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diff --git a/8700-h/old/chap12.html b/8700-h/old/chap12.html new file mode 100644 index 0000000..40cb6a2 --- /dev/null +++ b/8700-h/old/chap12.html @@ -0,0 +1,809 @@ +<!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> XII<br> +<br> +<b>EMBRYONIC SHIELD AND GERMINATIVE AREA</b></center> + +<br> + + +<p class="one">The three higher classes of vertebrates which we +call the amniotes—the mammals, birds, and reptiles—are +notably distinguished by a number of peculiarities of their +development from the five lower classes of the stem—the +animals without an amnion (the <i>anamnia</i>). All the amniotes +have a distinctive embryonic membrane known as the amnion (or +“water-membrane”), and a special embryonic +appendage—the allantois. They have, further, a large +yelk-sac, which is filled with food-yelk in the reptiles and birds, +and with a corresponding clear fluid in the mammals. In consequence +of these later-acquired structures, the original features of the +development of the amniotes are so much altered that it is very +difficult to reduce them to the palingenetic embryonic processes of +the lower amnion-less vertebrates. The gastræa theory shows +us how to do this, by representing the embryology of the lowest +vertebrate, the skull-less amphioxus, as the original form, and +deducing from it, through a series of gradual modifications, the +gastrulation and cœlomation of the craniota.</p> + +<p>It was somewhat fatal to the true conception of the chief +embryonic processes of the vertebrate that all the older +embryologists, from Malpighi (1687) and Wolff (1750) to Baer (1828) +and Remak (1850), always started from the investigation of the +hen’s egg, and transferred to man and the other vertebrates +the impressions they gathered from this. This classical object of +embryological research is, as we have seen, a source of dangerous +errors. The large round food-yelk of the bird’s egg causes, +in the first place, a flat discoid expansion of the small gastrula, +and then so distinctive a development of this thin round embryonic +disk that the controversy as to its significance occupies a large +part of embryological literature.</p> + +<p>One of the most unfortunate errors that this led to was the idea +of an original</p> + +<br> +<hr> +<p class="page"><a name="page 116">[ 116 ]</a></p> + +<p> </p> + +<p class="one">antithesis of germ and yelk. The latter was regarded +as a foreign body, extrinsic to the real germ, whereas it is +properly a part of it, an embryonic organ of nutrition. Many +authors said there was no trace of the embryo until a later stage, +and outside the yelk; sometimes the two-layered embryonic disk +itself, at other times only the central portion of it (as +distinguished from the germinative area, which we will describe +presently), was taken to be the first outline of the embryo. In the +light of the gastræa theory it is hardly necessary to dwell +on the defects of this earlier view and the erroneous conclusions +drawn from it. In reality, the first segmentation-cell, and even +the stem-cell itself and all that issues therefrom, belong to the +embryo. As the large original yelk-mass in the undivided egg of the +bird only represents an inclosure in the greatly enlarged ovum, so +the later contents of its embryonic yelk-sac (whether yet segmented +or not) are only a part of the entoderm which forms the primitive +gut. This is clearly shown by the ova of the amphibia and +cyclostoma, which explain the transition from the yelk-less ova of +the amphioxus to the large yelk-filled ova of the reptiles and +birds.</p> + +<br> +<center> +<table class="capt" width="397" summary= +"Fig. 105. Severance of the discoid mammal embryo from the yelk-sac, in transverse section (diagrammatic)."> +<tr> +<td align="justify"><img src="images2/fig105.GIF" width="397" height="272" alt= +"Severance of the discoid mammal embryo from the yelk-sac, in transverse section (diagrammatic)."> +<br><br><a name="Fig. 105">Fig. +105</a>—<b>Severance of the discoid mammal embryo from the +yelk-sac, in transverse section</b> (diagrammatic). <i>A</i> The +germinal disk (<i>h, hf</i>) lies flat on one side of the +branchial-gut vesicle (<i>kb</i>). <i>B</i> In the middle of the +germinal disk we find the medullary groove (<i>mr</i>), and +underneath it the chorda (<i>ch</i>). <i>C</i> The gut-fibre-layer +(<i>df</i>) has been enclosed by the gut-gland-layer (<i>dd</i>). +<i>D</i> The skin-fibre-layer (<i>hf</i>) and gut-fibre-layer +(<i>df</i>) divide at the periphery; the gut (<i>d</i>) begins to +separate from the yelk-sac or umbilical vesicle (<i>nb</i>). <i> +E</i> The medullary tube (<i>mr</i>) is closed; the body-cavity +(<i>c</i>) begins to form. <i>F</i> The provertebræ +(<i>w</i>) begin to grow round the medullary tube (<i>mr</i>) and +the chorda (<i>ch</i>): the gut (<i>d</i>) is cut off from the +umbilical vesicle (<i>nb</i>). <i>H</i> The vertebræ +(<i>w</i>) have grown round the medullary tube (<i>mr</i>) and +chorda; the body-cavity is closed, and the umbilical vesicle has +disappeared. The amnion and serous membrane are omitted. The +letters have the same meaning throughout: <i>h</i> horn-plate, <i> +mr</i> medullary tube, <i>hf</i> skin-fibre-layer, <i>w</i> +provertebræ, <i>ch</i> chorda, <i>c</i> body-cavity or +cœloma, <i>df</i> gut-fibre-layer, <i>dd</i> gut-gland-layer, +<i>d</i> gut-cavity, <i>nb</i> umbilical vesicle.</td> +<?tr?> +</tr> +</table> +</center> + +<p>It is precisely in the study of these difficult features that we +see the incalculable value of phylogenetic considerations in +explaining complex ontogenetic facts, and the need of separating +cenogenetic phenomena from palingenetic. This is particularly clear +as regards the comparative embryology of the vertebrates, because +here the phylogenetic unity of the stem has been already +established by the well-known facts of paleontology and comparative +anatomy. If this unity of the stem, on the basis of the amphioxus, +were always borne in mind, we should not have these errors +constantly recurring.</p> + +<p>In many cases the cenogenetic relation of the embryo to the +food-yelk has until now given rise to a quite wrong idea of</p> + +<br> +<hr> +<p class="page"><a name="page 117">[ 117 ]</a></p> + +<p> </p> + +<p class="one">the first and most important embryonic processes in +the higher vertebrates, and has occasioned a number of false +theories in connection with them. Until thirty years ago the +embryology of the higher vertebrates always started from the +position that the first structure of the embryo is a flat, +leaf-shaped disk; it was for this reason that the cell-layers that +compose this germinal disk (also called germinative area) are +called “germinal layers.” This flat germinal disk, +which is round at first and then oval, and which is often described +as the tread or cicatricula in the laid hen’s egg, is found +at a certain part of the surface of the large globular food-yelk. I +am convinced that it is nothing else than the discoid, flattened +gastrula of the birds. At the beginning of germination the flat +embryonic disk curves outwards, and separates on the inner side +from the underlying large yelk-ball. In this way the flat layers +are converted into tubes, their edges folding and joining together +(Fig. 105). As the embryo grows at the expense of the food-yelk, +the latter becomes smaller and smaller; it is completely surrounded +by the germinal layers. Later still, the remainder of the food-yelk +only forms a small round sac, the yelk-sac or umbilical vesicle +(Fig. 105 <i>nb</i>). This is enclosed by the visceral layer, is +connected by a thin stalk, the yelk-duct, with the central part of +the gut-tube, and is finally, in most of the vertebrates, entirely +absorbed by this (<i>H</i>). The point at which this takes place, +and where the gut finally closes, is the visceral navel. In the +mammals, in which the remainder of the yelk-sac remains without and +atrophies, the yelk-duct at length penetrates the outer ventral +wall. At birth the umbilical cord proceeds from here, and the point +of closure remains throughout life in the skin as the navel.</p> + +<p>As the older embryology of the higher vertebrates was mainly +based on the chick, and regarded the antithesis of embryo (or +formative-yelk) and food-yelk (or yelk-sac) as original, it had +also to look upon the flat leaf-shaped structure of the germinal +disk as the primitive embryonic form, and emphasise the fact that +hollow grooves were formed of these flat layers by folding, and +closed tubes by the joining together of their edges.</p> + +<p>This idea, which dominated the whole treatment of the embryology +of the higher vertebrates until thirty years ago, was totally +false. The gastræa theory, which has its chief application +here, teaches us that it is the very reverse of the truth. The +cup-shaped gastrula, in the body-wall of which the two primary +germinal layers appear from the first as closed tubes, is the +original embryonic form of all the vertebrates, and all the +multicellular invertebrates; and the flat germinal disk with its +superficially expanded germinal layers is a later, secondary form, +due to the cenogenetic formation of the large food-yelk and the +gradual spread of the germ-layers over its surface. Hence the +actual folding of the germinal layers and their conversion into +tubes is not an original and primary, but a much later and +tertiary, evolutionary process. In the phylogeny of the vertebrate +embryonic process we may distinguish the following three +stages:—</p> + +<br> + + +<center> +<table class="text" border="1" cellspacing="0" cellpadding="4" +summary= +"Primary, secondary and tertiary stages in the phylogeny of the vertebrate embryonic process."> +<tr> +<td align="center" width="30%">A. First stage:<br> +<b>Primary</b><br> +(palingenic)<br> +embryonic process.</td> +<td align="center" width="30%">B. Second stage:<br> +<b>Secondary</b><br> +(cenogenetic)<br> +embryonic process.</td> +<td align="center" width="30%">C. Third stage:<br> +<b>Tertiary</b><br> +(cenogenetic)<br> +embryonic process.</td> +</tr> + +<tr> +<td align="justify" valign="top">The germinal layers form from the +first closed tubes, the one-layered blastula being converted into +the two-layered gastrula by invagination.<br> + No food-yelk.<br> + (<i>Amphioxus.</i>)</td> +<td align="justify" valign="top">The germinal layers spread out +leaf-wise, food-yelk gathering in the ventral entoderm, and a large +yelk-sac being formed from the middle of the gut-tube.<br> + (<i>Amphibia.</i>)</td> +<td align="justify" valign="top">The germinal layers form a flat +germinal disk, the borders of which join together and form closed +tubes, separating from the central yelk-sac.<br> + (<i>Amniotes.</i>)</td> +</tr> +</table> +</center> + +<br> + + +<p>As this theory, a logical conclusion from the gastræa +theory, has been fully substantiated by the comparative study of +gastrulation in the last few decades, we must exactly reverse the +hitherto prevalent mode of treatment. The yelk-sac is not to be +treated, as was done formerly, as if it were originally antithetic +to the embryo, but as an essential part of it, a part of its +visceral tube. The primitive gut of the gastrula has, on this view, +been divided into two parts in the higher animals as a result of +the cenogenetic formation of the food-yelk—the permanent gut +(<i>metagaster</i>), or permanent alimentary canal, and the +yelk-sac (<i>lecithoma</i>), or umbilical vesicle. This is very +clearly shown by the comparative ontogeny of the fishes and +amphibia. In these cases the whole yelk undergoes cleavage at +first, and forms a yelk-gland, composed of yelk-cells, in the +ventral wall</p> + +<br> +<hr> +<p class="page"><a name="page 118">[ 118 ]</a></p> + +<p> </p> + +<p class="one">of the primitive gut. But it afterwards becomes so +large that a part of the yelk does not divide, and is used up in +the yelk-sac that is cut off outside.</p> + +<p>When we make a comparative study of the embryology of the +amphioxus, the frog, the chick, and the rabbit, there cannot, in my +opinion, be any further doubt as to the truth of this position, +which I have held for thirty years. Hence in the light of the +gastræa theory we must regard the features of the amphioxus +as the only and real primitive structure among all the vertebrates, +departing very little from the palingenetic embryonic form. In the +cyclostoma and the frog these features are, on the whole, not much +altered cenogenetically, but they are very much so in the chick, +and most of all in the rabbit. In the bell-gastrula of the +amphioxus and in the hooded gastrula of the lamprey and the frog +the germinal layers are found to be closed tubes or vesicles from +the first. On the other hand, the chick-embryo (in the new laid, +but not yet hatched, egg) is a flat circular disk, and it was not +easy to recognise this as a real gastrula. Rauber and Goette have, +however, achieved this. As the discoid gastrula grows round the +large globular yelk, and the permanent gut then separates from the +outlying yelk-sac, we find all the processes which we have shown +(diagrammatically) in Figure 1.108—processes that were +hitherto regarded as principal acts, whereas they are merely +secondary.</p> + +<br> +<table class="capt" summary= +"Figs. 106 and 107. The visceral embryonnic vesicle (blastocystis or gastrocystis) of a rabbit."> +<tr> +<td><img src="images2/fig106.GIF" width="317" height="181" alt= +"The visceral embryonnic vesicle (blastocystis or gastrocystis) of a rabbit."> +</td> +<td align="left" valign="bottom"><a name="Fig. 106">Fig. +106</a>—<b>The visceral embryonic vesicle</b> +(<i>blastocystis</i> or <i>gastrocystis</i>) of a rabbit (the +“blastula” or <i>vesicula blastodermica</i> of other +writers), <i>a</i> outer envelope (ovolemma), <i>b</i> skin-layer +or ectoderm, forming the entire wall of the yelk-vesicle, <i>c</i> +groups of dark cells, representing the visceral layer or +entoderm.<br> +Fig. 107—<b>The same</b> in section. Letters as above. <i> +d</i> cavity of the vesicle. (From <i>Bischoff.</i>)</td> +</tr> +</table> + +<br> + + +<p>The oldest, oviparous mammals, the monotremes, behave in the +same way as the reptiles and birds. But the corresponding embryonic +processes in the viviparous mammals, the marsupials and placentals, +are very elaborate and distinctive. They were formerly quite +misinterpreted; it was not until the publication of the studies of +Edward van Beneden (1875) and the later research of Selenka, +Kuppfer, Rabl, and others, that light was thrown on them, and we +were in a position to bring them into line with the principles of +the gastræa theory and trace them to the embryonic forms of +the lower vertebrates. Although there is no independent food-yelk, +apart from the formative yelk, in the mammal ovum, and although its +segmentation is total on that account, nevertheless a large +yelk-sac is formed in their embryos, and the “embryo +proper” spreads leaf-wise over its surface, as in the +reptiles and birds, which have a large food-yelk and partial +segmentation. In the mammals, as well as in the latter, the flat, +leaf-shaped germinal disk separates from the yelk-sac, and its +edges join together and form tubes.</p> + +<p>How can we explain this curious anomaly? Only as a result of +very characteristic and peculiar cenogenetic modifications of the +embryonic process, the real causes of which must be sought in the +change in the rearing of the young on the part of the viviparous +mammals. These are clearly connected with the fact that the +ancestors of the viviparous mammals were oviparous amniotes like +the present monotremes, and only gradually became viviparous. This +can no longer be questioned now that it has been shown (1884) that +the monotremes, the lowest and oldest of the mammals, still lay +eggs, and that these develop like the ova of the reptiles and +birds. Their nearest descendants, the marsupials, formed the habit +of retaining the eggs, and developing them in the</p> + +<br> +<hr> +<p class="page"><a name="page 119">[ 119 ]</a></p> + +<p> </p> + +<p class="one">oviduct; the latter was thus converted into a womb +(uterus). A nutritive fluid that was secreted from its wall, and +passed through the wall of the blastula, now served to feed the +embryo, and took the place of the food-yelk. In this way the +original food-yelk of the monotremes gradually atrophied, and at +last disappeared so completely that the partial ovum-segmentation +of their descendants, the rest of the mammals, once more became +total. From the <i>discogastrula</i> of the former was evolved the +distinctive <i>epigastrula</i> of the latter.</p> + +<p>It is only by this phylogenetic explanation that we can +understand the formation and development of the peculiar, and +hitherto totally misunderstood, blastula of the mammal. The +vesicular condition of the mammal embryo was discovered 200 years +ago (1677) by Regner de Graaf. He found in the uterus of a rabbit +four days after impregnation small, round, loose, transparent +vesicles, with a double envelope. However, Graaf’s discovery +passed without recognition. It was not until 1827 that these +vesicles were rediscovered by Baer, and then more closely studied +in 1842 by Bischoff in the rabbit (Figs. 106, 107). They are found +in the womb of the rabbit, the dog, and other small mammals, a few +days after copulation. The mature ova of the mammal, when they have +left the ovary, are fertilised either here or in the oviduct +immediately afterwards by the invading sperm-cells.<sup>1</sup> (As +to the womb and oviduct see Chapter XXIX) The cleavage and +formation of the gastrula take place in the oviduct. Either here in +the oviduct or after the mammal gastrula has passed into the uterus +it is converted into the globular vesicle which is shown externally +in Fig. 106, and in section in Fig. 107. The thick, outer, +structureless envelope that encloses it is the original <i> +ovolemma</i> or <i>zona pellucida,</i> modified, and clothed with a +layer of albumin that has been deposited on the outside. From this +stage the envelope is called the external membrane, the <i>primary +chorion</i> or prochorion (<i>a</i>). The real wall of the vesicle +enclosed by it consists of a simple layer of ectodermic cells +(<i>b</i>), which are flattened by mutual pressure, and generally +hexagonal; a light nucleus shines through their fine-grained +protoplasm (Fig. 108). At one part (<i>c</i>) inside this hollow +ball we find a circular disc, formed of darker, softer, and rounder +cells, the dark-grained entodermic cells (Fig. 109).</p> + +<table class="capt" summary= +"Fig. 108. Four entodermic cells from the vesicle of the rabbit. Fig. 109. Two entodermic cells from the embryonic vesicle of the rabbit."> +<tr> +<td><img src="images2/fig108.GIF" width="267" height="165" alt= +"Fig. 108. Four entodermic cells from the vesicle of the rabbit. Fig. 109. Two entodermic cells from the embryonic vesicle of the rabbit."> +</td> +<td align="left" valign="bottom"><a name="Fig. 108">Fig. +108</a>—<b>Four entodermic cells</b> from the embryonic +vesicle of the rabbit.<br> +Fig. 109—<b>Two entodermic cells</b> from the embryonic +vesicle of the rabbit.</td> +</tr> +</table> + +<br> + + +<p>The characteristic embryonic form that the developing mammal now +exhibits has up to the present usually been called the +“blastula” (Bischoff), “sac-shaped embryo” +(Baer), “vesicular embryo” (<i>vesicula +blastodermica,</i> or, briefly, <i>blastosphæra</i>). The +wall of the hollow vesicle, which consists of a single layer of +cells, was called the “blastoderm,” and was supposed to +be equivalent to the cell-layer of the same name that forms the +wall of the real blastula of the amphioxus and many of the +invertebrates (such as <i>Monoxenia,</i> <a href= +"chap8.html#page 61">Fig. 29 <i>F, G</i>).</a> Formerly this real +blastula was generally believed to be equivalent to the embryonic +vesicle of the mammal. However, this is by no means the case. What +is called the “blastula” of the mammal and the real +blastula of the amphioxus and many of the invertebrates are totally +different embryonic structures. The latter (blastula) is +palingenetic, and precedes the formation of the gastrula. The +former (blastodermic vesicle) is cenogenetic, and follows +gastrulation. The globular wall of the blastula is a real +blastoderm, and consists of homogeneous (blastodermic) cells; it is +not yet differentiated into the two primary germinal layers. But +the globular wall of the mammal vesicle is the differentiated +ectoderm, and at one point in it we find a circular disk of quite +different cells—the entoderm. The round</p> + +<p class="fnote">1. In man and the other mammals the fertilisation +of the ova probably takes place, as a rule, in the oviduct; here +the ova, which issue from the female ovary in the shape of the +Graafian follicle, and enter the inner aperture of the oviduct, +encounter the mobile sperm-cells of the male seed, which pass into +the uterus at copulation, and from this into the external aperture +of the oviduct. Impregnation rarely takes place in the ovary or in +the womb.</p> + +<br> +<hr> +<p class="page"><a name="page 120">[ 120 ]</a></p> + +<p> </p> + +<p class="one">cavity, filled with fluid, inside the real blastula +is the segmentation-cavity. But the similar cavity within the +mammal vesicle is the yelk-sac cavity, which is connected with the +incipient gut-cavity. This primitive gut-cavity passes directly +into the segmentation-cavity in the mammals, in consequence of the +peculiar cenogenetic changes in their gastrulation, which we have +considered previously (Chapter IX). For these reasons it is very +necessary to recognise the secondary embryonic vesicle in the +mammal (<i>gastrocystis</i> or <i>blastocystis</i>) as a +characteristic structure peculiar to this class, and distinguish it +carefully from the primary blastula of the amphioxus and the +invertebrates.</p> + +<br> +<center> +<table class="capt" width="437" summary= +"Fig. 110. Ovum of a rabbit from the uterus, one-sixth of an inch in diameter. Fig. 111. The same ovum, seen in profile. Fig. 112. Ovum of a rabbit from the uterus, one-fourth of an inch in diameter. Fig. 113. The same ovum, seen in profile. Fig. 114. Ovum of a rabbit from the uterus, one-third of an inch in diameter."> +<tr> +<td align="justify"> +<img src="images2/fig110.GIF" width="437" height="270" alt= +"Fig. 110. Ovum of a rabbit from the uterus, one-sixth of an inch in diameter. Fig. 111. The same ovum, seen in profile. Fig. 112. Ovum of a rabbit from the uterus, one-fourth of an inch in diameter. Fig. 113. The same ovum, seen in profile."> +<br><br><a name="Fig. 110">Fig. 110</a>—<b>Ovum +of a rabbit</b> from the uterus, one sixth of an inch in diameter. +The embryonic vesicle (<i>b</i>) has withdrawn a little from the +smooth ovolemma (<i>a</i>). In the middle of the ovolemma we see +the round germinal disk (blastodiscus, <i>c</i>), at the edge of +which (at <i>d</i>) the inner layer of the embryonic vesicle is +already beginning to expand. (Figs. 110–114 from <i> +Bischoff.</i><br> +Fig. 111—<b>The same ovum,</b> seen in profile. Letters as in +Fig. 110.<br> +Fig. 112—<b>Ovum of a rabbit from the uterus,</b> one-fourth +of an inch in diameter. The blastoderm is already for the most part +two-layered (<i>b</i>). The ovolemma, or outer envelope, is tufted +(<i>a</i>).<br> +Fig. 113—<b>The same ovum,</b> seen in profile. Letters as in +Fig. 112.<br> +Fig. 114—<b>Ovum of a rabbit from the uterus,</b> one-third +of an inch in diameter. The embryonic vesicle is now nearly +everywhere two-layered (<i>k</i>) only remaining one-layered below +(at <i>d</i>).</td> +</tr> +</table> +</center> + +<br> +<hr> +<p class="page"><a name="page 121">[ 121 ]</a></p> + +<p> </p> + +<p>The small, circular, whitish, and opaque spot which the gastric +disk <a href="#Fig. 106">(Fig. 106)</a> forms at a certain part of +the surface of the clear and transparent embryonic vesicle has long +been known to science, and compared to the germinal disk of the +birds and reptiles. Sometimes it has been called the germinal disk, +sometimes the germinal spot, and usually the germinative area. From +the area the further development of the embryo proceeds. However, +the larger part of the embryonic vesicle of the mammal is not +directly used for building up the later body, but for the +construction of the temporary umbilical vesicle. The embryo +separates from this in proportion as it grows at its expense; the +two are only connected by the yelk-duct (the stalk of the +yelk-sac), and this maintains the direct communication between the +cavity of the umbilical vesicle and the forming visceral cavity <a +href="#Fig. 105">(Fig. 105).</a></p> + +<br> + + +<center> +<table class="capt" width="329" summary= +"Fig. 115. Round germinative area of the rabbit. Fig. 116. Oval area, with the opaque whitish border of the dark area without."> +<tr> +<td align="justify"><img src="images2/fig115.GIF" width="329" height="234" alt= +"Fig. 115. Round germinative area of the rabbit. Fig. 116. Oval area, with the opaque whitish border of the dark area without."> +<br><br><a name="Fig. 115">Fig. 115</a>—<b>Round +germinative area of the rabbit,</b> divided into the central light +area (<i>area pellucida</i>) and the peripheral dark area (<i>area +opaca</i>). The light area seems darker on account of the dark +ground appearing through it.<br> +Fig. 116—<b>Oval area,</b> with the opaque whitish border of +the dark area without.</td> +</tr> +</table> +</center> + +<p>The germinative area or gastric disk of the animal consists at +first (like the germinal disk of birds and reptiles) merely of the +two primary germinal layers, the ectoderm and entoderm. But soon +there appears in the middle of the circular disk between the two a +third stratum of cells, the rudiment of the middle layer or fibrous +layer (<i>mesoderm</i>). This middle germinal layer consists from +the first, as we have seen in Chapter X, of two separate epithelial +plates, the two layers of the cœlom-pouches (parietal and +visceral). However, in all the amniotes (on account of the large +formation of yelk) these thin middle plates are so firmly pressed +together that they seem to represent a single layer. It is thus +peculiar to the amniotes that the middle of the germinative area is +composed of four germinal layers, the two limiting (or primary) +layers and the middle layers between them <a href= +"chap10.html#Fig. 96">(Figs. 96, 97).</a> These four secondary +germinal layers can be clearly distinguished as soon as what is +called the sickle-groove (or “embryonic sickle”) is +seen at the hind border of the germinative area. At the borders, +however, the germinative area of the mammal only consists of two +layers. The rest of the wall of the embryonic vesicle consists at +first (but only for a short time in most of the mammals) of a +single layer, the outer germinal layer.</p> + +<p>From this stage, however, the whole wall of the embryonic +vesicle becomes two-layered. The middle of the germinative area is +much thickened by the growth of the cells of the middle layers, and +the inner layer expands at the same time, and increases at the +border of the disk all round. Lying close on the outer layer +throughout, it grows over its inner surface at all points, covers +first the upper and then the lower hemisphere, and at last closes +in the middle of the inner layer (Figs. 110–114). The wall of +the embryonic vesicle now consists throughout of two layers of +cells, the ectoderm without and the entoderm within. It is only in +the centre of the circular area, which becomes thicker and thicker +through the growth of the middle layers, that it is made up of all +four layers. At the same time, small structureless tufts or warts +are deposited on the surface of the outer</p> + +<br> +<hr> +<p class="page"><a name="page 122">[ 122 ]</a></p> + +<p> </p> + +<p class="one">ovolemma or prochorion, which has been raised above +the embryonic vesicle (Figs. 112–114 <i>a</i>).</p> + +<p>We may now disregard both the outer ovolemma and the greater +part of the vesicle, and concentrate our attention on the +germinative area and the four-layered embryonic disk. It is here +alone that we find the important changes which lead to the +differentiation of the first organs. It is immaterial whether we +examine the germinative area of the mammal (the rabbit, for +instance) or the germinal disk of a bird or a reptile (such as a +lizard or tortoise). The embryonic processes we are now going to +consider are essentially the same in all members of the three +higher classes of vertebrates which we call the amniotes. Man is +found to agree in this respect with the rabbit, dog, ox, etc.; and +in all these animals the germinative area undergoes essentially the +same changes as in the birds and reptiles. They are most frequently +and accurately studied in the chick, because we can have incubated +hens’ eggs in any quantity at any stage of development. +Moreover, the round germinal disk of the chick passes immediately +after the beginning of incubation (within a few hours) from the +two-layered to the four-layered stage, the two-layered mesoderm +developing from the median primitive groove between the ectoderm +and entoderm <a href="chap10.html#Fig. 81">(Figs. +82–95).</a></p> + +<br> +<center> +<table class="capt" width="272" summary= +"Fig. 117. Oval germinal disk of the rabbit, magnified. Fig. 118. Pear-shaped germinal shield of the rabbit (eight days old), magnified."> +<tr> +<td align="justify"> +<img src="images2/fig117.GIF" width="272" height="212" alt= +"Fig. 117. Oval germinal disk of the rabbit, magnified. Fig. 118. Pear-shaped germinal shield of the rabbit (eight days old), magnified."> +<br><a name="Fig. 117">Fig. 117</a>—<b>Oval +germinal disk of the rabbit,</b> magnified. As the delicate, +half-transparent disk lies on a black ground, the pellucid area +looks like a dark ring, and the opaque area (lying outside it) like +a white ring. The oval shield in the centre also looks whitish, and +in its axis we see the dark medullary groove. (From <i> +Bischoff.</i>)<br> +Fig. 118—<b>Pear-shaped germinal shield of the rabbit</b> +(eight days old), magnified. <i>rf</i> medullary groove. <i>pr</i> +primitive groove (primitive mouth). (From <i> +Kölliker.</i></td> +</tr> +</table> +</center> + +<p>The first change in the round germinal disk of the chick is that +the cells at its edges multiply more briskly, and form darker +nuclei in their protoplasm. This gives rise to a dark ring, more or +less sharply set off from the lighter centre of the germinal disk +(Fig. 115). From this point the latter takes the name of the +“light area” (<i>area pellucida</i>), and the darker +ring is called the “dark area” (<i>area opaca</i>). (In +a strong light, as in Figs. 115–117, the light area seems +dark, because the dark ground is seen through it; and the dark area +seems whiter). The circular shape of the area now changes into +elliptic, and then immediately into oval (Figs. 116, 117). One end +seems to be broader and blunter, the other narrower and more +pointed; the former corresponds to the anterior and the latter to +the posterior section of the subsequent body. At the same time, we +can already trace the characteristic bilateral form of the body, +the antithesis of right and left, before and behind. This will be +made clearer by the “primitive streak,” which appears +at the posterior end.</p> + +<p>At an early stage an opaque spot is seen in the middle of the +clear germinative</p> + +<br> +<hr> +<p class="page"><a name="page 123">[ 123 ]</a></p> + +<p> </p> + +<p class="one">area, and this also passes from a circular to an +oval shape. At first this shield-shaped marking is very delicate +and barely perceptible; but it soon becomes clearer, and now stands +out as an oval shield, surrounded by two rings or areas (Fig. 117). +The inner and brighter ring is the remainder of the pellucid area, +and the dark outer ring the remainder of the opaque area; the +opaque shield-like spot itself is the first rudiment of the dorsal +part of the embryo. We give it briefly the name of embryonic shield +or dorsal shield. In most works this embryonic shield is described +as “the first rudiment or trace of the embryo,” or +“primitive embryo.” But this is wrong, though it rests +on the authority of Baer and Bischoff.</p> + +<br> +<center> +<table class="capt" width="376" summary= +"Fig. 119. Median longitudinal section of the gastrula of four vertebrates."> +<tr> +<td align="justify"> +<img src="images2/fig119.GIF" width="376" height="337" alt= +"Median longitudinal section of the gastrula of four vertebrates."> +<br><br><a name="Fig. 119">Fig. 119</a>—<b>Median +longitudinal section of the gastrula of four vertebrates.</b> (From +<i>Rabl.</i>) <i>A</i> discogastrula of a shark +(<i>Pristiurus</i>). <i>B</i> amphigastrula of a sturgeon +(<i>Accipenser</i>). <i>C</i> amphigastrula of an amphibium +(<i>Triton</i>). <i>D</i> epigastrula of an amniote (diagram). <i> +a</i> ventral, <i>b</i> dorsal lip of the primitive mouth.</td> +</tr> +</table> +</center> + +<p>As a matter of fact, we already have the embryo in the +stem-cell, the gastrula, and all the subsequent stages. The +embryonic shield is simply the first rudiment of the dorsal part, +which is the earliest to develop. As the older names of +“embryonic rudiment” and “germinative area” +are used in many different senses—and this has led to a fatal +confusion in embryonic literature—we must explain very +clearly the real significance of these important embryonic parts of +the amniote. It will be useful to do so in a series of formal +principles:—</p> + +<p>1. The so-called ”first trace of the embryo” in the +amniotes, or the embryonic shield, in the centre of the pellucid +area, consists merely of an early differentiation and formation of +the middle dorsal parts.</p> + +<p>2. Hence the best name for it is ”the dorsal +shield,” as I proposed long ago.</p> + +<p>3. The germinative area, in which the first embryonic +blood-vessels appear at an early stage, is not opposed as an +external area to the ”embryo proper,” but is a part of +it.</p> + +<p>4. In the same way, the yelk-sac or the umbilical vesicle is not +a foreign external</p> + +<br> +<hr> +<p class="page"><a name="page 124">[ 124 ]</a></p> + +<p> </p> + +<p class="one">appendage of the embryo, but an outlying part of its +primitive gut.</p> + +<p>5. The dorsal shield gradually separates from the germinative +area and the yelk-sac, its edges growing downwards and folding +together to form ventral plates.</p> + +<p>6. The yelk-sac and vessels of the germinative area, which soon +spread over its whole surface, are, therefore, real embryonic +organs, or temporary parts of the embryo, and have a transitory +importance in connection with the nutrition of the growing later +body; the latter may be called the ”permanent body” in +contrast to them.</p> + +<p>The relation of these cenogenetic features of the amniotes to +the palingenetic structures of the older non-amniotic vertebrates +may be expressed in the following theses: The original gastrula, +which completely passes into the embryonic body in the acrania, +cyclostoma, and amphibia, is early divided into two parts in the +amniotes—the embryonic shield, which represents the dorsal +outline of the permanent body; and the temporary embryonic organs +of the germinative area and its blood-vessels, which soon grow over +the whole of the yelk-sac. The differences which we find in the +various classes of the vertebrate stem in these important +particulars can only be fully understood when we bear in mind their +phylogenetic relations on the one hand, and, on the other, the +cenogenetic modifications of structure that have been brought about +by changes in the rearing of the young and the variation in the +mass of the food-yelk.</p> + +<p>We have already described in Chapter IX the changes which this +increase and decrease of the nutritive yelk causes in the form of +the gastrula, and especially in the situation and shape of the +primitive mouth. The primitive mouth or prostoma is originally a +simple round aperture at the lower pole of the long axis; its +dorsal lip is above and ventral lip below. In the amphioxus this +primitive mouth is a little eccentric, or shifted to the dorsal +side <a href="chap8.html#Fig. 39">(Fig. 39).</a> The aperture +increases with the growth of the food-yelk in the cyclostoma and +ganoids; in the sturgeon it lies almost on the equator of the round +ovum, the ventral lip (<i>a</i>) in front and the dorsal lip +(<i>b</i>) behind (Fig. 119 <i>b</i>). In the wide-mouthed, +circular discoid gastrula of the selachii or primitive fishes, +which spreads quite flat on the large food-yelk, the anterior +semi-circle of the border of the disk is the ventral, and the +posterior semicircle the dorsal lip (Fig. 119 <i>A</i>). The +amphiblastic amphibia are directly connected with their earlier +fish-ancestors, the dipneusts and ganoids, and further the oldest +selachii (<i>Cestracion</i>); they have retained their total +unequal segmentation, and their small primitive mouth (Fig. 119 <i> +C, ab</i>), blocked up by the yelk-stopper, lies at the limit of +the dorsal and ventral surface of the embryo (at the lower pole of +its equatorial axis), and there again has an upper dorsal and a +lower ventral lip (<i>a, b</i>). The formation of a large food-yelk +followed again in the stem-forms of the amniotes, the protamniotes +or proreptilia, descended from the amphibia (Fig. 119 <i>D</i>). +But here the accumulation of the food-yelk took place only in the +ventral wall of the primitive-gut, so that the narrow primitive +mouth lying behind was forced upwards, and came to lie on the back +of the discoid ”epigastrula” in the shape of the +”primitive groove”; thus (in contrast to the case of +the selachii, Fig. 119 <i>A</i>) the dorsal lip (<i>b</i>) had to +be in front, and the ventral lip (<i>a</i>) behind (Fig. 119 <i> +D</i>). This feature was transmitted to all the amniotes, whether +they retained the large food-yelk (reptiles, birds, and +monotremes), or lost it by atrophy (the viviparous mammals).</p> + +<p>This phylogenetic explanation of gastrulation and +cœlomation, and the comparative study of them in the various +vertebrates, throw a clear and full light on many ontogenetic +phenomena, as to which the most obscure and confused opinions were +prevalent thirty years ago. In this we see especially the high +scientific value of the biogenetic law and the careful separation +of palingenetic from cenogenetic processes. To the opponents of +this law the real explanation of these remarkable phenomena is +impossible. Here, and in every other part of embryology, the true +key to the solution lies in phylogeny.</p> + +<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="chap11.html">Chapter XI</a><br> +<a href="chap13.html">Chapter XIII</a><br> +<a href="Title.html#Illustrations">Figs. 1–209</a><br> +<a href="title2.html#Illustrations">Figs. 210–408</a></p> +</body> +</html> + |