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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/chap9.html b/8700-h/old/chap9.html new file mode 100644 index 0000000..4118686 --- /dev/null +++ b/8700-h/old/chap9.html @@ -0,0 +1,1569 @@ +<!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> IX<br> +<br> +<b>THE GASTRULATION OF THE VERTEBRATE<sup>1</sup></b></center> + +<br> + + +<p class="one">The remarkable processes of gastrulation, +ovum-segmentation, and formation of germinal layers present a most +conspicuous variety. There is to-day only the lowest of the +vertebrates, the amphioxus, that exhibits the original form of +those processes, or the palingenetic gastrulation which we have +considered in the preceding chapter, and which culminates in the +formation of the archigastrula <a href="chap8.html#Fig. 38">(Fig. +38).</a> In all other extant vertebrates these fundamental +processes have been more or less modified by adaptation to the +conditions of embryonic development (especially by changes in the +food-yelk); they exhibit various cenogenetic types of the formation +of germinal layers. However, the different classes vary +considerably from each other. In order to grasp the unity that +underlies the manifold differences in these phenomena and their +historical connection, it is necessary to bear in mind always the +unity of the vertebrate-stem. This “phylogenetic +unity,” which I developed in my <i>General Morphology</i> in +1866, is now generally admitted. All impartial zoologists agree +to-day that all the vertebrates, from the amphioxus and the fishes +to the ape and man, descend from a common ancestor, “the +primitive vertebrate.” Hence the embryonic processes, by +which each individual vertebrate is developed, must also be capable +of being reduced to one common type of embryonic development; and +this primitive type is most certainly exhibited to-day by the +amphioxus.</p> + +<p>It must, therefore, be our next task to make a comparative study +of the various forms of vertebrate gastrulation, and trace them +backwards to that of the lancelet. Broadly speaking, they fall +first into two groups: the older cyclostoma, the earliest fishes, +most of the amphibia, and the viviparous mammals, have <i> +holoblastic</i> ova—that is to say, ova with total, unequal +segmentation; while the younger cyclostoma, most of the fishes, the +cephalopods, reptiles, birds, and monotremes, have <i> +meroblastic</i> ova, or ova with partial discoid segmentation. A +closer study of them shows, however, that these two groups do not +present a natural unity, and that the historical relations between +their several divisions are very complicated. In order to +understand them properly, we must first consider the various +modifications of gastrulation in these classes. We may begin with +that of the amphibia.</p> + +<p>The most suitable and most available objects of study in this +class are the eggs of our indigenous amphibia, the tailless frogs +and toads, and the tailed salamander. In spring they are to be +found in clusters in every pond, and careful examination of the ova +with a lens is sufficient to show at least the external features of +the segmentation. In order to understand the whole process rightly +and follow the formation of the germinal layers and the gastrula, +the ova of the frog and salamander must be carefully hardened; then +the thinnest possible sections must be made of the hardened ova +with the microtome, and the tinted sections must be very closely +compared under a powerful microscope.</p> + +<p>The ova of the frog or toad are globular in shape, about the +twelfth of an inch in diameter, and are clustered in jelly-like +masses, which are lumped together in the case of the frog, but form +long strings in the case of the toad. When we examine the opaque, +grey, brown, or blackish ova closely, we find that the upper half +is darker than the lower. The middle of the upper half is in many +species black, while the middle of the lower half is +white.<sup>2</sup> In this way we get a definite axis of the ovum +with two poles. To give a clear</p> + +<p class="fnote">1. Cf. Balfour’s <i>Manual of Comparative +Embryology,</i> vol. ii; Theodore Morgan’s <i>The Development +of the Frog’s Egg.</i><br> +2. The colouring of the eggs of the amphibia is caused by the +accumulation of dark-colouring matter at the animal pole of the +ovum. In consequence of this, the animal cells of the ectoderm are +darker than the vegetal cells of the entoderm. We find the reverse +of this in the case of most animals, the protoplasm of the entoderm +cells being usually darker and coarser-grained.</p> + +<br> +<hr> +<p class="page"><a name="page 72">[ 72 ]</a></p> + +<p> </p> + +<p class="one">idea of the segmentation of this ovum, it is best to +compare it with a globe, on the surface of which are marked the +various parallels of longitude and latitude. The superficial +dividing lines between the different cells, which come from the +repeated segmentation of the ovum, look like deep furrows on the +surface, and hence the whole process has been given the name of +furcation. In reality, however, this “furcation,” which +was formerly regarded as a very mysterious process, is nothing but +the familiar, repeated cell-segmentation. Hence also the +segmentation-cells which result from it are real cells.</p> + +<br> + + +<center> +<table class="capt" width="354" summary= +"Fig. 40. The cleavage of the frog's ovum."> +<tr> +<td align="justify"> +<img src="images/fig40.GIF" width="354" height="291" alt= +"Fig. 40. The cleavage of the frog's ovum."> +<br><br><a name="Fig. 40">Fig. 40</a>—<b>The +cleavage of the frog’s ovum</b> (magnified). A stem-cell. <i> +B</i> the first two segmentation-cells. <i>C</i> four cells. <i> +D</i> eight cells (4 animal and 4 vegetative). <i>E</i> twelve +cells (8 animal and 4 vegetative). <i>F</i> sixteen cells (8 animal +and 8 vegetative). <i>G</i> twenty-four cells (16 animal and 8 +vegetative). <i>H</i> thirty-two cells. <i>I</i> forty-eight cells. +<i>K</i> sixty-four cells. <i>L</i> ninety-six cells. <i>M</i> 160 +cells (128 animal and 32 vegetative).</td> +</tr> +</table> +</center> + +<br> + + +<p>The unequal segmentation which we observe in the ovum of the +amphibia has the special feature of beginning at the upper and +darker pole (the north pole of the terrestrial globe in our +illustration), and slowly advancing towards the lower and brighter +pole (the south pole). Also the upper and darker hemisphere remains +in this position throughout the course of the segmentation, and its +cells multiply much more briskly. Hence the cells of the lower +hemisphere are found to be larger and less numerous. The cleavage +of the stem-cell (Fig. 40 <i>A</i>) begins with the formation of a +complete furrow, which starts from the north pole and reaches to +the south (<i>B</i>). An hour later a second furrow arises in the +same way, and this cuts the first at a right angle (Fig. 40 <i> +C</i>). The ovum is thus divided into four equal parts. Each of +these four “segmentation cells” has an upper and darker +and a lower, brighter half. A few hours later a third furrow +appears, vertically to the first two (Fig. 40 <i>D</i>). The +globular germ now consists of eight cells, four smaller ones above +(northern) and four larger ones below (southern). Next, each of the +four upper ones divides into two halves by a cleavage beginning +from the north pole, so that we now have eight above and four below +(Fig. 40 <i>E</i>). Later, the</p> + +<br> +<hr> +<p class="page"><a name="page 73">[ 73 ]</a></p> + +<p> </p> + +<center> +<table class="capt" width="286" summary= +"Figs. 41-44. Four vertical sections of the fertilised ovum of the toad, in four successive stages of development."> +<tr> +<td align="justify"> +<img src="images/fig41.GIF" width="286" height="328" alt= +"Figs. 41-44. Four vertical sections of the fertilised ovum of the toad, in four successive stages of development."> +<br><br><a name="Fig. 41">Figs. +41–44</a>—<b>Four vertical sections of the fertilised +ovum of the toad,</b> in four successive stages of development. The +letters have the same meaning throughout: <i>F</i> +segmentation-cavity. <i>D</i> covering of same (<i>D</i> dorsal +half of the embryo, <i>P</i> ventral half). <i>P</i> yelk-stopper +(white round field at the lower pole). <i>Z</i> yelk-cells of the +entoderm (Remak’s “glandular embryo”). <i>N</i> +primitive gut cavity (progaster or Rusconian alimentary cavity). +The primitive mouth (prostoma) is closed by the yelk-stopper, <i>P. +s</i> partition between the primitive gut cavity (<i>N</i>) and the +segmentation cavity (<i>F</i>). <i>k k',</i> section of the large +circular lip-border of the primitive mouth (the Rusconian anus). +The line of dots between <i>k</i> and <i>k'</i> indicates the +earlier connection of the yelk-stopper (<i>P</i>) with the central +mass of the yelk-cells (<i>Z</i>). In Fig. 44 the ovum has turned +90°, so that the back of the embryo is uppermost and the +ventral side down. (From <i>Stricker.</i>).</td> +</tr> +</table> +</center> + +<p class="one">four new longitudinal divisions extend gradually to +the lower cells, and the number rises from twelve to sixteen +(<i>F</i>). Then a second circular furrow appears, parallel to the +first, and nearer to the north pole, so that we may compare it to +the north polar circle. In this way we get twenty-four +segmentation-cells—sixteen upper, smaller, and darker ones, +and eight smaller and brighter ones below (<i>G</i>). Soon, +however, the latter also sub-divide into sixteen, a third or +“meridian of latitude” appearing, this time in the +southern hemisphere: this makes thirty-two cells altogether +(<i>H</i>). Then eight new longitudinal lines are formed at the +north pole, and these proceed to divide, first the darker cells +above and afterwards the lighter southern cells, and finally reach +the south pole. In this way we get in succession forty, +forty-eight, fifty-six, and at last sixty-four cells (<i>I, K</i>). +In the meantime, the two hemispheres differ more and more from each +other. Whereas the sluggish lower hemisphere long remains at +thirty-two cells, the lively northern hemisphere briskly +sub-divides twice, producing first sixty-four and then 128 cells +(<i>L, M</i>). Thus we reach a stage in which we count on the +surface of the ovum 128 small cells in the upper half and +thirty-two large ones in the lower half, or 160 altogether. The +dissimilarity of the two halves increases: while the northern +breaks up into a great number of small cells, the southern consists +of a much smaller number of larger cells. Finally, the dark cells +of the upper half grow almost over the surface of the ovum, leaving +only a small circular spot</p> + +<br> +<hr> +<p class="page"><a name="page 74">[ 74 ]</a></p> + +<p> </p> + +<p class="one">at the south pole, where the large and clear cells +of the lower half are visible. This white region at the south pole +corresponds, as we shall see afterwards, to the primitive mouth of +the gastrula. The whole mass of the inner and larger and clearer +cells (including the white polar region) belongs to the entoderm or +ventral layer. The outer envelope of dark smaller cells forms the +ectoderm or skin-layer.</p> + +<table class="capt" width="193" align="left" summary="Blastula of the water-salamander."> +<tr> +<td align="center"><img src="images/fig45.GIF" width="193" height="181" alt= +"Fig. 45. Blastula of the water-salamander."> +<a name="Fig. 45">Fig. +45</a>—<b>Blastula of the water-salamander</b> +(<i>Triton</i>). <i>fh</i> segmentation-cavity, <i>dz</i> yelk-cells, <i>rz</i> +border-zone. +(From <i>Hertwig.</i>)</td> +</tr> +</table> + +<p>In the meantime, a large cavity, full of fluid, has been formed +within the globular body—the segmentation-cavity or embryonic +cavity (<i>blastocœl,</i> <a href="#Fig. 41">Figs. +41–44 <i>F</i>).</a> It extends considerably as the cleavage +proceeds, and afterwards assumes an almost semi-circular form (Fig. +41 <i>F</i>). The frog-embryo now represents a modified embryonic +vesicle or <i>blastula,</i> with hollow animal half and solid +vegetal half.</p> + +<p>Now a second, narrower but longer, cavity arises by a process of +folding at the lower pole, and by the falling away from each other +of the white entoderm-cells (Figs. 41–44 <i>N</i>). This is +the primitive gut-cavity or the gastric cavity of the gastrula, +progaster or archenteron. It was first observed in the ovum of the +amphibia by Rusconi, and so called the Rusconian cavity. The reason +of its peculiar narrowness here is that it is, for the most part, +full of yelk-cells of the entoderm. These also stop up the whole of +the wide opening of the primitive mouth, and form what is known as +the “yelk-stopper,” which is seen freely at the white +round spot at the south pole (<i>P</i>). Around it the ectoderm is +much thicker, and forms the border of the primitive mouth, the most +important part of the embryo (Fig. 44 <i>k, k'</i>). Soon the +primitive gut-cavity stretches further and further at the expense +of the segmentation-cavity (<i>F</i>), until at last the latter +disappears altogether. The two cavities are only separated by a +thin partition (Fig. 43 <i>s</i>). With the formation of the +primitive gut our frog-embryo has reached the gastrula stage, +though it is clear that this cenogenetic amphibian gastrula is very +different from the real palingenetic gastrula we have considered +(Figs. 30–36).</p> + +<p>In the growth of this hooded gastrula we cannot sharply mark off +the various stages which we distinguish successively in the +bell-gastrula as morula and gastrula. Nevertheless, it is not +difficult to reduce the whole cenogenetic or disturbed development +of this amphigastrula to the true palingenetic formation of the +archigastrula of the amphioxus.</p> + + +<table class="capt" width="187" align="left" summary="Embryonic vesicle of triton."> +<tr> +<td align="justify"><img src="images/fig46.GIF" width="187" height="109" alt= +"Fig. 46. Embryonic vesicle of triton."> +<a name="Fig. 46">Fig. +46</a>—<b>Embryonic vesicle of triton</b> (<i>blastula</i>), +outer view, with the transverse fold of the primitive mouth +(<i>u</i>). (From <i>Hertwig.</i>)</td> +</tr> +</table> + +<p class="pic">This reduction becomes easier if, after considering the +gastrulation of the tailless amphibia (frogs and toads), we glance +for a moment at that of the tailed amphibia, the salamanders. In +some of the latter, that have only recently been carefully studied, +and that are phylogenetically older, the process is much simpler +and clearer than is the case with the former and longer known. Our +common water-salamander (<i>Triton taeniatus</i>) is a particularly +good subject for observation. Its nutritive yelk is much smaller +and its formative yelk less obscured with black pigment-cells than +in the case of the frog; and its gastrulation has better retained +the original palingenetic character. It was first described by +Scott and Osborn (1879), and Oscar Hertwig especially made a +careful study of it (1881), and rightly pointed out its great +importance in helping us to understand the vertebrate development. +Its globular blastula (Fig. 45) consists of loosely-aggregated,</p> + +<br> +<hr> +<p class="page"><a name="page 75">[ 75 ]</a></p> + +<p> </p> + +<p class="one">yelk-filled entodermic cells or yelk-cells +(<i>dz</i>) in the lower vegetal half; the upper, animal half +encloses the hemispherical segmentation-cavity (<i>fh</i>), the +curved roof of which is formed of two or three strata of small +ectodermic cells. At the point where the latter pass into the +former (at the equator of the globular vesicle) we have the border +zone (<i>rz</i>). The folding which leads to the formation of the +gastrula takes place at a spot in this border zone, the primitive +mouth (Fig. 46 <i>u</i>).</p> + + +<table class="capt" width="211" align="left" summary="Sagittal section of a hooded-embryo (depula) of triton."> +<tr> +<td align="justify"><img src="images/fig47.GIF" width="211" height="186" alt= +"Fig. 47. Sagittal section of a hooded-embryo (depula) of triton."> +<a name="Fig. 47">Fig. +47</a>—<b>Sagittal section of a hooded-embryo</b> +(<i>depula</i>) <b>of triton</b> (blastula at the commencement of +gastrulation). <i>ak</i> outer germinal layer, <i>ik</i> inner +germinal layer, <i>fh</i> segmentation-cavity, ud primitive gut, +<i>u</i> primitive mouth, <i>dl</i> and <i>vl</i> dorsal and +ventral lips of the mouth, <i>dz</i> yelk-cells. (From <i> +Hertwig.</i>)</td> +</tr> +</table> + +<p>Unequal segmentation takes place in some of the cyclostoma and +in the oldest fishes in just the same way as in most of the +amphibia. Among the cyclostoma (“round-mouthed”) the +familiar lampreys are particularly interesting. In respect of +organisation and development they are half-way between the acrania +(lancelet) and the lowest real fishes (<i>Selachii</i>); hence I +divided the group of the cyclostoma in 1886 from the real fishes +with which they were formerly associated, and formed of them a +special class of vertebrates. The ovum-segmentation in our common +river-lamprey (<i>Petromyzon fluviatilis</i>) was described by Max +Schultze in 1856, and afterwards by Scott (1882) and Goette +(1890).</p> + +<p>Unequal total segmentation follows the same lines in the oldest +fishes, the selachii and ganoids, which are directly descended from +the cyclostoma. The primitive fishes (<i>Selachii</i>), which we +must regard as the ancestral group of the true fishes, were +generally considered, until a short time ago, to be discoblastic. +It was not until the beginning of the twentieth century that +Bashford Dean made the important discovery in Japan that one of the +oldest living fishes of the shark type (<i>Cestracion +japonicus</i>) has the same total unequal segmentation as the +amphiblastic plated fishes (<i>ganoides</i>).<sup>1</sup> This is +particularly interesting in connection with our subject, because +the few remaining survivors of this division, which was so numerous +in paleozoic times, exhibit three different types of gastrulation.</p> + +<table class="capt" width="215" align="left" summary= +"Sagittal section of the gastrula of the water-salamander."> +<tr> +<td align="justify"><img src="images/fig48.GIF" width="215" height="179" alt= +"Fig. 48. Sagittal section of the gastrula of the water-salamander."> +<a name="Fig. 48">Fig. +48</a>—<b>Sagittal section of the gastrula of the +water-salamander</b> (<i>Triton</i>). (From <i>Hertwig.</i>) +Letters as in Fig. 47; except—<i>p</i> yelk-stopper, <i> +mk</i> beginning of the middle germinal layer.)</td> +</tr> +</table> +<p> +The oldest and most conservative forms of the modern ganoids are +the scaly sturgeons (Sturiones), plated fishes of great +evolutionary importance, the eggs of which are eaten as caviar; +their cleavage is not essentially different from that of the +lampreys and the amphibia. On the other hand, the most modern of +the plated fishes, the beautifully scaled bony pike of the North +American rivers (Lepidosteus), approaches the osseous fishes, and +is discoblastic like them. A third genus (Amia) is midway between +the sturgeons and the latter.</p> +<p>The group of the lung-fishes (<i>Dipneusta</i> or <i>Dipnoi</i>) +is closely connected with the older ganoids. In respect of their +whole organisation they are midway between the gill-breathing +fishes and the lung-breathing amphibia; they share with the former +the shape of the body and limbs, and with the latter the form of +the heart</p> + +<p class="fnote">1. Bashford Dean, <i>Holoblastic Cleavage in the +Egg of a Shark, Cestracion japonicus Macleay. Annotationes +zoologicae japonenses,</i> vol. iv, Tokio, 1901.</p> + +<br><br> +<hr> +<p class="page"><a name="page 76">[ 76 ]</a></p> + +<p> </p> + +<p class="one">and lungs. Of the older dipnoi +(<i>Paladipneusta</i>) we have now only one specimen, the +remarkable Ceratodus of East Australia; its amphiblastic +gastrulation has been recently explained by Richard Semon (cf. +Chapter XXI). That of the two modern dipneusta, of which <i> +Protopterus</i> is found in Africa and <i>Lepidosiren</i> in +America, is not materially different. (Cf. Fig. 51.)</p> + +<center> +<table class="capt" summary= +"Fig. 49. Ovum-segmentation in the lamprey."> +<tr> +<td width="306" align="justify"><img src="images/fig49.GIF" width="306" height="100" alt= +"Ovum-segmentation in the lamprey."> +<br><br><a name="Fig. 49">Fig. +49</a>—<b>Ovum-segmentation of the lamprey</b> (<i>Petromyzon +fluviatalis</i>), in four successive stages. The small cells of the +upper (animal) hemisphere divide much more quickly than the cells +of the lower (vegetal) hemisphere.</td> +</tr> +</table> + +<br> +<table class="capt" summary= +"Fig. 50. Gastrulation of the lamprey."> +<tr> +<td width="468" align="justify"> +<img src="images/fig50.GIF" width="468" height="152" alt= +"Gastrulation of the lamprey."> +<br><br><a name="Fig. 50">Fig. +50</a>—<b>Gastrulation of the lamprey</b> (<i>Petromyzon +fluviatilis</i>). A blastula, with wide embryonic cavity +(blastocoel, <i>bl</i>), <i>g</i> incipient invagination. <i>B</i> +depula, with advanced invagination, from the primitive mouth +(<i>g</i>). <i>C</i> gastrula, with complete primitive gut: the +embryonic cavity has almost disappeared in consequence of +invagination.</td> +</tr> +</table> +</center> + +<p>All these amphiblastic vertebrates, <i>Petromyzon</i> and <i> +Cestracion, Accipenser</i> and <i>Ceratodus,</i> and also the +salamanders and batrachia, belong to the old, conservative groups +of our stem. Their unequal ovum-segmentation and gastrulation have +many peculiarities in detail, but can always be reduced with +comparative ease to the original cleavage and gastrulation of the +lowest vertebrate, the amphioxus; and this is little removed, as we +have seen, from the very simple archigastrula of the <i>Sagitta</i> +and <i>Monoxenia</i> (see <a href="chap8.html#Fig. 29">Fig. +29–36</a>). All these and many other classes of animals +generally agree in the circumstance that in segmentation their</p> + +<br> +<hr> +<p class="page"><a name="page 77">[ 77 ]</a></p> + +<p> </p> + +<p class="one">ovum divides into a large number of cells by +repeated cleavage. All such ova have been called, after Remak, +“whole-cleaving” (<i>holoblasta</i>), because their +division into cells is complete or total.</p> + +<br> + + +<center> +<table class="capt" width="314" summary= +"Fig. 51. Gastrulation of ceratodus."> +<tr> +<td align="justify"> +<img src="images/fig51.GIF" width="314" height="332" alt= +"Gastrulation of ceratodus."> +<br><br><a name="Fig. 51">Fig. +51</a>—<b>Gastrulation of ceratodus</b> (from <i>Semon</i>). +<i>A</i> and <i>C</i> stage with four cells, <i>B</i> and <i>D</i> +with sixteen cells. <i>A</i> and <i>B</i> are seen from above, <i> +C</i> and <i>D</i> sideways. <i>E</i> stage with thirty-two cells; +<i>F</i> blastula; <i>G</i> gastrula in longitudinal section. <i> +fh</i> segmentation-cavity. <i>gh</i> primitive gut or gastric +cavity.</td> +</tr> +</table> +</center> + +<p>In a great many other classes of animals this is not the case, +as we find (in the vertebrate stem) among the birds, reptiles, and +most of the fishes; among the insects and most of the spiders and +crabs (of the articulates); and the cephalopods (of the molluscs). +In all these animals the mature ovum, and the stem-cell that arises +from it in fertilisation, consist of two different and separate +parts, which we have called formative yelk and nutritive yelk. The +formative yelk alone consists of living protoplasm, and is the +active, evolutionary, and nucleated part of the ovum; this alone +divides in segmentation, and produces the numerous cells which make +up the embryo. On the other hand, the nutritive yelk is merely a +passive part of the contents of the ovum, a subordinate element +which contains nutritive material (albumin, fat, etc.), and so +represents in a sense the provision-store of the developing embryo. +The latter takes a quantity of food out of this store, and finally +consumes it all. Hence the nutritive yelk is of great indirect +importance in embryonic development, though it has no direct share +in it. It either does not divide at all, or only later on, and does +not generally consist of cells. It is sometimes large and sometimes +small, but generally many times larger than the formative yelk; and +hence it is</p> + +<br> +<hr> +<p class="page"><a name="page 78">[ 78 ]</a></p> + +<p> </p> + +<p class="one">that it was formerly thought the more important of +the two. As the respective significance of these two parts of the +ovum is often wrongly described, it must be borne in mind that the +nutritive yelk is only a secondary addition to the primary cell, it +is an inner enclosure, not an external appendage. All ova that have +this independent nutritive yelk are called, after Remak, +“partially-cleaving” (<i>meroblasta</i>). Their +segmentation is incomplete or partial.</p> + + +<table class="capt" width="202" align="left" summary= +"Fig. 52. Ovum of a deep-sea bony fish."> +<tr> +<td align="justify"><img src="images/fig52.GIF" width="202" height= +"123" alt="Ovum of a deep-sea bony fish."> +<a name="Fig. 52">Fig. +52</a>—<b>Ovum of a deep-sea bony fish.</b> <i>b</i> +protoplasm of the stem-cell, <i>k</i> nucleus of same, <i>d</i> +clear globule of albumin, the nutritive yelk, <i>f</i> fat-globule +of same, <i>c</i> outer membrane of the ovum, or ovolemma.)</td> +</tr> +</table> + +<p>There are many difficulties in the way of understanding this +partial segmentation and the gastrula that arises from it. We have +only recently succeeded, by means of comparative research, in +overcoming these difficulties, and reducing this cenogenetic form +of gastrulation to the original palingenetic type. This is +comparatively easy in the small meroblastic ova which contain +little nutritive yelk—for instance, in the marine ova of a +bony fish, the development of which I observed in 1875 at Ajaccio +in Corsica. I found them joined together in lumps of jelly, +floating on the surface of the sea; and, as the little ovula were +completely transparent, I could easily follow the development of +the germ step by step. These ovula are glossy and colourless +globules of little more than the 50th of an inch. Inside a +structureless, thin, but firm membrane (<i>ovolemma,</i> Fig. 52 +<i>c</i>) we find a large, quite clear, and transparent globule of +albumin (<i>d</i>). At both poles of its axis this globule has a +pit-like depression. In the pit at the upper, animal pole (which is +turned downwards in the floating ovum) there is a bi-convex lens +composed of protoplasm, and this encloses the nucleus (<i>k</i>); +this is the formative yelk of the stem-cell, or the germinal disk +(<i>b</i>). The small fat-globule (<i>f</i>) and the large +albumin-globule (<i>d</i>) together form the nutritive yelk. Only +the formative yelk undergoes cleavage, the nutritive yelk not +dividing at all at first.</p> + +<p>The segmentation of the lens-shaped formative yelk (<i>b</i>) +proceeds quite independently of the nutritive yelk, and in perfect +geometrical order.</p> + +<p>When the mulberry-like cluster of cells has been formed, the +border-cells of the lens separate from the rest and travel into the +yelk and the border-layer. From this the blastula is developed; the +regular bi-convex lens being converted into a disk, like a +watch-glass, with thick borders. This lies on the upper and less +curved polar surface of the nutritive yelk like the watch glass on +the yelk. Fluid gathers between the outer layer and the border, and +the segmentation-cavity is formed. The gastrula is then formed by +invagination, or a kind of turning-up of the edge of the +blastoderm. In this process the segmentation-cavity disappears.</p> + +<p>The space underneath the entoderm corresponds to the primitive +gut-cavity, and is filled with the decreasing food-yelk (<i>n</i>). +Thus the formation of the gastrula of our fish is complete. In +contrast to the two chief forms of gastrula we considered +previously, we give the name of discoid gastrula +(<i>discogastrula,</i> <a href="#Fig. 54">Fig. 54</a>) to this +third principal type.</p> + +<p>Very similar to the discoid gastrulation of the bony fishes is +that of the hags or myxinoida, the remarkable cyclostomes that live +parasitically in the body-cavity of fishes, and are distinguished +by several notable peculiarities from their nearest relatives, the +lampreys. While the amphiblastic ova of the latter are small and +develop like those of the amphibia, the cucumber-shaped ova of the +hag are about an inch long, and form a discoid gastrula. Up to the +present it has only been observed in one species (<i>Bdellostoma +Stouti</i>), by Dean and Doflein (1898).</p> + +<p>It is clear that the important features which distinguish the +discoid gastrula from the other chief forms we have considered are +determined by the large food-yelk. This takes no direct part in the +building of the germinal layers, and completely fills the primitive +gut-cavity of the gastrula, even protruding at the mouth-opening. +If we imagine the original bell-gastrula (Figs. 30–36) trying +to swallow a</p> + +<br> +<hr> +<p class="page"><a name="page 79">[ 79 ]</a></p> + +<p> </p> + +<p class="one">ball of food which is much bigger than itself, it +would spread out round it in discoid shape in the attempt, just as +we find to be the case here (Fig. 54). Hence we may derive the +discoid gastrula from the original bell-gastrula, through the +intermediate stage of the hooded gastrula. It has arisen through +the accumulation of a store of food-stuff at the vegetal pole, a +“nutritive yelk” being thus formed in contrast to the +“formative yelk.” Nevertheless, the gastrula is formed +here, as in the previous cases, by the folding or invagination of +the blastula. We can, therefore, reduce this cenogenetic form of +the discoid segmentation to the palingenetic form of the primitive +cleavage.</p> + +<br> + + +<center> +<table class="capt" summary= +"Fig. 53. Ovum-segmentation of a bony fish."> +<tr> +<td width="420" align="justify"> +<img src="images/fig53.GIF" width="420" height="153" alt= +"Ovum-segmentation of a bony fish."> +<br><br><a name="Fig. 53">Fig. +53</a>—<b>Ovum-segmentation of a bony fish.</b> <i>A</i> +first cleavage of the stem-cell (<i>cytula</i>), <i>B</i> division +of same into four segmentation-cells (only two visible), <i>C</i> +the germinal disk divides into the blastoderm (<i>b</i>) and the +periblast (<i>p</i>). <i>d</i> nutritive yelk, <i>f</i> +fat-globule, <i>c</i> ovolemma, <i>z</i> space between the ovolemma +and the ovum, filled with a clear fluid.)</td> +</tr> +</table> +</center> + +<p>This reduction is tolerably easy and confident in the case of +the small ovum of our deep-sea bony fish, but it becomes difficult +and uncertain in the case of the large ova that we find in the +majority of the other fishes and in all the reptiles and birds. In +these cases the food-yelk is, in the first place, comparatively +colossal, the formative yelk being almost invisible beside it; and, +in the second place, the food-yelk contains a quantity of different +elements, which are known as “yelk-granules, yelk-globules, +yelk-plates, yelk-flakes, yelk-vesicles,” and so on. +Frequently these definite elements in the yelk have been described +as real cells, and it has been wrongly stated that a portion of the +embryonic body is built up from these cells. This is by no means +the case. In every case, however large it is—and even when +cell-nuclei travel into it during the cleavage of the +border—the nutritive yelk remains a dead accumulation of +food, which is taken into the gut during embryonic development and +consumed by the embryo. The latter develops solely from the living +formative yelk of the stem-cell. This is equally true of the ova of +our small bony fishes and of the colossal ova of the primitive +fishes, reptiles, and birds.</p> + +<p>The gastrulation of the primitive fishes or selachii (sharks and +rays) has been carefully studied of late years by Ruckert, Rabl, +and H.E. Ziegler in particular, and is very important in the sense +that this group is the oldest among living fishes, and their +gastrulation can be derived directly from that of the cyclostoma by +the accumulation of a large quantity of food-yelk. The oldest +sharks (<i>Cestracion</i>) still have the unequal segmentation +inherited from the cyclostoma. But while in this case, as in the +case of the amphibia, the small ovum completely divides into cells +in segmentation, this is no longer so in the great majority of the +selachii (or <i>Elasmobranchii</i>). In these the contractility of +the active protoplasm no longer suffices to break up the huge mass +of the passive deutoplasm completely into cells; this is only +possible in the upper or dorsal part, but not in the lower or +ventral section. Hence we find in the primitive fishes a blastula +with a small eccentric segmentation-cavity <a href="#Fig. 55">(Fig. +55 <i>b</i>),</a> the wall of which varies greatly in composition. +The circular border of the germinal disk which connects the roof +and floor of the segmentation-cavity corresponds to the border-zone +at the equator of the amphibian ovum. In the middle of its hinder +border we have the beginning of the invagination of the primitive +gut</p> + +<br> +<hr> +<p class="page"><a name="page 80">[ 80 ]</a></p> + +<p> </p> + +<p class="one"><a href="#Fig. 56">(Fig. 56 <i>ud</i>)</a>; it +extends gradually from this spot (which corresponds to the +Rusconian anus of the amphibia) forward and around, so that the +primitive mouth becomes first crescent-shaped and then circular, +and, as it opens wider, surrounds the ball of the larger +food-yelk.</p> + + +<table class="capt" width="201" align="left" summary= +"Fig. 54. Discoid gastrula (discogastrula) of a bony fish."> +<tr> +<td align="justify"><img src="images/fig54.GIF" width="201" height="130" alt= +"Discoid gastrula (discogastrula) of a bony fish."> +<a name="Fig. 54">Fig. +54</a>—<b>Discoid gastrula</b> (<i>discogastrula</i>) <b>of a +bony fish.</b> <i>e</i> ectoderm, <i>i</i> entoderm, <i>w</i> +border-swelling or primitive mouth, <i>n</i> albuminous globule of +the nutritive yelk, <i>f</i> fat-globule of same, <i>c</i> external +membrane (ovolemma), <i>d</i> partition between entoderm and +ectoderm (earlier the segmentation-cavity.)</td> +</tr> +</table> + +<p>Essentially different from the wide-mouthed discoid gastrula of +most of the selachii is the narrow-mouthed discoid gastrula (or <i> +epigastrula</i>) of the amniotes, the reptiles, birds, and +monotremes; between the two—as an intermediate stage—we +have the <i>amphigastrula</i> of the amphibia. The latter has +developed from the amphigastrula of the ganoids and dipneusts, +whereas the discoid amniote gastrula has been evolved from the +amphibian gastrula by the addition of food-yelk. This change of +gastrulation is still found in the remarkable ophidia +(<i>Gymnophiona, Cœcilia,</i> or <i>Peromela</i>), +serpent-like amphibia that live in moist soil in the tropics, and +in many respects represent the transition from the gill-breathing +amphibia to the lung-breathing reptiles. Their embryonic +development has been explained by the fine studies of the brothers +Sarasin of <i>Ichthyophis glutinosa</i> at Ceylon (1887), and those +of August Brauer of the <i>Hypogeophis rostrata</i> in the +Seychelles (1897). It is only by the historical and comparative +study of these that we can understand the difficult and obscure +gastrulation of the amniotes.</p> + +<table class="capt" width="210" align="left" summary= +"Fig. 55. Longitudinal section through the blastula of a shark (Pristiuris)."> +<tr> +<td><img src="images/fig55.GIF" width="210" height="121" alt= +"Longitudinal section through the blastula of a shark."> +<a name="Fig. 55">Fig. +55</a>—<b>Longitudinal section through the blastula of a +shark</b> (<i>Pristiuris</i>). (From <i>Ruckert.</i>) (Looked at +from the left; to the right is the hinder end, <i>H,</i> to the +left the fore end, <i>V.</i>) <i>B</i> segmentation-cavity, <i> +kz</i> cells of the germinal membrane, <i>dk</i> yelk-nuclei.</td> +</tr> +</table> + +<p>The bird’s egg is particularly important for our purpose, +because most of the chief studies of the development of the +vertebrates are based on observations of the hen’s egg during +hatching. The mammal ovum is much more difficult to obtain and +study, and for this practical and obvious reason very rarely +thoroughly investigated. But we can get hens’ eggs in any +quantity at any time, and, by means of artificial incubation, +follow the development of the embryo step by step. The bird’s +egg differs considerably from the tiny mammal ovum in size, a large +quantity of food-yelk accumulating within the original yelk or the +protoplasm of the ovum. This is the yellow ball which we commonly +call the yolk of the egg. In order to understand the bird’s +egg aright—for it is very often quite wrongly +explained—we must examine it in its original condition, and +follow it from the very beginning of its development in the +bird’s ovary. We then see that the original ovum is a quite +small, naked, and simple cell with a nucleus, not differing in +either size or shape from the original ovum of the mammals and +other animals (cf. <a href="chap6.html#Fig. 13">Fig. 13 <i> +E</i></a>). As in the case of all the craniota (animals with a +skull), the original or primitive ovum (<i>protovum</i>) is covered +with a continuous layer of small cells. This membrane is the +follicle, from which the ovum afterwards issues. Immediately +underneath it the structureless yelk-membrane is secreted from the +yelk.</p> + +<p>The small primitive ovum of the bird begins very early to take +up into itself a quantity of food-stuff through the yelk-membrane, +and work it up into the “yellow yelk.” In this way the +ovum</p> + +<br> +<hr> +<p class="page"><a name="page 81">[ 81 ]</a></p> + +<p> </p> + +<p class="one">enters on its second stage (the metovum), which is +many times larger than the first, but still only a single enlarged +cell. Through the accumulation of the store of yellow yelk within +the ball of protoplasm the nucleus it contains (the germinal +vesicle) is forced to the surface of the ball. Here it is +surrounded by a small quantity of protoplasm, and with this forms +the lens-shaped formative yelk <a href="chap6.html#Fig. 15">(Fig. +15 <i>b</i>).</a> This is seen on the yellow yelk-ball, at a +certain point of the surface, as a small round white spot—the +“tread” (<i>cicatricula</i>). From this point a +thread-like column of white nutritive yelk (<i>d</i>), which +contains no yellow yelk-granules, and is softer than the yellow +food-yelk, proceeds to the middle of the yellow yelk-ball, and +forms there a small central globule of white yelk (Fig. 15 <i> +d</i>). The whole of this white yelk is not sharply separated from +the yellow yelk, which shows a slight trace of concentric layers in +the hard-boiled egg (Fig. 15 <i>c</i>). We also find in the +hen’s egg, when we break the shell and take out the yelk, a +round small white disk at its surface which corresponds to the +tread. But this small white “germinal disk” is now +further developed, and is really the gastrula of the chick. The +body of the chick is formed from it alone. The whole white and +yellow yelk-mass is without any significance for the formation of +the embryo, it being merely used as food by the developing chick. +The clear, glarous mass of albumin that surrounds the yellow yelk +of the bird’s egg, and also the hard chalky shell, are only +formed within the oviduct round the impregnated ovum.</p> + +<br> + + +<table class="capt" summary= +"Fig. 56. Longitudinal section of the blastula of a shark (Pristiurus) at the beginning of gastrulation."> +<tr> +<td><img src="images/fig56.GIF" width="329" height="121" alt= +"Longitudinal section of the blastula of a shark (Pristiurus) at the beginning of gastrulation."> +</td> +<td align="left" valign="bottom"><a name="Fig. 56">Fig. +56</a>—<b>Longitudinal section of the blastula of a shark</b> +(<i>Pristiurus</i>) at the beginning of gastrulation. (From <i> +Ruckert.</i>) (Seen from the left.) <i>V</i> fore end, <i>H</i> +hind end, <i>B</i> segmentation-cavity, <i>ud</i> first trace of +the primitive gut, <i>dk</i> yelk-nuclei, <i>fd</i> fine-grained +yelk, <i>gd</i> coarse-grained yelk.</td> +</tr> +</table> + +<p>When the fertilisation of the bird’s ovum has taken place +within the mother’s body, we find in the lens-shaped +stem-cell the progress of flat, discoid segmentation <a href= +"#Fig. 57">(Fig. 57).</a> First two equal segmentation-cells +(<i>A</i>) are formed from the ovum. These divide into four +(<i>B</i>), then into eight, sixteen (<i>C</i>), thirty-two, +sixty-four, and so on. The cleavage of the cells is always preceded +by a division of their nuclei. The cleavage surfaces between the +segmentation-cells appear at the free surface of the tread as +clefts. The first two divisions are vertical to each other, in the +form of a cross (<i>B</i>). Then there are two more divisions, +which cut the former at an angle of forty-five degrees. The tread, +which thus becomes the germinal disk, now has the appearance of an +eight-rayed star. A circular cleavage next taking place round the +middle, the eight triangular cells divide into sixteen, of which +eight are in the middle and eight distributed around (<i>C</i>). +Afterwards circular clefts and radial clefts, directed towards the +centre, alternate more or less irregularly (<i>D, E</i>). In most +of the amniotes the formation of concentric and radial clefts is +irregular from the very first; and so also in the hen’s egg. +But the final outcome of the cleavage-process is once more the +formation of a large number of small cells of a similar nature. As +in the case of the fish-ovum, these segmentation-cells form a +round, lens-shaped disk, which corresponds to the morula, and is +embedded in a small depression of the white yelk. Between the +lens-shaped disk of the morula-cells and the underlying white yelk +a small cavity is now formed by the accumulation of fluid, as in +the fishes. Thus we get the peculiar and not easily recognisable +blastula of the bird <a href="#Fig. 58">(Fig. 58).</a> The small +segmentation-cavity (<i>fh</i>) is very flat and much compressed. +The upper or dorsal wall (<i>dw</i>) is formed of a single layer of +clear, distinctly separated cells; this</p> + +<br> +<hr> +<p class="page"><a name="page 82">[ 82 ]</a></p> + +<p> </p> + +<p class="one">corresponds to the upper or animal hemisphere of the +triton-blastula <a href="#Fig. 45">(Fig. 45).</a> The lower or +ventral wall of the flat dividing space (<i>vw</i>) is made up of +larger and darker segmentation-cells; it corresponds to the lower +or vegetal hemisphere of the blastula of the water-salamander (Fig. +45 <i>dz</i>). The nuclei of the yelk-cells, which are in this case +especially numerous at the edge of the lens-shaped blastula, travel +into the white yelk, increase by cleavage, and contribute even to +the further growth of the germinal disk by furnishing it with +food-stuff.</p> + +<br> + + +<center> +<table class="capt" width="314" summary= +"Diagram of discoid segmentation in the bird's ovum."> +<tr> +<td align="justify"><img src="images/fig57.GIF" width="314" height="206" alt= +"Diagram of discoid segmentation in the bird's ovum."> +<br><br><a name="Fig. 57">Fig. 57</a>—<b>Diagram +of discoid segmentation in the bird’s ovum</b> (magnified). +Only the formative yelk (the tread) is shown in these six figures +(<i>A</i> to <i>F</i>), because cleavage only takes place in this. +The much larger food-yelk, which does not share in the cleavage, is +left out and merely indicated by the dark ring without.</td> +</tr> +</table> +</center> + +<p>The invagination or the folding inwards of the bird-blastula +takes place in this case also at the hinder pole of the subsequent +chief axis, in the middle of the hind border of the round germinal +disk <a href="#Fig. 58">(Fig. 59 <i>s</i>).</a> At this spot we +have the most brisk cleavage of the cells; hence the cells are more +numerous and smaller here than in the fore-half of the germinal +disk. The border-swelling or thick edge of the disk is less clear +but whiter behind, and is more sharply separated from contiguous +parts. In the middle of its hind border there is a white, +crescent-shaped groove—Koller’s sickle-groove (Fig. 59 +<i>s</i>); a small projecting process in the centre of it is called +the sickle-knob (<i>sk</i>). This important cleft is the primitive +mouth, which was described for a long time as the “primitive +groove.” If we make a vertical section through this part, we +see that a flat and broad cleft stretches under the germinal disk +forwards from the primitive mouth; this is the primitive gut (Fig. +60 <i>ud</i>). Its roof or dorsal wall is formed by the folded +upper part of the blastula, and its floor or ventral wall by the +white yelk (<i>wd</i>), in which a number of yelk-nuclei +(<i>dk</i>) are distributed. There is a brisk multiplication of +these at the edge of the germinal disk, especially in the +neighbourhood of the sickle-shaped primitive mouth.</p> + +<p>We learn from sections through later stages of this discoid +bird-gastrula that the primitive gut-cavity, extending forward from +the primitive mouth as a flat pouch, undermines the whole region of +the round flat lens-shaped blastula <a href="#Fig. 61">(Fig. 61 <i> +ud</i>).</a> At the same time, the segmentation-cavity gradually +disappears altogether, the folded inner germinal layer (<i>ik</i>) +placing itself from underneath on the overlying outer germinal +layer (<i>ak</i>). The typical process of invagination, though +greatly disguised, can thus be clearly seen in this case, as Goette +and Rauber, and more recently Duval (Fig. 61), have shown.</p> + +<p>The older embryologists (Pander, Baer, Remak), and, in recent +times especially,</p> + +<br> +<hr> +<p class="page"><a name="page 83">[ 83 ]</a></p> + +<p> </p> + +<center> +<table class="capt" width="385" summary= +"Fig. 58. Vertical section of the bastula of a hen. Fig. 59. The germinal disk of the hen's ovum at the beginning of gastrulation. Fig. 60. Longitudinal section of the germinal disk of a siskin."> +<tr> +<td align="justify"><img src="images/fig58.GIF" width="385" height="285" alt= +"Fig. 58. Vertical section of the bastula of a hen. Fig. 59. The germinal disk of the hen's ovum at the beginning of gastrulation. Fig. 60. Longitudinal section of the germinal disk of a siskin."> +<br><br><a name="Fig. 58">Fig. 58</a>—<b>Vertical +section of the blastula of a hen</b> (<i>discoblastula</i>). <i> +fh</i> segmentation-cavity, <i>dw</i> dorsal wall of same, <i> +vw</i> ventral wall, passing directly into the white yelk +(<i>wd</i>). (From <i>Duval.</i>)<br> +Fig. 59—<b>The germinal disk of the hen’s ovum at the +beginning of gastrulation;</b> <i>A</i> before incubation, <i>B</i> +in the first hour of incubation. (From <i>Koller.</i>) <i>ks</i> +germinal-disk, <i>V</i> its fore and <i>H</i> its hind border; <i> +es</i> embryonic shield, <i>s</i> sickle-groove, <i>sk</i> sickle +knob, <i>d</i> yelk.<br> +Fig. 60—<b>Longitudinal section of the germinal disk of a +siskin</b> (<i>discogastrula</i>). (From <i>Duval.</i>) <i>ud</i> +primitive gut, <i>vl, hl</i> fore and hind lips of the primitive +mouth (or sickle-edge); <i>ak</i> outer germinal layer, <i>ik</i> +inner germinal layer, <i>dk</i> yelk-nuclei, <i>wd</i> white +yelk.</td> +</tr> +</table> + +<br> +<table class="capt" width="443" summary= +"Fig. 61. Longitudinal section of the discoid gastrula of the nightingale."> +<tr> +<td align="justify"> +<img src="images/fig61.GIF" width="443" height="87" alt= +"Longitudinal section of the discoid gastrula of the nightingale."> +<br><br><a name="Fig. 61">Fig. +61</a>—<b>Longitudinal section of the discoid gastrula of the +nightingale.</b> (From <i>Duval.</i>) <i>ud</i> primitive gut, <i> +vl, hl</i> fore and hind lips of the primitive mouth; <i>ak, ik</i> +outer and inner germinal layers; <i>vr</i> fore-border of the +discogastrula.</td> +</tr> +</table> +</center> + +<p>His, Kölliker, and others, said that the two primary +germinal layers of the hen’s ovum—the oldest and most +frequent subject of observation!—arose by horizontal cleavage +of a simple germinal disk. In opposition to this accepted view, I +affirmed in my <i>Gastræa Theory</i> (1873) that the discoid +bird-gastrula, like that of all other vertebrates, is formed by +folding (or invagination), and that this typical process is merely +altered in a peculiar way and disguised by the immense accumulation +of food-yelk and the flat spreading of the discoid blastula at one +part of its surface. I endeavoured to establish this view by the +derivation of the vertebrates from one source, and especially by +proving that the birds descend from the reptiles, and these from +the amphibia. If this is correct, the discoid gastrula of the +amniotes must have been formed by the folding-in of a hollow +blastula, as has been shown by Remak and Rusconi of the discoid +gastrula of the amphibia, their direct ancestors. The accurate and +extremely careful observations of the authors I have mentioned +(Goette, Rauber, and Duval) have decisively proved this</p> + +<br> +<hr> +<p class="page"><a name="page 84">[ 84 ]</a></p> + +<p> </p> + +<p class="one">recently for the birds; and the same has been done +for the reptiles by the fine studies of Kupffer, Beneke, Wenkebach, +and others. In the shield-shaped germinal disk of the lizard (Fig. +62), the crocodile, the tortoise, and other reptiles, we find in +the middle of the hind border (at the same spot as the sickle +groove in the bird) a transverse furrow (<i>u</i>), which leads +into a flat, pouch-like, blind sac, the primitive gut. The fore +(dorsal) and hind (ventral) lips of the transverse furrow +correspond exactly to the lips of the primitive mouth (or +sickle-groove) in the birds.</p> + + +<table class="capt" width="246" align="left" summary= +"Fig. 62. Germinal disk of the lizard."> +<tr> +<td align="justify"><img src="images/fig62.GIF" width="246" height="221" alt= +"Germinal disk of the lizard."> +<a name="Fig. 62">Fig. +62</a>—<b>Germinal disk of the lizard</b> (<i>Lacerta +agilis</i>). (From <i>Kupffer.</i>) <i>u</i> primitive mouth, <i> +s</i> sickle, <i>es</i> embryonic shield, <i>hf</i> and <i>df</i> +light and dark germinative area.</td> +</tr> +</table> + +<p>The gastrulation of the mammals must be derived from this +special embryonic development of the reptiles and birds. This +latest and most advanced class of the vertebrates has, as we shall +see afterwards, evolved at a comparatively recent date from an +older group of reptiles; and all these amniotes must have come +originally from a common stem-form. Hence the distinctive embryonic +process of the mammal must have arisen by cenogenetic modifications +from the older form of gastrulation of the reptiles and birds. +Until we admit this thesis we cannot understand the formation of +the germinal layers in the mammal, and therefore in man.</p> + +<p>I first advanced this fundamental principle in my essay <i>On +the Gastrulation of Mammals</i> (1877), and sought to show in this +way that I assumed a gradual degeneration of the food-yelk and the +yelk-sac on the way from the proreptiles to the mammals. “The +cenogenetic process of adaptation,” I said, “which has +occasioned the atrophy of the rudimentary yelk-sac of the mammal, +is perfectly clear. It is due to the fact that the young of the +mammal, whose ancestors were certainly oviparous, now remain a long +time in the womb. As the great store of food-yelk, which the +oviparous ancestors gave to the egg, became superfluous in their +descendants owing to the long carrying in the womb, and the +maternal blood in the wall of the uterus made itself the chief +source of nourishment, the now useless yelk-sac was bound to +atrophy by embryonic adaptation.”</p> + +<p>My opinion met with little approval at the time; it was +vehemently attacked by Kölliker, Hensen, and His in +particular. However, it has been gradually accepted, and has +recently been firmly established by a large number of excellent +studies of mammal gastrulation, especially by Edward Van +Beneden’s studies of the rabbit and bat, Selenka’s on +the marsupials and rodents, Heape’s and +Lieberkühn’s on the mole, Kupffer and Keibel’s on +the rodents, Bonnet’s on the ruminants, etc. From the general +comparative point of view, Carl Rabl in his theory of the mesoderm, +Oscar Hertwig in the latest edition of his Manual (1902), and +Hubrecht in his <i>Studies in Mammalian Embryology</i> (1891), have +supported the opinion, and sought to derive the peculiarly modified +gastrulation of the mammal from that of the reptile.</p> + +<p>In the meantime (1884) the studies of Wilhelm Haacke and +Caldwell provided a proof of the long-suspected and very +interesting fact, that the lowest mammals, the monotremes, <i>lay +eggs,</i> like the birds and reptiles, and are not viviparous like +the other mammals. Although the gastrulation of the monotremes was +not really known until studied by Richard</p> + +<br> +<hr> +<p class="page"><a name="page 85">[ 85 ]</a></p> + +<p> </p> + +<p class="one">Semon in 1894, there could be little doubt, in view +of the great size of their food-yelk, that their ovum-segmentation +was discoid, and led to the formation of a sickle-mouthed +discogastrula, as in the case of the reptiles and birds. Hence I +had, in 1875 (in my essay on <i>The Gastrula and Ovum-segmentation +of Animals</i>), counted the monotremes among the discoblastic +vertebrates. This hypothesis was established as a fact nineteen +years afterwards by the careful observations of Semon; he gave in +the second volume of his great work, <i>Zoological Journeys in +Australia</i> (1894), the first description and correct explanation +of the discoid gastrulation of the monotremes. The fertilised ova +of the two living monotremes (<i>Echidna</i> and <i> +Ornithorhynchus</i>) are balls of one-fifth of an inch in diameter, +enclosed in a stiff shell; but they grow considerably during +development, so that when laid the egg is three times as large. The +structure of the plentiful yelk, and especially the relation of the +yellow and the white yelk, are just the same as in the reptiles and +birds. As with these, partial cleavage takes place at a spot on the +surface at which the small formative yelk and the nucleus it +encloses are found. First is formed a lens-shaped circular germinal +disk. This is made up of several strata of cells, but it spreads +over the yelk-ball, and thus becomes a one-layered blastula.</p> + + +<table class="capt" width="202" align="left" summary= +"Fig. 63. Ovum of the opossum (Didelphys) divided into four."> +<tr> +<td align="justify"><img src="images/fig63.GIF" width="202" height="205" alt= +"Ovum of the opossum (Didelphys) divided into four."> +<a name="Fig. 63">Fig. +63</a>—<b>Ovum of the opossum</b> (<i>Didelphys</i>) <b> +divided into four.</b> (From <i>Selenka.</i>) <i>b</i> the four +segmentation-cells, <i>r</i> directive body, <i>c</i> unnucleated +coagulated matter, <i>p,</i> albumin-membrane.</td> +</tr> +</table> + +<p>If we then imagine the yelk it contains to be dissolved and replaced by a +clear liquid, we have the characteristic blastula of the higher +mammals. In these the gastrulation proceeds in two phases, as Semon +rightly observes: firstly, formation of the entoderm by cleavage at +the centre and further growth at the edge; secondly, invagination. +In the monotremes more primitive conditions have been retained +better than in the reptiles and birds. In the latter, before the +commencement of the gastrula-folding, we have, at least at the +periphery, a two-layered embryo forming from the cleavage. But in +the monotremes the formation of the cenogenetic entoderm does not +precede the invagination; hence in this case the construction of +the germinal layers is less modified than in the other amniota.</p> + +<table class="capt" width="198" align="left" summary= +"Fig. 64. Blastula of the opossum (Didelphys)."> +<tr> +<td align="justify"><img src="images/fig64.GIF" width="198" height="169" alt= +"Blastula of the opossum (Didelphys)."> +<a name="Fig. 64">Fig. +64</a>—<b>Blastula of the opossum</b> (<i>Didelphys</i>). +(From <i>Selenka.</i>) <i>a</i> animal pole of the blastula, <i> +v</i> vegetal pole, <i>en</i> mother-cell of the entoderm, <i> +ex</i> ectodermic cells, <i>s</i> spermia, <i>ib</i> unnucleated +yelk-balls (remainder of the food-yelk), <i>p</i> albumin +membrane.</td> +</tr> +</table> + +<p>The marsupials, a second sub-class, come next to the oviparous +monotremes, the oldest of the mammals. But as in their case the +food-yelk is already atrophied, and the little ovum develops within +the mother’s body, the partial cleavage has been reconverted +into total. One section of the marsupials still show points of +agreement with the monotremes, while another section of them, +according to the splendid investigations of Selenka, form a +connecting-link between these and the placentals.</p> + +<p>The fertilised ovum of the opossum (<i>Didelphys</i>) divides, +according to Selenka, first into two, then four, then eight equal +cells; hence the segmentation is at first equal or homogeneous. But +in the course of the cleavage a larger cell, distinguished by its +less clear plasm and its containing more yelk-granules (the mother +cell of the entoderm, Fig. 64 <i>en</i>),</p> + +<br> +<hr> +<p class="page"><a name="page 86">[ 86 ]</a></p> + +<p> </p> + +<p class="one">separates from the others; the latter multiply more +rapidly than the former. As, further, a quantity of fluid gathers +in the morula, we get a round blastula, the wall of which is of +varying thickness, like that of the amphioxus <a href= +"chap8.html#Fig. 38">(Fig. 38 <i>E</i>)</a> and the amphibia <a +href="#Fig. 45">(Fig. 45).</a> The upper or animal hemisphere is +formed of a large number of small cells; the lower or vegetal +hemisphere of a small number of large cells. One of the latter, +distinguished by its size (Fig. 64 <i>en</i>), lies at the vegetal +pole of the blastula-axis, at the point where the primitive mouth +afterwards appears. This is the mother-cell of the entoderm; it now +begins to multiply by cleavage, and the daughter-cells (Fig. 65 <i> +i</i>) spread out from this spot over the inner surface of the +blastula, though at first only over the vegetal hemisphere. The +less clear entodermic cells (<i>i</i>) are distinguished at first +by their rounder shape and darker nuclei from the higher, clearer, +and longer entodermic cells (<i>e</i>), afterwards both are greatly +flattened, the inner blastodermic cells more than the outer.</p> + +<br> + + +<center> +<table class="capt" width="361" summary= +"Fig. 65. Blastula of the opossum (Didelphys) at the beginning of gastrulation. Fig. 66. Oval gastrula of the opossum (Didelphys), about eight hours old."> +<tr> +<td align="justify"><img src="images/fig65.GIF" width="361" height="235" alt= +"Fig. 65. Blastula of the opossum (Didelphys) at the beginning of gastrulation. Fig. 66. Oval gastrula of the opossum (Didelphys), about eight hours old."> +<br><br><a name="Fig. 65">Fig. 65</a>—<b>Blastula +of the opossum</b> (<i>Didelphys</i>) at the beginning of +gastrulation. (From <i>Selenka.</i>) <i>e</i> ectoderm, <i>i</i> +entoderm; <i>a</i> animal pole, <i>u</i> primitive mouth at the +vegetal pole, <i>f</i> segmentation-cavity, <i>d</i> unnucleated +yelk-balls (relics of the reduced food-yelk), c nucleated curd +(without yelk-granules)<br> +Fig. 66—<b>Oval gastrula of the opossum</b> +(<i>Didelphys</i>), about eight hours old. (From <i>Selenka</i>) +(external view).)</td> +</tr> +</table> +</center> + +<p>The unnucleated yelk-balls and curd (Fig. 65 <i>d</i>) that we +find in the fluid of the blastula in these marsupials are very +remarkable; they are the relics of the atrophied food-yelk, which +was developed in their ancestors, the monotremes, and in the +reptiles.</p> + +<p>In the further course of the gastrulation of the opossum the +oval shape of the gastrula (Fig. 66) gradually changes into +globular, a larger quantity of fluid accumulating in the vesicle. +At the same time, the entoderm spreads further and further over the +inner surface of the ectoderm (<i>e</i>). A globular vesicle is +formed, the wall of which consists of two thin simple strata of +cells; the cells of the outer germinal layer are rounder, and those +of the inner layer flatter. In the region of the primitive mouth +(<i>p</i>) the cells are less flattened, and multiply briskly. From +this point—from the hind (ventral) lip of the primitive +mouth, which extends in a central cleft, the primitive +groove—the construction of the mesoderm proceeds.</p> + +<p>Gastrulation is still more modified and curtailed +cenogenetically in the placentals than in the marsupials. It was +first accurately known to us by the distinguished investigations of +Edward Van Beneden in 1875, the first object of study being the +ovum of the rabbit. But as man also belongs to this sub-class, and +as his as yet unstudied gastrulation cannot be materially different +from that of the other placentals, it merits the closest attention. +We have, in the first place, the peculiar feature that the two +first segmentation-cells that proceed from the cleavage of the +fertilised ovum <a href="#Fig. 68">(Fig. 68)</a> are of different +sizes and natures; the difference is sometimes greater, sometimes +less (Fig. 69). One of these first daughter-cells of the ovum is a +little</p> + +<br> +<hr> +<p class="page"><a name="page 87">[ 87 ]</a></p> + +<p> </p> + +<p class="one">larger, clearer, and more transparent than the +other. Further, the smaller cell takes a colour in carmine, osmium, +etc., more strongly than the larger. By repeated cleavage of it a +morula is formed, and from this a blastula, which changes in a very +characteristic way into the greatly modified gastrula. When the +number of the segmentation-cells in the mammal embryo has reached +ninety-six (in the rabbit, about seventy hours after impregnation) +the fœtus assumes a form very like the archigastrula <a href= +"#Fig. 72">(Fig. 72).</a> The spherical embryo consists of a +central mass of thirty-two soft, round cells with dark nuclei, +which are flattened into polygonal shape by mutual pressure, and +colour dark-brown with osmic acid (Fig. 72 <i>i</i>). This dark +central group of cells is surrounded by a lighter spherical +membrane, consisting of sixty-four cube-shaped, small, and +fine-grained cells which lie close together in a single stratum, +and only colour slightly in osmic acid (Fig. 72 <i>e</i>). The +authors who regard this embryonic form as the primary gastrula of +the placental conceive the outer layer as the ectoderm and the +inner as the entoderm. The entodermic membrane is only interrupted +at one spot, one, two, or three of the ectodermic cells being loose +there. These form the yelk-stopper, and fill up the mouth of the +gastrula (<i>a</i>). The central primitive gut-cavity (<i>d</i>) is +full of entodermic cells. The uni-axial type of the mammal gastrula +is accentuated in this way. However, opinions still differ +considerably as to the real nature of this “provisional +gastrula” of the placental and its relation to the blastula +into which it is converted.</p> + +<p>As the gastrulation proceeds a large spherical blastula is +formed from this peculiar solid amphigastrula of the placental, as +we saw in the case of the marsupial. The accumulation of fluid in +the solid gastrula <a href="#Fig. 73">(Fig. 73 A)</a> leads to the +formation of an eccentric cavity, the group of the darker +entodermic cells (<i>hy</i>) remaining directly attached at one +spot with the round enveloping stratum of the lighter ectodermic +cells (<i>ep</i>). This spot corresponds to the original primitive +mouth (prostoma or blastoporus). From this important spot the inner +germinal layer spreads all round on the inner surface of the outer +layer, the cell-stratum of which forms the wall of the hollow +sphere; the extension proceeds from the vegetal towards the animal +pole.</p> + +<table class="capt" width="198" align="left" summary= +"Fig. 67. Longitudinal section through the oval gastrula of the opossum."> +<tr> +<td align="justify"><img src="images/fig67.GIF" width="198" height="209" alt= +"Longitudinal section through the oval gastrula of the opossum."> +<a name="Fig. 67">Fig. +67</a>—<b>Longitudinal section through the oval gastrula of +the opossum</b> (Fig. 69). (From <i>Selenka.</i>) <i>p</i> +primitive mouth, <i>e</i> ectoderm, <i>i</i> entoderm, <i>d</i> +yelk remains in the primitive gut-cavity (<i>u</i>).</td> +</tr> +</table> + +<p>The cenogenetic gastrulation of the placental has been greatly +modified by secondary adaptation in the various groups of this most +advanced and youngest sub-class of the mammals. Thus, for instance, +we find in many of the rodents (guinea-pigs, mice, etc.) <i> +apparently</i> a temporary inversion of the two germinal layers. +This is due to a folding of the blastodermic wall by what is called +the “girder,” a plug-shaped growth of Rauber’s +“roof-layer.” It is a thin layer of flat epithelial +cells, that is freed from the surface of the blastoderm in some of +the rodents; it has no more significance in connection with the +general course of placental gastrulation than the conspicuous +departure from the usual globular shape in the blastula of some of +the ungulates. In some pigs and ruminants it grows into a +thread-like, long and thin tube.</p> + +<p>Thus the gastrulation of the placentals, which diverges most +from that of the amphioxus, the primitive form, is reduced to the +original type, the invagination of a modified blastula. Its chief +peculiarity is that the folded part of the blastoderm does not form +a completely closed (only open at the primitive mouth) blind sac, +as is usual; but this blind sac has a wide opening at the ventral +curve (opposite to the dorsal mouth); and through this opening the +primitive gut communicates from the first with the embryonic cavity +of the blastula. The folded crest-shaped</p> + +<br> +<hr> +<p class="page"><a name="page 88">[ 88 ]</a></p> + +<p> </p> + +<p class="one">entoderm grows with a free circular border on the +inner surface of the entoderm towards the vegetal pole; when it has +reached this, and the inner surface of the blastula is completely +grown over, the primitive gut is closed. This remarkable direct +transition of the primitive gut-cavity into the segmentation-cavity +is explained simply by the assumption that in most of the mammals +the yelk-mass, which is still possessed by the oldest forms of the +class (the monotremes) and their ancestors (the reptiles), is +atrophied. This proves the essential unity of gastrulation in all +the vertebrates, in spite of the striking differences in the +various classes.</p><br> + +<center> +<table class="capt" summary= +"Fig. 68. Stem-cell of the mammal ovum (from the rabbit). Fig. 69. Incipient cleavage of the mammal ovum (from the rabbit). Fig. 70. The first four segmentation-cells of the mammal ovum (from the rabbit). Fig. 71. Mammal ovum with eight segmentation-cells (from the rabbit)."> +<tr> +<td width="170" align="left"><img src="images/fig68.GIF" width= +"170" height="170" alt= +"Stem-cell of the mammal ovum (from the rabbit)."> <a name= +"Fig. 68">Fig. 68</a>—<b>Stem-cell of the mammal ovum</b> +(from the rabbit).<br> +<i>k</i> stem-nucleus, <i>n</i> nuclear corpuscle,<br> +<i>p</i> protoplasm of the stem-cell,<br> + <i>z</i> modified zona pellucida, <i>h</i> outer albuminous +membrane, <i>s</i> dead sperm-cells.</td> +<td width="20"> </td> +<td width="170" align="left"><img src="images/fig70.GIF" width= +"170" height="170" alt= +"The first four segmentation-cells of the mammal ovum (from the rabbit)."> +<br>Fig. 70—<b>The first four segmentation-cells of the mammal +ovum</b> (from the rabbit).<br> + <i>e</i> the two larger (and lighter) cells,<br> + <i>i</i> the two smaller (and darker) cells,<br> + <i>z</i> zona pellucida, <i>h</i> outer albuminous membrane.</td> +</tr> + +<tr> +<td width="170" align="left"><img src="images/fig69.GIF" width= +"170" height="170" alt= +"Incipient cleavage of the mammal ovum (from the rabbit)."><br>Fig. +69—<b>Incipient cleavage of the mammal ovum</b> (from the +rabbit). The stem-cell has divided into two unequal cells, one +lighter (<i>e</i>) and one darker (<i>i</i>). <i>z</i> zona +pellucida, <i>h</i> outer albuminous membrane, <i>s</i> dead +sperm-cell.</td> +<td> </td> +<td width="170" align="left"><img src="images/fig71.GIF" width= +"170" height="170" alt= +"Mammal ovum with eight segmentation-cells (from the rabbit)."><br> +Fig. +71—<b>Mammal ovum with eight segmentation-cells</b> (from the +rabbit). <i>e</i> four larger and lighter cells, <i>i</i> four +smaller and darker cells, <i>z</i> zona pellucida, <i>h</i> outer +albuminous membrane.</td> +</tr> +</table> +</center> + +<p>In order to complete our consideration of the important +processes of segmentation and gastrulation, we will, in conclusion, +cast a brief glance at the fourth chief type—superficial +segmentation. In the vertebrates this form is not found at all. But +it plays the chief part in the large stem of the +articulates—the insects,</p> + +<br> +<hr> +<p class="page"><a name="page 89">[ 89 ]</a></p> + +<p> </p> + +<p class="one">spiders, myriapods, and crabs. The distinctive form +of gastrula that comes of it is the “vesicular +gastrula” (<i>Perigastrula</i>).</p> + +<p>In the ova which undergo this superficial cleavage the formative +yelk is sharply divided from the nutritive yelk, as in the +preceding cases of the ova of birds, reptiles, fishes, etc.; the +formative yelk alone undergoes cleavage. But while in the ova with +discoid gastrulation the formative yelk is not in the centre, but +at one pole of the uni-axial ovum, and the food-yelk gathered at +the other pole, in the ova with superficial cleavage we find the +formative yelk spread over the whole surface of the ovum; it +encloses spherically the food-yelk, which is accumulated in the +middle of the ova. As the segmentation only affects the former and +not the latter, it is bound to be entirely +“superficial”; the store of food in the middle is quite +untouched by it. As a rule, it proceeds in regular geometrical +progression. In the end the whole of the formative yelk divides +into a number of small and homogeneous cells, which lie close +together in a single stratum on the entire surface of the ovum, and +form a superficial blastoderm. This blastoderm is a simple, +completely closed vesicle, the internal cavity of which is entirely +full of food-yelk. This real blastula only differs from that of the +primitive ova in its chemical composition. In the latter the +content is water or a watery jelly; in the former it is a thick +mixture, rich in food-yelk, of albuminous and fatty substances. As +this quantity of food-yelk fills the centre of the ovum before +cleavage begins, there is no difference in this respect between the +morula and the blastula. The two stages rather agree in this.</p> + +<p>When the blastula is fully formed, we have again in this case +the important folding or invagination that determines gastrulation. +The space between the skin-layer and the gut-layer (the remainder +of the segmentation-cavity) remains full of food-yelk, which is +gradually used up. This is the only material difference between our +vesicular gastrula (<i>perigastrula</i>) and the original form of +the bell-gastrula (<i>archigastrula</i>). Clearly the one has been +developed from the other in the course of time, owing to the +accumulation of food-yelk in the centre of the +ovum.<sup>1</sup></p> + +<p>We must count it an important advance that we are thus in a +position to reduce all the various embryonic phenomena in the +different groups of animals to these four principal forms of +segmentation and gastrulation. Of these four forms we must regard +one only as the original palingenetic, and the other three as +cenogenetic and derivative. The unequal, the discoid, and the +superficial segmentation have all clearly arisen by secondary +adaptation from the primary segmentation; and the chief cause of +their development has been the gradual formation of the food-yelk, +and the increasing antithesis between animal and vegetal halves of +the ovum, or between ectoderm (skin-layer) and entoderm +(gut-layer).</p> + +<table class="capt" width="206" align="left" summary= +"Fig. 72. Gastrula of the placental mammal (epigastrula from the rabbit), longitudinal section through the axis."> +<tr> +<td align="justify"><img src="images/fig72.GIF" width="206" height="182" alt= +"Gastrula of the placental mammal (epigastrula from the rabbit), longitudinal section through the axis."> +<a name="Fig. 72">Fig. +72</a>—<b>Gastrula of the placental mammal</b> (epigastrula +from the rabbit), longitudinal section through the axis. <i>e</i> ectodermic cells (sixty-four, lighter and smaller), <i>i</i> entodermic cells (thirty-two, darker and larger), <i> +d</i> central entodermic cell, filling the primitive gut-cavity, +<i>o</i> peripheral entodermic cell, stopping up the opening of the +primitive mouth (yelk-stopper in the Rusconian anus).</td> +</tr> +</table> + +<p>The numbers of careful studies of animal gastrulation that have +been made in the last few decades have completely established the +views I have expounded, and which I first advanced in the years +1872–76. For a time they were greatly disputed by many +embryologists. Some said that the original embryonic form of the +metazoa was not the gastrula, but the “planula”—a +double-walled vesicle with closed cavity and without +mouth-aperture; the latter was supposed to pierce through +gradually. It was afterwards shown that this planula (found in +several sponges, etc.) was a later evolution from the gastrula.</p><br> + +<p class="fnote">1. On the reduction of all forms of gastrulation +to the original palingenetic form see especially the lucid +treatment of the subject in Arnold Lang’s <i>Manual of +Comparative Anatomy</i> (1888), Part I.</p> + +<br> +<hr> +<p class="page"><a name="page 90">[ 90 ]</a></p> + +<p> </p> + +<center> +<table class="capt" width="307" summary= +"Fig. 73. Gastrula of the rabbit."> +<tr> +<td align="justify"><img src="images/fig73.GIF" width="307" height="172" alt= +"Gastrula of the rabbit."> +<br><br><a name="Fig. 73">Fig. 73</a>—<b>Gastrula +of the rabbit.</b> A as a solid, spherical cluster of cells, B +changing into the embryonic vesicle, <i>bp</i> primitive mouth, <i> +ep</i> ectoderm, <i>hy</i> entoderm.</td> +</tr> +</table> +</center> + +<p class="one">It was also shown that what is called +delamination—the rise of the two primary germinal layers by +the folding of the surface of the blastoderm (for instance, in the +<i>Geryonidæ</i> and other medusæ)—was a +secondary formation, due to cenogenetic variations from the +original invagination of the blastula. The same may be said of what +is called “immigration,” in which certain cells or +groups of cells are detached from the simple layer of the +blastoderm, and travel into the interior of the blastula; they +attach themselves to the inner wall of the blastula, and form a +second internal epithelial layer—that is to say, the +entoderm. In these and many other controversies of modern +embryology the first requisite for clear and natural explanation is +a careful and discriminative distinction between palingenetic +(hereditary) and cenogenetic (adaptive) processes. If this is +properly attended to, we find evidence everywhere of the biogenetic +law.</p> + +<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="chap8.html">Chapter VIII</a><br> +<a href="chap10.html">Chapter X</a><br> +<a href="Title.html#Illustrations">Figs. 1–209</a><br> +<a href="title2.html#Illustrations">Figs. 210–408</a></p> +</body> +</html> + |