, especially the Monera (Figs. 226–228) and the +unicellular Amœbæ (Fig. 17). There the simple individual is a single plastid. +As soon as it has reached a certain limit of size by continuous feeding and +normal growth, it cannot pass it, but divides, by simple cleavage, into two +equal halves. Each of these halves then continues its independent life, and +grows on until it in turn reaches the limit of growth, and divides. In each of +these acts of self-cleavage two new centres of attraction are formed for the +particles of bodies, the foundations of the two new-formed individuals. There +is no such thing as immortality even in these unicellulars. +<span class='pagenum'><a name="Page_331" id="Page_331"></a></span> +The individual as such is annihilated in the act of cleavage (cf. p. 48). +</p> + +<p> +In many other Protozoa reproduction takes place not by cleavage, but by budding +(gemmation). In this case the growth that determines reproduction is not total +(as in segmentation), but partial. Hence in gemmation also we may oppose the +local growth-product, that becomes a new individual in the bud, as a +child-organism to the parent-organism from which it is formed. The latter is +older and larger than the former. In cleavage the two products are equal in age +and morphological value. Next to gemmation we have, as other forms of asexual +reproduction, the forming of embryonic buds and the forming of embryonic cells. +But the latter leads us at once to sexual generation, the distinctive feature +of which is the separation of the sexes. I have dealt fully with these various +types of reproduction in my <i>History of Creation</i> (chap. viii) and my +<i>Wonders of Life</i> (chap. xi). +</p> + +<p> +The earliest ancestors of man and the higher animals had no faculty of sexual +reproduction, but multiplied solely by asexual means—cleavage, gemmation, +or the formation of embryonic buds or cells, as many Protozoa still do. The +differentiation of the sexes came at a later stage. We see this most plainly in +the Protists, in which the union of two individuals precedes the continuous +cleavage of the unicellular organism (transitory conjugation and permanent +copulation of the Infusoria). We may say that in this case the growth (the +condition of reproduction) is attained by the coalescence of two full-grown +cells into a single, disproportionately large individual. At the same time, the +mixture of the two plastids causes a rejuvenation of the plasm. At first the +copulating cells are quite homogeneous; but natural selection soon brings about +a certain contrast between them—larger female cells (<i>macrospores</i>) +and smaller male cells (<i>microspores</i>). It must be a great advantage in +the struggle for life for the new individual to have inherited different +qualities from the two cellular parents. The further advance of this contrast +between the generating cells led to sexual differentiation. One cell became the +female ovum (<i>macrogonidion</i>), and the other the male sperm-cell +(<i>microgonidion</i>). +</p> + +<p> +The simplest forms of sexual reproduction among the living Metazoa are seen in +the Gastræads p. 233, the lower sponges, the common fresh-water polyp +(<i>Hydra</i>), and other Cœlenteria of the lowest rank. Prophysema (Fig. 234), +Olynthus (Fig. 238), Hydra, etc., have very simple tubular bodies, the thin +wall of which consists (as in the original gastrula) only of the two primary +germinal layers. As soon as the body reaches sexual maturity, a number of the +cells in its wall become female ova, and others male sperm-cells: the former +become very large, as they accumulate a considerable quantity of yelk-granules +in their protoplasm (Fig. 235 <i>e</i>); the latter are very small on account +of their repeated cleavage, and change into mobile cone-shaped spermatozoa +(Fig. 20). Both kinds of cells detach from their source of origin, the primary +germinal layers, fall either into the surrounding water or into the cavity of +the gut, and unite there by fusing together. This is the momentous process of +fecundation, which we have examined in Chapter VII (cf. Figs. 23–29). +</p> + +<p> +From these simplest forms of sexual propagation, as we can observe them to-day +in the lowest Zoophytes, the Gastræads, Sponges, and Polyps, we gather most +important data. In the first place, we learn that, properly speaking, nothing +is required for sexual reproduction except the fusion or coalescence of two +different cells—a female ovum and male sperm-cell. All other features, +and all the very complex phenomena that accompany the sexual act in the higher +animals, are of a subordinate and secondary character, and are later additions +to this simple, primary process of copulation and fecundation. But if we bear +in mind how extremely important a part this relation of the two sexes plays in +the whole of organic nature, in the life of plants, of animals, and of man; how +the mutual attraction of the sexes, love, is the mainspring of the most +remarkable processes—in fact, one of the chief mechanical causes of the +highest development of life—we cannot too greatly emphasise this tracing +of love to its source, the attractive force of two erotic cells. +</p> + +<p> +Throughout the whole of living nature the greatest effects proceed from this +very small cause. Consider the part that the flowers, the sexual organs of the +flowering plants, play in nature; or the exuberance of wonderful phenomena that +sexual selection produces in animal life; or the +<span class='pagenum'><a name="Page_332" id="Page_332"></a></span> +momentous influence of love in the life of man. In every case the fusion of two +cells is the sole original motive power; in every case this invisible process +profoundly affects the development of the most varied structures. We may say, +indeed, that no other organic process can be compared to it for a moment in +comprehensiveness and intensity of action. Are not the Semitic myth of Adam and +Eve, the old Greek legend of Paris and Helena, and so many other famous +traditions, only the poetic expression of the vast influence that love and +sexual selection have exercised over the course of history ever since the +differentiation of the sexes? All the other passions that agitate the heart of +man are far outstripped in their joint influence by this sense-inflaming and +mind-benumbing Eros. On the one hand, we look to love with gratitude as the +source of the greatest artistic achievements—the noblest creations of +poetry, plastic art, and music; we see in it the chief factor in the moral +advance of humanity, the foundation of family life, and therefore of social +advance. On the other hand, we dread it as the devouring flame that brings +destruction on so many, and has caused more misery, vice, and crime than all +the other evils of human life put together. So wonderful is love and so +momentous its influence on the life of the soul, or on the different functions +of the medullary tube, that here more than anywhere else the +“supernatural” result seems to mock any attempt at natural +explanation. Yet comparative evolution leads us clearly and indubitably to the +first source of love—the affinity of two different erotic cells, the +sperm-cell and ovum.<a href="#linknote-34" name="linknoteref-34" id="linknoteref-34"><sup>[34]</sup></a> +</p> + +<p class="footnote"> +<a name="linknote-34" id="linknote-34"></a> <a href="#linknoteref-34">[34]</a> +The sensual perception (probably related to smell) of the two copulating +sex-cells, which causes their mutual attraction, is a little understood, but +very interesting, chemical function of the cell-soul (cf. p. 58 and <i>The +Riddle of the Universe,</i> chap. ix.) +</p> + +<p> +The lowest Metazoa throw light on this very simple origin of the intricate +phenomena of reproduction, and they also teach us that the earliest sexual form +was hermaphrodism, and that the separation of the sexes (by division of labour) +is a secondary and later phenomenon. Hermaphrodism predominates in the most +varied groups of the lower animals; each sexually-mature individual, each +person, contains female and male sexual cells, and is therefore able to +fertilise itself and reproduce. Thus we find ova and sperm-cells in the same +individual, not only in the lowest Zoophytes (Gastræads, Sponges, and many +Polyps), but also in many worms (leeches and earthworms), many of the snails +(the common garden and vineyard snails), all the Tunicates, and many other +invertebrate animals. All man’s earlier invertebrate ancestors, from the +Gastræads up to the Prochordonia, were hermaphrodites; possibly even the +earliest Acrania. We have an instructive proof of this in the remarkable +circumstance that many genera of fishes are still hermaphrodites, and that it +is occasionally found in the higher Vertebrates of all classes (as atavism). We +may conclude from this that gonochorism (separation of the sexes) was a later +stage in our development. At first, male and female individuals differ only in +the possession of one or other kind of gonads; in other respects they were +identical, as we still find in the Amphioxus and the Cyclostomes. Afterwards, +accessory organs (ducts, etc.) are associated with the primary sexual glands; +and much later again sexual selection has given rise to the secondary sexual +characters—those differences between the sexes which do not affect the +sexual organs themselves, but other parts of the body (such as the man’s +beard or the woman’s breast). +</p> + +<p> +The third important fact that we learn from the lower Zoophytes relates to the +earliest origin of the two kinds of sexual cells. As in the Gastræads (the +lowest sponges and hydroids), in which we find the first beginnings of sexual +differentiation, the whole body consists merely of the two primary germinal +layers, it follows that the sexual cells also must have proceeded from the +cells of these primary layers, either the inner or outer, or from both. This +simple fact is extremely important, because the first trace of the ova as well +as the spermatozoa is found in the middle germinal layer or mesoderm in the +higher animals, especially the Vertebrates. This arrangement is a later +development from the preceding (in connection with the secondary formation of +the mesoderm). +</p> + +<p> +If we trace the phylogeny of the sexual organs in our earliest Metazoa +ancestors, as the comparative anatomy and ontogeny of the lowest Cœlenteria +(<i>Cnidaria, Platodaria</i>) exhibit it to us, we find that the first step in +advance is the localisation or concentration of the two kinds of sexual +<span class='pagenum'><a name="Page_333" id="Page_333"></a></span> +cells scattered in the epithelium into definite groups. In the Sponges and +lowest Hydropolyps isolated cells are detached from the cell-strata of the two +primary germinal layers, and become free sexual cells; but in the Cnidaria and +Platodes we find these associated in groups which we call sexual glands +(<i>gonads</i>). We can now for the first time speak of sexual organs in the +morphological sense. The female germinative glands, which in this simplest form +are merely groups of homogeneous cells, are the ovaries (Fig. 241 <i>c</i>). +The male germinative glands, which also in their first form consist of a +cluster of sperm-cells, are the testicles (Fig. 241 <i>h</i>). In the medusæ, +which descend, both ontogenetically and phylogenetically, from the more simply +organised Polyps, we find these simple sexual glands sometimes as gastric +pouches, sometimes as outgrowths of the radial canals that proceed from the +stomach. Particularly interesting in connection with the question of the first +origin of the gonads are the lowest forms of the Platodes, the +<i>Cryptocœla</i> that have of late been separated as a special class +(<i>Platodaria</i>) from the Turbellaria proper (Fig. 239). In these very +primitive Platodes the two pairs of sexual glands are merely two pairs of rows +of differentiated cells in the entodermic wall of the primitive gut—two +median ovaries (<i>o</i>) within, and two lateral spermaries (<i>s</i>) +without. The mature sexual cells are ejected by the posterior outlets; the +female (<i>f</i>) lies in front of the male (<i>m</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus383"></a> +<img src="images/fig383.gif" width="343" height="142" alt="Fig.383. Embryos of +Sagitta, in three earlier stages of development." /> +<p class="caption">Fig. 383—<b>Embryos of Sagitta,</b> in three +earlier stages of development. (From <i>Hertwig.</i>) <i>A</i> gastrula, +<i>B</i> cœlomula with open primitive mouth, <i>C</i> the same primitive mouth +closed, <i>ua</i> primitive gut, <i>bl</i> primitive mouth, <i>g</i> progonidia +(hermaphroditic primitive sexual cells), <i>cs</i> cœlom-pouches, <i>pm</i> +parietal layer, <i>vm</i> visceral layer of same, <i>d</i> permanent gut +(enteron), <i>st</i> mouth-pit (stomodæum).</p> +</div> + +<p> +In the great majority of the Bilateria or Cœlomaria it is the mesoderm from +which the gonads develop. Probably the first traces of them are the two large +cells that appear at the edge of the primitive mouth (right and left), as a +rule during gastrulation or immediately afterwards—the important +promesoblasts, or “polar cells of the mesoderm,” or +“primitive cells of the middle germinal layer” (p. 194). In the +real Enterocœla, in which the mesoderm appears from the first in the shape of a +couple of cœlom-pouches, these are very probably the original gonads (p. 194). +This is seen very clearly in the arrow-worm (<i>Sagitta</i>). In the gastrula +of Sagitta (Fig. 383 <i>A</i>) we find at an early stage a couple of entodermic +cells of an unusual size (<i>g</i>) at the base of the primitive gut +(<i>ud</i>). These primitive sexual cells (<i>progonidia</i>) are symmetrically +placed to the right and left of the middle plane, like the two promesoblasts of +the bilateral gastrula of the Amphioxus (Fig. 38 <i>p</i>). A little outwards +from them the two cœlom pouches (<i>B, cs</i>) are developed out of the +primitive gut, and each progonidion divides into a male and a female sexual +cell (<i>B, g</i>). The two male cells (at first rather the larger) lie close +together within, and are the parent-cells of the testicles +(<i>prospermaria</i>). The two female cells lie outwards from these, and are +the parent-cells of the ovary (<i>protovaria</i>). Afterwards, when the +cœlom-pouches have detached from the permanent gut (<i>C, d</i>) and the +primitive mouth (<i>A, bl</i>) is closed, the female cells advance towards the +mouth (<i>C, st</i>), and the male towards the rear. The foremost pair of +ovaries are then separated by a transverse partition from the hind pair. Thus +the first structures of the sexual glands of the Sagitta are a couple of +hermaphroditic entodermic cells; each of these divides +<span class='pagenum'><a name="Page_334" id="Page_334"></a></span> +into a male and a female cell; and these four cells are the parent-cells of the +four sexual glands. Probably the two promesoblasts of the Amphioxus-gastrula +(Fig. 38) are also hermaphroditic primitive sexual cells in the same sense, +inherited by this earliest vertebrate from its ancient bilateral gastræad +ancestors. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus384"></a> +<img src="images/fig384.gif" width="149" height="358" alt="Fig.384. A, Part of the +kidneys of Bdellostoma. B Portion of same, highly magnified." /> +<p class="caption">Fig. 384—<b><i>A,</i> Part of the kidneys of +Bdellostoma.</b> <i>a</i> prorenal duct (nephroductus), <i>b</i> segmental or +primitive urinary canals (pronephridia), <i>c</i> renal or Malpighian capsules. +<b><i>B</i> Portion of same,</b> highly magnified. <i>c</i> renal capsules with +the glomerulus, <i>d</i> afferent artery, <i>e</i> efferent artery. From +<i>Johannes Müller</i> (Myxinoides).</p> +</div> + +<p> +The sexually-mature Amphioxus is not hermaphroditic, as its nearest +invertebrate relatives, the Tunicates, are, and as the long-extinct +pre-Silurian Primitive Vertebrate (<i>Prospondylus,</i> Figs. 98–102) +probably was. The actual lancelet has gonochoristic structures of a very +interesting kind. As we saw in the anatomy of the Amphioxus, we find the +ovaries of the female and the spermaries of the male in the shape of twenty to +thirty pairs of elliptical or roundish four-cornered sacs, which lie on either +side of the gut on the parietal surface of the respiratory pore (Fig. 219 +<i>g</i>). According to the important discovery of Rückert (1888), the sexual +glands of the earliest fishes, the Selachii, are similarly arranged. They only +unite afterwards to form a pair of simple gonads. These have been transmitted +by heredity to all the rest of the Craniotes. In every case they lie originally +on each side of the mesentery, underneath the chorda, at the bottom of the +body-cavity. The first traces of them are found in the cœlom-epithelium, at the +spot where the skin-fibre layer and gut-fibre layer meet in the middle of the +mesenteric plate (Fig. 93 <i>mp</i>). At this point we observe at an early +stage in all craniote embryos a small string-like cluster of cells, which we +may call, with Waldeyer, the “germ epithelium,” or (in harmony with +the other plate-shaped rudimentary organs) the <i>sexual plate</i> (Fig. 173 +<i>g</i>). This germinal or sexual plate is found in the fifth week in the +human embryo, in the shape of a couple of long whitish streaks, on the inner +side of the primitive kidneys (Fig. 183 <i>t</i>). The cells of this sexual +plate are distinguished by their cylindrical form and chemical composition from +the rest of the cœlom-cells; they have a different purport from the flat cells +which line the rest of the body-cavity. As the germ epithelium of the sexual +plate becomes thicker, and supporting tissue grows into it from the mesoderm, +it becomes a rudimentary sexual gland. This ventral gonad then develops into +the ovary in the female Craniotes, and the testicles in the male. +</p> + +<p> +In the formation of the gonidia or erotic sexual cells and their conjunction at +fecundation we have the sole essential features of sexual reproduction; but in +the great majority of animals we find other organs taking part in it. The chief +of these secondary sexual organs are the gonoducts, which serve to convey the +mature sexual cells out of the body, and the copulative organs, which bring the +fecundating male sperm into touch with the ovum-bearing female. The latter +organs are, as a rule, only found in the higher animals, and are much less +widely distributed than the gonoducts. But these also are secondary formations, +and are wanting in many animals of the lower groups. +</p> + +<p> +In the lower animals the mature sexual cells are generally ejected directly +from +<span class='pagenum'><a name="Page_335" id="Page_335"></a></span> +the body. Sometimes they pass out immediately through the skin (Hydra and many +hydroids); sometimes they fall into the gastric cavity, and are evacuated by +the mouth (gastræads, sponges, many medusæ, and corals); sometimes they fall +into the body-cavity, and are ejected by a special pore (<i>porus +genitalis</i>) in the ventral wall. The latter procedure is found in many of +the worms, and also in the lowest Vertebrates. Amphioxus has the peculiar +feature that the mature sexual products fall first into the mantle-cavity; from +there they are either evacuated by the respiratory pore, or else they pass +through the gill-clefts into the branchial gut, and so out by the mouth (p. +185). In the Cyclostomes they fall into the body-cavity, and are ejected by a +genital pore in its wall; so also in some of the fishes. From these we gather +the features of our earlier ancestors in this respect. On the other hand, in +all the higher and most of the lower Vertebrates (and most of the higher +Invertebrates) we find in both sexes special tubular passages of the sexual +gland, which are called “gonoducts.” In the female they conduct the +ova from the ovary, and so are called “oviducts,” or +“Fallopian tubes.” In the male they convey the spermatozoa from the +testicles, and are called “spermaducts,” or <i>vasa deferentia.</i> +</p> + +<div class="fig" style="width:100%;"> +<a name="illus385"></a> +<img src="images/fig385.gif" width="425" height="127" alt="Fig.385. Transverse +section of the embryonic shield of a chick, forty-two hours old." /> +<p class="caption">Fig. 385—<b>Transverse section of the embryonic +shield of a chick,</b> forty-two hours old. (From <i>Kölliker.</i>) <i>mr</i> +medullary tube, <i>ch</i> chorda, <i>h</i> horny plate (skin-sense layer), +<i>ung</i> nephroduct, <i>vw</i> episomites (dorsal primitive segments), +<i>hp</i> skin-fibre layer (parietal layer of the hyposomites), <i>dfp</i> +gut-fibre layer (visceral layer of hyposomites), <i>ao</i> aorta, <i>g</i> +vessels. (Cf. transverse section of duck-embryo, Fig. 152.)</p> +</div> + +<p> +The original and genetic relation of these two kinds of ducts is just the same +in man as in the rest of the higher Vertebrates, and quite different from what +we find in most of the Invertebrates. In the latter, as a rule, the gonoducts +develop directly from the embryonic glands or from the outer skin; but in the +Vertebrates an independent organic system is employed to convey the sexual +products, and this had originally a totally different function—namely, +the system of urinary organs. These organs have primarily the sole duty of +removing unusable matter from the body in a fluid form. Their liquid excretory +product, the urine, is either evacuated directly through the skin or through +the last section of the gut. It is only at a later stage that the tubular +urinary passages also convey the sexual products from the body. In this way +they become “urogenital ducts.” This remarkable secondary +conjunction of the urinary and sexual organs into a common urogenital system is +very characteristic of the Gnathostomes, the six higher classes of Vertebrates. +It is wanting in the lower classes. In order to appreciate it fully, we must +give a comparative glance at the structure of the urinary organs. +</p> + +<p> +The renal or urinary system is one of the oldest and most important systems of +organs in the differentiated animal body, as I have pointed out on several +previous occasions (cf. Chapter XVII). We find it not only in the higher stems, +but also very generally distributed in the earlier group of the Vermalia. Here +we meet it in the lowest worms, the Rotatoria (Gastrotricha, Fig. 242), and in +the instructive stem of the Platodes. It consists of a pair of simple or +branching canals, which are lined with one layer of cells, absorb unusable +juices from the tissue, and eject them by an outlet in the outer skin (Fig. 240 +<i>nm</i>). Not only the free-living Turbellaria, but also the parasitic +Suctoria, and even the still more degenerate tapeworms, which have lost their +alimentary canal in consequence of their parasitic life, are equipped with +these renal canals +<span class='pagenum'><a name="Page_336" id="Page_336"></a></span> +or nephridia. In the first embryonic structure they are merely a pair of simple +cutaneous glands, or depressions in the ectoderm. They are generally described +as excretory organs in the worms, but formerly often as “water +vessels.” They may be conceived as largely-developed tubular cutaneous +glands, formed by invagination of the cutaneous layer. According to another +view, they owe their origin to a later rupture of the body-cavity outwards. In +most of the Vermalia each nephridium has an inner opening (with cilia) into the +body-cavity and an outer one on the epidermis. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus386"></a> +<img src="images/fig386.gif" width="213" height="269" alt="Fig.386. Rudimentary primitive kidneys of a +dog-embryo." /> +<p class="caption">Fig. 386—<b>Rudimentary primitive kidneys of a +dog-embryo.</b> The hind end of the embryonic body is seen from the ventral +side and covered with the visceral layer of the yelk-sac, which is torn away +and folded down in front in order to show the nephroducts with the primitive +urinary canals (<i>a</i>). <i>b</i> primitive vertebræ, <i>c</i> spinal cord, +<i>d</i> entrance into the pelvic-gut cavity. (From <i>Bischoff.</i>)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus387"></a> +<img src="images/fig387.gif" width="127" height="255" alt="Fig. 387. Primitive kidneys of a human embryo." /> +<p class="caption">Fig. 387—<b>Primitive kidneys of a human +embryo.</b> <i>u</i> the urinary canals of the primitive kidneys, <i>w</i> +Wolffian duct, <i>w′</i> uppermost end of the same (Morgagni’s +hydatid), <i>m</i> Mullerian duct. <i>m′</i> uppermost end of same +(Fallopian hydatid), <i>g</i> gonad (sexual gland). (From +<i>Kobelt.</i>)</p> +</div> + +<p> +In these lowest, unsegmented worms, and in the unsegmented Molluscs, there is +only one pair of renal canals. They are more numerous in the higher +Articulates. In the Annelids, the body of which is composed of a large number +of joints, there is a pair of these pronephridia in each segment (hence they +are called segmental canals or organs). Even here they are still simple tubes; +on account of their coiled or looped form they are often called “looped +canals.” In most of the Annelids, and many of the Vermalia, we can +distinguish three sections in the nephridium—an outer muscular duct, a +glandular middle part, and an inner part that opens by a ciliated funnel into +the body-cavity. This opening is furnished with whirling cilia, and can, +therefore, take up the juices to be excreted directly from the body-cavity and +convey them from the body. But in these worms the sexual cells, which develop +in very primitive form on the inner surface of the body-cavity, also fall into +it when mature, and are sucked up by the funnel-shaped inner ciliated openings +of the renal canals, and ejected with the urine. Thus the urine-forming looped +canals, or pronephridia, serve as oviducts in the female Annelids and as +spermaducts in the male. +</p> + +<p> +The renal system of the Vertebrates is similar to, yet materially different +from, these segmental canals of the Annelids. The peculiar development of it +and its relations to the sexual organs are among the most difficult problems in +the morphology of our stem. If we examine briefly the vertebrate renal system +from the phylogenetic point of view, as confirmed by recent discoveries, we may +distinguish three forms of it: (1) Fore-kidneys or head-kidneys +(<i>pronephros</i>); (2) primitive or middle kidneys (mesonephros); (3) +permanent kidneys (<i>metanephros</i>). These three systems of kidneys are not +fundamentally and completely distinct, as earlier students (such as Semper) +wrongly supposed; they represent three different generations of one and the +same excretory apparatus; they correspond to three phylogenetic stages, +<span class='pagenum'><a name="Page_337" id="Page_337"></a></span> +and succeed each other in the stem-history of the Vertebrates in such wise that +each younger and more advanced generation develops farther behind in the body, +and replaces the older and less advanced generation that preceded it in time +and space. The <i>fore kidneys,</i> first accurately described by Wilhelm +Müller in 1875 in the Cyclostomes and Ichthyoda, form the sole excretory organ +of the Acrania (Amphioxus); they continue in the Cyclostomes and some of the +fishes, but are found only in slight traces and for a time in the embryos of +the six other classes of Vertebrates. The <i>primitive kidneys</i> are first +found in the Cyclostomes, behind the fore kidneys; they have been transmitted +from the Selachii to all the Gnathostomes. In the <i>Anamnia</i> they act +permanently as urinary glands; in the <i>Amniotes</i> their anterior part +(“germinal kidneys”) changes into organs of the sexual apparatus, +while the third generation develops from the end of their posterior part +(“urinal kidneys”)—the characteristic after or permanent +kidneys of the three higher classes of Vertebrates. The order in which the +three renal systems succeed each other in the embryo of man and the higher +Vertebrates corresponds to their phylogenetic succession in the history of our +stem, and, consequently, in the natural classification of the Vertebrates. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus388"></a> +<img src="images/fig388.gif" width="168" height="247" alt="Fig.388. Pig-embryo, three-fifths of an inch +long, seen from the ventral side." /> +<p class="caption">Fig. 388—<b>Pig-embryo,</b> three-fifths of an +inch long, seen from the ventral side. <i>a</i> fore leg, <i>z</i> hind leg, +<i>b</i> ventral wall, <i>r</i> sexual prominence, <i>w</i> nephroduct, +<i>n</i> primitive kidneys, <i>n1</i> their inner part. (From <i>Oscar +Schultze.</i>)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus389"></a> +<img src="images/fig389.gif" width="187" height="363" alt="Fig. 389. Human embryo of the fifth week, +two-fifths of an inch long, seen from the ventral side." /> +<p class="caption">Fig. 389—<b>Human embryo</b> of the fifth week, +two-fifths of an inch long, seen from the ventral side (the anterior ventral +wall, <i>b,</i> is removed, the body-cavity, <i>c,</i> opened). <i>d</i> gut +(cut off), <i>f</i> frontal process, <i>g</i> cerebrum, <i>m</i> middle brain, +<i>e</i> after brain, <i>h</i> heart, <i>k</i> first gill-cleft, <i>l</i> +pulmonary sac, <i>n</i> primitive kidneys, <i>r</i> sexual region, <i>p</i> +phallus (sexual prominences), <i>s</i> tail. (From <i>Kollmann.</i>)</p> +</div> + +<p> +As in the morphology of any other system of organs, so in the case of the +urinary and sexual organs the Amphioxus is the real typical primitive +Vertebrate; it affords the key to the mysteries of the structure of man and the +higher Vertebrates. The kidneys of the Amphioxus—first discovered by +Boveri in 1890—are typical “fore kidneys,” composed of a +double row of short segmental canals (Fig. 217 <i>x</i>). The inner aperture of +these pronephridia opens into the mesodermic body-cavity (the middle part of +the cœloma, <i>B</i>); the external aperture into the ectodermic mantle or +peribranchial cavity (<i>C</i>). Their position, their +<span class='pagenum'><a name="Page_338" id="Page_338"></a></span> +structure, and their relation to the branchial vessel make it clear that these +segmental pronephridia correspond to the rudimentary fore kidneys of the +Craniotes. The mantle-cavity into which they open seems to correspond to the +prorenal duct of the latter. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus390"></a> +<img src="images/fig390.gif" width="223" height="172" alt="Fig.390, 391, 392. Primitive kidneys and +rudimentary sexual organs." /> +<p class="caption">Figs. 390, 391, +392—<b>Primitive kidneys and rudimentary sexual organs.</b> Figs. 390 +and 391 of Amphibia (frog-larvæ); Fig. 390 earlier, 391 later stage. Fig. 392 +of a mammal (ox-embryo). <i>u</i> primitive kidney, <i>k</i> sexual gland +(rudiment of testicle and ovary). The primary nephroduct (<i>ug</i> in Fig. +390) divides (in Figs. 391 and 392) into the two secondary +nephroducts—the Mullerian (<i>m</i>) and Wolffian (<i>ug′</i>) +ducts, joined together behind in the genital cord (<i>g</i>). <i>l</i> ligament +of the primitive kidneys. (From Gegenbaur.)</p> +</div> + +<div class="fig" style="width:100%;"> +<a name="illus393"></a> +<img src="images/fig393.gif" width="176" height="377" alt="Fig.393, 394. Urinary +and sexual organs of an Amphibian (water salamander or Triton). Fig. 393 of a +female, 394 of a male." /> +<p class="caption">Figs. 393, 394—<b>Urinary and sexual organs of an +Amphibian</b> (water salamander or Triton). Fig. 393 of a female, 394 of a +male. <i>r</i> primitive kidney, <i>ov</i> ovary, <i>od</i> oviduct and +<i>c</i> Rathke’s duct, both developed from the Müllerian duct, <i>u</i> +primitive ureter (also acting as spermaduct [<i>ve</i>] in the male, opening +below into the Wolffian duct [<i>u</i> apostrophe]), <i>ms</i> mesovarium. +(From <i>Gegenbaur.</i>)</p> +</div> + +<p> +The next higher Vertebrates, the Cyclostomes, yield some very interesting data. +Both orders of this class, the hags and lampreys, have still the fore kidneys +inherited from the Acrania—the former permanently, the latter in their +earlier stages. Behind these the primitive kidneys soon develop, and in a very +characteristic form. The remarkable structure of the mesonephros of the +Cyclostomes, discovered by Johannes Müller, explains the intricate formation of +the kidneys in the higher Vertebrates. We find in the hag-fishes +(<i>Bdellostoma</i>) a long tube, the prorenal duct (<i>nephroductus,</i> Fig. +384 <i>a</i>). This opens with its anterior end into the cœloma by a ciliated +aperture, and externally with its posterior end by an outlet in the skin. +Inside it open a large number of small transverse canals (“segmental or +primitive urinary canals,” <i>b</i>). Each of these terminates blindly in +a vesicular capsule (<i>c</i>), and this encloses a coil of blood-vessel +(<i>glomerulus,</i> an arterial network, Fig. 384 <i>B, c</i>). Afferent +branches of arteries conduct arterial blood into the coiled branches of the +glomerulus (<i>d</i>), and efferent arterial branches conduct it away from the +net (<i>c</i>). The primitive renal canals (mesonephridia) are distinguished by +this net-formation from their predecessors. +</p> + +<p> +In the Selachii also we find a longitudinal row of segmental canals on each +side, which open outwards into the primitive renal ducts (<i>nephrotomes,</i> +p. 149. The segmental canals (a pair in each segment of the middle part of the +body) open internally by a ciliated funnel into the body-cavity. From the +posterior group of these organs a compact primitive kidney is formed, the +anterior group taking part in the construction of the sexual organs. +</p> + +<p> +In the same simple form that remains +<span class='pagenum'><a name="Page_339" id="Page_339"></a></span> +throughout life in the Myxinoides and partly in the Selachii we find the +primitive kidney first developing in the embryo of man and the higher Craniotes +(Figs. 386, 387). Of the two parts that compose the comb-shaped primitive +kidney the longitudinal channel, or nephroduct, is always the first to appear; +afterwards the transverse “canals,” the excreting nephridia, are +formed in the mesoderm; and after this again the Malpighian capsules with their +arterial coils are associated with these as cœlous outgrowths. The primitive +renal duct, which appears first, is found in all craniote embryos at the early +stage in which the differentiation of the medullary tube takes place in the +ectoderm, the severance of the chorda from the visceral layer in the entoderm, +and the first trace of the cœlom-pouches arises between the limiting layers +(Fig. 385). The nephroduct (<i>ung</i>) is seen on each side, directly under +the horny plate, in the shape of a long, thin, thread-like string of cells. It +presently hollows out and becomes a canal, running straight from front to back, +and clearly showing in the transverse section of the embryo its original +position in the space between horny plate (<i>h</i>), primitive segments +(<i>uw</i>), and lateral plates (<i>hpl</i>). As the originally very short +urinary canals lengthen and multiply, each of the two primitive kidneys assumes +the form of a half-feathered leaf (Fig. 387). The lines of the leaf are +represented by the urinary canals (<i>u</i>), and the rib by the outlying +nephroduct (<i>w</i>). At the inner edge of the primitive kidneys the rudiment +of the ventral sexual gland (<i>g</i>) can now be seen as a body of some size. +The hindermost end of the nephroduct opens right behind into the last section +of the rectum, thus making a cloaca of it. However, this opening of the +nephroducts into the intestine must be regarded as a secondary formation. +Originally they open, as the Cyclostomes clearly show, quite independently of +the gut, in the external skin of the abdomen. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus395"></a> +<img src="images/fig395.gif" width="269" height="233" alt="Fig.395. Primitive +kidneys and germinal glands of a human embryo, three inches in length +(beginning of the sixth week)." /> +<p class="caption">Fig. 395—<b>Primitive kidneys and germinal glands +of a human embryo,</b> three inches in length (beginning of the sixth week), +magnified. <i>k</i> germinal gland, <i>u</i> primitive kidney, <i>z</i> +diaphragmatic ligament of same, <i>w</i> Wolffian duct (opened on the right), +<i>g</i> directing ligament (gubernaculum), <i>a</i> allantoic duct. (From +<i>Kollmann.</i>)</p> +</div> + +<p> +In the Myxinoides the primitive kidneys retain this simple comb-shaped +structure, and a part of it is preserved in the Selachii; but in all the other +Craniotes it is only found for a short time in the embryo, as an ontogenetic +reproduction of the earlier phylogenetic structure. In these the primitive +kidney soon assumes the form (by the rapid growth, lengthening, increase, and +serpentining of the urinary canals) of a large compact gland, of a long, oval +or spindle-shaped character, which passes through the greater part of the +embryonic body-cavity (Figs. 183 <i>m,</i> 184 <i>m,</i> 388 <i>n</i>). It lies +near the middle line, directly under the primitive vertebral column, and +reaches from the cardiac region to the cloaca. The right and left kidneys are +parallel to each other, quite close together, and only separated by the +mesentery—the thin narrow layer that attaches the middle gut to the under +surface of the vertebral column. The passage of each primitive kidney, the +nephroduct, runs towards the back on the lower and outer side of the gland, and +opens in the cloaca, close to the starting-point of the allantois; it +afterwards opens into the allantois itself. +</p> + +<p> +The primitive or primordial kidneys of the amniote embryo were formerly called +the “Wolffian bodies,” and sometimes “Oken’s +bodies.” They act for a time as +<span class='pagenum'><a name="Page_340" id="Page_340"></a></span> +kidneys, absorbing unusable juices from the embryonic body and conducting them +to the cloaca—afterwards to the allantois. There the primitive urine +accumulates, and thus the allantois acts as bladder or urinary sac in the +embryos of man and the other Amniotes. It has, however, no genetic connection +with the primitive kidneys, but is a pouch-like growth from the anterior wall +of the rectum (Fig. 147 <i>u</i>). Thus it is a product of the visceral layer, +whereas the primitive kidneys are a product of the middle layer. +Phylogenetically we must suppose that the allantois originated as a pouch-like +growth from the cloaca-wall in consequence of the expansion caused by the urine +accumulated in it and excreted by the kidneys. It is originally a blind sac of +the rectum. The real bladder of the vertebrate certainly made its first +appearance among the Dipneusts (in Lepidosiren), and has been transmitted from +them to the Amphibia, and from these to the Amniotes. In the embryo of the +latter it protrudes far out of the not yet closed ventral wall. It is true that +many of the fishes also have a “bladder.” But this is merely a +local enlargement of the lower section of the nephroducts, and so totally +different in origin and composition from the real bladder. The two structures +can be compared from the physiological point of view, and so are +<i>analogous,</i> as they have the same function; but not from the +morphological point of view, and are therefore not <i>homologous.</i> The false +bladder of the fishes is a mesodermic product of the nephroducts; the true +bladder of the Dipneusts, Amphibia, and Amniotes is an entodermic blind sac of +the rectum. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus396"></a> +<img src="images/fig396.gif" width="344" height="288" alt="Figs. 396-398. Urinary +and sexual organs of ox-embryos." /> +<p class="caption">Figs. 396–398—<b>Urinary and sexual organs +of ox-embryos.</b> Fig. 396, female embryo one and a half inches long; Fig. +397, male embryo, one and a half inches long. Fig. 398 female embryo two and a +half inches long. <i>w</i> primitive kidney, <i>wg</i> Wolffian duct, <i>m</i> +Müllerian duct, <i>m′</i> upper end of same (opened at <i>t</i>), +<i>i</i> lower and thicker part of same (rudiment of uterus), <i>g</i> genital +cord, <i>h</i> testicle, (<i>h′,</i> lower and <i>h″,</i> upper +testicular ligament), <i>o</i> ovary, <i>o′</i> lower ovarian ligament, +<i>i</i> inguinal ligament of primitive kidney, <i>d</i> diaphragmatic ligament +of primitive kidney, <i>nn</i> accessory kidneys, <i>n</i> permanent kidneys, +under them the S-shaped ureters, between these the rectum, <i>v</i> bladder, +<i>a</i> umbilical artery. (From <i>Kölliker.</i>)</p> +</div> + +<p> +In all the Anamnia (the lower amnionless Craniotes, Cyclostomes, Fishes, +Dipneusts, and Amphibia) the urinary organs remain at a lower stage of +development to this extent, that the primitive kidneys (<i>protonephri</i>) act +permanently as urinary glands. This is only so as a passing phase of the early +embryonic life in the three higher classes of Vertebrates, the Amniotes. In +these the permanent or after or secondary (really <i>tertiary</i>) kidneys +(<i>renes</i> or <i>metanephri</i>) that are distinctive of these three classes +soon make their appearance. They represent the third and last generation of the +vertebrate kidneys. The permanent kidneys do not arise (as was long supposed) +as independent glands from the alimentary tube, but from the last section of +the primitive kidneys and the nephroduct. Here a simple tube, the secondary +renal duct, develops, near the point of its entry into the cloaca; and this +tube grows considerably forward. With its blind upper or anterior end is +connected a glandular renal growth, that owes its origin to a differentiation +of the last part of the primitive kidneys. This rudiment of the +<span class='pagenum'><a name="Page_341" id="Page_341"></a></span> +permanent kidneys consists of coiled urinary canals with Malpighian capsules +and vascular coils (without ciliated funnels), of the same structure as the +segmental mesonephridia of the primitive kidneys. The further growth of these +metanephridia gives rise to the compact permanent kidneys, which have the +familiar bean-shape in man and most of the higher mammals, but consist of a +number of separate folds in the lower mammals, birds, and reptiles. As the +permanent kidneys grow rapidly and advance forward, their passage, the ureter, +detaches altogether from its birth-place, the posterior end of the nephroduct; +it passes to the posterior surface of the allantois. At first in the oldest +Amniotes this ureter opens into the cloaca together with the last section of +the nephroduct, but afterwards separately from this, and finally into the +permanent bladder apart from the rectum altogether. The bladder originates from +the hindmost and lowest part of the allantoic pedicle (<i>urachus</i>), which +enlarges in spindle shape before the entry into the cloaca. The anterior or +upper part of the pedicle, which runs to the navel in the ventral wall of the +embryo, atrophies subsequently, and only a useless string-like relic of it is +left as a rudimentary organ; that is the single vesico-umbilical ligament. To +the right and left of it in the adult male are a couple of other rudimentary +organs, the lateral vesico-umbilical ligaments. These are the degenerate +string-like relics of the earlier umbilical arteries. +</p> + +<p> +Though in man and all the other Amniotes the primitive kidneys are thus early +replaced by the permanent kidneys, and these alone then act as urinary organs, +all the parts of the former are by no means lost. The nephroducts become very +important physiologically by being converted into the passages of the sexual +glands. In all the Gnathostomes—or all the Vertebrates from the fishes up +to man—a second similar canal develops beside the nephroduct at an early +stage of embryonic evolution. The latter is usually called the Müllerian duct, +after its discoverer, Johannes Müller, while the former is called the Wolffian +duct. The origin of the Müllerian duct is still obscure; comparative anatomy +and ontogeny seem to indicate that it originates by differentiation from the +Wolffian duct. Perhaps it would be best to say: “The original primary +nephroduct divides by differentiation (or longitudinal cleavage) into two +secondary nephroducts, the Wolffian and the Müllerian ducts.” The latter +(Fig. 387 <i>m</i>) lies just on the inner side of the former (Fig. 387 +<i>w</i>). Both open behind into the cloaca. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus399"></a> +<img src="images/fig399.gif" width="154" height="197" alt="Fig.399. Female sexual +organs of a Monotreme (Ornithorhynchus, Fig. 269)." /> +<p class="caption">Fig. 399—<b>Female sexual organs of a +Monotreme</b> (<i>Ornithorhynchus,</i> Fig. 269). <i>o</i> ovaries, <i>t</i> +oviducts, <i>u</i> womb, <i>sug</i> urogenital sinus; at <i>u′</i> is the +outlet of the two wombs, and between them the bladder (<i>vu</i>). <i>cl</i> +cloaca. (From <i>Gegenbaur.</i>)</p> +</div> + +<p> +However uncertain the origin of the nephroduct and its two products, the +Müllerian and the Wolffian ducts, may be, its later development is clear +enough. In all the Gnathostomes the Wolffian duct is converted into the +spermaduct, and the Müllerian duct into the oviduct. Only one of them is +retained in each sex; the other either disappears altogether, or only leaves +relics in the shape of rudimentary organs. In the male sex, in which the two +Wolffian ducts become the spermaducts, we often find traces of the Müllerian +ducts, which I have called “Rathke’s canals” (Fig. 394 +<i>c</i>). In the female sex, in which the two Müllerian ducts form the +oviducts, there are relics of the Wolffian ducts, which are called “the +ducts of Gaertner.” +</p> + +<p> +We obtain the most interesting information with regard to this remarkable +evolution of the nephroducts and their association with the sexual glands from +the Amphibia (Figs. 390–395). The first structure of the nephroduct and +its differentiation into Müllerian and Wolffian ducts are just the same in both +sexes in the Amphibia, as in the mammal embryos (Figs. 392, 396). In the female +Amphibia +<span class='pagenum'><a name="Page_342" id="Page_342"></a></span> +the Müllerian duct develops on either side into a large oviduct (Fig. 393 +<i>od</i>), while the Wolffian duct acts permanently as ureter (<i>u</i>). In +the male Amphibia the Müllerian duct only remains as a rudimentary organ +without any functional significance, as Rathke’s canal (Fig. 394 +<i>c</i>); the Wolffian duct serves also as ureter, but at the same time as +spermaduct, the sperm-canals (<i>ve</i>) that proceed from the testicles +(<i>t</i>) entering the fore part of the primitive kidneys and combining there +with the urinary canals. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus400"></a> +<img src="images/fig400.gif" width="217" height="173" alt="Figs. 400, 401. Original position of the sexual +glands in the ventral cavity of the human embryo (three months old)." /> +<p class="caption">Figs. 400, 401—<b>Original position of the sexual +glands in the ventral cavity of the human embryo</b> (three months old). Fig. +400, male. <i>h</i> testicles, <i>gh</i> conducting ligament of the testicles, +<i>wg</i> spermaduct, <i>h</i> bladder, <i>uh</i> inferior vena cava, <i>nn</i> +accessory kidneys, <i>n</i> kidneys. Fig. 401, female. <i>r</i> round maternal +ligament (underneath it the bladder, over it the ovaries). <i>r′</i> +kidneys, <i>s</i> accessory kidneys, <i>c</i> cæcum, <i>o</i> small reticle, +<i>om</i> large reticle (stomach between the two), <i>l</i> spleen. (From +<i>Kölliker.</i>)</p> +</div> + +<p> +In the mammals these permanent amphibian features are only seen as brief phases +of the earlier period of embryonic development (Fig. 392). Here the primitive +kidneys, which act as excretory organs of urine throughout life in the +amnion-less Vertebrates, are replaced in the mammals by the permanent kidneys. +The real primitive kidneys disappear for the most part at an early stage of +development, and only small relics of them remain. In the male mammal the +<i>epididymis</i> develops from the uppermost part of the primitive kidney; in +the female a useless rudimentary organ, the <i>epovarium,</i> is formed from +the same part. The atrophied relic of the former is known as the +<i>paradidymis,</i> that of the latter as the <i>parovarium.</i> +</p> + +<div class="fig" style="width:100%;"> +<a name="illus402"></a> +<img src="images/fig402.gif" width="226" height="231" alt="Fig.402. Urogenital +system of a human embryo of three inches in length." /> +<p class="caption">Fig. 402—<b>Urogenital system of a human +embryo</b> of three inches in length. <i>h</i> testicles, <i>wg</i> +spermaducts, <i>gh</i> conducting ligament, <i>p</i> processus vaginalis, +<i>b</i> bladder, <i>au</i> umbilical arteries, <i>m</i> mesorchium, <i>d</i> +intestine, <i>u</i> ureter, <i>n</i> kidney, <i>nn</i> accessory kidney. (From +<i>Kollman.</i>)</p> +</div> + +<p> +The Müllerian ducts undergo very important changes in the female mammal. The +oviducts proper are developed only from their upper part; the lower part +dilates into a spindle-shaped tube with thick muscular wall, in which the +impregnated ovum develops into the embryo. This is the womb (<i>uterus</i>). At +first the two wombs (Fig. 399 <i>u</i>) are completely separate, and open into +the cloaca on either side of the bladder (<i>vu</i>), as is still the case in +the lowest living mammals, the Monotremes. But in the Marsupials a +communication is opened between the two Müllerian ducts, and in the Placentals +they combine below with the rudimentary Wolffian ducts to form a single +“genital cord.” The original independence of the two wombs and the +vaginal canals formed from their lower ends are retained in many of the lower +Placentals, but in the higher they gradually blend and form a single organ. The +conjunction proceeds from below (or behind) upwards (or forwards). In many of +the Rodents (such as the rabbit and squirrel) two separate wombs still open +into the simple and single vaginal canal; but in others, and in the Carnivora, +Cetacea, and Ungulates, the lower halves of the wombs have already fused into a +single piece, though the upper halves (or “horns”) are still +separate (“two-horned” womb, <i>uteris bicornis</i>). In the bats +and lemurs the “horns” are +<span class='pagenum'><a name="Page_343" id="Page_343"></a></span> +very short, and the lower common part is longer. Finally, in the apes and in +man the blending of the two halves is complete, and there is only the one +simple, pear-shaped uterine pouch, into which the oviducts open on each side. +This simple uterus is a late evolutionary product, and is found <i>only</i> in +the ape and man. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus403"></a> +<a name="illus404"></a> +<img src="images/fig403.gif" width="365" height="435" alt="Figs. 403-406. Origin +of human ova in the female ovary." /> +<p class="caption">Figs. 403–406—<b>Origin of human ova in the +female ovary.</b> Fig. 403. <b>Vertical section of the ovary</b> of a new-born +female infant, <i>a</i> ovarian epithelium, <i>b</i> rudimentary string of ova, +<i>c</i> young ova in the epithelium, <i>d</i> long string of ova with +follicle-formation (Pflüger’s tube), <i>e</i> group of young follicles, +<i>f</i> isolated young follicle, <i>g</i> blood-vessels in connective tissue +(stroma) of the ovary. In the strings the young ova are distinguished by their +considerable size from the surrounding follicle-cells. (From +<i>Waldeyer.</i>)<br/> Fig. 404—<b>Two young Graafian follicles,</b> +isolated. In <i>1</i> the follicle-cells still form a simple, and in <i>2</i> a +double, stratum round the young ovum; in <i>2</i> they are beginning to form +the ovolemma or the zona pellucida (<i>a</i>).<br/> Figs. 405 and +406—<b>Two older Graafian follicles,</b> in which fluid is beginning to +accumulate inside the eccentrically thickened epithelial mass of the +follicle-cells (Fig. 405 with little, 406 with much, follicle-water). <i>ei</i> +the young ovum, with embryonic vesicle and spot, <i>zp</i> ovolemma or zona +pellucida, <i>dp</i> discus proligerus, formed of an accumulation of +follicle-cells, which surround the ovum, <i>ff</i> follicle-liquid (<i>liquor +folliculi</i>), gathered inside the stratified follicle-epithelium (<i>fe</i>), +<i>fk</i> connective-tissue fibrous capsule of the Graafian follicle (<i>theca +folliculi</i>).</p> +</div> + +<p> +In the male mammals there is the same fusion of the Müllerian and Wolffian +ducts at their lower ends. Here again they form a single genital cord (Fig. 397 +<i>g</i>), and this opens similarly into the +<span class='pagenum'><a name="Page_344" id="Page_344"></a></span> +original urogenital sinus, which develops from the lowest section of the +bladder (<i>v</i>). But while in the male mammal the Wolffian ducts develop +into the permanent spermaducts, there are only rudimentary relics left of the +Müllerian ducts. The most notable of these is the “male womb” +(<i>uterus masculinus</i>), which originates from the lowest fused part of the +ducts, and corresponds to the female uterus. It is a small, flask-shaped +vesicle without any physiological significance, which opens into the ureter +between the two spermaducts and the prostate folds (<i>vesicula +prostatica</i>). +</p> + +<div class="fig" style="width:100%;"> +<a name="illus407"></a> +<img src="images/fig407.gif" width="313" height="268" alt="Fig.407. A ripe human Graafian follicle." /> +<p class="caption">Fig. 407—<b>A ripe human Graafian follicle.</b> +<i>a</i> the mature ovum, <i>b</i> the surrounding follicle-cells, <i>c</i> the +epithelial cells of the follicle, <i>d</i> the fibrous membrane of the +follicle, <i>e</i> its outer surface.</p> +</div> + +<p> +The internal sexual organs of the mammals undergo very distinctive changes of +position. At first the germinal glands of both sexes lie deep inside the +ventral cavity, at the inner edge of the primitive kidneys (Figs. 386 <i>g</i>, +392 <i>k</i>), attached to the vertebral column by a short mesentery +(<i>mesorchium</i> in the male, <i>mesovarium</i> in the female). But this +primary arrangement is retained permanently only in the Monotremes (and the +lower Vertebrates). In all other mammals (both Marsupials and Placentals) they +leave their original cradle and travel more or less far down (or behind), +following the direction of a ligament that goes from the primitive kidneys to +the inguinal region of the ventral wall. This is the inguinal ligament of the +primitive kidneys, known in the male as the Hunterian ligament (Fig. 400 +<i>gh</i>), and in the female as the “round maternal ligament” +(Fig. 401 <i>r</i>). In woman the ovaries travel more or less towards the small +pelvis, or enter into it altogether. In the male the testicles pass out of the +ventral cavity, and penetrate by the inguinal canal into a sac-shaped fold of +the outer skin. When the right and left folds (“sexual swellings”) +join together they form the <i>scrotum.</i> The various mammals bring before us +the successive stages of this displacement. In the elephant and the whale the +testicles descend very little, and remain underneath the kidneys. In many of +the rodents and carnassia they enter the inguinal canal. In most of the higher +mammals they pass through this into the scrotum. As a rule, the inguinal canal +closes up. When it remains open the testicles may periodically pass into the +scrotum, and withdraw into the ventral cavity again in time of rut (as in many +of the marsupials, rodents, bats, etc.). +</p> + +<p> +The structure of the external sexual organs, the copulative organs that convey +the fecundating sperm from the male to the female organism in the act of +copulation, is also peculiar to the mammals. There are no organs of this +character in most of the other Vertebrates. In those that live in water (such +as the Acrania and Cyclostomes, and most of the fishes) the ova and sperm-cells +are simply ejected into the water, where their conjunction and fertilisation +are left to chance. But in many of the fishes and amphibia, which are +viviparous, there is a direct conveyance of the male sperm into the female +body; and this is the case with all the Amniotes (reptiles, birds, and +mammals). In these the urinary and sexual organs always open originally into +the last section of the rectum, which thus forms a cloaca +<span class='pagenum'><a name="Page_345" id="Page_345"></a></span> +(p. 249). Among the mammals this arrangement is permanent only in the +Monotremes, which take their name from it (Fig. 399 <i>cl</i>). In all the +other mammals a frontal partition is developed in the cloaca (in the human +embryo about the beginning of the third month), and this divides it into two +cavities. The anterior cavity receives the urogenital canal, and is the sole +outlet of the urine and the sexual products; the hind or anus-cavity passes the +excrements only. +</p> + +<p> +Even before this partition has been formed in the Marsupials and Placentals, we +see the first trace of the external sexual organs. First a conical protuberance +rises at the anterior border of the cloaca-outlet—the sexual prominence +(<i>phallus,</i> Fig. 402 <i>A, e, B, e</i>). At the tip it is swollen in the +shape of a club (“acorn” <i>glans</i>). On its under side there is +a furrow, the sexual groove (<i>sulcus genitalis, f</i>), and on each side of +this a fold of skin, the “sexual pad” (<i>torus genitalis, h +l</i>). The sexual protuberance or phallus is the chief organ of the sexual +sense (p. 282); the sexual nerves spread on it, and these are the principal +organs of the specific sexual sensation. As erectile bodies (<i>corpora +cavernosa</i>) are developed in the male phallus by peculiar modifications of +the blood-vessels, it becomes capable of erecting periodically on a strong +accession of blood, becoming stiff, so as to penetrate into the female vagina +and thus effect copulation. In the male the phallus becomes the penis; in the +female it becomes the much smaller clitoris; this is only found to be very +large in certain apes (<i>Ateles</i>). A prepuce (“foreskin”) is +developed in both sexes as a protecting fold on the anterior surface of the +phallus. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus408"></a> +<img src="images/fig408.gif" width="278" height="265" alt="Fig.408. The human ovum +after issuing from the Graafian follicle, surrounded by the clinging cells of +the discus proligerus (in two radiating crowns)." /> +<p class="caption">Fig. 408—<b>The human ovum</b> after issuing from +the Graafian follicle, surrounded by the clinging cells of the <i>discus +proligerus</i> (in two radiating crowns). <i>z</i> ovolemma (zona pellucida, +with radial porous canals), <i>p</i> cytosoma (protoplasm of the cell-body, +darker within, lighter without), <i>k</i> nucleus of the ovum (embryonic +vesicle). (From <i>Nagel.</i>) (Cf. Figs. 1 and 14.)</p> +</div> + +<p> +The external sexual member (<i>phallus</i>) is found at various stages of +development within the mammal class, both in regard to size and shape, and the +differentiation and structure of its various parts; this applies especially to +the terminal part of the phallus, the glans, both the larger <i>glans penis</i> +of the male and the smaller <i>glans clitoridis</i> of the female. The part of +the cloaca from the upper wall of which it forms belongs to the +<i>proctodæum,</i> the ectodermic invagination of the rectum (p. 311); hence +its epithelial covering can develop the same horny growths as the corneous +layer of the epidermis. Thus the glans, which is quite smooth in man and the +higher apes, is covered with spines in many of the lower apes and in the cat, +and in many of the rodents with hairs (marmot) or scales (guinea-pig) or solid +horny warts (beaver). Many of the Ungulates have a free conical projection on +the glans, and in many of the Ruminants this “phallus-tentacle” +grows into a long cone, bent hook-wise at the base (as in the goat, antelope, +gazelle, etc.). The different forms of the phallus are connected with +variations in the structure and distribution of the sensory +corpuscles—<i>i.e.</i> the real organs of the sexual sense, which develop +in certain papillæ of the corium of the phallus, and have been evolved from +ordinary tactile corpuscles of the corium by erotic adaptation (p. 282). +</p> + +<p> +<span class='pagenum'><a name="Page_346" id="Page_346"></a></span> +The formation of the <i>corpora cavernosa,</i> which cause the stiffness of the +phallus and its capability of penetrating the vagina, by certain special +structures of their spongy vascular spaces, also shows a good deal of variety +within the vertebrate stem. This stiffness is increased in many orders of +mammals (especially the carnassia and rodents) by the ossification of a part of +the fibrous body (<i>corpus fibrosum</i>). This penis-bone (<i>os priapi</i>) +is very large in the badger and dog, and bent like a hook in the marten; it is +also very large in some of the lower apes, and protrudes far out into the +glans. It is wanting in most of the anthropoid apes; it seems to have been lost +in their case (and in man) by atrophy. +</p> + +<p> +The sexual groove on the under side of the phallus receives in the male the +mouth of the urogenital canal, and is changed into a continuation of this, +becoming a closed canal by the juncture of its parallel edges, the male +urethra. In the female this only takes place in a few cases (some of the +lemurs, rodents, and moles); as a rule, the groove remains open, and the +borders of this “vestibule of the vagina” develop into the smaller +labia (<i>nymphæ</i>). The large labia of the female develop from the sexual +pads (<i>tori genitales</i>), the two parallel folds of the skin that are found +on each side of the genital groove. They join together in the male, and form +the closed scrotum. These striking differences between the two sexes cannot yet +be detected in the human embryo of the ninth week. We begin to trace them in +the tenth week of development, and they are accentuated in proportion as the +difference of the sexes develops. +</p> + +<p> +Sometimes the normal juncture of the two sexual pads in the male fails to take +place, and the sexual groove may also remain open (<i>hypospadia</i>). In these +cases the external male genitals resemble the female, and they are often +wrongly regarded as cases of hermaphrodism. Other malformations of various +kinds are not infrequently found in the human external sexual organs, and some +of them have a great morphological interest. The reverse of hypospadia, in +which the penis is split open below, is seen in <i>epispadia,</i> in which the +urethra is open above. In this case the urogenital canal opens above at the +dorsal root of the penis; in the former case down below. These and similar +obstructions interfere with a man’s generative power, and thus +prejudicially affect his whole development. They clearly prove that our history +is not guided by a “kind Providence,” but left to the play of blind +chance. +</p> + +<p> +We must carefully distinguish the rarer cases of real hermaphrodism from the +preceding. This is only found when the essential organs of reproduction, the +genital glands of both kinds, are united in one individual. In these cases +either an ovary is developed on the right and a testicle on the left (or +<i>vice versa</i>); or else there are testicles and ovaries on both sides, some +more and others less developed. As hermaphrodism was probably the original +arrangement in all the Vertebrates, and the division of the sexes only followed +by later differentiation of this, these curious cases offer no theoretical +difficulty. But they are rarely found in man and the higher mammals. On the +other hand, we constantly find the original hermaphrodism in some of the lower +Vertebrates, such as the Myxinoides, many fishes of the perch-type +(<i>serranus</i>), and some of the Amphibia (ringed snake, toad). In these +cases the male often has a rudimentary ovary at the fore end of the testicle; +and the female sometimes has a rudimentary, inactive testicle. In the carp also +and some other fishes this is found occasionally. We have already seen how +traces of the earlier hemaphrodism can be traced in the passages of the +Amphibia. +</p> + +<p> +Man has faithfully preserved the main features of his stem-history in the +ontogeny of his urinary and sexual organs. We can follow their development step +by step in the human embryo in the same advancing gradation that is presented +to us by the comparison of the urogenital organs in the Acrania, Cyclostomes; +Fishes, Amphibia, Reptiles, and then (within the mammal series) in the +Monotremes, Marsupials, and the various Placentals. All the peculiarities of +urogenital structure that distinguish the mammals from the rest of the +Vertebrates are found in man; and in all special structural features he +resembles the apes, particularly the anthropoid apes. In proof of the fact that +the special features of the mammals have been inherited by man, I will, in +conclusion, point out the identical way in which the ova are formed in the +ovary. In all the mammals the mature ova are contained in special capsules, +which are known as the <i>Graafian</i> +<span class='pagenum'><a name="Page_347" id="Page_347"></a></span> +<i>follicles,</i> after their discoverer, Roger de Graaf (1677). They were +formerly supposed to be the ova themselves; but Baer discovered the ova within +the follicles (p. 16). Each follicle (Fig. 407) consists of a round fibrous +capsule (<i>d</i>), which contains fluid and is lined with several strata of +cells (<i>c</i>). The layer is thickened like a knob at one point (<i>b</i>); +this ovum-capsule encloses the ovum proper (<i>a</i>). The mammal ovary is +originally a very simple oval body (Fig. 387 <i>g</i>), formed only of +connective tissue and blood-vessels, covered with a layer of cells, the ovarian +epithelium or the female germ epithelium. From this germ epithelium strings of +cells grow out into the connective tissue or “stroma” of the ovary +(Fig. 403 <i>b</i>). Some of the cells of these strings (or Pflüger’s +tubes) grow larger and become ova (primitive ova, <i>c</i>); but the great +majority remain small, and form a protective and nutritive stratum of cells +round each ovum—the “follicle-epithelium” (<i>e</i>). +</p> + +<p> +The follicle-epithelium of the mammal has at first one stratum (Fig. 404 +<i>1</i>), but afterwards several (<i>2</i>). It is true that in all the other +Vertebrates the ova are enclosed in a membrane, or “follicle,” that +consists of smaller cells. But it is only in the mammals that fluid accumulates +between the growing follicle-cells, and distends the follicle into a large +round capsule, on the inside wall of which the ovum lies, at one side (Figs. +405, 406). There again, as in the whole of his morphology, man proves +indubitably his descent from the mammals. +</p> + +<p> +In the lower Vertebrates the formation of ova in the germ-epithelium of the +ovary continues throughout life; but in the higher it is restricted to the +earlier stages, or even to the period of embryonic development. In man it seems +to cease in the first year; in the second year we find no new-formed ova or +chains of ova (Pflüger’s tubes). However, the number of ova in the two +ovaries is very large in the young girl; there are calculated to be 72,000 in +the sexually-mature maiden. In the production of the ova men resemble most of +the anthropoid apes. +</p> + +<p> +Generally speaking, the natural history of the human sexual organs is one of +those parts of anthropology that furnish the most convincing proofs of the +animal origin of the human race. Any man who is acquainted with the facts and +impartially weighs them will conclude from them alone that we have been evolved +from the lower Vertebrates. The larger and the detailed structure, the action, +and the embryological development of the sexual organs are just the same in man +as in the apes. This applies equally to the male and the female, the internal +and the external organs. The differences we find in this respect between man +and the anthropoid apes are much slighter than the differences between the +various species of apes. But all the apes have certainly a common origin, and +have been evolved from a long-extinct early-Tertiary stem-form, which we must +trace to a branch of the lemurs. If we had this unknown pithecoid stem-form +before us, we should certainly put it in the order of the true apes in the +primate system; but within this order we cannot, for the anatomic and +ontogenetic reasons we have seen, separate man from the group of the anthropoid +apes. Here again, therefore, on the ground of the pithecometra-principle, +comparative anatomy and ontogeny teach with full confidence the descent of man +from the ape. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap30"></a> +<span class='pagenum'><a name="Page_348" id="Page_348"></a></span> +Chapter XXX.<br/> +RESULTS OF ANTHROPOGENY</h2> + +<p> +Now that we have traversed the wonderful region of human embryology and are +familiar with the principal parts of it, it will be well to look back on the +way we have come, and forward to the further path to truth to which it has led +us. We started from the simplest facts of ontogeny, or the development of the +individual—from observations that we can repeat and verify by microscopic +and anatomic study at any moment. The first and most important of these facts +is that every man, like every other animal, begins his existence as a simple +cell. This round ovum has the same characteristic form and origin as the ovum +of any other mammal. From it is developed in the same manner in all the +Placentals, by repeated cleavage, a multicellular blastula. This is converted +into a gastrula, and this in turn into a blastocystis (or embryonic vesicle). +The two strata of cells that compose its wall are the primary germinal layers, +the skin-layer (ectoderm), and gut-layer (entoderm). This two-layered embryonic +form is the ontogenetic reproduction of the extremely important phylogenetic +stem-form of all the Metazoa, which we have called the Gastræa. As the human +embryo passes through the gastrula-form like that of all the other Metazoa, we +can trace its phylogenetic origin to the Gastræa. +</p> + +<p> +As we continued to follow the embryonic development of the two-layered +structure, we saw that first a third, or middle layer (mesoderm), appears +between the two primary layers; when this divides into two, we have the four +secondary germinal layers. These have just the same composition and genetic +significance in man as in all the other Vertebrates. From the skin-sense layer +are developed the epidermis, the central nervous system, and the chief part of +the sense-organs. The skin-fibre layer forms the corium and the motor +organs—the skeleton and the muscular system. From the gut-fibre layer are +developed the vascular system, the muscular wall of the gut, and the sexual +glands. Finally, the gut-gland layer only forms the epithelium, or the inner +cellular stratum of the mucous membrane of the alimentary canal and glands +(lungs, liver, etc.). +</p> + +<p> +The manner in which these different systems of organs arise from the secondary +germinal layers is essentially the same from the start in man as in all the +other Vertebrates. We saw, in studying the embryonic development of each organ, +that the human embryo follows the special lines of differentiation and +construction that are only found otherwise in the Vertebrates. Within the +limits of this vast stem we have followed, step by step, the development both +of the body as a whole and of its various parts. This higher development +follows in the human embryo the form that is peculiar to the mammals. Finally, +we saw that, even within the limits of this class, the various phylogenetic +stages that we distinguish in a natural classification of the mammals +correspond to the ontogenetic stages that the human embryo passes through in +the course of its evolution. We were thus in a position to determine precisely +the position of man in this class, and so to establish his relationship to the +different orders of mammals. +</p> + +<p> +The line of argument we followed in this explanation of the ontogenetic facts +was simply a consistent application of the biogenetic law. In this we have +throughout taken strict account of the distinction between palingenetic and +cenogenetic phenomena. Palingenesis (or “synoptic development”) +alone enables us to draw conclusions from the observed embryonic form to the +stem-form preserved by heredity. Such inference becomes more or less precarious +when there has been cenogenesis, or disturbance of development, owing to fresh +adaptations. We cannot understand embryonic development unless we appreciate +this very important distinction. Here we stand at the very limit that separates +the older and the new science or philosophy of nature. The whole of the results +of recent morphological research compel us irresistibly +<span class='pagenum'><a name="Page_349" id="Page_349"></a></span> +to recognise the biogenetic law and its far-reaching consequences. These are, +it is true, irreconcilable with the legends and doctrines of former days, that +have been impressed on us by religious education. But without the <i>biogenetic +law,</i> without the distinction between <i>palingenesis</i> and +<i>cenogenesis,</i> and without the theory of <i>evolution</i> on which we base +it, it is quite impossible to understand the facts of organic development; +without them we cannot cast the faintest gleam of explanation over this +marvellous field of phenomena. But when we recognise the causal correlation of +ontogeny and phylogeny expressed in this law, the wonderful facts of embryology +are susceptible of a very simple explanation; they are found to be the +necessary mechanical effects of the evolution of the stem, determined by the +laws of heredity and adaptation. The correlative action of these laws under the +universal influence of the struggle for existence, or—as we may say in a +word, with Darwin—“natural selection,” is entirely adequate +to explain the whole process of embryology in the light of phylogeny. It is the +chief merit of Darwin that he explained by his theory of selection the +correlation of the laws of heredity and adaptation that Lamarck had recognised, +and pointed out the true way to reach a causal interpretation of evolution. +</p> + +<p> +The phenomenon that it is most imperative to recognise in this connection is +the inheritance of functional variations. Jean Lamarck was the first to +appreciate its fundamental importance in 1809, and we may therefore justly give +the name of Lamarckism to the theory of descent he based on it. Hence the +radical opponents of the latter have very properly directed their attacks +chiefly against the former. One of the most distinguished and most +narrow-minded of these opponents, Wilhelm His, affirms very positively that +“characteristics acquired in the life of the individual are not +inherited.” +</p> + +<p> +The inheritance of acquired characters is denied, not only by thorough +opponents of evolution, but even by scientists who admit it and have +contributed a good deal to its establishment, especially Weismann, Galton, Ray +Lankester, etc. Since 1884 the chief opponent has been August Weismann, who has +rendered the greatest service in the development of Darwin’s theory of +selection. In his work on <i>The Continuity of the Germ-plasm,</i> and in his +recent excellent <i>Lectures on the Theory of Descent</i> (1902), he has with +great success advanced the opinion that “only those characters can be +transmitted to subsequent generations that were contained in rudimentary form +in the embryo.” However, this germ-plasm theory, with its attempt to +explain heredity, is merely a “provisional molecular hypothesis”; +it is one of those metaphysical speculations that attribute the evolutionary +phenomena exclusively to internal causes, and regard the influence of the +environment as insignificant. Herbert Spencer, Theodor Eimer, Lester Ward, +Hering, and Zehnder have pointed out the untenable consequences of this +position. I have given my view of it in the tenth edition of the <i>History of +Creation</i> (pp. 192, 203). I hold, with Lamarck and Darwin, that the +hereditary transmission of acquired characters is one of the most important +phenomena in biology, and is proved by thousands of morphological and +physiological experiences. It is an indispensable foundation of the theory of +evolution. +</p> + +<p> +Of the many and weighty arguments for the truth of this conception of evolution +I will for the moment merely point to the invaluable evidence of dysteleology, +the science of rudimentary organs. We cannot insist too often or too strongly +on the great morphological significance of these remarkable organs, which are +completely useless from the physiological point of view. We find some of these +useless parts, inherited from our lower vertebrate ancestors, in every system +of organs in man and the higher Vertebrates. Thus we find at once on the skin a +scanty and rudimentary coat of hair, only fully developed on the head, under +the shoulders, and at a few other parts of the body. The short hairs on the +greater part of the body are quite useless and devoid of physiological value; +they are the last relic of the thicker hairy coat of our simian ancestors. The +sensory apparatus presents a series of most remarkable rudimentary organs. We +have seen that the whole of the shell of the external ear, with its cartilages, +muscles, and skin, is in man a useless appendage, and has not the physiological +importance that was formerly ascribed to it. It is the degenerate remainder of +the pointed, freely moving, and more advanced mammal ear, the muscles of which +we still have, but cannot work them. We found at the +<span class='pagenum'><a name="Page_350" id="Page_350"></a></span> +inner corner of our eye a small, curious, semi-lunar fold that is of no use +whatever to us, and is only interesting as the last relic of the nictitating +membrane, the third, inner eye-lid that had a distinct physiological purpose in +the ancient sharks, and still has in many of the Amniotes. +</p> + +<p> +The motor apparatus, in both the skeleton and muscular systems, provides a +number of interesting dysteleological arguments. I need only recall the +projecting tail of the human embryo, with its rudimentary caudal vertebræ and +muscles; this is totally useless in man, but very interesting as the degenerate +relic of the long tail of our simian ancestors. From these we have also +inherited various bony processes and muscles, which were very useful to them in +climbing trees, but are useless to us. At various points of the skin we have +cutaneous muscles which we never use—remnants of a strongly-developed +cutaneous muscle in our lower mammal ancestors. This “panniculus +carnosus” had the function of contracting and creasing the skin to chase +away the flies, as we see every day in the horse. Another relic in us of this +large cutaneous muscle is the frontal muscle, by which we knit our forehead and +raise our eye-brows; but there is another considerable relic of it, the large +cutaneous muscle in the neck (<i>platysma myoides</i>), over which we have no +voluntary control. +</p> + +<p> +Not only in the systems of animal organs, but also in the vegetal apparatus, we +find a number of rudimentary organs, many of which we have already noticed. In +the alimentary apparatus there are the thymus-gland and the thyroid gland, the +seat of goitre and the relic of a ciliated groove that the Tunicates and +Acrania still have in the gill-pannier; there is also the vermiform appendix to +the cæcum. In the vascular system we have a number of useless cords which +represent relics of atrophied vessels that were once active as +blood-canals—the <i>ductus Botalli</i> between the pulmonary artery and +the aorta, the <i>ductus venosus Arantii</i> between the portal vein and the +vena cava, and many others. The many rudimentary organs in the urinary and +sexual apparatus are particularly interesting. These are generally developed in +one sex and rudimentary in the other. Thus the spermaducts are formed from the +Wolffian ducts in the male, whereas in the female we have merely rudimentary +traces of them in Gaertner’s canals. On the other hand, in the female the +oviducts and womb are developed from the Mullerian ducts, while in the male +only the lowest ends of them remain as the “male womb” (<i>vesicula +prostatica</i>). Again, the male has in his nipples and mammary glands the +rudiments of organs that are usually active only in the female. +</p> + +<p> +A careful anatomic study of the human frame would disclose to us numbers of +other rudimentary organs, and these can only be explained on the theory of +evolution. Robert Wiedersheim has collected a large number of them in his work +on <i>The Human Frame as a Witness to its Past.</i> They are some of the +weightiest proofs of the truth of the mechanical conception and the strongest +disproofs of the teleological view. If, as the latter demands, man or any other +organism had been designed and fitted for his life-purposes from the start and +brought into being by a creative act, the existence of these rudimentary organs +would be an insoluble enigma; it would be impossible to understand why the +Creator had put this useless burden on his creatures to walk a path that is in +itself by no means easy. But the theory of evolution gives the simplest +possible explanation of them. It says: The rudimentary organs are parts of the +body that have fallen into disuse in the course of centuries; they had definite +functions in our animal ancestors, but have lost their physiological +significance. On account of fresh adaptations they have become superfluous, but +are transmitted from generation to generation by heredity, and gradually +atrophy. +</p> + +<p> +We have inherited not only these rudimentary parts, but all the organs of our +body, from the mammals—proximately from the apes. The human body does not +contain a single organ that has not been inherited from the apes. In fact, with +the aid of our biogenetic law we can trace the origin of our various systems of +organs much further, down to the lowest stages of our ancestry. We can say, for +instance, that we have inherited the oldest organs of the body, the external +skin and the internal coat of the alimentary system, from the Gastræads; the +nervous and muscular systems from the Platodes; the vascular system, the +body-cavity, and the blood from the Vermalia; the chorda and the branchial gut +from the Prochordonia; +<span class='pagenum'><a name="Page_351" id="Page_351"></a></span> +the articulation of the body from the Acrania; the primitive skull and the +higher sense-organs from the Cyclostomes; the limbs and jaws from the Selachii; +the five-toed foot from the Amphibia; the palate from the Reptiles; the hairy +coat, the mammary glands, and the external sexual organs from the Pro-mammals. +When we formulated “the law of the ontogenetic connection of +systematically related forms,” and determined the relative age of organs, +we saw how it was possible to draw phylogenetic conclusions from the +ontogenetic succession of systems of organs. +</p> + +<p> +With the aid of this important law and of comparative anatomy we were also +enabled to determine “man’s place in nature,” or, as we put +it, assign to man his position in the classification of the animal kingdom. In +recent zoological classification the animal world is divided into twelve stems +or phyla, and these are broadly sub-divided into about sixty classes, and these +classes into at least 300 orders. In his whole organisation man is most +certainly, in the first place, a member of one of these stems, the vertebrate +stem; secondly, a member of one particular class in this stem, the Mammals; and +thirdly, of one particular order, the order of Primates. He has all the +characteristics that distinguish the Vertebrates from the other eleven animal +stems, the Mammals from the other sixty classes, and the Primates from the 300 +other orders of the animal kingdom. We may turn and twist as we like, but we +cannot get over this fact of anatomy and classification. Of late years this +fact has given rise to a good deal of discussion, and especially of controversy +as to the particular anatomic relationship of man to the apes. The most curious +opinions have been advanced on this “ape-question,” or +“pithecoid-theory.” It is as well, therefore, to go into it once +more and distinguish the essential from the unessential. (Cf. pp. 261–5.) +</p> + +<p> +We start from the undisputed fact that man is in any case—whether we +accept or reject his special blood-relationship to the apes—a true +mammal; in fact, a placental mammal. This fundamental fact can be proved so +easily at any moment from comparative anatomy that it has been universally +admitted since the separation of the Placentals from the lower mammals +(Marsupials and Monotremes). But for every consistent subscriber to the theory +of evolution it must follow at once that man descends from a common stem-form +with all the other Placentals, the stem-ancestor of the Placentals, just as we +must admit a common mesozoic ancestor of all the mammals. This is, however, to +settle decisively the great and burning question of man’s place in +nature, whether or no we go on to admit a nearer or more distant relationship +to the apes. Whether man is or is not a member of the ape-order (or, if you +prefer, the primate-order.) in the phylogenetic sense, in any case his direct +blood-relationship to the rest of the mammals, and especially the Placentals, +is established. It is possible that the affinities of the various orders of +mammals to each other are different from what we hypothetically assume to-day. +But, in any case, the common descent of man and all the other mammals from one +stem-form is beyond question. This long-extinct Promammal was probably evolved +from Proreptiles during the Triassic period, and must certainly be regarded as +the monotreme and oviparous ancestor of <i>all</i> the mammals. +</p> + +<p> +If we hold firmly to this fundamental and most important thesis, we shall see +the “ape-question” in a very different light from that in which it +is usually regarded. Little reflection is then needed to see that it is not +nearly so important as it is said to be. The origin of the human race from a +series of mammal ancestors, and the historic evolution of these from an earlier +series of lower vertebrate ancestors, together with all the weighty conclusions +that every thoughtful man deduces therefrom, remain untouched; so far as these +are concerned, it is immaterial whether we regard true “apes” as +our nearest ancestors or not. But as it has become the fashion to lay the chief +stress in the whole question of man’s origin on the “descent from +the apes,” I am compelled to return to it once more, and recall the facts +of comparative anatomy and ontogeny that give a decisive answer to this +“ape-question.” +</p> + +<p> +The shortest way to attain our purpose is that followed by Huxley in 1863 in +his able work, which I have already often quoted, <i>Man’s Place in +Nature</i>—the way of comparative anatomy and ontogeny. We have to +compare impartially all man’s organs with the same organs in the higher +apes, and then to examine if the differences between the two are greater +<span class='pagenum'><a name="Page_352" id="Page_352"></a></span> +than the corresponding differences between the higher and the lower apes. The +indubitable and incontestable result of this comparative-anatomical study, +conducted with the greatest care and impartiality, was the +pithecometra-principle, which we have called the Huxleian law in honour of its +formulator—namely, that the differences in organisation between man and +the most advanced apes we know are much slighter than the corresponding +differences in organisation between the higher and lower apes. We may even give +a more precise formula to this law, by excluding the Platyrrhines or American +apes as distant relatives, and restricting the comparison to the narrower +family-circle of the Catarrhines, the apes of the Old World. Within the limits +of this small group of mammals we found the structural differences between the +lower and higher catarrhine apes—for instance, the baboon and the +gorilla—to be much greater than the differences between the anthropoid +apes and man. If we now turn to ontogeny, and find, according to our “law +of the ontogenetic connection of systematically related forms,” that the +embryos of the anthropoid apes and man retain their resemblance for a longer +time than the embryos of the highest and the lowest apes, we are forced, +whether we like it or no, to recognise our descent from the order of apes. We +can assuredly construct an approximate picture in the imagination of the form +of our early Tertiary ancestors from the foregoing facts of comparative +anatomy; however we may frame this in detail, it will be the picture of a true +ape, and a distinct catarrhine ape. This has been shown so well by Huxley +(1863) that the recent attacks of Klaatsch, Virchow, and other anthropologists, +have completely failed (cf. pp.263–264). All the structural characters +that distinguish the Catarrhines from the Platyrrhines are found in man. Hence +in the genealogy of the mammals we must derive man immediately from the +catarrhine group, and locate the origin of the human race in the Old World. +Only the early root-form from which both descended was common to them. +</p> + +<p> +It is, therefore, established beyond question for all impartial scientific +inquiry that the human race comes directly from the apes of the Old World; but, +at the same time, I repeat that this is not so important in connection with the +main question of the origin of man as is commonly supposed. Even if we entirely +ignore it, all that we have learned from the zoological facts of comparative +anatomy and ontogeny as to the placental character of man remains untouched. +These prove beyond all doubt the common descent of man and all the rest of the +mammals. Further, the main question is not in the least affected if it is said: +“It is true that man is a mammal; but he has diverged at the very root of +the class from all the other mammals, and has no closer relationship to any +living group of mammals.” The affinity is more or less close in any case, +if we examine the relation of the mammal class to the sixty other classes of +the animal world. Quite certainly the whole of the mammals, including man, have +had a common origin; and it is equally certain that their common stem-forms +were gradually evolved from a long series of lower Vertebrates. +</p> + +<p> +The resistance to the theory of a descent from the apes is clearly due in most +men to feeling rather than to reason. They shrink from the notion of such an +origin just because they see in the ape organism a caricature of man, a +distorted and unattractive image of themselves, because it hurts man’s +æsthetic complacency and self-ennoblement. It is more flattering to think we +have descended from some lofty and god-like being; and so, from the earliest +times, human vanity has been pleased to believe in our origin from gods or +demi-gods. The Church, with that sophistic reversal of ideas of which it is a +master, has succeeded in representing this ridiculous piece of vanity as +“Christian humility”; and the very men who reject with horror the +notion of an animal origin, and count themselves “children of God,” +love to prate of their “humble sense of servitude.” In most of the +sermons that have poured out from pulpit and altar against the doctrine of +evolution human vanity and conceit have been a conspicuous element; and, +although we have inherited this very characteristic weakness from the apes, we +must admit that we have developed it to a higher degree, which is entirely +repudiated by sound and normal intelligence. We are greatly amused at all the +childish follies that the ridiculous pride of ancestry has maintained from the +Middle Ages to our own time; yet there is a large amount of this empty feeling +in +<span class='pagenum'><a name="Page_353" id="Page_353"></a></span> +most men. Just as most people much prefer to trace their family back to some +degenerate baron or some famous prince rather than to an unknown peasant, so +most men would rather have as parent of the race a sinful and fallen Adam than +an advancing, and vigorous ape. It is a matter of taste, and to that extent we +cannot quarrel over these genealogical tendencies. Personally, the notion of +ascent is more congenial to me than that of descent. It seems to me a finer +thing to be the advanced offspring of a simian ancestor, that has developed +progressively from the lower mammals in the struggle for life, than the +degenerate descendant of a god-like being, made from a clod, and fallen for his +sins, and an Eve created from one of his ribs. Speaking of the rib, I may add +to what I have said about the development of the skeleton, that the number of +ribs is just the same in man and woman. In both of them the ribs are formed +from the middle germinal layer, and are, from the phylogenetic point of view, +lower or ventral vertebral arches. +</p> + +<p> +But it is said: “That is all very well, as far as the human body is +concerned; on the facts quoted it is impossible to doubt that it has really and +gradually been evolved from the long ancestral series of the Vertebrates. But +it is quite another thing as regards man’s mind, or soul; this cannot +possibly have been developed from the vertebrate-soul.”<a href="#linknote-35" name="linknoteref-35" id="linknoteref-35"><sup>[35]</sup></a> Let +us see if we cannot meet this grave stricture from the well-known facts of +comparative anatomy, physiology, and embryology. It will be best to begin with +a comparative study of the souls of various groups of Vertebrates. Here we find +such an enormous variety of vertebrate souls that, at first sight, it seems +quite impossible to trace them all to a common “Primitive +Vertebrate.” Think of the tiny Amphioxus, with no real brain but a simple +medullary tube, and its whole psychic life at the very lowest stage among the +Vertebrates. The following group of the Cyclostomes are still very limited, +though they have a brain. When we pass on to the fishes, we find their +intelligence remaining at a very low level. We do not see any material advance +in mental development until we go on to the Amphibia and Reptiles. There is +still greater advance when we come to the Mammals, though even here the minds +of the Monotremes and of the stupid Marsupials remain at a low stage. But when +we rise from these to the Placentals we find within this one vast group such a +number of important stages of differentiation and progress that the psychic +differences between the least intelligent (such as the sloths and armadillos) +and the most intelligent Placentals (such as the dogs and apes) are much +greater than the psychic differences between the lowest Placentals and the +Marsupials or Monotremes. Most certainly the differences are far greater than +the differences in mental power between the dog, the ape, and man. Yet all +these animals are genetically-related members of a single natural class. +</p> + +<p class="footnote"> +<a name="linknote-35" id="linknote-35"></a> <a href="#linknoteref-35">[35]</a> +The English reader will recognise here the curious position of Dr. Wallace and +of the late Dr. Mivart.—Translator. +</p> + +<p> +We see this to a still more astonishing extent in the comparative psychology of +another class of animals, that is especially interesting for many +reasons—the insect class. It is well known that we find in many insects a +degree of intelligence that is found in man alone among the Vertebrates. +Everybody knows of the famous communities and states of bees and ants, and of +the very remarkable social arrangements in them, such as we find among the more +advanced races of men, but among no other group of animals. I need only mention +the social organisation and government of the monarchic bees and the republican +ants, and their division into different conditions—queen, drone-nobles, +workers, educators, soldiers, etc. One of the most remarkable phenomena in this +very interesting province is the cattle-keeping of the ants, which rear +plant-lice as milch-cows and regularly extract their honeyed juice. Still more +remarkable is the slave-holding of the large red ants, which steal the young of +the small black ants and bring them up as slaves. It has long been known that +these political and social arrangements of the ants are due to the deliberate +cooperation of the countless citizens, and that they understand each other. A +number of recent observers, especially Fritz Müller, Sir J. Lubbock (Lord +Avebury), and August Forel, have put the astonishing degree of intelligence of +these tiny Articulates beyond question. +</p> + +<p> +Now, compare with these the mental life of many of the lower, especially the +parasitic insects, as Darwin did. There is, for instance, the cochineal insect +<span class='pagenum'><a name="Page_354" id="Page_354"></a></span> +(<i>Coccus</i>), which, in its adult state, has a motionless, shield-shaped +body, attached to the leaves of plants. Its feet are atrophied. Its snout is +sunk in the tissue of the plants of which it absorbs the sap. The whole psychic +life of these inert female parasites consists in the pleasure they experience +from sucking the sap of the plant and in sexual intercourse with the males. It +is the same with the maggot-like females of the fan-fly (<i>Strepsitera</i>), +which spend their lives parasitically and immovably, without wings or feet, in +the abdomen of wasps. There is no question here of higher psychic action. If we +compare these sluggish parasites with the intelligent and active ants, we must +admit that the psychic differences between them are much greater than the +psychic differences between the lowest and highest mammals, between the +Monotremes, Marsupials, and armadillos on the one hand, and the dog, ape, or +man on the other. Yet all these insects belong to the same class of +Articulates, just as all the mammals belong to one and the same class. And just +as every consistent evolutionist must admit a common stem-form for all these +insects, so he must also for all the mammals. +</p> + +<p> +If we now turn from the comparative study of psychic life in different animals +to the question of the organs of this function, we receive the answer that in +all the higher animals they are always bound up with certain groups of cells, +the ganglionic cells or neurona that compose the nervous system. All scientists +without exception are agreed that the central nervous system is the organ of +psychic life in the animal, and it is possible to prove this experimentally at +any moment. When we partially or wholly destroy the central nervous system, we +extinguish in the same proportion, partially or wholly, the “soul” +or psychic activity of the animal. We have, therefore, to examine the features +of the psychic organ in man. The reader already knows the incontestable answer +to this question. Man’s psychic organ is, in structure and origin, just +the same organ as in all the other Vertebrates. It originates in the shape of a +simple medullary tube from the outer membrane of the embryo—the +skin-sense layer. The simple cerebral vesicle that is formed by the expansion +of the head-part of this medullary tube divides by transverse constrictions +into five, and these pass through more or less the same stages of construction +in the human embryo as in the rest of the mammals. As these are undoubtedly of +a common origin, their brain and spinal cord must also have a common origin. +</p> + +<p> +Physiology teaches us further, on the ground of observation and experiment, +that the relation of the “soul” to its organ, the brain and spinal +cord, is just the same in man as in the other mammals. The one cannot act at +all without the other; it is just as much bound up with it as muscular movement +is with the muscles. It can only develop in connection with it. If we are +evolutionists at all, and grant the causal connection of ontogenesis and +phylogenesis, we are forced to admit this thesis: The human soul or psyche, as +a function of the medullary tube, has developed along with it; and just as +brain and spinal cord now develop from the simple medullary tube in every human +individual, so the human mind or the psychic life of the whole human race has +been gradually evolved from the lower vertebrate soul. Just as to-day the +intricate structure of the brain proceeds step by step from the same rudiment +in every human individual—the same five cerebral vesicles—as in all +the other Craniotes; so the human soul has been gradually developed in the +course of millions of years from a long series of craniote-souls. Finally, just +as to-day in every human embryo the various parts of the brain differentiate +after the special type of the ape-brain, so the human psyche has proceeded +historically from the ape-soul. +</p> + +<p> +It is true that this Monistic conception is rejected with horror by most men, +and the Dualistic idea, which denies the inseparable connection of brain and +mind, and regards body and soul as two totally different things, is still +popular. But how can we reconcile this view with the known facts of evolution? +It meets with difficulties equally great and insuperable in embryology and in +phylogeny. If we suppose with the majority of men that the soul is an +independent entity, which has nothing to do with the body originally, but +merely inhabits it for a time, and gives expression to its experiences through +the brain just as the pianist does through his instrument, we must assign a +point in human embryology at which the soul enters into the brain; and at death +again we must assign a moment at which it abandons the body. As, further, each +human individual has inherited certain +<span class='pagenum'><a name="Page_355" id="Page_355"></a></span> +personal features from each parent, we must suppose that in the act of +conception pieces were detached from their souls and transferred to the embryo. +A piece of the paternal soul goes with-the spermatozoon, and a piece of the +mother’s soul remains in the ovum. At the moment of conception, when +portions of the two nuclei of the copulating cells join together to form the +nucleus of the stem-cell, the accompanying fragments of the immaterial souls +must also be supposed to coalesce. +</p> + +<p> +On this Dualistic view the phenomena of psychic development are totally +incomprehensible. Everybody knows that the new-born child has no consciousness, +no knowledge of itself and the surrounding world. Every parent who has +impartially followed the mental development of his children will find it +impossible to deny that it is a case of biological evolutionary processes. Just +as all other functions of the body develop in connection with their organs, so +the soul does in connection with the brain. This gradual unfolding of the soul +of the child is, in fact, so wonderful and glorious a phenomenon that every +mother or father who has eyes to observe is never tired of contemplating it. It +is only our manuals of psychology that know nothing of this development; we are +almost tempted to think sometimes that their authors can never have had +children themselves. The human soul, as described in most of our psychological +works, is merely the soul of a learned philosopher, who has read a good many +books, but knows nothing of evolution, and never even reflects that his own +soul has had a development. +</p> + +<p> +When these Dualistic philosophers are consistent they must assign a moment in +the phylogeny of the human soul at which it was first “introduced” +into man’s vertebrate body. Hence, at the time when the human body was +evolved from the anthropoid body of the ape (probably in the Tertiary period), +a specific human psychic element—or, as people love to say, “a +spark of divinity”—must have been suddenly infused or breathed into +the anthropoid brain, and been associated with the ape-soul already present in +it. I need not insist on the enormous theoretical difficulties of this idea. I +will only point out that this “spark of divinity,” which is +supposed to distinguish the soul of man from that of the other animals, must be +itself capable of development, and has, as a matter of fact, progressively +developed in the course of human history. As a rule, reason is taken to be this +“spark of divinity,” and is supposed to be an exclusive possession +of humanity. But comparative psychology shows us that it is quite impossible to +set up this barrier between man and the brute. Either we take the word +“reason” in the wider sense, and then it is found in the higher +mammals (ape, dog, elephant, horse) just as well as in most men; or else in the +narrower sense, and then it is lacking in most men just as much as in the +majority of animals. On the whole, we may still say of man’s reason what +Goethe’s Mephistopheles said:— +</p> + +<p class="poem"> +Life somewhat better might content him<br/> +But for the gleam of heavenly light that Thou hast given him.<br/> +He calls it reason; thence his power’s increased<br/> +To be still beastlier than any beast. +</p> + +<p> +If, then, we must reject these popular and, in some respects, agreeable +Dualistic theories as untenable, because inconsistent with the genetic facts, +there remains only the opposite or Monistic conception, according to which the +human soul is, like any other animal soul, a function of the central nervous +system, and develops in inseparable connection therewith. We see this +<i>ontogenetically</i> in every child. The biogenetic law compels us to affirm +it <i>phylogenetically.</i> Just as in every human embryo the skin-sense layer +gives rise to the medullary tube, from the anterior end of which the five +cerebral vesicles of the Craniotes are developed, and from these the mammal +brain (first with the characters of the lower, then with those of the higher +mammals); and as the whole of this ontogenetic process is only a brief, +hereditary reproduction of the same process in the phylogenesis of the +Vertebrates; so the wonderful spiritual life of the human race through many +thousands of years has been evolved step by step from the lowly psychic life of +the lower Vertebrates, and the development of every child-soul is only a brief +repetition of that long and complex phylogenetic process. From all these facts +sound reason must conclude that the still prevalent belief in the immortality +of the soul is an untenable superstition. I have shown its inconsistency with +modern science in the eleventh chapter of <i>The Riddle of the Universe.</i> +</p> + +<p> +Here it may also be well to point out +<span class='pagenum'><a name="Page_356" id="Page_356"></a></span> +the great importance of anthropogeny, in the light of the biogenetic law, for +the purposes of philosophy. The speculative philosophers who take cognizance of +these ontogenetic facts, and explain them (in accordance with the law) +phylogenetically, will advance the great questions of philosophy far more than +the most distinguished thinkers of all ages have yet succeeded in doing. Most +certainly every clear and consistent thinker must derive from the facts of +comparative anatomy and ontogeny we have adduced a number of suggestive ideas +that cannot fail to have an influence on the progress of philosophy. Nor can it +be doubted that the candid statement and impartial appreciation of these facts +will lead to the decisive triumph of the philosophic tendency that we call +“Monistic” or “Mechanical,” as opposed to the +“Dualistic” or “Teleological,” on which most of the +ancient, medieval, and modern systems of philosophy are based. The Monistic or +Mechanical philosophy affirms that all the phenomena of human life and of the +rest of nature are ruled by fixed and unalterable laws; that there is +everywhere a necessary causal connection of phenomena; and that, therefore, the +whole knowable universe is a harmonious unity, a <i>monon.</i> It says, +further, that all phenomena are due solely to mechanical or efficient causes, +not to final causes. It does not admit free-will in the ordinary sense of the +word. In the light of the Monistic philosophy the phenomena that we are wont to +regard as the freest and most independent, the expressions of the human will, +are subject just as much to rigid laws as any other natural phenomenon. As a +matter of fact, impartial and thorough examination of our “free” +volitions shows that they are never really free, but always determined by +antecedent factors that can be traced to either heredity or adaptation. We +cannot, therefore, admit the conventional distinction between nature and +spirit. There is spirit everywhere in nature, and we know of no spirit outside +of nature. Hence, also, the common antithesis of natural science and mental or +moral science is untenable. Every science, as such, is both natural and mental. +That is a firm principle of Monism, which, on its religious side, we may also +denominate Pantheism. Man is not above, but in, nature. +</p> + +<p> +It is true that the opponents of evolution love to misrepresent the Monistic +philosophy based on it as “Materialism,” and confuse the +philosophic tendency of this name with a wholly unconnected and despicable +moral materialism. Strictly speaking, it would be just as proper to call our +system Spiritualism as Materialism. The real Materialistic philosophy affirms +that the phenomena of life are, like all other phenomena, effects or products +of matter. The opposite extreme, the Spiritualistic philosophy, says, on the +contrary, that matter is a product of energy, and that all material forms are +produced by free and independent forces. Thus, according to one-sided +Materialism, the matter is antecedent to the living force; according to the +equally one-sided view of the Spiritist, it is the reverse. Both views are +Dualistic, and, in my opinion, both are false. For us the antithesis disappears +in the Monistic philosophy, which knows neither matter without force nor force +without matter. It is only necessary to reflect for some time over the question +from the strictly scientific point of view to see that it is impossible to form +a clear idea of either hypothesis. As Goethe said, “Matter can never +exist or act without spirit, nor spirit without matter.” +</p> + +<p> +The human “spirit” or “soul” is merely a force or form +of energy, inseparably bound up with the material sub-stratum of the body. The +thinking force of the mind is just as much connected with the structural +elements of the brain as the motor force of the muscles with their structural +elements. Our mental powers are functions of the brain as much as any other +force is a function of a material body. We know of no matter that is devoid of +force, and no forces that are not bound up with matter. When the forces enter +into the phenomenon as movements we call them living or active forces; when +they are in a state of rest or equilibrium we call them latent or potential. +This applies equally to inorganic and organic bodies. The magnet that attracts +iron filings, the powder that explodes, the steam that drives the locomotive, +are living inorganics; they act by living force as much as the sensitive Mimosa +does when it contracts its leaves at touch, or the venerable Amphioxus that +buries itself in the sand of the sea, or man when he thinks. Only in the latter +cases the combinations of the different forces that appear as +“movement” in the +<span class='pagenum'><a name="Page_357" id="Page_357"></a></span> +phenomenon are much more intricate and difficult to analyse than in the former. +</p> + +<p> +Our study has led us to the conclusion that in the whole evolution of man, in +his embryology and in his phylogeny, there are no living forces at work other +than those of the rest of organic and inorganic nature. All the forces that are +operative in it could be reduced in the ultimate analysis to growth, the +fundamental evolutionary function that brings about the forms of both the +organic and the inorganic. But growth itself depends on the attraction and +repulsion of homogeneous and heterogeneous particles. Seventy-five years ago +Carl Ernst von Baer summed up the general result of his classic studies of +animal development in the sentence: “The evolution of the individual is +the history of the growth of individuality in every respect.” And if we +go deeper to the root of this law of growth, we find that in the long run it +can always be reduced to that attraction and repulsion of animated atoms which +Empedocles called the “love and hatred” of the elements. +</p> + +<p> +Thus the evolution of man is directed by the same “eternal, iron +laws” as the development of any other body. These laws always lead us +back to the same simple principles, the elementary principles of physics and +chemistry. The various phenomena of nature only differ in the degree of +complexity in which the different forces work together. Each single process of +adaptation and heredity in the stem-history of our ancestors is in itself a +very complex physiological phenomenon. Far more intricate are the processes of +human embryology; in these are condensed and comprised thousands of the +phylogenetic processes. +</p> + +<p> +In my <i>General Morphology,</i> which appeared in 1866, I made the first +attempt to apply the theory of evolution, as reformed by Darwin, to the whole +province of biology, and especially to provide with its assistance a mechanical +foundation for the science of organic forms. The intimate relations that exist +between all parts of organic science, especially the direct causal nexus +between the two sections of evolution—ontogeny and phylogeny—were +explained in that work for the first time by transformism, and were interpreted +philosophically in the light of the theory of descent. The anthropological part +of the <i>General Morphology</i> (Book vii) contains the first attempt to +determine the series of man’s ancestors (vol. ii, p. 428). However +imperfect this attempt was, it provided a starting-point for further +investigation. In the thirty-seven years that have since elapsed the biological +horizon has been enormously widened; our empirical acquisitions in +paleontology, comparative anatomy, and ontogeny have grown to an astonishing +extent, thanks to the united efforts of a number of able workers and the +employment of better methods. Many important biological questions that then +appeared to be obscure enigmas seem to be entirely settled. Darwinism arose +like the dawn of a new day of clear Monistic science after the dark night of +mystic dogmatism, and we can say now, proudly and gladly, that there is +daylight in our field of inquiry. +</p> + +<p> +Philosophers and others, who are equally ignorant of the empirical sources of +our evidence and the phylogenetic methods of utilising it, have even lately +claimed that in the matter of constructing our genealogical tree nothing more +has been done than the discovery of a “gallery of ancestors,” such +as we find in the mansions of the nobility. This would be quite true if the +genealogy given in the second part of this work were merely the juxtaposition +of a series of animal forms, of which we gathered the genetic connection from +their external physiognomic resemblances. As we have sufficiently proved +already, it is for us a question of a totally different thing—of the +morphological and historical proof of the phylogenetic connection of these +ancestors on the basis of their identity in internal structure and embryonic +development; and I think I have sufficiently shown in the first part of this +work how far this is calculated to reveal to us their inner nature and its +historical development. I see the essence of its significance precisely in the +proof of historical connection. I am one of those scientists who believe in a +real “natural history,” and who think as much of an historical +knowledge of the past as of an exact investigation of the present. The +incalculable value of the historical consciousness cannot be sufficiently +emphasised at a time when historical research is ignored and neglected, and +when an “exact” school, as dogmatic as it is narrow, would +substitute for it physical experiments and mathematical formulæ. Historical +knowledge cannot be replaced by any other branch of science. +</p> + +<p> +<span class='pagenum'><a name="Page_358" id="Page_358"></a></span> +It is clear that the prejudices that stand in the way of a general recognition +of this “natural anthropogeny” are still very great; otherwise the +long struggle of philosophic systems would have ended in favour of Monism. But +we may confidently expect that a more general acquaintance with the genetic +facts will gradually destroy these prejudices, and lead to the triumph of the +natural conception of “man’s place in nature.” When we hear +it said, in face of this expectation, that this would lead to retrogression in +the intellectual and moral development of mankind, I cannot refrain from saying +that, in my opinion, it will be just the reverse; that it will promote to an +enormous extent the advance of the human mind. All progress in our knowledge of +truth means an advance in the higher cultivation of the human intelligence; and +all progress in its application to practical life implies a corresponding +improvement of morality. The worst enemies of the human race—ignorance +and superstition—can only be vanquished by truth and reason. In any case, +I hope and desire to have convinced the reader of these chapters that the true +scientific comprehension of the human frame can only be attained in the way +that we recognise to be the sole sound and effective one in organic science +generally—namely, the way of Evolution. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap31"></a> +<span class='pagenum'><a name="Page_359" id="Page_359"></a></span> +INDEX</h2> + +<p class="noindent"> + +<b>A</b><br/><br/> +Abiogenesis, <a href="#Page_26">26</a><br/> + +<i>Accipenser</i>, <a href="#Page_234">234</a><br/> + +Abortive ova, <a href="#Page_55">55</a><br/> + +Achromatin, <a href="#Page_42">42</a><br/> + +Achromin, <a href="#Page_42">42</a><br/> + +Acœla, <a href="#Page_221">221</a><br/> + +Acoustic nerve, the, <a href="#Page_289">289,</a> <a href="#Page_290">290</a><br/> + +Acquired characters, inheritance of, <a href="#Page_349">349</a><br/> + +Acrania, the, <a href="#Page_182">182</a><br/> + +Acroganglion, the, <a href="#Page_268">268,</a> <a href="#Page_275">275</a><br/> + +Adam’s apple, the, <a href="#Page_184">184</a><br/> + +Adapida, <a href="#Page_257">257</a><br/> + +Adaptation, <a href="#Page_3">3,</a> <a href="#Page_5">5,</a> <a href="#Page_27">27</a><br/> + +After-birth, the, <a href="#Page_167">167</a><br/> + +Agassiz, L., <a href="#Page_34">34</a><br/> + +Age of life, <a href="#Page_200">200</a><br/> + +Alimentary canal, evolution of the, <a href="#Page_13">13,</a> <a href="#Page_14">14,</a> <a href="#Page_133">133,</a> <a href="#Page_308">308–17</a><br/> +— — structure of the, <a href="#Page_169">169,</a> <a href="#Page_308">308–10</a><br/> + +Allantoic circulation, the, <a href="#Page_171">171</a><br/> + +Allantois, development of the, <a href="#Page_166">166</a><br/> + +Allmann, <a href="#Page_20">20</a><br/> + +<i>Amblystoma,</i> <a href="#Page_243">243</a><br/> + +Amitotic cleavage, <a href="#Page_40">40</a><br/> + +Ammoconida, <a href="#Page_217">217</a><br/> + +<i>Ammolynthus,</i> <a href="#Page_217">217</a><br/> + +Amnion, the, <a href="#Page_115">115</a><br/> +— formation of the, <a href="#Page_134">134,</a> <a href="#Page_244">244</a><br/> + +Amniotic fluid, the, <a href="#Page_134">134</a><br/> + +Amœba, the, <a href="#Page_47">47–9,</a> <a href="#Page_210">210</a><br/> + +Amphibia, the, <a href="#Page_239">239</a><br/> + +<i>Amphichœrus,</i> <a href="#Page_221">221</a><br/> + +Amphigastrula, <a href="#Page_80">80</a><br/> + +Amphioxus, the, <a href="#Page_105">105,</a> <a href="#Page_181">181–95</a><br/> +— circulation of the, <a href="#Page_184">184</a><br/> +— cœlomation of the, <a href="#Page_95">95</a><br/> +— embryology of the, <a href="#Page_191">191–95</a><br/> +— structure of the, <a href="#Page_183">183–88</a><br/> + +Amphirhina, <a href="#Page_230">230</a><br/> + +Anamnia, the, <a href="#Page_115">115</a><br/> + +Anatomy, comparative, <a href="#Page_208">208</a><br/> + +Animalculists, <a href="#Page_12">12</a><br/> + +Animal layer, the, <a href="#Page_16">16</a><br/> + +Annelids, the, <a href="#Page_142">142,</a> <a href="#Page_219">219</a><br/> + +Annelid theory, the, <a href="#Page_142">142</a><br/> + +Anomodontia, <a href="#Page_246">246</a><br/> + +Ant, intelligence of the, <a href="#Page_353">353</a><br/> + +<i>Anthropithecus,</i> <a href="#Page_174">174,</a> <a href="#Page_262">262</a><br/> + +Anthropogeny, <a href="#Page_1">1</a><br/> + +Anthropoid apes, the, <a href="#Page_166">166,</a> <a href="#Page_173">173,</a> <a href="#Page_262">262</a><br/> + +Anthropology, <a href="#Page_1">1,</a> <a href="#Page_35">35</a><br/> + +Anthropozoic period, <a href="#Page_203">203</a><br/> + +Antimera, <a href="#Page_107">107</a><br/> + +Anura, <a href="#Page_243">243</a><br/> + +Anus, the, <a href="#Page_317">317</a><br/> + +Anus, formation of the, <a href="#Page_139">139</a><br/> + +Aorta, the, <a href="#Page_327">327</a><br/> +— development of the, <a href="#Page_170">170</a><br/> + +Ape and man, <a href="#Page_157">157,</a> <a href="#Page_164">164,</a> <a href="#Page_261">261,</a> <a href="#Page_307">307,</a> <a href="#Page_351">351</a><br/> + +Ape-man, the, <a href="#Page_263">263</a><br/> + +Apes, the, <a href="#Page_257">257–60</a><br/> + +<i>Aphanocapsa,</i> <a href="#Page_210">210</a><br/> + +<i>Aphanostomum,</i> <a href="#Page_221">221</a><br/> + +Appendicaria, <a href="#Page_197">197</a><br/> + +Appendix vermiformis, the, <a href="#Page_32">32</a><br/> + +Aquatic life, early prevalence of, <a href="#Page_235">235</a><br/> + +Ararat, Mount, <a href="#Page_24">24</a><br/> + +Archenteron, <a href="#Page_64">64,</a> <a href="#Page_74">74</a><br/> + +Archeolithic age, <a href="#Page_203">203</a><br/> + +Archicaryon, <a href="#Page_55">55</a><br/> + +Archicrania, <a href="#Page_230">230</a><br/> + +Archigastrula, <a href="#Page_65">65,</a> <a href="#Page_193">193</a><br/> + +<i>Archiprimas,</i> <a href="#Page_263">263</a><br/> + +Arctopitheca, <a href="#Page_261">261</a><br/> + +Area, the germinative, <a href="#Page_121">121</a><br/> + +Aristotle, <a href="#Page_9">9</a><br/> + +Arm, structure of the, <a href="#Page_306">306</a><br/> + +Arrow-worm, the, <a href="#Page_191">191</a><br/> + +Arterial arches, the, <a href="#Page_325">325–26</a><br/> +— cone, the, <a href="#Page_324">324</a><br/> + +Arteries, evolution of the, <a href="#Page_170">170,</a> <a href="#Page_323">323–24</a><br/> + +Articulates, the, <a href="#Page_142">142,</a> <a href="#Page_219">219</a><br/> +— skeleton of the, <a href="#Page_294">294</a><br/> + +Articulation, <a href="#Page_141">141–42</a><br/> + +Aryo-Romanic languages, the, <a href="#Page_203">203</a><br/> + +Ascidia, the, <a href="#Page_181">181,</a> <a href="#Page_188">188–90</a><br/> +— embryology of the, <a href="#Page_196">196–98</a><br/> + +Ascula, <a href="#Page_217">217</a><br/> + +Asexual reproduction, <a href="#Page_51">51</a><br/> + +Atlas, the, <a href="#Page_247">247</a><br/> + +Atrium, the, <a href="#Page_183">183,</a> <a href="#Page_185">185</a><br/> +— (heart), the, <a href="#Page_326">326</a><br/> + +Auditory nerve, the, <a href="#Page_289">289,</a> <a href="#Page_290">290</a><br/> + +Auricles of the heart, <a href="#Page_325">325</a><br/> + +<i>Autolemures,</i> <a href="#Page_257">257</a><br/> + +Axolotl, the, <a href="#Page_243">243</a><br/> +<br/> +<b>B</b><br/><br/> +Bacteria, <a href="#Page_38">38,</a> <a href="#Page_210">210</a><br/> + +Baer, K. E. von, <a href="#Page_15">15–17</a><br/> + +Balanoglossus, <a href="#Page_226">226</a><br/> + +Balfour, F., <a href="#Page_21">21</a><br/> + +Batrachia, <a href="#Page_241">241</a><br/> + +<i>Bdellostoma Stouti,</i> <a href="#Page_78">78</a><br/> + +Bee, generation of the, <a href="#Page_9">9</a><br/> + +Beyschlag, W., on evolution, <a href="#Page_50">50</a><br/> + +Bilateral symmetry, <a href="#Page_66">66</a><br/> +— — origin of, <a href="#Page_221">221</a><br/> + +Bimana, <a href="#Page_258">258</a><br/> + +Biogenetic law, the, <a href="#Page_2">2,</a> <a href="#Page_21">21,</a> <a href="#Page_23">23,</a> <a href="#Page_179">179,</a> <a href="#Page_349">349</a><br/> + +Biogeny, <a href="#Page_2">2</a><br/> + +Bionomy, <a href="#Page_33">33</a><br/> + +Bird, evolution of the, <a href="#Page_245">245</a><br/> +— ovum of the, <a href="#Page_44">44–6,</a> <a href="#Page_80">80–1</a><br/> + +Bischoff, W., <a href="#Page_17">17</a><br/> + +Bladder, evolution of the, <a href="#Page_244">244,</a> <a href="#Page_339">339</a><br/> + +Blastæa, the, <a href="#Page_206">206,</a> <a href="#Page_213">213</a><br/> + +Blastocœl, the, <a href="#Page_62">62,</a> <a href="#Page_74">74</a><br/> + +Blastocrene, the, <a href="#Page_99">99</a><br/> + +Blastocystis, the, <a href="#Page_62">62,</a> <a href="#Page_119">119,</a> <a href="#Page_120">120</a><br/> + +Blastoderm, the, <a href="#Page_62">62</a><br/> + +Blastodermic vesicle, the, <a href="#Page_119">119</a><br/> + +Blastoporus, the, <a href="#Page_64">64</a><br/> + +Blastosphere, the, <a href="#Page_62">62,</a> <a href="#Page_119">119</a><br/> + +Blastula, the, <a href="#Page_62">62,</a> <a href="#Page_74">74</a><br/> +— the mammal, <a href="#Page_119">119</a><br/> + +Blood, importance of the, <a href="#Page_318">318</a><br/> +— recent experiments in mixture of, <a href="#Page_172">172</a><br/> +— structure of the, <a href="#Page_319">319</a><br/> + +Blood-cells, the, <a href="#Page_319">319</a><br/> + +Blood-vessels, the, <a href="#Page_318">318–25</a><br/> +— development of the, <a href="#Page_168">168</a><br/> +— of the vertebrate, <a href="#Page_110">110</a><br/> +— origin of the, <a href="#Page_320">320–21</a><br/> + +Boniface VIII, Bull of, <a href="#Page_10">10</a><br/> + +Bonnet, <a href="#Page_13">13</a><br/> + +Borneo nosed-ape, the, <a href="#Page_164">164</a><br/> + +Boveri, Theodor, <a href="#Page_185">185</a><br/> + +Brachytarsi, <a href="#Page_257">257</a><br/> + +Brain and mind, <a href="#Page_278">278,</a> <a href="#Page_354">354–56</a><br/> +— evolution of the, <a href="#Page_8">8,</a> <a href="#Page_275">275–80</a><br/> +— in the fish, <a href="#Page_276">276</a><br/> +— in the lower animals, <a href="#Page_275">275</a><br/> +— structure of the, <a href="#Page_273">273–74</a><br/> + +Branchial arches, evolution of the, <a href="#Page_303">303</a><br/> +— cavity, the, <a href="#Page_183">183,</a> <a href="#Page_189">189</a><br/> +— system, the, <a href="#Page_110">110</a><br/> + +Branchiotomes, <a href="#Page_149">149</a><br/> + +Breasts, the, <a href="#Page_113">113</a><br/> + +Bulbilla, <a href="#Page_184">184</a><br/> + +<br/><b>C</b><br/><br/> + +<i>Calamichthys,</i> <a href="#Page_234">234</a><br/> + +<i>Calcolynthus,</i> <a href="#Page_217">217</a><br/> + +Capillaries, the, <a href="#Page_323">323</a><br/> + +Caracoideum, the, <a href="#Page_249">249</a><br/> + +Carboniferous strata, <a href="#Page_202">202</a><br/> + +<i>Carcharodon,</i> <a href="#Page_234">234</a><br/> + +Cardiac cavity, the, <a href="#Page_170">170</a><br/> + +Cardiocœl, the, <a href="#Page_328">328</a><br/> + +Caryobasis, <a href="#Page_38">38,</a> <a href="#Page_54">54</a><br/> + +Caryokinesis, <a href="#Page_42">42</a><br/> + +Caryolymph, <a href="#Page_38">38,</a> <a href="#Page_54">54</a><br/> + +Caryolyses, <a href="#Page_42">42</a><br/> + +Caryon, <a href="#Page_37">37</a><br/> + +Caryoplasm, <a href="#Page_37">37</a><br/> + +Catallacta, <a href="#Page_213">213</a><br/> + +Catarrhinæ, the, <a href="#Page_173">173,</a> <a href="#Page_261">261</a><br/> + +Catastrophic theory, the, <a href="#Page_24">24</a><br/> + +Caudate cells, <a href="#Page_53">53</a><br/> + +Cell, life of the, <a href="#Page_41">41–3</a><br/> +— nature of the, <a href="#Page_36">36–7</a><br/> +— size of the, <a href="#Page_38">38</a><br/> + +Cell theory, the, <a href="#Page_18">18,</a> <a href="#Page_36">36</a><br/> + +Cenogenesis, <a href="#Page_4">4</a><br/> + +Cenogenetic structures, <a href="#Page_4">4</a><br/> + +Cenozoic period, the, <a href="#Page_203">203</a><br/> + +Central body, the, <a href="#Page_38">38,</a> <a href="#Page_42">42</a><br/> + +Central nervous system, the, <a href="#Page_273">273</a><br/> + +Centrolecithal ova, <a href="#Page_68">68</a><br/> + +Centrosoma, the, <a href="#Page_38">38,</a> <a href="#Page_42">42</a><br/> + +Ceratodus, the, <a href="#Page_76">76,</a> <a href="#Page_237">237</a><br/> + +Cerebellum, the, <a href="#Page_274">274</a><br/> + +Cerebral vesicles, evolution of the, <a href="#Page_276">276</a><br/> + +Cerebrum, the, <a href="#Page_273">273</a><br/> + +<i>Cestracion Japonicus,</i> <a href="#Page_75">75,</a> <a href="#Page_79">79</a><br/> + +Chætognatha, <a href="#Page_94">94</a><br/> + +Chick, importance of the, in embryology, <a href="#Page_11">11,</a> <a href="#Page_16">16</a><br/> + +Child, mind of the, <a href="#Page_8">8,</a> <a href="#Page_355">355</a><br/> + +Chimpanzee, the, <a href="#Page_174">174,</a> <a href="#Page_262">262</a><br/> + +<i>Chiromys,</i> <a href="#Page_257">257</a><br/> + +Chiroptera, <a href="#Page_258">258</a><br/> + +<i>Chirotherium,</i> <a href="#Page_239">239</a><br/> + +Chondylarthra, <a href="#Page_257">257</a><br/> + +Chorda, the, <a href="#Page_17">17,</a> <a href="#Page_95">95,</a> <a href="#Page_107">107,</a> <a href="#Page_183">183</a><br/> +— evolution of the, <a href="#Page_296">296</a><br/> + +<i>Chordæa,</i> the, <a href="#Page_97">97</a><br/> + +Chordalemma, the, <a href="#Page_296">296</a><br/> + +Chordaria, <a href="#Page_97">97</a><br/> + +Chordula, the, <a href="#Page_3">3,</a> <a href="#Page_96">96,</a> <a href="#Page_191">191</a><br/> + +Choriata, the, <a href="#Page_166">166</a><br/> + +Chorion, the, <a href="#Page_119">119</a><br/> +— development of the, <a href="#Page_165">165–6</a><br/> +— frondosum, <a href="#Page_255">255</a><br/> +— læve, <a href="#Page_255">255</a><br/> + +Choroid coat, the, <a href="#Page_286">286</a><br/> + +Chorology, <a href="#Page_33">33</a><br/> + +Chromacea, <a href="#Page_209">209</a><br/> + +Chromatin, <a href="#Page_42">42</a><br/> + +Chroococcacea, <a href="#Page_210">210</a><br/> + +<i>Chroococcus,</i> the, <a href="#Page_210">210</a><br/> + +Church, opposition of, to science in Middle Ages, <a href="#Page_10">10</a><br/> + +Chyle, <a href="#Page_318">318</a><br/> + +Chyle-vessels, <a href="#Page_324">324</a><br/> + +Cicatricula, the, <a href="#Page_45">45,</a> <a href="#Page_81">81</a><br/> + +Ciliated cells, <a href="#Page_53">53,</a> <a href="#Page_193">193</a><br/> + +Cinghalese gynecomast, <a href="#Page_114">114</a><br/> + +Circulation in the lancelet, <a href="#Page_184">184</a><br/> + +Circulatory system, evolution of the, <a href="#Page_321">321–25</a><br/> +— — structure of the, <a href="#Page_318">318</a><br/> + +Classification, <a href="#Page_103">103</a><br/> +— evolutionary value of, <a href="#Page_33">33</a><br/> + +Clitoris, the, <a href="#Page_345">345</a><br/> + +Cloaca, the, <a href="#Page_249">249,</a> <a href="#Page_317">317</a><br/> + +Cnidaria, <a href="#Page_217">217</a><br/> + +Coccyx, the, <a href="#Page_295">295</a><br/> + +Cochineal insect, the, <a href="#Page_354">354</a><br/> + +Cochlea, the, <a href="#Page_289">289</a><br/> + +Cœcilia, <a href="#Page_241">241</a><br/> + +Cœcum, the, <a href="#Page_310">310,</a> <a href="#Page_317">317</a><br/> + +Cœlenterata, <a href="#Page_20">20,</a> <a href="#Page_91">91,</a> <a href="#Page_93">93,</a> <a href="#Page_104">104</a><br/> + +Cœlenteria, <a href="#Page_221">221</a><br/> + +Cœloma, the, <a href="#Page_21">21,</a> <a href="#Page_64">64,</a> <a href="#Page_91">91</a><br/> + +Cœlomæa, the, <a href="#Page_98">98</a><br/> + +Cœlomaria, <a href="#Page_21">21,</a> <a href="#Page_91">91,</a> <a href="#Page_104">104,</a> <a href="#Page_221">221</a><br/> + +Cœlomation, <a href="#Page_93">93–4</a><br/> + +Cœlom-theory, the, <a href="#Page_21">21,</a> <a href="#Page_93">93</a><br/> + +Cœlomula, the, <a href="#Page_98">98</a><br/> + +Colon, the, <a href="#Page_310">310,</a> <a href="#Page_317">317</a><br/> + +Comparative anatomy, <a href="#Page_31">31</a><br/> + +Conception, nature of, <a href="#Page_51">51</a><br/> + +Conjunctiva, the, <a href="#Page_286">286</a><br/> + +<i>Conocyema,</i> <a href="#Page_215">215</a><br/> + +<i>Convoluta,</i> <a href="#Page_221">221</a><br/> + +Copelata, the, <a href="#Page_197">197</a><br/> + +Copulative organs, evolution of the, <a href="#Page_344">344–45</a><br/> + +Corium, the, <a href="#Page_108">108,</a> <a href="#Page_268">268</a><br/> + +Cornea, the, <a href="#Page_286">286</a><br/> + +Corpora cavernosa, the, <a href="#Page_345">345,</a> <a href="#Page_346">346</a><br/> + +Corpora quadrigemina, <a href="#Page_274">274</a><br/> + +Corpora striata, <a href="#Page_274">274</a><br/> + +Corpus callosum, the, <a href="#Page_274">274</a><br/> + +Corpus vitreum, the, <a href="#Page_285">285</a><br/> + +Corpuscles of the blood, <a href="#Page_319">319</a><br/> + +Craniology, <a href="#Page_303">303</a><br/> + +Craniota, the, <a href="#Page_182">182,</a> <a href="#Page_229">229</a><br/> + +Cranium, the, <a href="#Page_299">299</a><br/> + +Creation, <a href="#Page_23">23–4</a><br/> + +Cretaceous strata, <a href="#Page_202">202</a><br/> + +Crossopterygii, <a href="#Page_234">234</a><br/> + +Crustacea, the, <a href="#Page_142">142,</a> <a href="#Page_219">219</a><br/> + +Cryptocœla, <a href="#Page_221">221</a><br/> + +Cryptorchism, <a href="#Page_114">114</a><br/> + +Crystalline lens, the, <a href="#Page_285">285</a><br/> +— — development of the, <a href="#Page_287">287</a><br/> + +Cutaneous glands, <a href="#Page_268">268</a><br/> + +Cuttlefish, embryology of the, <a href="#Page_9">9</a><br/> + +Cuvier, G., <a href="#Page_17">17,</a> <a href="#Page_24">24</a><br/> + +Cyanophycea, <a href="#Page_209">209</a><br/> + +Cyclostoma, the, <a href="#Page_188">188,</a> <a href="#Page_230">230–32</a><br/> +— ova of the, <a href="#Page_75">75</a><br/> + +Cyemaria, <a href="#Page_214">214</a><br/> + +Cynopitheca, <a href="#Page_262">262</a><br/> + +<i>Cynthia,</i> <a href="#Page_191">191,</a> <a href="#Page_196">196</a><br/> + +Cytoblastus, the, <a href="#Page_37">37</a><br/> + +Cytodes, <a href="#Page_40">40</a><br/> + +Cytoplasm, <a href="#Page_37">37,</a> <a href="#Page_38">38</a><br/> + +Cytosoma, <a href="#Page_37">37</a><br/> + +Cytula, the, <a href="#Page_54">54</a><br/> +<br/><b>D</b><br/><br/> +Dalton, <a href="#Page_15">15</a><br/> + +Darwin, C., <a href="#Page_2">2,</a> <a href="#Page_5">5,</a> <a href="#Page_23">23,</a> <a href="#Page_28">28–9</a><br/> + +Darwin, E., <a href="#Page_28">28</a><br/> + +Darwinism, <a href="#Page_5">5,</a> <a href="#Page_28">28</a><br/> + +Decidua, the, <a href="#Page_167">167</a><br/> + +Deciduata, <a href="#Page_255">255</a><br/> + +Deduction, nature of, <a href="#Page_208">208</a><br/> + +Degeneration theory, the, <a href="#Page_219">219</a><br/> + +Dentition of the ape and man, <a href="#Page_259">259</a><br/> + +Depula, <a href="#Page_62">62</a><br/> + +<i>Descent of Man,</i> <a href="#Page_30">30</a><br/> + +Design in organisms, <a href="#Page_33">33</a><br/> + +Deutoplasm, <a href="#Page_44">44</a><br/> + +Devonian strata, <a href="#Page_202">202</a><br/> + +Diaphragm, the, <a href="#Page_309">309</a><br/> +— evolution of the, <a href="#Page_328">328</a><br/> + +<i>Dicyema,</i> <a href="#Page_215">215</a><br/> + +Dicyemida, <a href="#Page_215">215</a><br/> + +Didelphia, <a href="#Page_248">248</a><br/> + +Digonopora, <a href="#Page_223">223</a><br/> + +Dinosauria, <a href="#Page_202">202</a><br/> + +Dipneumones, <a href="#Page_238">238</a><br/> + +Dipneusta, <a href="#Page_235">235–38</a><br/> +— ova of the, <a href="#Page_75">75</a><br/> + +Dipnoa, <a href="#Page_236">236</a><br/> + +Directive bodies, <a href="#Page_54">54</a><br/> + +Discoblastic ova, <a href="#Page_68">68</a><br/> + +Discoplacenta, <a href="#Page_255">255</a><br/> + +<i>Dissatyrus,</i> <a href="#Page_174">174</a><br/> + +Dissection, medieval decrees against, <a href="#Page_10">10</a><br/> + +Dohrn, Anton, <a href="#Page_219">219</a><br/> + +Döllinger, <a href="#Page_15">15</a><br/> + +Dorsal furrow, the, <a href="#Page_125">125</a><br/> +— shield, the, <a href="#Page_123">123</a><br/> +— zone, the, <a href="#Page_129">129</a><br/> + +<i>Dromatherium,</i> <a href="#Page_248">248</a><br/> + +Dualism, <a href="#Page_6">6</a><br/> + +Dubois, Eugen, <a href="#Page_263">263</a><br/> + +<i>Ductus Botalli,</i> the, <a href="#Page_350">350</a><br/> + +<i>Ductus venosus Arantii,</i> <a href="#Page_350">350</a><br/> + +Duodenum, the, <a href="#Page_309">309,</a> <a href="#Page_317">317</a><br/> + +Duration of embryonic development, <a href="#Page_199">199</a><br/> +— of man’s history, <a href="#Page_199">199</a><br/> + +Dysteleology, <a href="#Page_32">32</a><br/> +— proofs of, <a href="#Page_349">349</a><br/> + +<br/><b>E</b><br/><br/> + +Ear, evolution of the, <a href="#Page_288">288–92</a><br/> +— structure of the, <a href="#Page_288">288</a><br/> +— uselessness of the external, <a href="#Page_32">32</a><br/> + +Ear-bones, the, <a href="#Page_289">289</a><br/> + +Earth, age of the, <a href="#Page_200">200–201</a><br/> + +<i>Echidna hystrix,</i> <a href="#Page_249">249</a><br/> + +Ectoblast, <a href="#Page_20">20,</a> <a href="#Page_64">64</a><br/> + +Ectoderm, the, <a href="#Page_20">20,</a> <a href="#Page_64">64</a><br/> + +Edentata, <a href="#Page_250">250</a><br/> + +Efficient causes, <a href="#Page_6">6</a><br/> + +Egg of the bird, <a href="#Page_44">44–6,</a> <a href="#Page_81">81</a><br/> +— or the chick, priority of the, <a href="#Page_211">211</a><br/> + +Elasmobranchs, the, <a href="#Page_79">79</a><br/> + +Embryo, human, development of the, <a href="#Page_158">158</a><br/> + +Embryology, <a href="#Page_2">2</a><br/> +— evolutionary value of, <a href="#Page_34">34</a><br/> + +Embryonic development, duration of, <a href="#Page_199">199</a><br/> +— disk, the, <a href="#Page_121">121–22</a><br/> +— spot, the, <a href="#Page_125">125</a><br/> + +Encephalon, the, <a href="#Page_273">273</a><br/> + +Endoblast, <a href="#Page_20">20,</a> <a href="#Page_64">64</a><br/> + +Endothelia, <a href="#Page_321">321</a><br/> + +Enterocœla, <a href="#Page_93">93,</a> <a href="#Page_223">223</a><br/> + +Enteropneusta, <a href="#Page_226">226</a><br/> + +Entoderm, the, <a href="#Page_20">20,</a> <a href="#Page_64">64</a><br/> + +Eocene strata, <a href="#Page_203">203</a><br/> + +Eopitheca, <a href="#Page_259">259</a><br/> + +Epiblast, <a href="#Page_20">20,</a> <a href="#Page_64">64</a><br/> + +Epidermis, the, <a href="#Page_108">108,</a> <a href="#Page_268">268</a><br/> + +Epididymis, the, <a href="#Page_342">342</a><br/> + +Epigastrula, <a href="#Page_80">80</a><br/> + +Epigenesis, <a href="#Page_11">11,</a> <a href="#Page_13">13</a><br/> + +Epiglottis, the, <a href="#Page_309">309</a><br/> + +Epiphysis, the, <a href="#Page_108">108</a><br/> + +Episoma, <a href="#Page_129">129</a><br/> + +Episomites, <a href="#Page_130">130,</a> <a href="#Page_194">194</a><br/> + +Epispadia, <a href="#Page_346">346</a><br/> + +Epithelia, <a href="#Page_37">37</a><br/> + +Epitheria, <a href="#Page_243">243,</a> <a href="#Page_253">253</a><br/> + +Epovarium, the, <a href="#Page_342">342</a><br/> + +Equilibrium, sense of, <a href="#Page_291">291</a><br/> + +Esthonychida, <a href="#Page_257">257</a><br/> + +Eustachian tube, the, <a href="#Page_289">289</a><br/> + +Eutheria, <a href="#Page_253">253</a><br/> + +Eve, <a href="#Page_12">12</a><br/> + +Evolution theory, the, <a href="#Page_11">11,</a> <a href="#Page_208">208</a><br/> +— inductive nature of, <a href="#Page_30">30</a><br/> + +Eye, evolution of the, <a href="#Page_285">285–88</a><br/> +— structure of the, <a href="#Page_285">285</a><br/> + +Eyelid, the third, <a href="#Page_32">32</a><br/> + +Eyelids, evolution of the, <a href="#Page_288">288</a><br/> + +<br/><b>F</b><br/><br/> + +Fabricius ab Aquapendente, <a href="#Page_10">10</a><br/> + +Face, embryonic development of the, <a href="#Page_284">284</a><br/> + +Fat glands in the skin, <a href="#Page_269">269</a><br/> + +Feathers, evolution of, <a href="#Page_270">270</a><br/> + +Fertilisation, <a href="#Page_51">51</a><br/> +— place of, <a href="#Page_119">119</a><br/> + +Fin, evolution of the, <a href="#Page_239">239,</a> <a href="#Page_304">304</a><br/> + +Final causes, <a href="#Page_6">6</a><br/> + +Flagellate cells, <a href="#Page_193">193</a><br/> + +Floating bladder, the, <a href="#Page_233">233,</a> <a href="#Page_241">241</a><br/> +— — evolution of the, <a href="#Page_314">314</a><br/> + +Fœtal circulation, <a href="#Page_170">170–71</a><br/> + +Food-yelk, the, <a href="#Page_67">67,</a><br/> + +Foot, evolution of the, <a href="#Page_241">241,</a> <a href="#Page_304">304–6</a><br/> +— of the ape and man, <a href="#Page_258">258–59</a><br/> + +Fore brain, the, <a href="#Page_278">278</a><br/> + +Fore kidneys, the, <a href="#Page_336">336,</a> <a href="#Page_337">337</a><br/> + +Fossiliferous strata, list of, <a href="#Page_201">201</a><br/> + +Fossils, <a href="#Page_180">180</a><br/> +— scarcity of, <a href="#Page_208">208</a><br/> + +Free will, <a href="#Page_356">356</a><br/> + +Friedenthal, experiments of, <a href="#Page_172">172</a><br/> + +Frog, the, <a href="#Page_241">241–42</a><br/> +— ova of the, <a href="#Page_71">71–2</a><br/> + +Frontonia, <a href="#Page_224">224</a><br/> + +Function and structure, <a href="#Page_7">7</a><br/> + +Furcation of ova, <a href="#Page_72">72</a><br/> + +<br/><b>G</b><br/><br/> + +Gaertner’s duct, <a href="#Page_341">341,</a> <a href="#Page_350">350</a><br/> + +Ganglia, commencement of, <a href="#Page_268">268</a><br/> + +Ganglionic cell, the, <a href="#Page_39">39</a><br/> + +Ganoids, <a href="#Page_233">233,</a> <a href="#Page_234">234</a><br/> + +Gastræa, the, <a href="#Page_3">3,</a> <a href="#Page_20">20,</a> <a href="#Page_206">206</a><br/> +— formation of the, <a href="#Page_213">213</a><br/> + +Gastræa theory, the, <a href="#Page_20">20,</a> <a href="#Page_64">64,</a> <a href="#Page_69">69</a><br/> + +Gastræads, <a href="#Page_69">69,</a> <a href="#Page_214">214</a><br/> + +Gastremaria, <a href="#Page_214">214</a><br/> + +Gastrocystis, the, <a href="#Page_62">62,</a> <a href="#Page_119">119,</a> <a href="#Page_120">120</a><br/> + +<i>Gastrophysema,</i> <a href="#Page_215">215</a><br/> + +Gastrotricha, <a href="#Page_224">224</a><br/> + +Gastrula, the, <a href="#Page_3">3,</a> <a href="#Page_20">20,</a> <a href="#Page_62">62</a><br/> + +Gastrulation, <a href="#Page_62">62</a><br/> + +Gegenbaur, Carl, <a href="#Page_220">220</a><br/> +— on evolution, <a href="#Page_32">32</a><br/> +— on the skull, <a href="#Page_300">300–1</a><br/> + +Gemmation, <a href="#Page_331">331</a><br/> + +<i>General Morphology,</i> <a href="#Page_8">8,</a> <a href="#Page_29">29</a><br/> + +<i>Genesis,</i> <a href="#Page_23">23</a><br/> + +Genital pore, the, <a href="#Page_335">335</a><br/> + +Geological evolution, length of, <a href="#Page_200">200</a><br/> +— periods, <a href="#Page_201">201</a><br/> + +Geology, methods of, <a href="#Page_180">180</a><br/> +— rise of, <a href="#Page_24">24</a><br/> + +Germ-plasm, theory of, <a href="#Page_349">349</a><br/> + +Germinal disk, <a href="#Page_46">46,</a> <a href="#Page_81">81</a><br/> +— layers, the, <a href="#Page_14">14,</a> <a href="#Page_16">16</a><br/> +— — scheme of the, <a href="#Page_92">92</a><br/> +— spot, the, <a href="#Page_44">44</a><br/> +— vesicle, the, <a href="#Page_43">43,</a> <a href="#Page_54">54</a><br/> + +Germinative area, the, <a href="#Page_121">121</a><br/> + +Giant gorilla, the, <a href="#Page_176">176</a><br/> + +Gibbon, the, <a href="#Page_173">173,</a> <a href="#Page_262">262</a><br/> + +Gill-clefts and arches, <a href="#Page_110">110</a><br/> +— formation of the, <a href="#Page_151">151–52,</a> <a href="#Page_303">303</a><br/> + +Gill-crate, the, <a href="#Page_183">183,</a> <a href="#Page_189">189</a><br/> + +Gills, disappearance of the, <a href="#Page_244">244</a><br/> + +Glœocapsa, <a href="#Page_210">210</a><br/> + +Gnathostoma, <a href="#Page_230">230,</a> <a href="#Page_232">232</a><br/> + +Goethe as an evolutionist, <a href="#Page_27">27,</a> <a href="#Page_299">299</a><br/> + +Goitre, <a href="#Page_110">110</a><br/> + +Gonads, the, <a href="#Page_111">111</a><br/> +— formation of the, <a href="#Page_149">149–50</a><br/> + +Gonidia, <a href="#Page_334">334</a><br/> + +Gonochorism, beginning of, <a href="#Page_322">322</a><br/> + +Gonoducts, <a href="#Page_335">335</a><br/> + +Gonotomes, <a href="#Page_146">146,</a> <a href="#Page_149">149</a><br/> + +Goodsir, <a href="#Page_189">189</a><br/> + +Gorilla, the, <a href="#Page_174">174,</a> <a href="#Page_176">176,</a> <a href="#Page_262">262</a><br/> + +Graafian follicles, the, <a href="#Page_17">17,</a> <a href="#Page_119">119,</a> <a href="#Page_347">347</a><br/> + +Gregarinæ, <a href="#Page_211">211</a><br/> + +Gullet-ganglion, the, <a href="#Page_190">190</a><br/> + +Gut, evolution of the, <a href="#Page_310">310–17</a><br/> + +<i>Gyrini,</i> <a href="#Page_242">242</a><br/> + +Gynecomastism, <a href="#Page_114">114</a><br/> + +<br/><b>H</b><br/><br/> + +Hag-fish, the, <a href="#Page_188">188</a><br/> + +Hair, evolution of the, <a href="#Page_270">270</a><br/> +— on the human embryo and infant, <a href="#Page_271">271</a><br/> + +Hair, restriction of, by sexual selection, <a href="#Page_271">271</a><br/> + +<i>Haliphysema,</i> <a href="#Page_215">215</a><br/> + +Halisauria, <a href="#Page_202">202</a><br/> + +Haller, Albrecht, <a href="#Page_12">12</a><br/> + +<i>Halosphæra viridis,</i> <a href="#Page_213">213</a><br/> + +Hand, evolution of the, <a href="#Page_250">250,</a> <a href="#Page_304">304–6</a><br/> +— of the ape and man, <a href="#Page_258">258</a><br/> + +Hapalidæ, <a href="#Page_261">261</a><br/> + +Harderian gland, the, <a href="#Page_288">288</a><br/> + +Hare-lip, <a href="#Page_284">284</a><br/> + +Harrison, Granville, <a href="#Page_161">161</a><br/> + +Hartmann, <a href="#Page_262">262</a><br/> + +Harvey, <a href="#Page_10">10</a><br/> + +Hatschek, <a href="#Page_192">192</a><br/> + +Hatteria, <a href="#Page_243">243,</a> <a href="#Page_246">246</a><br/> + +Head-cavity, the, <a href="#Page_138">138</a><br/> + +Head-plates, the, <a href="#Page_149">149</a><br/> + +Heart, development of the, <a href="#Page_7">7,</a> <a href="#Page_10">10,</a> <a href="#Page_111">111,</a> <a href="#Page_151">151,</a> <a href="#Page_170">170,</a> <a href="#Page_322">322,</a> <a href="#Page_324">324–27</a><br/> +— of the ascidia, <a href="#Page_190">190</a><br/> +— position of the, <a href="#Page_327">327</a><br/> + +Helmholtz, <a href="#Page_207">207</a><br/> + +Helminthes, <a href="#Page_223">223</a><br/> + +Hepatic gut, the, <a href="#Page_109">109,</a> <a href="#Page_316">316</a><br/> + +Heredity, nature of, <a href="#Page_3">3,</a> <a href="#Page_5">5,</a> <a href="#Page_27">27,</a> <a href="#Page_56">56–7,</a> <a href="#Page_349">349</a><br/> + +Hermaphrodism, <a href="#Page_9">9,</a> <a href="#Page_23">23,</a> <a href="#Page_114">114,</a> <a href="#Page_218">218,</a> <a href="#Page_322">322,</a> <a href="#Page_346">346</a><br/> + +Hertwig, <a href="#Page_21">21</a><br/> + +Hesperopitheca, <a href="#Page_259">259</a><br/> + +His, W., <a href="#Page_19">19</a><br/> + +Histogeny, <a href="#Page_18">18,</a> <a href="#Page_19">19</a><br/> + +<i>History of Creation,</i> <a href="#Page_6">6,</a> <a href="#Page_30">30</a><br/> + +Holoblastic ova, <a href="#Page_67">67,</a> <a href="#Page_71">71,</a> <a href="#Page_77">77</a><br/> + +<i>Homœosaurus,</i> <a href="#Page_244">244,</a> <a href="#Page_246">246</a><br/> + +Homology of the germinal layers, <a href="#Page_20">20</a><br/> + +Hoof, evolution of the, <a href="#Page_270">270</a><br/> + +Hunterian ligament, the, <a href="#Page_344">344</a><br/> + +Huxleian law, the, <a href="#Page_171">171,</a> <a href="#Page_257">257,</a> <a href="#Page_262">262</a><br/> + +Huxley, T. H., <a href="#Page_7">7,</a> <a href="#Page_20">20,</a> <a href="#Page_29">29</a><br/> + +Hydra, the, <a href="#Page_69">69,</a> <a href="#Page_217">217</a><br/> + +Hydrostatic apparatus in the fish, <a href="#Page_315">315</a><br/> + +<i>Hylobates,</i> <a href="#Page_173">173,</a> <a href="#Page_262">262</a><br/> + +<i>Hylodes Martinicensis,</i> <a href="#Page_241">241</a><br/> + +Hyoid bone, the, <a href="#Page_299">299</a><br/> + +Hypermastism, <a href="#Page_113">113</a><br/> + +Hyperthelism, <a href="#Page_113">113</a><br/> + +Hypoblast, <a href="#Page_20">20,</a> <a href="#Page_64">64</a><br/> + +Hypobranchial groove, the, <a href="#Page_110">110,</a> <a href="#Page_184">184,</a> <a href="#Page_226">226,</a> <a href="#Page_316">316</a><br/> + +Hypodermis, the, <a href="#Page_268">268</a><br/> + +Hypopsodina, <a href="#Page_257">257</a><br/> + +Hyposoma, the, <a href="#Page_129">129</a><br/> + +Hyposomites, <a href="#Page_130">130,</a> <a href="#Page_194">194</a><br/> + +Hypospadia, <a href="#Page_346">346</a><br/> + +<br/><b>I</b><br/><br/> + +Ichthydina, <a href="#Page_224">224</a><br/> + +<i>Ichthyophis glutinosa,</i> <a href="#Page_80">80</a><br/> + +Ictopsida, <a href="#Page_257">257</a><br/> + +Ileum, the, <a href="#Page_310">310</a><br/> + +Immortality, Aristotle on, <a href="#Page_10">10</a><br/> + +Immortality of the soul, <a href="#Page_58">58</a><br/> + +Impregnation-rise, the, <a href="#Page_55">55</a><br/> + +Indecidua, <a href="#Page_255">255</a><br/> + +Indo-Germanic languages, <a href="#Page_203">203</a><br/> + +Induction and deduction, <a href="#Page_31">31,</a> <a href="#Page_208">208</a><br/> + +Inheritance of acquired characters, <a href="#Page_349">349</a><br/> + +Insects, intelligence of, <a href="#Page_353">353</a><br/> + +Interamniotic cavity, the, <a href="#Page_165">165</a><br/> + +Intestines, the, <a href="#Page_309">309,</a> <a href="#Page_316">316–17</a><br/> + +Invagination, <a href="#Page_62">62</a><br/> + +Iris, the, <a href="#Page_286">286</a><br/> + +<br/><b>J</b><br/><br/> + +<i>Jacchus,</i> <a href="#Page_261">261</a><br/> + +Java, ape-man of, <a href="#Page_263">263,</a> <a href="#Page_264">264</a><br/> + +Jaws, evolution of the, <a href="#Page_301">301</a><br/> + +Jurassic strata, <a href="#Page_202">202</a><br/> + +<br/><b>K</b><br/><br/> + +Kant, dualism of, <a href="#Page_25">25</a><br/> + +Kelvin, Lord, on the origin of life, <a href="#Page_207">207</a><br/> + +Kidneys, the, <a href="#Page_111">111</a><br/> +— formation of the, <a href="#Page_150">150–51,</a> <a href="#Page_336">336–42</a><br/> + +Klaatsch, <a href="#Page_262">262</a><br/> + +Kölliker, <a href="#Page_21">21</a><br/> + +Kowalevsky, <a href="#Page_191">191</a><br/> + +<br/><b>L</b><br/><br/> + +Labia, the, <a href="#Page_346">346</a><br/> + +Labyrinth, the, <a href="#Page_290">290</a><br/> + +Lachrymal glands, <a href="#Page_269">269</a><br/> + +Lamarck, J., <a href="#Page_23">23,</a> <a href="#Page_25">25–7</a><br/> +— theories of, <a href="#Page_26">26,</a> <a href="#Page_349">349</a><br/> + +Lamprey, the, <a href="#Page_230">230</a><br/> +— ova of the, <a href="#Page_75">75</a><br/> + +Lancelet, the, <a href="#Page_60">60,</a> <a href="#Page_181">181–95</a><br/> +— description of the, <a href="#Page_105">105</a><br/> + +Languages, evolution of, <a href="#Page_203">203</a><br/> + +Lanugo of the embryo, <a href="#Page_271">271</a><br/> + +Larynx, the, <a href="#Page_309">309</a><br/> +— evolution of the, <a href="#Page_314">314</a><br/> + +Latebra, the, <a href="#Page_45">45</a><br/> + +Lateral plates, the, <a href="#Page_129">129</a><br/> + +Laurentian strata, <a href="#Page_201">201</a><br/> + +Lecithoma, the, <a href="#Page_117">117</a><br/> + +Leg, evolution of the, <a href="#Page_304">304</a><br/> +— structure of the, <a href="#Page_306">306</a><br/> + +Lemuravida, <a href="#Page_257">257</a><br/> + +Lemurogona, <a href="#Page_257">257</a><br/> + +Lemurs, the, <a href="#Page_257">257</a><br/> + +<i>Lepidosiren,</i> <a href="#Page_257">257</a><br/> + +Leucocytes, <a href="#Page_319">319</a><br/> + +Life, age of, <a href="#Page_200">200</a><br/> + +Limbs, evolution of the, <a href="#Page_152">152,</a> <a href="#Page_239">239,</a> <a href="#Page_304">304</a><br/> + +Limiting furrow, the, <a href="#Page_133">133</a><br/> + +Linin, <a href="#Page_42">42</a><br/> + +Liver, the, <a href="#Page_309">309,</a> <a href="#Page_317">317</a><br/> + +Long-nosed ape, the, <a href="#Page_164">164</a><br/> + +Love, importance of in nature, <a href="#Page_332">332</a><br/> + +Lungs, the, <a href="#Page_110">110</a><br/> +— evolution of the, <a href="#Page_241">241,</a> <a href="#Page_314">314–15</a><br/> + +Lyell, Sir C., <a href="#Page_24">24</a><br/> + +Lymphatic vessels, the, <a href="#Page_318">318</a><br/> + +Lymph-cells, the, <a href="#Page_319">319</a><br/> + +<br/><b>M</b><br/><br/> + +Macrogonidion, <a href="#Page_331">331</a><br/> + +Macrospores, <a href="#Page_331">331</a><br/> + +<i>Magosphæra planula,</i> <a href="#Page_213">213</a><br/> + +Male womb, the, <a href="#Page_344">344,</a> <a href="#Page_350">350</a><br/> + +Mallochorion, the, <a href="#Page_166">166</a><br/> + +Mallotheria, <a href="#Page_257">257</a><br/> + +Malpighian capsules, <a href="#Page_339">339,</a> <a href="#Page_341">341</a><br/> + +Mammal, characters of the, <a href="#Page_112">112</a><br/> +— gastrulation of the, <a href="#Page_84">84</a><br/> + +Mammals, unity of the, <a href="#Page_247">247–48</a><br/> + +Mammary glands, the, <a href="#Page_113">113,</a> <a href="#Page_269">269</a><br/> + +Man and the ape, relation of, <a href="#Page_262">262,</a> <a href="#Page_351">351</a><br/> +— origin of, <a href="#Page_29">29</a><br/> + +<i>Man’s Place in Nature,</i> <a href="#Page_7">7,</a> <a href="#Page_29">29,</a> <a href="#Page_351">351</a><br/> + +Mantle, the, <a href="#Page_189">189</a><br/> + +Mantle-folds, the, <a href="#Page_185">185</a><br/> + +Marsupials, the, <a href="#Page_250">250–52</a><br/> +— ova of the, <a href="#Page_85">85</a><br/> + +Materialism, <a href="#Page_356">356</a><br/> + +Mathematical method, the, <a href="#Page_30">30</a><br/> + +Mechanical causes, <a href="#Page_6">6</a><br/> +— embryology, <a href="#Page_8">8,</a> <a href="#Page_19">19,</a> <a href="#Page_22">22</a><br/> + +Meckel’s cartilage, <a href="#Page_304">304</a><br/> + +<i>Medulla capitis,</i> the, <a href="#Page_273">273</a><br/> +— <i>oblongata,</i> the, <a href="#Page_274">274</a><br/> +— <i>spinalis,</i> the, <a href="#Page_273">273</a><br/> + +Medullary groove, the, <a href="#Page_125">125</a><br/> +— tube, the, <a href="#Page_107">107,</a> <a href="#Page_128">128</a><br/> +— — formation of the, <a href="#Page_131">131,</a> <a href="#Page_133">133,</a> <a href="#Page_227">227,</a> <a href="#Page_267">267,</a> <a href="#Page_276">276</a><br/> + +Mehnert, E., on the biogenetic law, <a href="#Page_5">5</a><br/> + +Meroblastic ova, <a href="#Page_67">67,</a> <a href="#Page_71">71,</a> <a href="#Page_78">78</a><br/> + +Merocytes, <a href="#Page_68">68,</a> <a href="#Page_321">321</a><br/> + +Mesentery, the, <a href="#Page_98">98,</a> <a href="#Page_109">109,</a> <a href="#Page_310">310,</a> <a href="#Page_316">316</a><br/> + +Mesocardium, the, <a href="#Page_327">327</a><br/> + +Mesoderm, the, <a href="#Page_20">20,</a> <a href="#Page_64">64,</a> <a href="#Page_90">90,</a> <a href="#Page_93">93</a><br/> + +Mesogastria, <a href="#Page_215">215</a><br/> + +Mesonephridia, the, <a href="#Page_338">338</a><br/> + +Mesonephros, the, <a href="#Page_336">336</a><br/> + +Mesorchium, the, <a href="#Page_344">344</a><br/> + +Mesovarium, the, <a href="#Page_344">344</a><br/> + +Mesozoic period, the, <a href="#Page_202">202</a><br/> + +Metogaster, the, <a href="#Page_64">64</a><br/> + +Metagastrula, the, <a href="#Page_67">67</a><br/> + +Metamerism, <a href="#Page_142">142</a><br/> + +Metanephridia, the, <a href="#Page_341">341</a><br/> + +Metanephros, the, <a href="#Page_336">336</a><br/> + +Metaplasm, <a href="#Page_39">39</a><br/> + +Metastoma, <a href="#Page_64">64,</a> <a href="#Page_222">222</a><br/> + +Metatheria, <a href="#Page_248">248</a><br/> + +Metazoa, <a href="#Page_20">20,</a> <a href="#Page_62">62</a><br/> + +Metovum, the, <a href="#Page_81">81</a><br/> + +Microgonidian, <a href="#Page_331">331</a><br/> + +Microspores, <a href="#Page_331">331</a><br/> + +Middle ear, the, <a href="#Page_291">291</a><br/> + +Migration, effect of, <a href="#Page_33">33</a><br/> + +Milk, secretion of the, <a href="#Page_269">269</a><br/> + +Mind, evolution of, <a href="#Page_353">353–54</a><br/> +— in the lower animals, <a href="#Page_353">353</a><br/> + +Miocene strata, <a href="#Page_203">203</a><br/> + +Mitosis, <a href="#Page_40">40,</a> <a href="#Page_41">41</a><br/> + +Monera, <a href="#Page_40">40,</a> <a href="#Page_206">206,</a> <a href="#Page_209">209</a><br/> + +Monism, <a href="#Page_6">6,</a> <a href="#Page_356">356</a><br/> + +Monodelphia, <a href="#Page_248">248</a><br/> + +Monogonopora, <a href="#Page_223">223</a><br/> + +Monopneumones, <a href="#Page_238">238</a><br/> + +Monotremes, <a href="#Page_118">118,</a> <a href="#Page_249">249</a><br/> +— ova of the, <a href="#Page_84">84</a><br/> + +<i>Monoxenia Darwinii,</i> <a href="#Page_60">60</a><br/> + +Morea, the, <a href="#Page_212">212</a><br/> + +Morphology, <a href="#Page_2">2,</a> <a href="#Page_27">27</a><br/> + +Morula, the, <a href="#Page_62">62,</a> <a href="#Page_212">212</a><br/> + +Motor-germinative layer, the, <a href="#Page_19">19</a><br/> + +Mouth, development of the, <a href="#Page_124">124,</a> <a href="#Page_139">139</a><br/> +— structure of the, <a href="#Page_308">308</a><br/> + +Mucous layer, the, <a href="#Page_16">16</a><br/> + +Müllerian duct, the, <a href="#Page_341">341</a><br/> + +Muscle-layer, the, <a href="#Page_16">16</a><br/> + +Muscles, evolution of the, <a href="#Page_307">307</a><br/> +— of the ear, rudimentary, <a href="#Page_292">292</a><br/> + +Myotomes, <a href="#Page_108">108,</a> <a href="#Page_146">146</a><br/> + +Myxinoides, the, <a href="#Page_188">188,</a> <a href="#Page_230">230</a><br/> + +<br/><b>N</b><br/><br/> + +Nails, evolution of the, <a href="#Page_270">270</a><br/> + +Nasal pits, <a href="#Page_284">284</a><br/> + +Natural philosophy, <a href="#Page_25">25</a><br/> +— selection, <a href="#Page_26">26,</a> <a href="#Page_28">28,</a> <a href="#Page_349">349</a><br/> + +Navel, the, <a href="#Page_117">117,</a> <a href="#Page_134">134</a><br/> + +Necrolemurs, <a href="#Page_257">257</a><br/> + +Nectocystis, the, <a href="#Page_314">314</a><br/> + +Nemertina, <a href="#Page_224">224–26</a><br/> + +Nephroduct, evolution of the, <a href="#Page_338">338–39</a><br/> + +Nephrotomes, <a href="#Page_149">149,</a> <a href="#Page_338">338</a><br/> + +Nerve-cell, the, <a href="#Page_39">39</a><br/> + +Nerves, animals without, <a href="#Page_267">267</a><br/> + +Nervous system, evolution of the, <a href="#Page_7">7,</a> <a href="#Page_267">267</a><br/> + +Neurenteric canal, the, <a href="#Page_127">127</a><br/> + +Nictitating membrane, the, <a href="#Page_32">32,</a> <a href="#Page_286">286,</a> <a href="#Page_288">288</a><br/> + +Nose, the, in man and the ape, <a href="#Page_164">164</a><br/> +— development of the, <a href="#Page_282">282–85</a><br/> +— structure of the, <a href="#Page_283">283</a><br/> + +Notochorda, the, <a href="#Page_107">107</a><br/> + +Nuclein, <a href="#Page_37">37</a><br/> + +Nucleolinus, <a href="#Page_44">44</a><br/> + +Nucleolus, the, <a href="#Page_38">38,</a> <a href="#Page_44">44,</a> <a href="#Page_54">54</a><br/> + +Nucleus of the cell, <a href="#Page_37">37</a><br/> + +<br/><b>O</b><br/><br/> + +Œsophagus, the, <a href="#Page_309">309,</a> <a href="#Page_316">316</a><br/> + +Oken, <a href="#Page_5">5,</a> <a href="#Page_27">27,</a> <a href="#Page_300">300</a><br/> + +Oken’s bodies, <a href="#Page_339">339</a><br/> + +Oligocene strata, <a href="#Page_203">203</a><br/> + +<i>Olynthus,</i> <a href="#Page_217">217</a><br/> + +On the generation of animals, <a href="#Page_9">9</a><br/> + +Ontogeny, <a href="#Page_2">2,</a> <a href="#Page_23">23</a><br/> +— defective evidence of, <a href="#Page_208">208</a><br/> + +Opaque area, the, <a href="#Page_122">122</a><br/> + +Opossum, the, <a href="#Page_252">252</a><br/> +— ova of the, <a href="#Page_85">85</a><br/> + +Optic nerve, the, <a href="#Page_287">287</a><br/> + +Optic thalami, <a href="#Page_274">274</a><br/> +— vesicles, <a href="#Page_286">286</a><br/> + +Orang, the, <a href="#Page_174">174,</a> <a href="#Page_262">262</a><br/> + +Ornithodelphia, <a href="#Page_248">248</a><br/> + +<i>Ornithorhyncus,</i> <a href="#Page_85">85,</a> <a href="#Page_249">249</a><br/> + +Ornithostoma, <a href="#Page_249">249</a><br/> + +Ossicles of the ear, <a href="#Page_289">289</a><br/> + +Otoliths, <a href="#Page_289">289</a><br/> + +Ova, number of, <a href="#Page_347">347</a><br/> +— of the lancelet, <a href="#Page_192">192</a><br/> + +Ovaries, evolution of the, <a href="#Page_333">333–34</a><br/> + +Oviduct, origin of the, <a href="#Page_335">335,</a> <a href="#Page_342">342</a><br/> + +Ovolemma, the, <a href="#Page_44">44</a><br/> + +Ovulists, <a href="#Page_12">12</a><br/> + +Ovum, discovery of the, <a href="#Page_16">16</a><br/> +— nature of the, <a href="#Page_40">40<br/> +— size of the, <a href="#Page_44">44</a><br/> + +<br/><b>P</b><br/><br/> + +Pachylemurs, the, <a href="#Page_257">257</a><br/> + +Pacinian corpuscles, <a href="#Page_282">282</a><br/> + +Paleontology, <a href="#Page_2">2</a><br/> +— evolutionary evidence of, <a href="#Page_31">31</a><br/> +— incompleteness of, <a href="#Page_208">208</a><br/> +— rise of, <a href="#Page_24">24</a><br/> + +Paleozoic age, the, <a href="#Page_202">202</a><br/> + +Palingenesis, <a href="#Page_4">4</a><br/> + +Palingenetic structures, <a href="#Page_4">4</a><br/> + +<i>Palæhatteria,</i> <a href="#Page_244">244,</a> <a href="#Page_246">246</a><br/> + +<i>Panniculus carnosus,</i> the, <a href="#Page_350">350</a><br/> + +Paradidymis, the, <a href="#Page_342">342</a><br/> + +Parietal zone, the, <a href="#Page_129">129</a><br/> + +Parthenogenesis, <a href="#Page_9">9,</a> <a href="#Page_13">13</a><br/> + +Pastrana, Miss Julia, <a href="#Page_164">164</a><br/> + +Pedimana, <a href="#Page_252">252</a><br/> + +Pellucid area, the, <a href="#Page_122">122</a><br/> + +Pelvic cavity, the, <a href="#Page_138">138</a><br/> + +<i>Pemmatodiscus gastrulaceus,</i> <a href="#Page_215">215</a><br/> + +Penis-bone, the, <a href="#Page_346">346</a><br/> + +Penis, varieties of the, <a href="#Page_345">345</a><br/> + +Peramelida, <a href="#Page_254">254</a><br/> + +Periblastic ova, <a href="#Page_68">68</a><br/> + +Peribranchial cavity, the, <a href="#Page_185">185,</a> <a href="#Page_190">190</a><br/> + +Pericardial cavity, the, <a href="#Page_328">328</a><br/> + +Perichorda, the, <a href="#Page_108">108,</a> <a href="#Page_183">183</a><br/> +— formation of the, <a href="#Page_136">136</a><br/> + +Perigastrula, <a href="#Page_89">89</a><br/> + +Permian strata, <a href="#Page_202">202</a><br/> + +Petromyzontes, the, <a href="#Page_188">188,</a> <a href="#Page_230">230</a><br/> + +Phagocytes, <a href="#Page_49">49,</a> <a href="#Page_320">320</a><br/> + +Pharyngeal ganglion, the, <a href="#Page_275">275</a><br/> + +Pharynx, the, <a href="#Page_309">309</a><br/> + +Philology, comparison with, <a href="#Page_203">203</a><br/> + +<i>Philosophie Zoologique,</i> <a href="#Page_25">25</a><br/> + +Philosophy and evolution, <a href="#Page_6">6</a><br/> + +Phycochromacea, <a href="#Page_209">209</a><br/> + +Phylogeny, <a href="#Page_2">2,</a> <a href="#Page_23">23</a><br/> + +Physemaria, <a href="#Page_214">214</a><br/> + +Physiology, backwardness of, <a href="#Page_7">7</a><br/> + +Phytomonera, <a href="#Page_209">209</a><br/> + +Pineal eye, the, <a href="#Page_108">108</a><br/> + +Pinna, the, <a href="#Page_291">291</a><br/> + +<i>Pithecanthropus,</i> <a href="#Page_263">263,</a> <a href="#Page_264">264</a><br/> + +Pithecometra-principle, the, <a href="#Page_171">171</a><br/> + +Placenta, the, <a href="#Page_166">166,</a> <a href="#Page_253">253–54</a><br/> + +Placentals, the, <a href="#Page_166">166</a><br/> +— characters of the, <a href="#Page_253">253</a><br/> +— gastrulation of the, <a href="#Page_86">86</a><br/> + +Planocytes, <a href="#Page_49">49,</a> <a href="#Page_320">320</a><br/> + +Plant-louse, parthenogenesis of the, <a href="#Page_13">13</a><br/> + +Planula, the, <a href="#Page_89">89</a><br/> + +Plasma-products, <a href="#Page_38">38,</a> <a href="#Page_39">39</a><br/> + +Plasson, <a href="#Page_40">40,</a> <a href="#Page_59">59</a><br/> + +Plastids, <a href="#Page_36">36,</a> <a href="#Page_40">40,</a> <a href="#Page_209">209</a><br/> + +Plastidules, <a href="#Page_59">59</a><br/> + +Platodaria, <a href="#Page_221">221</a><br/> + +Platodes, the, <a href="#Page_221">221</a><br/> + +Platyrrhinæ, <a href="#Page_261">261</a><br/> + +Pleuracanthida, <a href="#Page_234">234</a><br/> + +Pleural ducts, <a href="#Page_328">328</a><br/> + +Pliocene strata, <a href="#Page_203">203</a><br/> + +Polar cells, <a href="#Page_54">54</a><br/> + +Polyspermism, <a href="#Page_58">58</a><br/> + +Preformation theory, the, <a href="#Page_11">11</a><br/> + +Primary period, the, <a href="#Page_202">202</a><br/> + +Primates, the, <a href="#Page_157">157,</a> <a href="#Page_257">257–60</a><br/> + +<i>Primatoid,</i> <a href="#Page_263">263</a><br/> + +Primitive groove, the, <a href="#Page_69">69,</a> <a href="#Page_82">82,</a> <a href="#Page_124">124,</a> <a href="#Page_125">125</a><br/> +— gut, the, <a href="#Page_20">20,</a> <a href="#Page_63">63,</a> <a href="#Page_214">214</a><br/> +— kidneys, the, <a href="#Page_111">111,</a> <a href="#Page_337">337</a><br/> +— mouth, the, <a href="#Page_20">20,</a> <a href="#Page_63">63</a><br/> +— segments, <a href="#Page_143">143</a><br/> +— streak, the, <a href="#Page_100">100,</a> <a href="#Page_122">122</a><br/> +— vertebræ, <a href="#Page_144">144,</a> <a href="#Page_195">195,</a> <a href="#Page_206">206,</a> <a href="#Page_229">229</a><br/> + +Primordial period, the, <a href="#Page_201">201</a><br/> + +Prochordata, <a href="#Page_192">192</a><br/> + +Prochordonia, the, <a href="#Page_192">192,</a> <a href="#Page_218">218</a><br/> + +Prochoriata, <a href="#Page_253">253</a><br/> + +Prochorion, the, <a href="#Page_44">44,</a> <a href="#Page_119">119</a><br/> + +<i>Proctodæum,</i> the, <a href="#Page_345">345</a><br/> + +<i>Procytella primordialis,</i> <a href="#Page_210">210</a><br/> + +Prodidelphia, <a href="#Page_256">256</a><br/> + +Progaster, the, <a href="#Page_20">20,</a> <a href="#Page_63">63</a><br/> + +Progonidia, <a href="#Page_333">333</a><br/> + +Promammalia, <a href="#Page_247">247</a><br/> + +Pronephridia, the, <a href="#Page_151">151</a><br/> + +Pronucleus femininus, <a href="#Page_54">54</a><br/> +— masculinus, <a href="#Page_54">54</a><br/> + +Properistoma, <a href="#Page_69">69</a><br/> + +Prorenal canals of the lancelet, <a href="#Page_186">186</a><br/> +— duct, the, <a href="#Page_132">132,</a> <a href="#Page_139">139,</a> <a href="#Page_186">186</a><br/> +— — evolution of the, <a href="#Page_338">338</a><br/> + +Proselachii, <a href="#Page_234">234</a><br/> + +Prosimiæ, the, <a href="#Page_257">257</a><br/> + +Prospermaria, <a href="#Page_333">333</a><br/> + +<i>Prospondylus,</i> <a href="#Page_105">105,</a> <a href="#Page_229">229</a><br/> + +Prostoma, <a href="#Page_20">20,</a> <a href="#Page_63">63,</a> <a href="#Page_222">222</a><br/> + +Protamniotes, <a href="#Page_243">243–44</a><br/> + +Protamœba, <a href="#Page_210">210</a><br/> + +Proterosaurus, the, <a href="#Page_202">202,</a> <a href="#Page_244">244</a><br/> + +Protists, <a href="#Page_36">36,</a> <a href="#Page_38">38</a><br/> + +Protonephros, <a href="#Page_111">111,</a> <a href="#Page_336">336</a><br/> + +Protophyta, <a href="#Page_210">210</a><br/> + +Protoplasm, <a href="#Page_37">37,</a> <a href="#Page_209">209</a><br/> + +<i>Protopterus,</i> <a href="#Page_238">238</a><br/> + +Prototheria, <a href="#Page_248">248</a><br/> + +Protovertebræ, <a href="#Page_142">142,</a> <a href="#Page_144">144</a><br/> + +Protozoa, <a href="#Page_20">20,</a> <a href="#Page_210">210</a><br/> + +Provertebral cavity, the, <a href="#Page_148">148</a><br/> +— plates, the, <a href="#Page_136">136,</a> <a href="#Page_144">144</a><br/> + +Pseudocœla, <a href="#Page_93">93,</a> <a href="#Page_221">221</a><br/> + +Pseudopodia, <a href="#Page_48">48</a><br/> + +Pseudova, <a href="#Page_13">13</a><br/> + +Psychic life, evolution of the, <a href="#Page_8">8</a><br/> + +Psychology, <a href="#Page_8">8</a><br/> + +Pterosauria, <a href="#Page_202">202</a><br/> + +Pylorus, the, <a href="#Page_309">309</a><br/> + +<br/><b>Q</b><br/><br/> + +Quadratum, the, <a href="#Page_247">247</a><br/> + +Quadrumana, <a href="#Page_258">258</a><br/> + +Quaternary period, <a href="#Page_203">203</a><br/> + +<br/><b>R</b><br/><br/> + +Rabbit, ova of the, <a href="#Page_86">86–7</a><br/> + +Radiates, the, <a href="#Page_103">103</a><br/> + +Rathke’s canals, <a href="#Page_341">341</a><br/> + +Rectum, the, <a href="#Page_317">317</a><br/> + +Regner de Graaf, <a href="#Page_119">119</a><br/> + +Renal system, evolution of the, <a href="#Page_335">335–42</a><br/> + +Reproduction, nature of, <a href="#Page_330">330–31</a><br/> + +Reptiles, <a href="#Page_245">245–47</a><br/> + +Respiratory organs, evolution of the, <a href="#Page_314">314–15</a><br/> +— pore, the, <a href="#Page_183">183,</a> <a href="#Page_189">189</a><br/> + +Retina, the, <a href="#Page_286">286</a><br/> + +Rhabdocœla, <a href="#Page_222">222</a><br/> + +Rhodocytes, <a href="#Page_319">319</a><br/> + +<i>Rhopalura,</i> <a href="#Page_215">215</a><br/> + +Rhyncocephala, <a href="#Page_243">243</a><br/> + +Ribs, the, <a href="#Page_295">295</a><br/> +— number of the, <a href="#Page_353">353</a><br/> + +Rudimentary ear-muscles, <a href="#Page_292">292</a><br/> +— organs, <a href="#Page_32">32</a><br/> +— — list of, <a href="#Page_349">349–50</a><br/> +— toes, <a href="#Page_306">306</a><br/> + +<br/><b>S</b><br/><br/> + +Sacculus, the, <a href="#Page_289">289</a><br/> + +<i>Sagitta,</i> <a href="#Page_65">65,</a> <a href="#Page_66">66,</a> <a href="#Page_191">191</a><br/> +— cœlomation of, <a href="#Page_93">93</a><br/> + +Salamander, the, <a href="#Page_241">241</a><br/> +— ova of the, <a href="#Page_74">74</a><br/> + +Sandal-shape of embryo, <a href="#Page_128">128–29</a><br/> + +<i>Satyrus,</i> <a href="#Page_174">174,</a> <a href="#Page_262">262</a><br/> + +Sauromammalia, <a href="#Page_246">246</a><br/> + +Sauropsida, <a href="#Page_245">245</a><br/> + +Scatulation theory, the, <a href="#Page_12">12</a><br/> + +Schizomycetes, <a href="#Page_210">210</a><br/> + +Schleiden, M., <a href="#Page_18">18,</a> <a href="#Page_36">36</a><br/> + +Schwann, T., <a href="#Page_18">18,</a> <a href="#Page_36">36</a><br/> + +Sclerotic coat, the, <a href="#Page_286">286</a><br/> + +Sclerotomes, <a href="#Page_108">108,</a> <a href="#Page_143">143,</a> <a href="#Page_148">148</a><br/> + +Scrotum, the, <a href="#Page_344">344</a><br/> + +<i>Scyllium,</i> nose of the, <a href="#Page_283">283</a><br/> + +Sea-squirt, the, <a href="#Page_181">181,</a> <a href="#Page_188">188–90</a><br/> + +Secondary period, the, <a href="#Page_202">202</a><br/> + +Segmentation, <a href="#Page_60">60,</a> <a href="#Page_141">141–42</a><br/> + +Segmentation-cells, <a href="#Page_54">54</a><br/> + +Segmentation-sphere, the, <a href="#Page_17">17</a><br/> + +Selachii, <a href="#Page_223">223</a><br/> +— skull of the, <a href="#Page_301">301</a><br/> + +Selection, theory of, <a href="#Page_28">28</a><br/> + +Selenka, <a href="#Page_166">166,</a> <a href="#Page_168">168</a><br/> + +Semnopitheci, <a href="#Page_262">262</a><br/> + +Sense-organs, evolution of the, <a href="#Page_151">151,</a> <a href="#Page_280">280</a><br/> +— number of the, <a href="#Page_281">281</a><br/> +— origin of the, <a href="#Page_281">281</a><br/> + +Sensory nerves, <a href="#Page_279">279</a><br/> + +Serocœlom, the, <a href="#Page_165">165</a><br/> + +Serous layer, the, <a href="#Page_16">16</a><br/> + +Sex-organs, early vertebrate form of the, <a href="#Page_111">111</a><br/> +— evolution of the, <a href="#Page_333">333</a><br/> + +Sexual reproduction, simplest forms of, <a href="#Page_331">331</a><br/> +— selection, <a href="#Page_30">30,</a> <a href="#Page_271">271–72</a><br/> + +Shark, the, <a href="#Page_233">233</a><br/> +— nose of the, <a href="#Page_283">283</a><br/> +— ova of the, <a href="#Page_75">75</a><br/> +— placenta of the, <a href="#Page_9">9</a><br/> +— skull of the, <a href="#Page_301">301</a><br/> + +Shoulder-blade, the, <a href="#Page_306">306</a><br/> + +Sickle-groove, the, <a href="#Page_82">82,</a> <a href="#Page_121">121</a><br/> + +Sieve-membrane, the, <a href="#Page_167">167</a><br/> + +Silurian strata, <a href="#Page_202">202</a><br/> + +Simiæ, the, <a href="#Page_257">257–60</a><br/> + +Siphonophoræ, embryology of the, <a href="#Page_21">21</a><br/> + +Skeleton, structure of the, <a href="#Page_294">294</a><br/> + +Skeleton-plate, the, <a href="#Page_148">148</a><br/> + +Skin, the, <a href="#Page_151">151</a><br/> +— evolution of, <a href="#Page_266">266–69</a><br/> +— function of the, <a href="#Page_269">269</a><br/> + +Skin-layer, the, <a href="#Page_16">16</a><br/> + +Skull, evolution of the, <a href="#Page_149">149,</a> <a href="#Page_299">299–303</a><br/> +— structure of the, <a href="#Page_299">299</a><br/> +— vertebral theory of the, <a href="#Page_300">300</a><br/> + +Smell, the sense of, <a href="#Page_282">282</a><br/> + +Soul, evolution of the, <a href="#Page_353">353–56</a><br/> +— nature of the, <a href="#Page_58">58,</a> <a href="#Page_356">356</a><br/> +— phylogeny of the, <a href="#Page_8">8</a><br/> +— seat of the, <a href="#Page_278">278</a><br/> + +Sound, sensations of, <a href="#Page_289">289–90</a><br/> + +Sozobranchia, <a href="#Page_242">242</a><br/> + +Space, sense of, <a href="#Page_291">291</a><br/> + +Species, nature of the, <a href="#Page_23">23,</a> <a href="#Page_34">34</a><br/> + +Speech, evolution of, <a href="#Page_264">264</a><br/> + +Spermaducts, <a href="#Page_335">335,</a> <a href="#Page_342">342</a><br/> + +Spermaries, evolution of the, <a href="#Page_333">333–34</a><br/> + +Spermatozoon, the, <a href="#Page_52">52–3</a><br/> +— discovery of the, <a href="#Page_12">12,</a> <a href="#Page_53">53</a><br/> + +Spinal cord, development of the, <a href="#Page_8">8</a><br/> +— structure of the, <a href="#Page_273">273</a><br/> + +Spirema, the, <a href="#Page_42">42</a><br/> + +Spiritualism, <a href="#Page_356">356</a><br/> + +Spleen, the, <a href="#Page_318">318</a><br/> + +Spondyli, <a href="#Page_142">142</a><br/> + +Sponges, classification of the, <a href="#Page_34">34</a><br/> +— ova of the, <a href="#Page_49">49</a><br/> + +Spontaneous generation, <a href="#Page_26">26,</a> <a href="#Page_206">206</a><br/> + +Stegocephala, <a href="#Page_239">239</a><br/> + +Stem-cell, the, <a href="#Page_54">54</a><br/> + +Stem-zone, the, <a href="#Page_129">129</a><br/> + +Stomach, evolution of the, <a href="#Page_311">311–14,</a> <a href="#Page_316">316</a><br/> +— structure of the human, <a href="#Page_309">309</a><br/> + +Strata, thickness of, <a href="#Page_200">200–201</a><br/> + +Struggle for life, the, <a href="#Page_28">28</a><br/> + +Subcutis, the, <a href="#Page_268">268</a><br/> + +Sweat glands, <a href="#Page_269">269</a><br/> + +<br/><b>T</b><br/><br/> + +Tactile corpuscles, <a href="#Page_268">268,</a> <a href="#Page_282">282</a><br/> + +Tadpole, the, <a href="#Page_242">242</a><br/> + +Tail, evolution of the, <a href="#Page_242">242–43</a><br/> +— rudimentary, in man, <a href="#Page_159">159,</a> <a href="#Page_295">295,</a> <a href="#Page_350">350</a><br/> + +Tailed men, <a href="#Page_160">160–61</a><br/> + +Taste, the sense of, <a href="#Page_282">282</a><br/> + +Teeth, evolution of the, <a href="#Page_314">314</a><br/> +— of the ape and man, <a href="#Page_259">259</a><br/> + +Teleostei, <a href="#Page_234">234</a><br/> + +Telolecithal ova, <a href="#Page_67">67,</a> <a href="#Page_68">68</a><br/> + +Temperature, sense of, <a href="#Page_282">282</a><br/> + +Terrestrial life, beginning of, <a href="#Page_235">235</a><br/> + +Tertiary period, the, <a href="#Page_203">203</a><br/> + +<i>Theoria generationis,</i> the, <a href="#Page_13">13</a><br/> + +Theories, value of, <a href="#Page_181">181</a><br/> + +Theromorpha, <a href="#Page_246">246</a><br/> + +Third eyelid, the, <a href="#Page_286">286,</a> <a href="#Page_288">288</a><br/> + +Thyroid gland, the, <a href="#Page_110">110,</a> <a href="#Page_184">184,</a> <a href="#Page_315">315</a><br/> + +Time-variations in ontogeny, <a href="#Page_5">5</a><br/> + +Tissues, primary and secondary, <a href="#Page_37">37</a><br/> + +Toad, the, <a href="#Page_241">241</a><br/> + +Tocosauria, <a href="#Page_246">246</a><br/> + +Toes, number of the, <a href="#Page_240">240</a><br/> + +<i>Tori genitales,</i> the, <a href="#Page_346">346</a><br/> + +Touch, the sense of, <a href="#Page_282">282</a><br/> + +Tracheata, <a href="#Page_142">142,</a> <a href="#Page_219">219</a><br/> + +Tread, the, <a href="#Page_45">45,</a> <a href="#Page_81">81</a><br/> + +Tree-frog, the, <a href="#Page_241">241</a><br/> + +Triassic strata, <a href="#Page_202">202</a><br/> + +<i>Triton tæniatus,</i> <a href="#Page_74">74</a><br/> + +Troglodytes, <a href="#Page_174">174</a><br/> + +Tunicates, the, <a href="#Page_189">189</a><br/> + +Turbellaria, <a href="#Page_222">222</a><br/> + +Turbinated bones, the, <a href="#Page_283">283</a><br/> + +Tympanic cavity, the, <a href="#Page_288">288</a><br/> + +<br/><b>U</b><br/><br/> + +Umbilical, cord, the, <a href="#Page_117">117</a><br/> +— vesicle, the, <a href="#Page_138">138</a><br/> + +Unicellular ancestor of all animals, <a href="#Page_47">47</a><br/> +— animals, <a href="#Page_38">38,</a> <a href="#Page_47">47</a><br/> + +Urachus, the, <a href="#Page_317">317,</a> <a href="#Page_341">341</a><br/> + +Urinary system, evolution of the, <a href="#Page_335">335–42</a><br/> + +Urogenital ducts, <a href="#Page_335">335</a><br/> + +<i>Uterus masculinus,</i> the, <a href="#Page_344">344,</a> <a href="#Page_350">350</a><br/> + +Utriculus, the, <a href="#Page_289">289</a><br/> + +<br/><b>V</b><br/><br/> + +<i>Vasa deferentia,</i> <a href="#Page_335">335</a><br/> + +Vascular layer, the, <a href="#Page_16">16,</a> <a href="#Page_168">168</a><br/> +— system, evolution of the, <a href="#Page_321">321–25</a><br/> +— — structure of the, <a href="#Page_318">318</a><br/> + +Vegetative layer, the, <a href="#Page_16">16</a><br/> + +Veins, evolution of the, <a href="#Page_323">323–24</a><br/> + +Ventral pedicle, the, <a href="#Page_166">166</a><br/> + +Ventricles of the heart, <a href="#Page_325">325</a><br/> + +Vermalia, <a href="#Page_220">220,</a> <a href="#Page_223">223</a><br/> + +Vermiform appendage, the, <a href="#Page_32">32,</a> <a href="#Page_310">310,</a> <a href="#Page_317">317</a><br/> + +Vertebræ, <a href="#Page_142">142,</a> <a href="#Page_294">294</a><br/> + +Vertebræa, <a href="#Page_105">105</a><br/> + +Vertebral arch, the, <a href="#Page_148">148,</a> <a href="#Page_295">295</a><br/> +— column, the, <a href="#Page_144">144</a><br/> +— — evolution of the, <a href="#Page_296">296</a><br/> +— — structure of the, <a href="#Page_294">294</a><br/> + +Vertebrates, character of the, <a href="#Page_104">104–10</a><br/> +— descent of the, <a href="#Page_219">219–20</a><br/> + +Vertebration, <a href="#Page_142">142</a><br/> + +Vesico-umbilical ligament, the, <a href="#Page_341">341</a><br/> + +<i>Vesicula prostatica,</i> the, <a href="#Page_344">344,</a> <a href="#Page_350">350</a><br/> + +Villi of the chorion, <a href="#Page_165">165</a><br/> + +Virchow, R., <a href="#Page_35">35</a><br/> +— on the ape-man, <a href="#Page_303">303</a><br/> +— on the evolution of man, <a href="#Page_264">264</a><br/> + +Virgin-birth, <a href="#Page_9">9,</a> <a href="#Page_13">13</a><br/> + +Vitalism, <a href="#Page_6">6</a><br/> + +Vitelline duct, the, <a href="#Page_138">138</a><br/> + +Volvocina, <a href="#Page_213">213</a><br/> + +<br/><b>W</b><br/><br/> + +Wallace, A. R., <a href="#Page_29">29</a><br/> + +Water, organic importance of, <a href="#Page_200">200</a><br/> + +Water vessels, <a href="#Page_336">336</a><br/> + +Weismann’s theories, <a href="#Page_349">349</a><br/> + +Wolff, C. F., <a href="#Page_13">13</a><br/> + +Wolffian bodies, <a href="#Page_339">339</a><br/> + +Wolffian duct, the, <a href="#Page_341">341</a><br/> + +Womb, evolution of the, <a href="#Page_342">342–43</a><br/> + +<br/><b>Y</b><br/><br/> + +Yelk, the, <a href="#Page_43">43,</a> <a href="#Page_45">45,</a> <a href="#Page_67">67</a><br/> + +Yelk-sac, the, <a href="#Page_117">117,</a> <a href="#Page_134">134</a><br/> + +<br/><b>Z</b><br/><br/> + +Zona pellucida, the, <a href="#Page_44">44</a><br/> + +Zonoplacenta, <a href="#Page_255">255</a><br/> + +Zoomonera, <a href="#Page_209">209</a><br/> + +Zoophytes, <a href="#Page_20">20,</a> <a href="#Page_64">64,</a> <a href="#Page_104">104</a><br/> +</p> + +<pre> + + + + + +End of the Project Gutenberg EBook of The Evolution of Man, by Ernst Haeckel + +*** END OF THIS PROJECT GUTENBERG EBOOK THE EVOLUTION OF MAN *** + +***** This file should be named 8700-h.htm or 8700-h.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/8/7/0/8700/ + +Produced by Produced by Sue Asscher and Derek Thompson + +Updated editions will replace the previous one--the old editions will +be renamed. + +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the United +States without permission and without paying copyright +royalties. 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