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| author | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:32:06 -0700 |
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| committer | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:32:06 -0700 |
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diff --git a/8700-h/old/chap10.html b/8700-h/old/chap10.html new file mode 100644 index 0000000..39738c6 --- /dev/null +++ b/8700-h/old/chap10.html @@ -0,0 +1,1091 @@ +<!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> X<br> +<br> +<b>THE CŒLOM THEORY</b></center> + +<br> + + +<p class="one">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</p> + +<br> +<hr> +<p class="page"><a name="page 91">[ 91 ]</a></p> + +<p> </p> + +<p class="one"> +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> + +<br> + + +<center> +<table class="capt" summary= +"Figs. 74 and 75. Diagram of the four secondary terminal layers."> +<tr> +<td width="363" align="justify"> +<img src="images/fig74.GIF" width="363" height="198" alt= +"Diagram of the four secondary terminal layers."> +<br><br><a name="Figs. 74 and 75">Figs. 74 +and 75</a>—<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.</td> +</tr> +</table> +</center> + +<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</p> + +<br> +<hr> +<p class="page"><a name="page 92">[ 92 ]</a></p> + +<p> </p> + +<p class="one">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> + + +<table class="capt" width="170" align="left" summary="Fig. 76. Coelomula of sagitta."> +<tr> +<td align="justify"><img src="images/fig76.GIF" width="170" height="138" alt= +"Coelumula of sagitta."> +<a name="Fig. 76">Fig. +76</a>—<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.</td> +</tr> +</table> + +<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> + +<br> + + +<center> +<table class="text" border="1" cellspacing="0" cellpadding="4" +summary= +"Names that have been given to the four secondary germinal layers."> +<tr> +<td align="left"><b>1. Skin-sense-layer</b><br> + (outer limiting layer).</td> +<td align="left"><b>I. Neural layer</b><br> + (<i>neuroblast</i>).</td> +<td rowspan="2" align="left" valign="middle">The two secondary +germinal<br> +layers of the body-wall:<br> +I. Epithelial.<br> +II. Fibrous.</td> +</tr> + +<tr> +<td align="left"><b>2. Skin-fibre-layer</b><br> + (outer middle layer).</td> +<td align="left"><b>II. Parietal layer</b><br> + (<i>myoblast</i>).</td> +</tr> + +<tr> +<td align="left"><b>3. Gut-fibre-layer</b><br> + (inner middle layer).</td> +<td align="left"><b>III. Visceral layer</b><br> + (<i>genoblast</i>).</td> +<td rowspan="2" align="left" valign="middle">The two secondary +germinal<br> +layers of the gut-wall:<br> +III. Fibrous.<br> +IV. Epithelial.</td> +</tr> + +<tr> +<td align="left"><b>4. Gut-gland-layer</b><br> + (inner limiting layer).</td> +<td align="left"><b>IV. Enteral layer</b><br> + (<i>enteroblast</i>)</td> +</tr> +</table> +</center> + +<br> + + +<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 <a href= +"#Figs. 74 and 75">(Figs. 74–76).</a></p> + + +<table class="capt" width="196" align="left" summary= +"Fig. 77. Coelomula of sagitta, in section."> +<tr> +<td align="justify"><img src="images/fig77.GIF" width="196" height="173" alt= +"Coelomula of sagitta, in section."> +<a name="Fig. 77">Fig. +77</a>—<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.</td> +</tr> +</table> + +<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</p> + +<br> +<hr> +<p class="page"><a name="page 93">[ 93 ]</a></p> + +<p> </p> + +<p class="one">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> + + +<table class="capt" width="180" align="left" summary="Fig. 78. Section of a young sagitta."> +<tr> +<td align="justify"><img src="images/fig78.GIF" width="180" height="122" alt= +"Section of a young sagitta."> +<a name="Fig. 78">Fig. +78</a>—<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.</td> +</tr> +</table> + +<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</p> + +<br> +<hr> +<p class="page"><a name="page 94">[ 94 ]</a></p> + +<p> </p> + +<p class="one">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> + +<br> + + +<center> +<table class="capt" summary= +"Figs. 79 and 80. Transverse section of amphioxus-larvae."> +<tr> +<td width="415" align="justify"><img src="images/fig79.GIF" width="415" height="185" alt= +"Transverse section of amphioxus-larvae."> +<br><a name="Fig. 79">Figs. 79 and +80.</a>—<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.)</td> +</tr> +</table> +</center> + +<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 <a href="#Fig. 76">(Fig. 76 <i> +pv</i>),</a> 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 <a href="#Fig. 78">(Fig. 78).</a> 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</p> + +<br> +<hr> +<p class="page"><a name="page 95">[ 95 ]</a></p> + +<p> </p> + +<p class="one">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> + +<br> +<center> +<table class="capt" summary= +"Figs. 81 and 82. Transverse section of amphioxus embryo."> +<tr> +<td width="347" align="justify"> +<img src="images/fig81.GIF" width="347" height="165" alt= +"Transverse section of amphioxus embryo."> +<br><a name="Fig. 81">Figs. 81 and +82.</a>—<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.</td> +</tr> +</table> +</center> + +<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</p> + +<br> +<hr> +<p class="page"><a name="page 96">[ 96 ]</a></p> + +<p> </p> + +<p class="one">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> + +<br> + + +<center> +<table class="capt" summary= +"Figs. 83 and 84. Chordula of the amphioxus."> +<tr> +<td width="401" align="justify"><img src="images/fig83.GIF" width="401" height="190" alt= +"Chordula of the amphioxus."> +<br><a name="Fig. 83">Figs. 83 and +84</a>—<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.</td> +</tr> + +<tr> +<td></td> +</tr> +</table> +</center> + +<br> +<center> +<table class="capt" summary= +"Figs. 85 and 86. Chordula of the amphibia (the ringed adder)."> +<tr> +<td width="401" align="justify"><img src="images/fig85.GIF" width="401" height="224" alt= +"Chordula of the amphibia (the ringed adder)."> +<br><a name="Fig. 85">Figs. 85 and +86</a>—<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.</td> +</tr> +</table> +</center> + +<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</p> + +<br> +<hr> +<p class="page"><a name="page 97">[ 97 ]</a></p> + +<p> </p> + +<center> +<table class="capt" summary= +"Figs. 87 and 88. Diagrammatic vertical section of coelomula-embryos of vertebrates."> +<tr> +<td width="430" align="justify"><img src="images/fig87.GIF" width="430" height="203" alt= +"Diagrammatic vertical section of coelomula-embryos of vertebrates."> +<br><a name="Fig. 87">Figs. 87 and +88</a>—<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.</td> +</tr> +</table> +</center> + +<br> +<center> +<table class="capt" summary= +"Figs. 89 and 90. Transverse section of coelomula embryos of triton."> +<tr> +<td width="430" align="justify"><img src="images/fig89.GIF" width="430" height="212" alt= +"Transverse section of coelomula embryos of triton."> +<br><a name="Fig. 89">Figs. 89 and +90</a>—<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.</td> +</tr> +</table> +</center> + +<br> + + +<p class="one">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</p> + +<br> +<hr> +<p class="page"><a name="page 98">[ 98 ]</a></p> + +<p> </p> + +<p class="one">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> + +<br> + + +<table class="capt" align="center" summary= +"Fig. 91 A, B, C. Vertical section of the dorsal part of three triton-embryos."> +<tr> +<td><img src="images/fig91.GIF" width="294" height="464" alt= +"Vertical section of the dorsal part of three triton-embryos."> +</td> +<td align="left" valign="bottom"><a name="Fig. 91">Fig. 91. <i>A, +B, C.</i></a>—<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.</td> +</tr> +</table> + +<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> <a href= +"#Fig. 81">(Fig. 81),</a> 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 <a +href="#Fig. 78">(Fig. 78 <i>dm, vm</i>);</a> 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</p> + +<br> +<hr> +<p class="page"><a name="page 99">[ 99 ]</a></p> + +<p> </p> + +<p class="one">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 <a href="#Fig. 85">(Figs. 85, 86)</a> 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 <a href= +"#Fig. 79">(Figs. 80, 81)</a> with the corresponding cenogenetic +form of the amphibia <a href="#Fig. 89">(Figs. 89–90),</a> +and construct the simple diagram that connects the two <a href= +"#Fig. 87">(Figs. 87, 88).</a> 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> + +<table class="capt" width="250" align="left" summary= +"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)."> +<tr> +<td align="justify"><img src="images/fig92.GIF" width="250" height= +"81" alt= +"Transverse section of the chordula-embryo of a bird (from a hen's egg at the close of the first day of incubation)."> +<a name="Fig. 92">Fig. +92</a>—<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.</td> +</tr> +</table> + +<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 <a +href="#Fig. 89">(Fig. 89)</a> 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>).</p> + +<br> +<hr> +<p class="page"><a name="page 100">[ 100 ]</a></p> + +<p> </p> + +<p class="one">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 +<a href="#Fig. 91">(Fig. 91 <i>A, B, C</i>).</a> 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> + +<br> +<center> +<table class="capt" width="327" summary= +"Fig. 93. Transverse section of the vertebrate-embryo of a bird (from a hen's egg on the second day of incubation)."> +<tr> +<td align="justify"><img src="images/fig93.GIF" width="327" height="95" alt= +"Transverse section of the vertebrate-embryo of a bird (from a hen's egg on the second day of incubation)."> +<br><br><a name="Fig. 93">Fig. +93</a>—<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.</td> +</tr> +</table> +</center> + +<br> + + +<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 <a href="#Fig. 92">(Fig. 92 <i>Rf</i>),</a> 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</p> + +<br> +<hr> +<p class="page"><a name="page 101">[ 101 ]</a></p> + +<p> </p> + +<p class="one">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> + +<br> +<center> +<table class="capt" width="319" summary= +"Transverse section of the primitive streak (primitive mouth) of the chick."> +<tr> +<td align="justify"> +<img src="images/fig94.GIF" width="319" height="214" alt= +"Transverse section of the primitive streak (primitive mouth) of the chick."> +<br><br><a name="Fig. 94">Figs. 94 and +95</a>—<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.</td> +</tr> +</table> +</center> + +<br> + + +<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 <a +href="#Fig. 96">(Fig. 96)</a> 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</p> + +<br> +<hr> +<p class="page"><a name="page 102">[ 102 ]</a></p> + +<p> </p> + +<p class="one">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> + +<br> +<center> +<table class="capt" width="304" summary= +"Fig. 96. Transverse section of the primitive groove (or primitive mouth) of a rabbit."> +<tr> +<td align="justify"><img src="images/fig96.GIF" width="304" height="165" alt= +"Transverse section of the primitive groove (or primitive mouth) of a rabbit."> +<br><br><a name="Fig. 96">Fig. +96</a>—<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.</td> +</tr> +</table> +</center> + +<br> +<center> +<table class="capt" width="274" summary= +"Fig. 97. Transverse section of the primitive mouth (or groove) of a human embryo (at the coelomula stage)."> +<tr> +<td align="justify"> +<img src="images/fig97.GIF" width="274" height="154" alt= +"Transverse section of the primitive mouth (or groove) of a human embryo (at the coelomula stage)."> +<br><br><a name="Fig. 97">Fig. +97</a>—<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.</td> +</tr> +</table> +</center> + +<br> + + +<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> + +<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="chap9.html">Chapter IX</a><br> +<a href="chap11.html">Chapter XI</a><br> +<a href="Title.html#Illustrations">Figs. 1–209</a><br> +<a href="title2.html#Illustrations">Figs. 210–408</a></p> +</body> +</html> + |