diff options
Diffstat (limited to '8700-h/old/chap8.html')
| -rw-r--r-- | 8700-h/old/chap8.html | 1039 |
1 files changed, 1039 insertions, 0 deletions
diff --git a/8700-h/old/chap8.html b/8700-h/old/chap8.html new file mode 100644 index 0000000..8675600 --- /dev/null +++ b/8700-h/old/chap8.html @@ -0,0 +1,1039 @@ +<!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> VIII<br> +<br> +<b>THE GASTRÆA THEORY</b></center> + +<br> + + +<p class="one">There is a substantial agreement throughout the +animal world in the first changes which follow the impregnation of +the ovum and the formation of the stem-cell; they begin in all +cases with the segmentation of the ovum and the formation of the +germinal layers. The only exception is found in the protozoa, the +very lowest and simplest forms of animal life; these remain +unicellular throughout life. To this group belong the amœbae, +gregarinæ, rhizopods, infusoria, etc. As their whole organism +consists of a single cell, they can never form germinal layers, or +definite strata of cells. But all the other animals—all the +tissue-forming animals, or <i>metazoa,</i> as we call them, in +contradistinction to the protozoa—construct real germinal +layers by the repeated cleavage of the impregnated ovum. This we +find in the lower cnidaria and worms, as well as in the more +highly-developed molluscs, echinoderms, articulates, and +vertebrates.</p> + +<p>In all these metazoa, or multicellular animals, the chief +embryonic processes are substantially alike, although they often +seem to a superficial observer to differ considerably. The +stem-cell that proceeds from the impregnated ovum always passes by +repeated cleavage into a number of simple cells. These cells are +all direct descendants of the stem-cell, and are, for reasons we +shall see presently, called segmentation-cells. The repeated +cleavage of the stem-cell, which gives rise to these +segmentation-spheres, has long been known as +“segmentation.” Sooner or later the segmentation-cells +join together to form a round (at first, globular) embryonic sphere +(<i>blastula</i>); they then form into two very different groups, +and arrange themselves</p> + +<br> +<hr> +<p class="page"><a name="page 60">[ 60 ]</a></p> + +<p> </p> + +<p class="one">in two separate strata—the two <i>primary +germinal layers.</i> These enclose a digestive cavity, the +primitive gut, with an opening, the primitive mouth. We give the +name of the <i>gastrula</i> to the important embryonic form that +has these primitive organs, and the name of <i>gastrulation</i> to +the formation of it. This ontogenetic process has a very great +significance, and is the real starting-point of the construction of +the multicellular animal body.</p> + +<p>The fundamental embryonic processes of the cleavage of the ovum +and the formation of the germinal layers have been very thoroughly +studied in the last thirty years, and their real significance has +been appreciated. They present a striking variety in the different +groups, and it was no light task to prove their essential identity +in the whole animal world. But since I formulated the gastræa +theory in 1872, and afterwards (1875) reduced all the various forms +of segmentation and gastrulation to one fundamental type, their +identity may be said to have been established. We have thus +mastered the law of unity which governs the first embryonic +processes in all the animals.</p> + +<p>Man is like all the other higher animals, especially the apes, +in regard to these earliest and most important processes. As the +human embryo does not essentially differ, even at a much later +stage of development—when we already perceive the cerebral +vesicles, the eyes, ears, gill-arches, etc.—from the similar +forms of the other higher mammals, we may confidently assume that +they agree in the earliest embryonic processes, segmentation and +the formation of germinal layers. This has not yet, it is true, +been established by observation. We have never yet had occasion to +dissect a woman immediately after impregnation and examine the +stem-cell or the segmentation-cells in her oviduct. However, as the +earliest human embryos we have examined, and the later and more +developed forms, agree with those of the rabbit, dog, and other +higher mammals, no reasonable man will doubt but that the +segmentation and formation of layers are the same in both +cases.</p> + +<p>But the special form of segmentation and layer formation which +we find in the mammal is by no means the original, simple, +palingenetic form. It has been much modified and cenogenetically +altered by a very complex adaptation to embryonic conditions. We +cannot, therefore, understand it altogether in itself. In order to +do this, we have to make a <i>comparative</i> study of segmentation +and layer-formation in the animal world; and we have especially to +seek the original, <i>palingenetic</i> form from which the modified +<i>cenogenetic</i> (see <a href="chap1.html#page 4">p. 4</a>) form +has gradually been developed.</p> + +<p>This original unaltered form of segmentation and layer-formation +is found to-day in only one case in the vertebrate-stem to which +man belongs—the lowest and oldest member of the stem, the +wonderful lancelet or amphioxus (cf. Chapters XVI and XVII). But we +find a precisely similar palingenetic form of embryonic development +in the case of many of the invertebrate animals, as, for instance, +the remarkable ascidia, the pond-snail (<i>Limnæus</i>), and +arrow-worm (<i>Sagitta</i>), and many of the echinoderms and +cnidaria, such as the common star-fish and sea-urchin, many of the +medusæ and corals, and the simpler sponges (<i>Olynthus</i>). +We may take as an illustration the palingenetic segmentation and +germinal layer-formation in an eight-fold insular coral, which I +discovered in the Red Sea, and described as <i>Monoxenia +Darwinii.</i></p> + +<p>The impregnated ovum of this coral (<a href="#Fig. 29">Fig. 29 +A, B</a>) first splits into two equal cells (C). First, the nucleus +of the stem-cell and its central body divide into two halves. These +recede from and repel each other, and act as centres of attraction +on the surrounding protoplasm; in consequence of this, the +protoplasm is constricted by a circular furrow, and, in turn, +divides into two halves. Each of the two segmentation-cells thus +produced splits in the same way into two equal cells. The four +segmentation-cells (grand-daughters of the stem-cell) lie in one +plane. Now, however, each of them subdivides into two equal halves, +the cleavage of the nucleus again preceding that of the surrounding +protoplasm. The eight cells which thus arise break into sixteen, +these into thirty-two, and then (each being constantly halved) into +sixty-four, 128, and so on.<sup>1</sup> The final result of +this</p> + +<p class="fnote">1. The number of segmentation-cells thus produced +increases geometrically in the original gastrulation, or the purest +palingenetic form of cleavage. However, in different animals the +number reaches a different height, so that the morula, and also the +blastula, may consist sometimes of thirty-two, sometimes of +sixty-four, and sometimes of 128, or more, cells.</p> + +<br> +<hr> +<p class="page"><a name="page 61">[ 61 ]</a></p> + +<p> </p> + +<center> +<table class="capt" width="279" summary="Gastrulation of a coral."> +<tr> +<td align="justify"> +<img src="images/fig29.GIF" width="279" height="476" alt= +"Gastrulation of a coral."><br><br> + +<a name="Fig. 29">Fig. 29—<b>Gastrulation +of a coral</b> (<i>Monoxenia Darwinii</i>). A, B, stem-cell +(cytula) or impregnated ovum. In Figure A (immediately after +impregnation) the nucleus is invisible. In Figure B (a little +later) it is quite clear. C two segmentation-cells. D four +segmentation-cells. E mulberry-formation (morula). F blastosphere +(blastula). G blastula (transverse section). H depula, or hollowed +blastula (transverse section). I gastrula (longitudinal section). K +gastrula, or cup-sphere, external appearance.)</a></td> +</tr> +</table> +</center> + +<br> +<hr> +<p class="page"><a name="page 62">[ 62 ]</a></p> + +<p> </p> + +<br class="one"> +<br> +<p class="one">repeated cleavage is the formation of a globular +cluster of similar segmentation-cells, which we call the +mulberry-formation or morula. The cells are thickly pressed +together like the parts of a mulberry or blackberry, and this gives +a lumpy appearance to the surface of the sphere (Fig. +E).<sup>1</sup></p> + +<p>When the cleavage is thus ended, the mulberry-like mass changes +into a hollow globular sphere. Watery fluid or jelly gathers inside +the globule; the segmentation-cells are loosened, and all rise to +the surface. There they are flattened by mutual pressure, and +assume the shape of truncated pyramids, and arrange themselves side +by side in one regular layer (Figs. F, G). This layer of cells is +called the germinal membrane (or blastoderm); the homogeneous cells +which compose its simple structure are called blastodermic cells; +and the whole hollow sphere, the walls of which are made of the +preceding, is called the <i>blastula</i> or <i> +blastosphere.</i><sup>2</sup></p> + +<p>In the case of our coral, and of many other lower forms of +animal life, the young embryo begins at once to move independently +and swim about in the water. A fine, long, thread-like process, a +sort of whip or lash, grows out of each blastodermic cell, and this +independently executes vibratory movements, slow at first, but +quicker after a time (Fig. F). In this way each blastodermic cell +becomes a ciliated cell. The combined force of all these vibrating +lashes causes the whole blastula to move about in a rotatory +fashion. In many other animals, especially those in which the +embryo develops within enclosed membranes, the ciliated cells are +only formed at a later stage, or even not formed at all. The +blastosphere may grow and expand by the blastodermic cells (at the +surface of the sphere) dividing and increasing, and more fluid is +secreted in the internal cavity. There are still to-day some +organisms that remain throughout life at the structural stage of +the blastula—hollow vesicles that swim about by a ciliary +movement in the water, the wall of which is composed of a single +layer of cells, such as the volvox, the magosphæra, synura, +etc. We shall speak further of the great phylogenetic significance +of this fact in Chapter XIX.</p> + +<p>A very important and remarkable process now +follows—namely, the curving or invagination of the blastula +(Fig. H). The vesicle with a single layer of cells for wall is +converted into a cup with a wall of two layers of cells (cf. Figs. +G, H, I). A certain spot at the surface of the sphere is flattened, +and then bent inward. This depression sinks deeper and deeper, +growing at the cost of the internal cavity. The latter decreases as +the hollow deepens. At last the internal cavity disappears +altogether, the inner side of the blastoderm (that which lines the +depression) coming to lie close on the outer side. At the same +time, the cells of the two sections assume different sizes and +shapes; the inner cells are more round and the outer more oval +(Fig. I). In this way the embryo takes the form of a cup or +jar-shaped body, with a wall made up of two layers of cells, the +inner cavity of which opens to the outside at one end (the spot +where the depression was originally formed). We call this very +important and interesting embryonic form the +“cup-embryo” or “cup-larva” +(<i>gastrula,</i> Fig. 29, I longitudinal section, K external +view). I have in my <i>Natural History of Creation</i> given the +name of <i>depula</i> to the remarkable intermediate form which +appears at the passage of the blastula into the gastrula. In this +intermediate stage there are two cavities in the embryo—the +original cavity (<i>blastocœl</i>) which is disappearing, and +the primitive gut-cavity (<i>progaster</i>) which is forming.</p> + +<p>I regard the gastrula as the most important and significant +embryonic form in the animal world. In all real animals (that is, +excluding the unicellular protists) the segmentation of the ovum +produces either a pure, primitive, palingenetic gastrula (Fig. 29 +I, K) or an equally instructive cenogenetic form, which has been +developed in time from the first, and can be directly reduced to +it. It is certainly a fact of the greatest interest and +instructiveness that animals of the most different +stems—vertebrates and tunicates, molluscs and articulates, +echinoderms and annelids, cnidaria and sponges—proceed from +one and the same embryonic form. In illustration I give a few</p> + +<p class="fnote">1. The segmentation-cells which make up the morula +after the close of the palingenetic cleavage seem usually to be +quite similar, and to present no differences as to size, form, and +composition. That, however, does not prevent them from +differentiating into animal and vegetative cells, even during the +cleavage.<br> +2. The blastula of the lower animals must not be confused with the +very different blastula of the mammal, which is properly called the +<i>gastrocystis</i> or <i>blastocystis.</i> This <i>cenogenetic</i> +gastrocystis and the <i>palingenetic</i> blastula are sometimes +very wrongly comprised under the common name of blastula or +vesicula blastodermica.</p> + +<br> +<hr> +<p class="page"><a name="page 63">[ 63 ]</a></p> + +<p> </p> + +<p class="one">pure gastrula forms from various groups of animals +(Figs. 30–35, explanation given below each).</p> + +<br> + + +<center> +<table class="capt" width="368" summary= +"Fig. 30--Gastrula of a very simple primitive-gut animal or gastræad. Fig. 31--Gastrula of a worm. Fig. 32--Gastrula of an echinoderm. Fig. 33--Gastrula of an arthropod. Fig. 34--Gastrula of a mollusc. Fig. 35--Gastrula of a vertebrate."> +<tr> +<td align="left"><img src="images/fig30.GIF" width="368" height="292" alt= +"Fig. 30--Gastrula of a very simple primitive-gut animal or gastræad. Fig. 31--Gastrula of a worm. Fig. 32--Gastrula of an echinoderm. Fig. 33--Gastrula of an arthropod. Fig. 34--Gastrula of a mollusc. Fig. 35--Gastrula of a vertebrate."><br><br> +<a name="Fig. 30">Fig. 30 +(<i>A</i>)</a>—<b>Gastrula of a very simple primitive-gut +animal</b> or <b>gastræad</b> (gastrophysema). +(<i>Haeckel.</i>)<br> +Fig. 31 (<i>B</i>)—<b>Gastrula of a worm</b> +(<i>Sagitta</i>). (From <i>Kowalevsky.</i>)<br> +Fig. 32 (<i>C</i>)—<b>Gastrula of an echinoderm</b> +(star-fish, <i>Uraster</i>), not completely folded in (depula). +(From <i>Alexander Agassiz.</i>)<br> +Fig. 33 (<i>D</i>)—<b>Gastrula of an arthropod</b> (primitive +crab, <i>Nauplius</i>) (as 32).<br> +Fig. 34 (<i>E</i>)—<b>Gastrula of a mollusc</b> (pond-snail, +<i>Linnæus</i>). (From <i>Karl Rabl.</i>)<br> +Fig. 35 (<i>F</i>)—<b>Gastrula of a vertebrate</b> (lancelet, +<i>Amphioxus</i>). (From <i>Kowalevsky.</i>) (Front view.)<br> +In each figure <i>d</i> is the +primitive-gut cavity, <i>o</i> primitive mouth,<br> <i>s</i> +segmentation-cavity, <i>i</i> entoderm (gut-layer), <i>e</i> +ectoderm (skin layer).</td> +</tr> +</table> +</center> + +<p>In view of this extraordinary significance of the gastrula, we +must make a very careful study of its original structure. As a +rule, the typical gastrula is very small, being invisible to the +naked eye, or at the most only visible as a fine point under very +favourable conditions, and measuring generally 1/500 to 1/250 of an +inch (less frequently 1/50 inch, or even more) in diameter. In +shape it is usually like a roundish drinking-cup. Sometimes it is +rather oval, at other times more ellipsoid or spindle-shaped; in +some cases it is half round, or even almost round, and in others +lengthened out, or almost cylindrical.</p> + +<p>I give the name of primitive gut (<i>progaster</i>) and +primitive mouth (<i>prostoma</i>) to the internal cavity of the +gastrula-body and its opening; because this cavity is the first +rudiment of the digestive cavity of the organism, and the opening +originally served to take food into it. Naturally, the primitive +gut and mouth change very considerably afterwards in the various +classes of animals. In most of the cnidaria and many of the +annelids (worm-like animals) they remain unchanged throughout life. +But in most of the</p> + +<br> +<hr> +<p class="page"><a name="page 64">[ 64 ]</a></p> + +<p> </p> + +<p class="one">higher animals, and so in the vertebrates, only the +larger central part of the later alimentary canal develops from the +primitive gut; the later mouth is a fresh development, the +primitive mouth disappearing or changing into the anus. We must +therefore distinguish carefully between the primitive gut and mouth +of the gastrula and the later alimentary canal and mouth of the +fully developed vertebrate.<sup>1</sup></p> + +<br> +<center> +<table class="capt" width="324" summary= +"Fig. 36--Gastrula of a lower sponge (olynthus)."> +<tr> +<td align="justify"> +<img src="images/fig36.GIF" width="324" height="192" alt= +"Fig. 36--Gastrula of a lower sponge (olynthus)."> +<br><br><a name="Fig. 36">Fig. 36</a>—<b>Gastrula +of a lower sponge</b> (lynthus). <i>A</i> external view, <i>B</i> +longitudinal section through the axis, <i>g</i> primitive-gut +cavity, a primitive mouth-aperture, <i>i</i> inner cell-layer +(entoderm, endoblast, gut-layer), <i>e</i> external cell-layer +(outer germinal layer, ectoderm, ectoblast, or skin-layer).</td> +</tr> +</table> +</center> + +<br> + + +<p>The two layers of cells which line the gut-cavity and compose +its wall are of extreme importance. These two layers, which are the +sole builders of the whole organism, are no other than the two +primary germinal layers, or the primitive germ-layers. I have +spoken in the introductory section (Chapter III) of their radical +importance. The outer stratum is the skin-layer, or <i>ectoderm</i> +<a href="#Fig. 30">(Figs. 30–35<i>e</i>);</a> the inner +stratum is the gut-layer, or <i>entoderm</i> (<i>i</i>). The former +is often also called the ectoblast, or epiblast, and the latter the +endoblast, or hypoblast. <i>From these two primary germinal layers +alone is developed the entire organism of all the metazoa or +multicellular animals.</i> The skin-layer forms the external skin, +the gut-layer forms the internal skin or lining of the body. +Between these two germinal layers are afterwards developed the +middle germinal layer (<i>mesoderma</i>) and the body-cavity +(<i>cœloma</i>) filled with blood or lymph.</p> + +<p>The two primary germinal layers were first distinguished by +Pander in 1817 in the incubated chick. Twenty years later (1849) +Huxley pointed out that in many of the lower zoophytes, especially +the medusæ, the whole body consists throughout life of these +two primary germinal layers. Soon afterwards (1853) Allman +introduced the names which have come into general use; he called +the outer layer the <i>ectoderm</i> (“outer-skin”), and +the inner the <i>entoderm</i> (“inner-skin”). But in +1867 it was shown, particularly by Kowalevsky, from comparative +observation, that even in invertebrates, also, of the most +different classes—annelids, molluscs, echinoderms, and +articulates—the body is developed out of the same two primary +layers. Finally, I discovered them (1872) in the lowest +tissue-forming animals, the sponges, and proved in my gastræa +theory that these two layers must be regarded as identical +throughout the animal world, from the sponges and corals to the +insects and vertebrates, including man. This fundamental +“homology</p> + +<p class="fnote">1. My distinction (1872) between the primitive gut +and mouth and the later permanent stomach (<i>metagaster</i>) and +mouth (<i>metastoma</i>) has been much criticised; but it is as +much justified as the distinction between the primitive kidneys and +the permanent kidneys. Professor E. Ray-Lankester suggested three +years afterwards (1875) the name <i>archenteron</i> for the +primitive gut, and <i>blastoporus</i> for the primitive mouth.</p> + +<br> +<hr> +<p class="page"><a name="page 65">[ 65 ]</a></p> + +<p> </p> + +<p class="one">[identity] of the primary germinal layers and the +primitive gut” has been confirmed during the last thirty +years by the careful research of many able observers, and is now +pretty generally admitted for the whole of the metazoa.</p> + +<p>As a rule, the cells which compose the two primary germinal +layers show appreciable differences even in the gastrula stage. +Generally (if not always) the cells of the skin-layer or ectoderm +(Figs. 36 <i>c</i> and 37 <i>e</i>) are the smaller, more numerous, +and clearer; while the cells of the gut-layer, or entoderm +(<i>i</i>), are larger, less numerous, and darker. The protoplasm +of the ectodermic (outer) cells is clearer and firmer than the +thicker and softer cell-matter of the entodermic (inner) cells; the +latter are, as a rule, much richer in yelk-granules (albumen and +fatty particles) than the former. Also the cells of the gut-layer +have, as a rule, a stronger affinity for colouring matter, and take +on a tinge in a solution of carmine, aniline, etc., more quickly +and appreciably than the cells of the skin-layer. The nuclei of the +entoderm-cells are usually roundish, while those of the +ectoderm-cells are oval.</p> + +<p>When the doubling-process is complete, very striking +histological differences between the cells of the two layers are +found (Fig. 37). The tiny, light ectoderm-cells (<i>e</i>) are +sharply distinguished from the larger and darker entoderm-cells +(<i>i</i>). Frequently this differentiation of the cell-forms sets +in at a very early stage, during the segmentation-process, and is +already very appreciable in the blastula.</p> + +<p>We have, up to the present, only considered that form of +segmentation and gastrulation which, for many and weighty reasons, +we may regard as the original, primordial, or palingenetic form. We +might call it “equal” or homogeneous segmentation, +because the divided cells retain a resemblance to each other at +first (and often until the formation of the blastoderm). We give +the name of the “bell-gastrula,” or <i> +archigastrula,</i> to the gastrula that succeeds it. In just the +same form as in the coral we considered (<i>Monoxenia,</i> Fig. +29), we find it in the lowest zoophyta (the gastrophysema, Fig. +30), and the simplest sponges (olynthus, Fig. 36); also in many of +the medusæ and hydrapolyps, lower types of worms of various +classes (brachiopod, arrow-worm, Fig. 31), tunicates (ascidia), +many of the echinoderms (Fig. 32), lower articulates (Fig. 33), and +molluscs (Fig. 34), and, finally, in a slightly modified form, in +the lowest vertebrate (the amphioxus, Fig. 35).</p> + + +<table class="capt" width="229" align="left" summary= +"Fig. 37--Cells from the two primary germinal layers."> +<tr> +<td align="justify"><img src="images/fig37.GIF" width="208" height="146" alt= +"Fig. 37--Cells from the two primary germinal layers"> +<a name="Fig. 37">Fig. 37</a>—<b>Cells from the two primary germinal layers</b> of the mammal (from both layers of the blastoderm). <i>i</i> larger and darker cells of the inner stratum, the vegetal layer or +entoderm. <i>e</i> smaller and clearer cells from the outer +stratum, the animal layer or ectoderm.</tr> +</table> + +<br> + + +<p class="pic">The gastrulation of the amphioxus is especially interesting +because this lowest and oldest of all the vertebrates is of the +highest significance in connection with the evolution of the +vertebrate stem, and therefore with that of man (compare Chapters +XVI and XVII). Just as the comparative anatomist traces the most +elaborate features in the structures of the various classes of +vertebrates to divergent development from this simple primitive +vertebrate, so comparative embryology traces the various secondary +forms of vertebrate gastrulation to the simple, primary formation +of the germinal layers in the amphioxus. Although this formation, +as distinguished from the cenogenetic modifications of the +vertebrate, may on the whole be regarded as palingenetic, it is +nevertheless different in some features from the quite primitive +gastrulation such as we have, for instance, in the <i>Monoxenia</i> +<a href="#Fig. 29">(Fig. 29)</a> and the <i>Sagitta.</i> Hatschek +rightly observes that the segmentation of the ovum in the amphioxus +is not strictly equal, but almost equal, and approaches the +unequal. The difference in size between the two groups of cells +continues to be very noticeable in the further course of the +segmentation; the smaller animal cells of the upper hemisphere +divide more quickly than the larger vegetal cells of the lower +(Fig. 38 <i>A, B</i>). Hence the blastoderm, which forms the +single-layer wall of the globular blastula at the end of the +cleavage-process, does not consist of</p> + +<br> +<hr> +<p class="page"><a name="page 66">[ 66 ]</a></p> + +<p> </p> + +<p class="one">homogeneous cells of equal size, as in the Sagitta +and the Monoxenia; the cells of the upper half of the blastoderm +(the mother-cells of the ectoderm) are more numerous and smaller, +and the cells of the lower half (the mother-cells of the entoderm) +less numerous and larger. Moreover, the segmentation-cavity of the +blastula (Fig. 38 <i>C, h</i>) is not quite globular, but forms a +flattened spheroid with unequal poles of its vertical axis. While +the blastula is being folded into a cup at the vegetal pole of its +axis, the difference in the size of the blastodermic cells +increases (Fig. 38 <i>D, E</i>); it is most conspicuous when the +invagination is complete and the segmentation-cavity has +disappeared (Fig. 38 <i>F</i>). The larger vegetal cells of the +entoderm are richer in granules, and so darker than the smaller and +lighter animal cells of the ectoderm.</p> + +<br> + + +<center> +<table class="capt" width="324" summary= +"Fig. 38--Gastrulation of the amphioxus."> +<tr> +<td align="justify"><img src="images/fig38.GIF" width="324" height="229" alt= +"Fig. 38--Gastrulation of the amphioxus."><br><br> +<a name="Fig. 38">Fig. +38</a>—<b>Gastrulation of the amphioxus,</b> from <i> +Hatschek</i> (vertical section through the axis of the ovum). <i>A, +B, C</i> three stages in the formation of the blastula; <i>D, E</i> +curving of the blastula; <i>F</i> complete gastrula. <i>h</i> +segmentation-cavity. <i>g</i> primitive gut-cavity.</td> +</tr> +</table> +</center> + +<p>But the unequal gastrulation of the amphioxus diverges from the +typical equal cleavage of the <i>Sagitta,</i> the <i>Monoxenia</i> +<a href="#Fig. 29">(Fig. 29),</a> and the <i>Olynthus</i> <a href= +"#Fig. 36">(Fig. 36),</a> in another important particular. The pure +archigastrula of the latter forms is uni-axial, and it is round in +its whole length in transverse section. The vegetal pole of the +vertical axis is just in the centre of the primitive mouth. This is +not the case in the gastrula of the amphioxus. During the folding +of the blastula the ideal axis is already bent on one side, the +growth of the blastoderm (or the increase of its cells) being +brisker on one side than on the other; the side that grows more +quickly, and so is more curved <a href="#Fig. 39">(Fig. 39 <i> +v</i>),</a> will be the anterior or belly-side, the opposite, +flatter side will form the back (<i>d</i>). The primitive mouth, +which at first, in the typical archigastrula, lay at the vegetal +pole of the main axis, is forced away to the dorsal side; and +whereas its two lips lay at first in a plane at right angles to the +chief axis, they are now so far thrust aside that their plane cuts +the axis at a sharp angle. The dorsal lip is therefore the upper +and more forward, the ventral lip the lower and hinder. In the +latter, at the ventral passage of the entoderm into the ectoderm, +there lie side by side a pair of very large cells, one to the right +and one to the left (Fig. 39 <i>p</i>): these are the important +polar cells of the primitive mouth, or “the primitive cells +of the mesoderm.” In consequence of these considerable +variations arising in the course of the gastrulation, the primitive +uni-axial form of the archigastrula in the amphioxus has already +become tri-axial, and thus the two-sidedness, or bilateral +symmetry, of the vertebrate body has already been determined. This +has been transmitted from the amphioxus to all the other modified +gastrula-forms of the vertebrate stem.</p> + +<p>Apart from this bilateral structure, the gastrula of the +amphioxus resembles the typical archigastrula of the lower animals +<a href="#Fig. 30">(Figs. 30–36)</a> in developing the two +primary germinal layers from a single layer of cells. This is +clearly the oldest and original form of the metazoic embryo. +Although the animals I have mentioned belong to the most diverse +classes, they nevertheless agree with each other, and many more +animal forms, in having retained to the present day, by a +conservative heredity, this palingenetic form of gastrulation which +they have from their</p> + +<br> +<hr> +<p class="page"><a name="page 67">[ 67 ]</a></p> + +<p> </p> + +<p class="one">earliest common ancestors. But this is not the case +with the great majority of the animals. With these the original +embryonic process has been gradually more or less altered in the +course of millions of years by adaptation to new conditions of +development. Both the segmentation of the ovum and the subsequent +gastrulation have in this way been considerably changed. In fact, +these variations have become so great in the course of time that +the segmentation was not rightly understood in most animals, and +the gastrula was unrecognised. It was not until I had made an +extensive comparative study, lasting a considerable time (in the +years 1866–75), in animals of the most diverse classes, that +I succeeded in showing the same common typical process in these +apparently very different forms of gastrulation, and tracing them +all to one original form. I regard all those that diverge from the +primary palingenetic gastrulation as secondary, modified, and +cenogenetic. The more or less divergent form of gastrula that is +produced may be called a secondary, modified gastrula, or a <i> +metagastrula.</i> The reader will find a scheme of these different +kinds of segmentation and gastrulation at the close of this +chapter.</p> + +<p>By far the most important process that determines the various +cenogenetic forms of gastrulation is the change in the nutrition of +the ovum and the accumulation in it of nutritive yelk. By this we +understand various chemical substances (chiefly granules of albumin +and fat-particles) which serve exclusively as reserve-matter or +food for the embryo. As the metazoic embryo in its earlier stages +of development is not yet able to obtain its food and so build up +the frame, the necessary material has to be stored up in the ovum. +Hence we distinguish in the ova two chief elements—the active +formative yelk (protoplasm) and the passive food-yelk (deutoplasm, +wrongly spoken of as “the yelk”). In the little +palingenetic ova, the segmentation of which we have already +considered, the yelk-granules are so small and so regularly +distributed in the protoplasm of the ovum that the even and +repeated cleavage is not affected by them. But in the great +majority of the animal ova the food-yelk is more or less +considerable, and is stored in a certain part of the ovum, so that +even in the unfertilised ovum the “granary” can clearly +be distinguished from the formative plasm. As a rule, the +formative-yelk (with the germinal vesicle) then usually gathers at +one pole and the food-yelk at the other. The first is the <i> +animal,</i> and the second the <i>vegetal,</i> pole of the vertical +axis of the ovum.</p> + +<table class="capt" width="208" align="left" summary= +"Fig. 39--Gastrula of the amphioxus, seen from left side."> +<tr> +<td align="justify"><img src="images/fig39.GIF" width="208" height= +"155" alt= +"Fig. 39--Gastrula of the amphioxus, seen from left side."> +<a name="Fig. 39">Fig. +39</a>—<b>Gastrula of the amphioxus, seen from left side</b> +(diagrammatic median section). (From <i>Hatschek.</i>) <i>g</i> +primitive gut, <i>u</i> primitive mouth, <i>p</i> peristomal +pole-cells, <i>i</i> entoderm, <i>e</i> ectoderm, <i>d</i> dorsal +side, <i>v</i> ventral side.</tr> +</table> + + + +<p class="pic">In these “telolecithal” ova, or ova with the yelk at +one end (for instance, in the cyclostoma and amphibia), the +gastrulation then usually takes place in such a way that in the +cleavage of the impregnated ovum the animal (usually the upper) +half splits up more quickly than the vegetal (lower). The +contractions of the active protoplasm, which effect this continual +cleavage of the cells, meet a greater resistance in the lower +vegetal half from the passive deutoplasm than in the upper animal +half. Hence we find in the latter more but smaller, and in the +former fewer but larger, cells. The animal cells produce the +external, and the vegetal cells the internal, germinal layer.</p> + +<p>Although this unequal segmentation of the cyclostoma, ganoids, +and amphibia seems at first sight to differ from the original equal +segmentation (for instance, in the monoxenia, Fig. 29), they both +have this in common, that the cleavage process throughout affects +the <i>whole</i> cell; hence Remak called it <i>total</i> +segmentation, and the ova in question <i>holoblastic,</i> or +“whole-cleaving.” It is otherwise with the second chief +group of ova, which he distinguished from these as <i> +meroblastic,</i> or “partially-cleaving ”: to this +class belong the familiar large eggs of birds and reptiles, and of +most fishes. The inert mass of the passive food-yelk is so</p> + +<br> +<hr> +<p class="page"><a name="page 68">[ 68 ]</a></p> + +<p> </p> + +<p class="one">large in these cases that the protoplasmic +contractions of the active yelk cannot effect any further cleavage. +In consequence, there is only a partial segmentation. While the +protoplasm in the animal section of the ovum continues briskly to +divide, multiplying the nuclei, the deutoplasm in the vegetal +section remains more or less undivided; it is merely consumed as +food by the forming cells. The larger the accumulation of food, the +more restricted is the process of segmentation. It may, however, +continue for some time (even after the gastrulation is more or less +complete) in the sense that the vegetal cell-nuclei distributed in +the deutoplasm slowly increase by cleavage; as each of them is +surrounded by a small quantity of protoplasm, it may afterwards +appropriate a portion of the food-yelk, and thus form a real +“yelk-cell” (<i>merocyte</i>). When this vegetal +cell-formation continues for a long time, after the two primary +germinal layers have been formed, it takes the name of the +“after-segmentation.”</p> + +<p>The meroblastic ova are only found in the larger and more highly +developed animals, and only in those whose embryo needs a longer +time and richer nourishment within the fœtal membranes. +According as the yelk-food accumulates at the centre or at the side +of the ovum, we distinguish two groups of dividing ova, periblastic +and discoblastic. In the periblastic the food-yelk is in the +centre, enclosed inside the ovum (hence they are also called +“centrolecithal” ova): the formative yelk surrounds the +food-yelk, and so suffers itself a superficial cleavage. This is +found among the articulates (crabs, spiders, insects, etc.). In the +discoblastic ova the food-yelk gathers at one side, at the vegetal +or lower pole of the vertical axis, while the nucleus of the ovum +and the great bulk of the formative yelk lie at the upper or animal +pole (hence these ova are also called “telolecithal”). +In these cases the cleavage of the ovum begins at the upper pole, +and leads to the formation of a dorsal discoid embryo. This is the +case with all meroblastic vertebrates, most fishes, the reptiles +and birds, and the oviparous mammals (the monotremes).</p> + +<p>The gastrulation of the discoblastic ova, which chiefly concerns +us, offers serious difficulties to microscopic investigation and +philosophic consideration. These, however, have been mastered by +the comparative embryological research which has been conducted by +a number of distinguished observers during the last few +decades—especially the brothers Hertwig, Rabl, Kupffer, +Selenka, Rückert, Goette, Rauber, etc. These thorough and +careful studies, aided by the most perfect modern improvements in +technical method (in tinting and dissection), have given a very +welcome support to the views which I put forward in my work, <i>On +the Gastrula and the Segmentation of the Animal Ovum</i> [not +translated], in 1875. As it is very important to understand these +views and their phylogenetic foundation clearly, not only as +regards evolution in general, but particularly in connection with +the genesis of man, I will give here a brief statement of them as +far as they concern the vertebrate-stem:—</p> + +<p>1. All the vertebrates, including man, are phylogenetically (or +genealogically) related—that is, are members of one single +natural stem.</p> + +<p>2. Consequently, the embryonic features in their individual +development must also have a genetic connection.</p> + +<p>3. As the gastrulation of the amphioxus shows the original +palingenetic form in its simplest features, that of the other +vertebrates must have been derived from it.</p> + +<p>4. The cenogenetic modifications of the latter are more +appreciable the more food-yelk is stored up in the ovum.</p> + +<p>5. Although the mass of the food-yelk may be very large in the +ova of the discoblastic vertebrates, nevertheless in every case a +blastula is developed from the morula, as in the holoblastic +ova.</p> + +<p>6. Also, in every case, the gastrula develops from the blastula +by curving or invagination.</p> + +<p>7. The cavity which is produced in the fœtus by this +curving is, in each case, the primitive gut (progaster), and its +opening the primitive mouth (prostoma).</p> + +<p>8. The food-yelk, whether large or small, is always stored in +the ventral wall of the primitive gut; the cells (called +“merocytes”) which may be formed in it subsequently (by +“after-segmentation”) also belong to the inner germinal +layer, like the cells which immediately enclose the primitive +gut-cavity.</p> + +<p>9. The primitive mouth, which at first lies below at the lower +pole of the vertical axis, is forced, by the growth of the yelk, +backwards and then upwards,</p> + +<br> +<hr> +<p class="page"><a name="page 69">[ 69 ]</a></p> + +<p> </p> + +<p class="one">towards the dorsal side of the embryo; the vertical +axis of the primitive gut is thus gradually converted into +horizontal.</p> + +<p>10. The primitive mouth is closed sooner or later in all the +vertebrates, and does not evolve into the permanent mouth-aperture; +it rather corresponds to the “properistoma,” or region +of the anus. From this important point the formation of the middle +germinal layer proceeds, between the two primary layers.</p> + +<p>The wide comparative studies of the scientists I have named have +further shown that in the case of the discoblastic higher +vertebrates (the three classes of amniotes) the primitive mouth of +the embryonic disc, which was long looked for in vain, is found +always, and is nothing else than the familiar “primitive +groove.” Of this we shall see more as we proceed. Meantime we +realise that gastrulation may be reduced to one and the same +process in all the vertebrates. Moreover, the various forms it +takes in the invertebrates can always be reduced to one of the four +types of segmentation described above. In relation to the +distinction between total and partial segmentation, the grouping of +the various forms is as follows:—</p> + +<br> +<center> +<table class="text" border="1" cellspacing="0" cellpadding="4" +summary= +"Grouping of various forms showing distinction between total and partial segmentation."> +<tr> +<td align="left" valign="middle">I. Palingenetic<br> + (primitive) segmentation.</td> +<td align="left">1. Equal segmentation<br> + (bell-gastrula).</td> +<td align="center" valign="middle" rowspan="2">A. Total +segmentation<br> +(without independent<br> +food-yelk).</td> +</tr> + +<tr> +<td align="left" valign="middle" rowspan="3">II. Cenogenetic +segmentation<br> + (modified by adaptation).</td> +<td align="left">2. Unequal segmentation<br> + (hooded gastrula).</td> +</tr> + +<tr> +<td align="left">3. Discoid segmentation<br> + (discoid gastrula).</td> +<td align="center" valign="middle" rowspan="2">B. Partial +segmentation<br> +(with independent<br> +food-yelk).</td> +</tr> + +<tr> +<td align="left">4. Superficial segmentation<br> + (spherical gastrula).</td> +</tr> +</table> +</center> + +<br> + + +<p>The lowest metazoa we know—namely, the lower zoophyta +(sponges, simple polyps, etc.)—remain throughout life at a +stage of development which differs little from the gastrula; their +whole body consists of two layers of cells. This is a fact of +extreme importance. We see that man, and also other vertebrates, +pass quickly through a stage of development in which they consist +of two layers, just as these lower zoophyta do throughout life. If +we apply our biogenetic law to the matter, we at once reach this +important conclusion. “Man and all the other animals which +pass through the two-layer stage, or gastrula-form, in the course +of their embryonic development, must descend from a primitive +simple stem-form, the whole body of which consisted throughout life +(as is the case with the lower zoophyta to-day) merely of two +cell-strata or germinal layers.” We will call this primitive +stem-form, with which we shall deal more fully later on, the <i> +gastræa</i>—that is to say, “primitive-gut +animal.”</p> + +<p>According to this gastræa-theory there was originally in +all the multicellular animals <i>one organ</i> with the same +structure and function. This was the primitive gut; and the two +primary germinal layers which form its wall must also be regarded +as identical in all. This important homology or identity of the +primary germinal layers is proved, on the one hand, from the fact +that the gastrula was originally formed in the same way in all +cases—namely, by the curving of the blastula; and, on the +other hand, by the fact that in every case the same fundamental +organs arise from the germinal layers. The outer or animal layer, +or ectoderm, always forms the chief organs of animal life—the +skin, nervous system, sense-organs, etc.; the inner or vegetal +layer, or entoderm, gives rise to the chief organs of vegetative +life—the organs of nourishment, digestion, blood-formation, +etc.</p> + +<p>In the lower zoophyta, whose body remains at the two-layer stage +throughout life, the gastræads, the simplest sponges +(<i>Olynthus</i>), and polyps (<i>Hydra</i>), these two groups of +functions, animal and vegetative, are strictly divided between the +two simple primary layers. Throughout life the outer or animal +layer acts simply as a covering for the body, and accomplishes its +movement and sensation. The inner or vegetative layer of cells acts +throughout life as a gut-lining, or nutritive layer of enteric +cells, and often also yields the reproductive cells.</p> + +<p>The best known of these “gastræads,” or +“gastrula-like animals,” is the common fresh-water +polyp (<i>Hydra</i>). This simplest of all the cnidaria has, it is +true, a crown of tentacles round its mouth. Also its outer germinal +layer has certain special modifications. But these are secondary +additions, and the inner germinal layer is a simple stratum of +cells. On the whole, the hydra has preserved to our day by heredity +the simple structure of our primitive ancestor, the <i> +gastræa</i> (cf. <a href="chap19.html">Chapter XIX</a>).</p> + +<br> +<hr> +<p class="page"><a name="page 70">[ 70 ]</a></p> + +<p> </p> + +<br class="one"> +<br> +<p>In all other animals, particularly the vertebrates, the gastrula +is merely a brief transitional stage. Here the two-layer stage of +the embryonic development is quickly succeeded by a three-layer, +and then a four-layer, stage. With the appearance of the four +superimposed germinal layers we reach again a firm and steady +standing-ground, from which we may follow the further, and much +more difficult and complicated, course of embryonic +development.</p> + +<br> + + +<center>SUMMARY OF THE CHIEF DIFFERENCES IN THE OVUM-SEGMENTATION +AND GASTRULATION OF ANIMALS.<br> +<br> +<p class="ind1">The animal stems are indicated by the letters <i> +a–g</i>: <i>a</i> Zoophyta. <i>b</i> Annelida.<br> +<i>c</i> Mollusca. <i>d</i> Echinoderma. <i>e</i> Articulata. <i> +f</i> Tunicata. <i>g</i> Vertebrata.</p> +</center> + +<br> + + +<table class="text" border="1" cellspacing="0" cellpadding="4" +summary= +"Summary of the chief differences in the ovum-segmentation and gastrulation of animals."> +<tr> +<td align="center" valign="middle" rowspan="2"><b>I.<br> +Total<br> +Segmentation.</b><br> +Holoblastic ova.<br> +<br> +<br> +<br> +<br> +<br> +<br> +<b>Gastrula without<br> +separate<br> +food-yelk.</b><br> +Hologastrula.</td> +<td align="center" valign="middle"><b>I. Primitive<br> +Segmentation.</b><br> +Archiblastic ova.<br> +<br> +<b>Bell-gastrula</b><br> +(archigastrula.)</td> +<td align="left"><i>a.</i> Many lower zoophyta (sponges,<br> + hydrapolyps, medusæ, simpler +corals).<br> +<i>b.</i> Many lower annelids (sagitta, phoronis,<br> + many nematoda, etc., terebratula, +argiope,<br> + pisidium).<br> +<i>c.</i> Some lower molluscs.<br> +<i>d.</i> Many echinoderms.<br> +<i>e.</i> A few lower articulata (some brachiopods,<br> + copepods: Tardigrades, pteromalina).<br> +<i>f.</i> Many tunicata.<br> +<i>g.</i> The acrania (amphioxus).</td> +</tr> + +<tr> +<td align="center" valign="middle"><b>II. Unequal<br> +Segmentation.</b><br> +Amphiblastic ova.<br> +<br> +<b>Hooded-gastrula</b><br> +(amphigastrula).</td> +<td align="left"><i>a.</i> Many zoophyta (sponges, +medusæ,<br> + corals, siphonophoræ, +ctenophora).<br> +<i>b.</i> Most worms.<br> +<i>c.</i> Most molluscs.<br> +<i>d.</i> Many echinoderms (viviparous species and<br> + some others).<br> +<i>e.</i> Some of the lower articulata (both crustacea<br> + and tracheata).<br> +<i>f.</i> Many tunicata.<br> +<i>g.</i> Cyclostoma, the oldest fishes, amphibia,<br> + mammals (not including man).</td> +</tr> + +<tr> +<td align="center" valign="middle" rowspan="2"><b>II.<br> +Partial Segmentation.</b><br> +Meroblastic ova.<br> +<br> +<b>Gastrula with<br> +separate<br> +food-yelk.</b><br> +Merogastrula.</td> +<td align="center" valign="middle"><b>III. Discoid<br> +Segmentation.</b><br> +Discoblastic ova.<br> +<br> +<b>Discoid gastrula.</b></td> +<td align="left"><i>c.</i> Cephalopods or cuttlefish.<br> +<i>e.</i> Many articulata, wood-lice, scorpions, etc.<br> +<i>g.</i> Primitive fishes, bony fishes, reptiles, birds,<br> + monotremes.</td> +</tr> + +<tr> +<td align="center" valign="middle"><b>IV. Superficial<br> +Segmentation.</b><br> +Periblastic ova.<br> +<b>Spherical-gastrula.</b></td> +<td align="left"><i>e.</i> The great majority of the articulata<br> + (crustaceans, myriapods, arachnids, +insects).</td> +</tr> +</table> + +<center><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="chap7.html">Chapter VII</a><br> +<a href="chap9.html">Chapter IX</a><br> +<a href="Title.html#Illustrations">Figs. 1–209</a><br> +<a href="title2.html#Illustrations">Figs. 210–408</a></p> +</center> +</body> +</html> + |