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diff --git a/8700-h/old/chap14.html b/8700-h/old/chap14.html new file mode 100644 index 0000000..2da8066 --- /dev/null +++ b/8700-h/old/chap14.html @@ -0,0 +1,1106 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Haeckel/The Evolution of Man --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Evolution of Man: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv="Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel="stylesheet" href="haeckel.css" type="text/css"> +</head> +<body> +<center>THE EVOLUTION OF MAN<br> +Volume I<br> +<br> +<hr noshade size="1" align="center" width="10%"> +<br> +C<font size="-2">HAPTER</font> XIV<br> +<br> +<b>THE ARTICULATION OF THE BODY<sup>1</sup></b></center> + +<br> + + +<p class="one">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</p> + +<p class="fnote">1. The term articulation is used in this chapter +to denote both “segmentation” and +“articulation” in the ordinary +sense.—Translator.</p> + +<br> +<hr> +<p class="page"><a name="page 142">[ 142 ]</a></p> + +<p> </p> + +<p class="one">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 <a href="chap10.html#Fig. 83">(Figs. +83–86),</a> 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.</p> + +<br> +<hr> +<p class="page"><a name="page 143">[ 143 ]</a></p> + +<p> </p> + +<p class="one">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> + +<br> + + +<center> +<table class="capt" width="383" summary= +"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."> +<tr> +<td align="justify"> +<img src="images2/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."> +<br><a name="Fig. 153">Figs. +153–155</a>—<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.</td> +</tr> +</table> +</center> + +<br> +<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</p> + +<br> +<hr> +<p class="page"><a name="page 144">[ 144 ]</a></p> + +<p> </p> + +<p class="one">middle of the sole-shaped embryonic shield, several +pairs of dark square spots, symmetrically distributed on both sides +of the chorda <a href="chap13.html#Fig. 131">(Figs. +131–135).</a> Transverse sections <a href= +"chap10.html#Fig. 93">(Fig. 93 <i>uw</i>)</a> 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> + +<table class="capt" width="149" align="left" summary= +"Fig. 156. Embryo of the amphioxus, sixteen hours old, seen from the back."> +<tr> +<td><img src="images2/fig156.GIF" width="149" height="198" alt= +"Embryo of the amphioxus, sixteen hours old, seen from the back."> +<a name="Fig. 156">Fig. +156</a>—<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.</td> +</tr> +</table> + + +<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 <a +href="chap13.html#Fig. 131">(Fig. 132),</a> 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> + +<table class="capt" width="249" align="left" summary= +"Fig. 157. Embryo of the amphioxus, twenty hours old, with five somites."> +<tr> +<td><img src="images2/fig157.GIF" width="249" height="147" alt= +"Embryo of the amphioxus, twenty hours old, with five somites."> +<a name="Fig. 157">Fig. +157</a>—<b>Embryo of the amphioxus, twenty hours old, with +five somites.</b> (Right view; for left view see <a href= +"chap13.html#Fig. 124">Fig. 124.)</a> (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.</td> +</tr> +</table> + + +<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</p> + +<br> +<hr> +<p class="page"><a name="page 145">[ 145 ]</a></p> + +<p> </p> + +<p class="one">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> + +<br> + + +<center> +<table class="capt" width="387" summary= +"Figs. 158-160> Embryo of the amphioxus, twenty four hours old, with eight somites."> +<tr> +<td align="justify"> +<img src="images2/fig158.GIF" width="387" height="330" alt= +"Figs. 158-160. Embryo of the amphioxus, twenty four hours old, with eight somites."> +<br><a name="Fig. 158">Figs. +158–160</a>—<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.</td> +</tr> +</table> +</center> + +<br> + + +<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</p> + +<br> +<hr> +<p class="page"><a name="page 146">[ 146 ]</a></p> + +<p> </p> + +<p class="one">backwards, and new cells are formed constantly (at +the primitive mouth) from the two primitive mesodermic cells (Figs. +159–160).</p> + +<br> + + +<center> +<table class="capt" width="395" summary= +"Figs. 161 and 162. Transverse section of shark-embryos (through the region of the kidneys)."> +<tr> +<td align="justify"> +<img src="images2/fig161.GIF" width="395" height="284" alt= +"Figs. 161 amd 162. Transverse section of shark-embryos (through the region of the kidneys)."> +<br><a name="Fig. 161">Figs. 161 and +162</a>—<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).</td> +</tr> +</table> +</center> + +<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>; <a href="chap10.html#Fig. 81">Fig. 82 <i>d,</i></a> +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</p> + +<br> +<hr> +<p class="page"><a name="page 147">[ 147 ]</a></p> + +<p> </p> + +<p class="one">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> + +<table class="capt" width="160" align="left" summary= +"Fig. 163. Frontal (or horizontal-longitudinal) section of a triton-embryo with three pairs of primitive segments."> +<tr> +<td align="center"><img src="images2/fig163.GIF" width="160" height="121" alt= +"Frontal (or horizontal-longitudinal) section of a triton-embryo with three pairs of primitive segments."> +<a name="Fig. 163">Fig. +163</a>—<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.</td> +</tr> +</table> + +<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. <a href="chap10.html#Fig. 91">Fig. 91, <i>A, +B, C</i>).</a> 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> + +<br> + + +<center> +<table class="capt" width="323" summary= +"Fig. 164. The third cervical vertebra (human)> Fig. 165. The sixth dorsal vertebra (human). Fig. 166. The second lumbar vertebra (human)"> +<tr> +<td align="center"> +<img src="images2/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)."> +<br><a name="Fig. 164">Fig. 164</a>—<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).</td> +</tr> +</table> +</center> + +<br> + + +<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</p> + +<br> +<hr> +<p class="page"><a name="page 148">[ 148 ]</a></p> + +<p> </p> + +<p class="one">“protovertebræ” <a href= +"chap13.html#Fig. 143">(Fig. 143 <i>uwh</i>).</a> 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 <a href="#Fig. 156">(Figs. 156–160)</a> 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> + +<br> + + +<center> +<table class="capt" width="372" summary= +"Fig. 167. Head of a shark embryo."> +<tr> +<td align="center"> +<img src="images2/fig167.GIF" width="372" height="293" alt= +"Head of a shark embryo."> +<br><a name="Fig. 167">Fig. 167</a>—<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."</td> +</tr> +</table> +</center> + + + +<p>The innermost median part of the primitive segment plates, which +lies immediately on the chorda <a href="chap13.html#Fig. 145">(Fig. +145 <i>ch</i>)</a> 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</p> + +<br> +<hr> +<p class="page"><a name="page 149">[ 149 ]</a></p> + +<p> </p> + +<p class="one">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> + +<table class="capt" width="202" align="left" summary= +"Figs. 168 and 169. Head of a chick embryo, of the third day."> +<tr> +<td><img src="images2/fig168.GIF" width="202" height="135" alt= +"Figs. 168 and 169. Head of a chick embryo, of the third day."> +<a name="Fig. 168">Fig. 168 and +169</a>—<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>)</td> +</tr> +</table> + +<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> + + +<table class="capt" width="170" align="left" summary= +"Fig. 170. Head of a dog embryo, seen from the front."> +<tr> +<td><img src="images2/fig170.GIF" width="170" height="172" alt= +"Head of a dog embryo, seen from the front."> +<a name="Fig. 170">Fig. +170</a>—<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>)</td> +</tr> +</table> + +<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</p> + +<br> +<hr> +<p class="page"><a name="page 150">[ 150 ]</a></p> + +<p> </p> + +<p class="one">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> + +<br> + + +<center> +<table class="capt" width="450" summary= +"Fig. 171. Human embryo of the fourth week (twenty-six days old)."> +<tr> +<td align="justify"> +<img src="images2/fig171.GIF" width="450" height="385" alt= +"Human embryo of the fourth week (twenty-six days old)."> +<br><a name="Fig. 171">Fig. 171</a>—<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>).</td> +</tr> +</table> +</center> + +<br> + + +<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</p> + +<br> +<hr> +<p class="page"><a name="page 151">[ 151 ]</a></p> + +<p> </p> + +<p class="one">(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 <a href="chap11.html#page 106"> +(Fig. 102 <i>n</i>);</a> 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> + +<table class="capt" width="219" align="left" summary= +"Fig. 172. Transverse section of the shoulder and fore-limb (wing) of a chick-embryo of the fourth day."> +<tr> +<td><img src="images2/fig172.GIF" width="219" height="231" alt= +"Transverse section of the shoulder and fore-limb (wing) of a chick-embryo of the fourth day."> +<a name="Fig. 172">Fig. +172</a>—<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>)</td> +</tr> +</table> + +<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 <a href= +"#Fig. 167">(Figs. 167–170 <i>f</i>).</a> They belong to the +characteristic and inalienable organs of the amniote-embryo, and +are found always in the same</p> + +<br> +<hr> +<p class="page"><a name="page 152">[ 152 ]</a></p> + +<p> </p> + +<p class="one">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> + + +<table class="capt" width="255" align="left" summary= +"Fig. 173. Transverse section of the pelvic region and hind legs of a chick-embryo of the fourth day."> +<tr> +<td><img src="images2/fig173.GIF" width="255" height="267" alt= +"Transverse section of the pelvic region and hind legs of a chick-embryo of the fourth day."> +<a name="Fig. 173">Fig. +173</a>—<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>)</td> +</tr> +</table> + +<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</p> + +<br> +<hr> +<p class="page"><a name="page 153">[ 153 ]</a></p> + +<p> </p> + +<p class="one">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> + +<br> + + +<center> +<table class="capt" width="449" summary= +"Fig. 174. Development of the lizard's legs."> +<tr> +<td align="justify"> +<img src="images2/fig174.GIF" width="449" height="401" alt= +"Development of the lizard's legs."> +<br><br><a name="Fig. 174">Fig. +174</a>—<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>)"</td> +</tr> +</table> +</center> + +<br> + + +<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.</p> + +<br> +<hr> +<p class="page"><a name="page 154">[ 154 ]</a></p> + +<p> </p> + +<p class="one">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> + +<br> + + +<center> +<table class="capt" width="401" summary= +"Fig. 175. Human embryo, five weeks old, half an inch long, seen from the right."> +<tr> +<td align="justify"> +<img src="images2/fig175.GIF" width="401" height="436" alt= +"Human embryo, five weeks old, half an inch long, seen from the right."> +<br><a name="Fig. 175">Fig. 175</a>—<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.</td> +</tr> +</table> + +<br> + + +<p>How the five fingers or toes with their</p> + +<br> +<hr> +<p class="page"><a name="page 155">[ 155 ]</a></p> + +<p> </p> + +<p class="one">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> + +<br> + + +<center> +<table class="capt" width="466" summary= +"Figs. 176-178. Embryos of the bat (Vespertilio murinus) at three different stages."> +<tr> +<td align="justify"> +<img src="images2/fig176.GIF" width="466" height="210" alt= +"Figs. 176-178. Embryos of the bat (Vespertilio murinus) at three different stages."> +<br><br><a name="Fig. 176">Figs. +176–178</a>—<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.</td> +</tr> +</table> +</center> +<br> + + +<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.<sup>1</sup></p> + +<p class="fnote">1. 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> + +<br> +<hr> +<p class="page"><a name="page 156">[ 156 ]</a></p> + +<p> </p> + +<p>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> + +<br> + + +<hr noshade align="left" size="1" width="20%"> +<p class="ref"><a href="Title.html">Title and Contents</a><br> +<a href="glossary.html">Glossary</a><br> +<a href="chap13.html">Chapter XIII</a><br> +<a href="chap15.html">Chapter XV</a><br> +<a href="Title.html#Illustrations">Figs. 1–209</a><br> +<a href="title2.html#Illustrations">Figs. 210–408</a></p> +</center> +</body> +</html> + |