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diff --git a/8700-h/old/chap17.html b/8700-h/old/chap17.html new file mode 100644 index 0000000..435e11b --- /dev/null +++ b/8700-h/old/chap17.html @@ -0,0 +1,657 @@ +<!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 II<br> +<br> +<hr noshade size="1" align="center" width="10%"> +<br> +C<font size="-2">HAPTER</font> XVII<br> +<br> +<b>EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT</b></center> + +<br> + + +<p class="one">The structural features that distinguish the +vertebrates from the invertebrates are so prominent that there was +the greatest difficulty in the earlier stages of classification in +determining the affinity of these two great groups. When scientists +began to speak of the affinity of the various animal groups in more +than a figurative—in a genealogical—sense, this +question came at once to the front, and seemed to constitute one of +the chief obstacles to the carrying-out of the evolutionary theory. +Even earlier, when they had studied the relations of the chief +groups, without any idea of real genealogical connection, they +believed they had found here and there among the invertebrates +points of contact with the vertebrates: some of the worms, +especially, seemed to approach the vertebrates in structure, such +as the marine arrow-worm (<i>Sagitta</i>). But on closer study the +analogies proved untenable. When Darwin gave an impulse to the +construction of a real stem-history of the animal kingdom by his +reform of the theory of evolution, the solution of this problem was +found to be particularly difficult. When I made the first attempt +in my <i>General Morphology</i> (1866) to work out the theory and +apply it to classification, I found no problem of phylogeny that +gave me so much trouble as the linking of the vertebrates with the +invertebrates.</p> + +<p>But just at this time the true link was discovered, and at a +point where it was least expected. Towards the end of 1866 two +works of the Russian zoologist, Kowalevsky, who had lived for some +time at Naples, and studied the embryology of the lower animals, +were issued in the publications of the St. Petersburg Academy. A +fortunate accident had directed the attention of this able observer +almost simultaneously to the embryology of the lowest vertebrate, +the Amphioxus, and that of an invertebrate, the close affinity of +which to the Amphioxus had been least suspected, the Ascidia. To +the extreme astonishment of all zoologists who were interested in +this important question, there turned out to be the utmost +resemblance in structure from the commencement of development +between these two very different animals—the lowest +vertebrate and the mis-shaped, sessile invertebrate. With this +undeniable identity of ontogenesis, which can be demonstrated to an +astounding extent, we had, in virtue of the biogenetic law, +discovered the long-sought genealogical link, and definitely +identified the invertebrate group that represents the nearest +blood-relatives of the vertebrates.</p> + +<br> +<hr> +<p class="page"><a name="page 192">[ 192 ]</a></p> + +<p> </p> + +<p class="one">The discovery was confirmed by other zoologists, and +there can no longer be any doubt that of all the classes of +invertebrates that of the Tunicates is most closely related to the +vertebrates, and of the Tunicates the nearest are the +Ascidiæ. We cannot say that the vertebrates are descended +from the Ascidiæ—and still less the reverse—but +we can say that of all the invertebrates it is the Tunicates, and, +within this group, the Ascidiæ, that are the nearest +blood-relatives of the ancient stem-form of the vertebrates. We +must assume as the common ancestral group of both stems an extinct +family of the extensive vermalia-stem, the <i>Prochordonia</i> or +<i>Prochordata</i> (“primitive chorda-animals”).</p> + +<p>In order to appreciate fully this remarkable fact, and +especially to secure the sound basis we seek for the genealogical +tree of the vertebrates, it is necessary to study thoroughly the +embryology of both these animals, and compare the individual +development of the Amphioxus step by step with that of the Ascidia. +We begin with the ontogeny of the Amphioxus.</p> + +<p>From the concordant observations of Kowalevsky at Naples and +Hatschek at Messina, it follows, firstly, that the +ovum-segmentation and gastrulation of the Amphioxus are of the +simplest character. They take place in the same way as we find them +in many of the lower animals of different invertebrate stems, which +we have already described as original or primordial; the +development of the Ascidia is of the same type. Sexually mature +specimens of the Amphioxus, which are found in great quantities at +Messina from April or May onwards, begin as a rule to eject their +sexual products in the evening; if you catch them about the middle +of a warm night and put them in a glass vessel with seawater, they +immediately eject through the mouth their accumulated sexual +products, in consequence of the disturbance. The males give out +masses of sperm, and the females discharge ova in such quantity +that many of them stick to the fibrils about their mouths. Both +kinds of cells pass first into the mantle-cavity after the opening +of the gonads, proceed through the gill-clefts into the branchial +gut, and are discharged from this through the mouth.</p> + +<p>The ova are simply round cells. They are only 1/250 of an inch +in diameter, and thus are only half the size of the mammal ova, and +have no distinctive features. The clear protoplasm of the mature +ovum is made so turbid by the numbers of dark granules of food-yelk +or deutoplasm scattered in it that it is difficult to follow the +process of fecundation and the behaviour of the two nuclei during +it <a href="chap7.html">(p. 51).</a> The active elements of the +male sperm, the cone-shaped spermatozoa, are similar to those of +most other animals (cf. Fig. 20). Fecundation takes place when +these lively ciliated cells of the sperm approach the ovum, and +seek to penetrate into the yelk-matter or the cellular substance of +the ovum with their head-part—the thicker part of the cell +that encloses the nucleus. Only one spermatozoon can bore its way +into the yelk at one pole of the ovum-axis; its head or nucleus +coalesces with the female nucleus, which remains after the +extrusion of the directive bodies from the germinal vesicle. Thus +is formed the “stem-nucleus,” or the nucleus of the +“stem-cell” <a href="chap6.html#Fig. 2">(cytula, Fig. +2).</a> This now undergoes total segmentation, dividing into two, +four, eight, sixteen, thirty-two cells, and so on. In this way we +get the spherical, mulberry-shaped body, which we call the <i> +morula.</i></p> + +<p>The segmentation of the Amphioxus is not entirely regular, as +was supposed after the first observations of Kowalevsky (1866). It +is not completely equal, but a little unequal. As Hatschek +afterwards found (1879), the segmentation-cells only remain equal +up to the morula-stage, the spherical body of which consists of +thirty-two cells. Then, as always happens in unequal segmentation, +the more sluggish vegetal cells are outstripped in the cleavage. At +the lower or vegetal pole of the ovum a crown of eight large +entodermic cells remains for a long time unchanged, while the other +cells divide, owing to the formation of a series of horizontal +circles, into an increasing number of crowns of sixteen cells each. +Afterwards the segmentation-cells get more or less irregularly +displaced, while the segmentation-cavity enlarges in the centre of +the morula; in the end the former all lie on the surface of the +latter, so that the fœtus attains the familiar blastula shape +and forms a hollow ball, the wall of which consists of a single +stratum of cells <a href="chap8.html#Fig. 38">(Fig. 38 <i> +A–C</i>).</a> This layer is the blastoderm, the simple +epithelium from the cells of which all the tissues of the body +proceed.</p> + +<br> +<hr> +<p class="page"><a name="page 193">[ 193 ]</a></p> + +<p> </p> + +<p>These important early embryonic processes take place so quickly +in the Amphioxus that four or five hours after fecundation, or +about midnight, the spherical blastula is completed. A pit-like +depression is then formed at the vegetal pole of it, and in +consequence of this the hollow sphere doubles on itself (Fig. 38 +<i>D</i>). This pit becomes deeper and deeper (Fig. 38 <i>E, +F</i>); at last the invagination (or doubling) is complete, and the +inner or folded part of the blastula-wall lies on the inside of the +outer wall. We thus get a hollow hemisphere, the thin wall of which +is made up of two layers of cells (Fig. 38 <i>E</i>). From +hemispherical the body soon becomes almost spherical once more, and +then oval, the internal cavity enlarging considerably and its mouth +growing narrower <a href="chap16.html#Fig. 212">(Fig. 213).</a> The +form which the Amphioxus-embryo has thus reached is a real +“cup-larva” or <i>gastrula,</i> of the original simple +type that we have previously described as the +“bell-gastrula” or <i>archigastrula</i> (Figs. +29–35).</p> + +<p>As in all the other animals that form an archigastrula, the +whole body is nothing but a simple gastric sac or stomach; its +internal cavity is the primitive gut (<i>progaster</i> or <i> +archenteron,</i> Fig. 38 <i>g,</i> 35 <i>d</i>), and its aperture +the primitive mouth (<i>prostoma</i> or <i>blastoporus, o</i>). The +wall is at once gut-wall and body-wall. It is composed of two +simple cell-layers, the familiar primary germinal layers. The inner +layer or the invaginated part of the blastoderm, which immediately +encloses the gut-cavity is the entoderm, the inner or vegetal +germ-layer, from which develop the wall of the alimentary canal and +all its appendages, the cœlom-pouches, etc. (Figs. 35, 36 <i> +i</i>). The outer stratum of cells, or the non-invaginated part of +the blastoderm, is the ectoderm, the outer or animal germ-layer, +which provides the outer skin (epidermis) and the nervous system +(<i>e</i>). The cells of the entoderm are much larger, darker, and +more fatty than those of the ectoderm, which are clearer and less +rich in fatty particles. Hence before and during invagination there +is an increasing differentiation of the inner from the outer layer. +The animal cells of the outer layer soon develop vibratory hairs; +the vegetal cells of the inner layer do so much later. A +thread-like process grows out of each cell, and effects continuous +vibratory movements. By the vibrations of these slender hairs the +gastrula of the Amphioxus swims about in the sea, when it has +pierced the thin ovolemma, like the gastrula of many other animals +(Fig. 36). As in many other lower animals, the cells have only one +whip-like hair each, and so are called <i>flagellate</i> (whip) +cells (in contrast with the <i>ciliated</i> cells, which have a +number of short lashes or cilia).</p> + +<p>In the further course of its rapid development the roundish +bell-gastrula becomes elongated, and begins to flatten on one side, +parallel to the long axis. The flattened side is the subsequent +dorsal side; the opposite or ventral side remains curved. The +latter grows more quickly than the former, with the result that the +primitive mouth is forced to the dorsal side (Fig. 39). In the +middle of the dorsal surface a shallow longitudinal groove or +furrow is formed (Fig. 79), and the edges of the body rise up on +each side of this groove in the shape of two parallel swellings. +This groove is, of course, the dorsal furrow, and the swellings are +the dorsal or medullary swellings; they form the first structure of +the central nervous system, the medullary tube. The medullary +swellings now rise higher; the groove between them becomes deeper +and deeper. The edges of the parallel swellings curve towards each +other, and at last unite, and the medullary tube is formed (Figs. +83 <i>m,</i> 84 <i>m</i>). Hence the formation of a medullary tube +out of the outer skin takes place in the naked dorsal surface of +the free-swimming larva of the Amphioxus in just the same way as we +have found in the embryo of man and the higher animals within the +fœtal membranes.</p> + +<p>Simultaneously with the construction of the medullary tube we +have in the Amphioxus-embryo the formation of the chorda, the +cœlom-pouches, and the mesoderm proceeding from their wall. +These processes also take place with characteristic simplicity and +clearness, so that they are very instructive to compare with the +vermalia on the one hand and with the higher vertebrates on the +other. While the medullary groove is sinking in the middle line of +the flat dorsal side of the oval embryo, and its parallel edges +unite to form the ectodermic neural tube, the single chorda is +formed directly underneath them, and on each side of this a +parallel longitudinal fold, from the dorsal wall of the primitive +gut. These longitudinal folds of the entoderm proceed from the +primitive mouth, or from its lower</p> + +<br> +<hr> +<p class="page"><a name="page 194">[ 194 ]</a></p> + +<p> </p> + +<p class="one">and hinder edge. Here we see at an early stage a +couple of large entodermic cells, which are distinguished from all +the others by their great size, round form, and fine-grained +protoplasm; they are the two promesoblasts, or polar cells of the +mesoderm <a href="chap10.html#Fig. 83">(Fig. 83 <i>p</i>).</a> They +indicate the original starting-point of the two +cœlom-pouches, which grow from this spot between the inner +and outer germinal layers, sever themselves from the primitive gut, +and provide the cellular material for the middle layer.</p> + +<p>Immediately after their formation the two cœlom-pouches of +the Amphioxus are divided into several parts by longitudinal and +transverse folds. Each of the primary pouches is divided into an +upper dorsal and a lower ventral section by a couple of lateral +longitudinal folds (Fig. 82). But these are again divided by +several parallel transverse folds into a number of successive sacs, +the primitive segments or somites (formerly called by the +unsuitable name of “primitive vertebræ”). They +have a different future above and below. The upper or dorsal +segments, the <i>episomites,</i> lose their cavity later on, and +form with their cells the muscular plates of the trunk. The lower +or ventral segments, the <i>hyposomites,</i> corresponding to the +lateral plates of the craniote-embryo, fuse together in the upper +part owing to the disappearance of their lateral walls, and thus +form the later body-cavity (metacœl); in the lower part they +remain separate, and afterwards form the segmental gonads.</p> + +<p>In the middle, between the two lateral cœlom-folds of the +primitive gut, a single central organ detaches from this at an +early stage in the middle line of its dorsal wall. This is the +dorsal chorda (Figs. 83, 84 <i>ch</i>). This axial rod, which is +the first foundation of the later vertebral column in all the +vertebrates, and is the only representative of it in the Amphioxus, +originates from the entoderm.</p> + +<p>In consequence of these important folding-processes in the +primitive gut, the simple entodermic tube divides into four +different sections:— I, underneath, at the ventral side, the +permanent alimentary canal or permanent gut; II, above, at the +dorsal side, the axial rod or chorda; and III, the two +cœlom-sacs, which immediately sub-divide into two +structures:—III<small>A</small>, above, on the dorsal side, +the <i>episomites,</i> the double row of primitive or muscular +segments; and III<small>B</small>, below, on each side of the gut, +the <i>hyposomites,</i> the two lateral plates that give rise to +the sex-glands, and the cavities of which partly unite to form the +body-cavity. At the same time, the neural or medullary tube is +formed above the chorda, on the dorsal surface, by the closing of +the parallel medullary swellings. All these processes, which +outline the typical structure of the vertebrate, take place with +astonishing rapidity in the embryo of the Amphioxus; in the +afternoon of the first day, or twenty-four hours after +fertilisation, the young vertebrate, the typical embryo, is formed; +it then has, as a rule, six to eight somites.</p> + +<p>The chief occurrence on the second day of development is the +construction of the two permanent openings of the gut—the +mouth and anus. In the earlier stages the alimentary tube is found +to be entirely closed, after the closing of the primitive mouth; it +only communicates behind by the neurenteric canal with the +medullary tube. The permanent mouth is a secondary formation, at +the opposite end. Here, at the end of the second day, we find a +pit-like depression in the outer skin, which penetrates inwards +into the closed gut. The anus is formed behind in the same way a +few hours later (in the vicinity of the additional gastrula-mouth). +In man and the higher vertebrates also the mouth and anus are +formed, as we have seen, as flat pits in the outer skin; they then +penetrate inwards, gradually becoming connected with the blind ends +of the closed gut-tube. During the second day the Amphioxus-embryo +undergoes few other changes. The number of primitive segments +increases, and generally amounts to fourteen, some forty-eight to +fifty hours after impregnation.</p> + +<p>Almost simultaneously with the formation of the mouth the first +gill-cleft breaks through in the fore section of the +Amphioxus-embryo (generally forty hours after the commencement of +development). It now begins to nourish itself independently, as the +food material stored up in the ovum is completely used up. The +further development of the free larvæ takes place very +slowly, and extends over several months. The body becomes much +longer, and is compressed at the sides, the head-end being +broadened in a sort of triangle. Two rudimentary sense-organs are +developed in it. Inside we find the first blood-vessels, an upper +or dorsal vessel, corresponding to the aorta, between the gut and +the dorsal cord, and a lower or ventral</p> + +<br> +<hr> +<p class="page"><a name="page 195">[ 195 ]</a></p> + +<p> </p> + +<p class="one">vessel, corresponding to the subintestinal vein, at +the lower border of the gut. Now, the gills or respiratory organs +also are formed at the fore-end of the alimentary canal. The whole +of the anterior or respiratory section of the gut is converted into +a gill-crate, which is pierced trellis-wise by numbers of +branchial-holes, as in the ascidia. This is done by the foremost +part of the gut-wall joining star-wise with the outer skin, and the +formation of clefts at the point of connection, piercing the wall +and leading into the gut from without. At first there are very few +of these branchial clefts; but there are soon a number of +them—first in one, then in two, rows. The foremost gill-cleft +is the oldest. In the end we have a sort of lattice work of fine +gill-clefts, supported on a number of stiff branchial rods; these +are connected in pairs by transverse rods.</p> + +<br> + + +<center> +<table class="capt" width="452" summary= +"Figs. 222-224. Transverse sections of young Amphioxus-larvae"> +<tr> +<td><img src="images3/fig222.GIF" width="452" height="261" alt= +"Transverse sections of young Amphioxus-larvae."><br> +<a name="Fig. 222">Figs. 222–224</a>—<b>Transverse +sections of young Amphioxus-larvæ</b> (diagrammatic, from <i> +Ralph.</i>) (Cf. also Fig. 216.) In Fig. 222 there is free +communication from without with the gut-cavity (<i>D</i>) through +the gill-clefts (<i>K</i>). In Fig. 223 the lateral folds of the +body-wall, or the gill-covers, which grow downwards, are formed. In +Fig. 224 these lateral folds have united underneath and joined +their edges in the middle line of the ventral side (<i>R</i> seam). +The respiratory water now passes from the gut-cavity (<i>D</i>) +into the mantle-cavity (<i>A</i>). The letters have the same +meaning throughout: <i>N</i> medullary tube, <i>Ch</i> chorda, <i> +M</i> lateral muscles, <i>Lh</i> body-cavity, <i>G</i> part of the +body-cavity in which the sexual organs are subsequently formed. <i> +D</i> gut-cavity, clothed with the gut-gland layer (<i>a</i>). A +mantle-cavity, <i>K</i> gill-clefts, <i>b</i>=<i>E</i> epidermis, +<i>E</i><sub>1</sub> the same as visceral epithelium of the +mantle-cavity, <i>E</i><sub>2</sub> as parietal epithelium of the +mantle-cavity.</td> +</tr> +</table> +</center> + +<br> + + +<p>At an early stage of embryonic development the structure of the +Amphioxus-larva is substantially the same as the ideal picture we +have previously formed of the “Primitive Vertebrate” +(Figs. 98–102). But the body afterwards undergoes various +modifications, especially in the fore-part. These modifications do +not concern us, as they depend on special adaptations, and do not +affect the hereditary vertebrate type. When the free-swimming +Amphioxus-larva is three months old, it abandons its pelagic habits +and changes into the young animal that lives in the sand. In spite +of its smallness (one-eighth of an inch), it has substantially the +same structure as the adult. As regards the remaining organs of the +Amphioxus, we need only mention that the gonads or sexual glands +are developed very late, immediately out of the inner cell-layer of +the</p> + +<br> +<hr> +<p class="page"><a name="page 196">[ 196 ]</a></p> + +<p> </p> + +<p class="one">body-cavity. Although we can find afterwards no +continuation of the body-cavity (Fig. 216 <i>U</i>) in the lateral +walls of the mantle-cavity, in the gill-covers or mantle-folds +(Fig. 224 <i>U</i>), there is one present in the beginning (Fig. +224 <i>Lh</i>). The sexual cells are formed below, at the bottom of +this continuation (Fig. 224 <i>S</i>). For the rest, the subsequent +development into the adult Amphioxus of the larva we have followed +is so simple that we need not go further into it here.</p> + +<p>We may now turn to the embryology of the Ascidia, an animal that +seems to stand so much lower and to be so much more simply +organised, remaining for the greater part of its life attached to +the bottom of the sea like a shapeless lump. It was a fortunate +accident that Kowalevsky first examined just those larger specimens +of the Ascidiæ that show most clearly the relationship of the +vertebrates to the invertebrates, and the larvæ of which +behave exactly like those of the Amphioxus in the first stages of +development. This resemblance is so close in the main features that +we have only to repeat what we have already said of the ontogenesis +of the Amphioxus.</p> + +<p>The ovum of the larger Ascidia (<i>Phallusia, Cynthia,</i> etc.) +is a simple round cell of 1/250 to 1/125 of an inch in diameter. In +the thick fine-grained yelk we find a clear round germinal vesicle +of about 1/750 of an inch in diameter, and this encloses a small +embryonic spot or nucleolus. Inside the membrane that surrounds the +ovum, the stem-cell of the Ascidia, after fecundation, passes +through just the same metamorphoses as the stem-cell of the +Amphioxus. It undergoes total segmentation; it divides into two, +four, eight, sixteen, thirty-two cells, and so on. By continued +total cleavage the morula, or mulberry-shaped cluster of cells, is +formed. Fluid gathers inside it, and thus we get once more a +globular vesicle (the blastula); the wall of this is a single +stratum of cells, the blastoderm. A real gastrula (a simple +bell-gastrula) is formed from the blastula by invagination, in the +same way as in the amphioxus.</p> + +<p>Up to this there is no definite ground in the embryology of the +Ascidiæ for bringing them into close relationship with the +Vertebrates; the same gastrula is formed in the same way in many +other animals of different stems. But we now find an embryonic +process that is peculiar to the Vertebrates, and that proves +irrefragably the affinity of the Ascidiæ to the Vertebrates. +From the epidermis of the gastrula a <i>medullary tube</i> is +formed on the dorsal side, and, between this and the primitive gut, +a <i>chorda</i>; these are the organs that are otherwise only found +in Vertebrates. The formation of these very important organs takes +place in the Ascidia-gastrula in precisely the same way as in that +of the Amphioxus. In the Ascidia (as in the other case) the oval +gastrula is first flattened on one side—the subsequent dorsal +side. A groove or furrow (the medullary groove) is sunk in the +middle line of the flat surface, and two parallel longitudinal +swellings arise on either side from the skin layer. These medullary +swellings join together over the furrow, and form a tube; in this +case, again, the neural or medullary tube is at first open in +front, and connected with the primitive gut behind by the +neurenteric canal. Further, in the Ascidia-larva also the two +permanent apertures of the alimentary canal only appear later, as +independent and new formations. The permanent mouth does not +develop from the primitive mouth of the gastrula; this primitive +mouth closes up, and the later anus is formed near it by +invagination from without, on the hinder end of the body, opposite +to the aperture of the medullary tube.</p> + +<p>During these important processes, that take place in just the +same way in the Amphioxus, a tail-like projection grows out of the +posterior end of the larva-body, and the larva folds itself up +within the round ovolemma in such a way that the dorsal side is +curved and the tail is forced on to the ventral side. In this tail +is developed—starting from the primitive gut—a +cylindrical string of cells, the fore end of which pushes into the +body of the larva, between the alimentary canal and the neural +canal, and is no other than the chorda dorsalis. This important +organ had hitherto been found only in the Vertebrates, not a single +trace of it being discoverable in the Invertebrates. At first the +chorda only consists of a single row of large entodermic cells. It +is afterwards composed of several rows of cells. In the +Ascidia-larva, also, the chorda develops from the dorsal middle +part of the primitive gut, while the two cœlom-pouches detach +themselves from it on both sides. The simple body-cavity is formed +by the coalescence of the two.</p> + +<p>When the Ascidia-larva has attained</p> + +<br> +<hr> +<p class="page"><a name="page 197">[ 197 ]</a></p> + +<p> </p> + +<p class="one">this stage of development it begins to move about in +the ovolemma. This causes the membrane to burst. The larva emerges +from it, and swims about in the sea by means of its oar-like tail. +These free-swimming larvæ of the Ascidia have been known for +a long time. They were first observed by Darwin during his voyage +round the world in 1833. They resemble tadpoles in outward +appearance, and use their tails as oars, as the tadpoles do. +However, this lively and highly-developed condition does not last +long. At first there is a progressive development; the foremost +part of the medullary tube enlarges into a brain, and inside this +two single sense-organs are developed, a dorsal auditory vesicle +and a ventral eye. Then a heart is formed on the ventral side of +the animal, or the lower wall of the gut, in the same simple form +and at the same spot at which the heart is developed in man and all +the other vertebrates. In the lower muscular wall of the gut we +find a weal-like thickening, a solid, spindle-shaped string of +cells, which becomes hollow in the centre; it begins to contract in +different directions, now forward and now backward, as is the case +with the adult Ascidia. In this way the sanguineous fluid +accumulated in the hollow muscular tube is driven in alternate +directions into the blood-vessels, which develop at both ends of +the cardiac tube. One principal vessel runs along the dorsal side +of the gut, another along its ventral side. The former corresponds +to the aorta and the dorsal vessel in the worms. The other +corresponds to the subintestinal vein and the ventral vessel of the +worms.</p> + +<table class="capt" width="147" align="left" summary= +"Fig. 225. An Appendicaria (Copelata), seen from the left."> +<tr> +<td><img src="images3/fig225.GIF" width="147" height="391" alt= +"An Appendicaria (Copelata), seen from the left."> +<a name="Fig. 225">Fig. +225</a>—<b>An Appendicaria (Copelata),</b> seen from the +left. <i>m</i> mouth, <i>k</i> branchial gut, <i>o</i> gullet, <i> +v</i> stomach, <i>a</i> anus, <i>n</i> brain (ganglion above the +gullet), <i>g</i> auditory vesicle, <i>f</i> ciliated groove under +the gills, <i>h</i> heart, <i>t</i> testicles, <i>e</i> ovary, <i> +c</i> chorda, <i>s</i> tail.</td> +</tr> +</table> + + +<p>With the formation of these organs the progressive development +of the Ascidia comes to an end, and degeneration sets in. The +free-swimming larva sinks to the floor of the sea, abandons its +locomotive habits, and attaches itself to stones, marine plants, +mussel-shells, corals, and other objects; this is done with the +part of the body that was foremost in movement. The attachment is +effected by a number of out-growths, usually three, which can be +seen even in the free-swimming larva. The tail is lost, as there is +no further use for it. It undergoes a fatty degeneration, and +disappears with the chorda dorsalis. The tailless body changes into +an unshapely tube, and, by the atrophy of some parts and the +modification of others, gradually assumes the appearance we have +already described.</p> + +<p>Among the living Tunicates there is a very interesting group of +small animals that remain throughout life at the stage of +development of the tailed, free Ascidia-larva, and swim about +briskly in the sea by means of their broad oar-tail. These are the +remarkable Copelata (<i>Appendicaria</i> and <i>Vexillaria,</i> +Fig. 225). They are the only living Vertebrates that have +throughout life a chorda dorsalis and a neural string above it; the +latter must be regarded as the prolongation of the cerebral +ganglion and the equivalent of the medullary tube. Their branchial +gut also opens directly outwards by a pair of</p> + +<br> +<hr> +<p class="page"><a name="page 198">[ 198 ]</a></p> + +<p> </p> + +<p class="one">branchial clefts. These instructive Copelata, +comparable to permanent Ascidia-larvæ, come next to the +extinct Prochordonia, those ancient worms which we must regard as +the common ancestors of the Tunicates and Vertebrates. The chorda +of the Appendicaria is a long, cylindrical string (Fig. 225 <i> +c</i>), and serves as an attachment for the muscles that work the +flat oar-tail.</p> + +<p>Among the various modifications which the Ascidia-larva +undergoes after its establishment at the sea-floor, the most +interesting (after the loss of the axial rod) is the atrophy of one +of its chief organs, the medullary tube. In the Amphioxus the +spinal marrow continues to develop, but in the Ascidia the tube +soon shrinks into a small and insignificant nervous ganglion that +lies above the mouth and the gill-crate, and is in accord with the +extremely slight mental power of the animal. This insignificant +relic of the medullary tube seems to be quite beyond comparison +with the nervous centre of the vertebrate, yet it started from the +same structure as the spinal cord of the Amphioxus. The +sense-organs that had been developed in the fore part of the neural +tube are also lost; no trace of which can be found in the adult +Ascidia. On the other hand, the alimentary canal becomes a most +extensive organ. It divides presently into two sections—a +wide fore or branchial gut that serves for respiration, and a +narrower hind or hepatic gut that accomplishes digestion. The +branchial or head-gut of the Ascidia is small at first, and opens +directly outwards only by a couple of lateral ducts or +gill-clefts—a permanent arrangement in the Copelata. The +gill-clefts are developed in the same way as in the Amphioxus. As +their number greatly increases we get a large gill-crate, pierced +like lattice work. In the middle line of its ventral side we find +the hypobranchial groove. The mantle or cloaca-cavity (the atrium) +that surrounds the gill-crate is also formed in the same way in the +Ascidia as in the Amphioxus. The ejection-opening of this +peribranchial cavity corresponds to the branchial pore of the +Amphioxus. In the adult Ascidia the branchial gut and the heart on +its ventral side are almost the only organs that recall the +original affinity with the vertebrates.</p> + +<p>The further development of the Ascidia in detail has no +particular interest for us, and we will not go into it. The chief +result that we obtain from its embryology is the complete agreement +with that of the Amphioxus in the earliest and most important +embryonic stages. They do not begin to diverge until after the +medullary tube and alimentary canal, and the axial rod with the +muscles between the two, have been formed. The Amphioxus continues +to advance, and resembles the embryonic forms of the higher +vertebrates; the Ascidia degenerates more and more, and at last, in +its adult condition, has the appearance of a very imperfect +invertebrate.</p> + +<p>If we now look back on all the remarkable features we have +encountered in the structure and the embryonic development of the +Amphioxus and the Ascidia, and compare them with the features of +man’s embryonic development which we have previously studied, +it will be clear that I have not exaggerated the importance of +these very interesting animals. It is evident that the Amphioxus +from the vertebrate side and the Ascidia from the invertebrate form +the bridge by which we can span the deep gulf that separates the +two great divisions of the animal kingdom. The radical agreement of +the lancelet and the sea-squirt in the first and most important +stages of development shows something more than their close +anatomic affinity and their proximity in classification; it shows +also their real blood-relationship and their common origin from one +and the same stem-form. In this way, it throws considerable light +on the oldest roots of man’s genealogical tree.</p> + +<br> + + +<hr noshade align="left" size="1" width="20%"> +<p class="ref"><a href="Title.html">Title and Contents</a><br> +<a href="title2.html">Vol. II Title and Contents</a><br> +<a href="glossary.html">Glossary</a><br> +<a href="chap16.html">Chapter XVI</a><br> +<a href="chap18.html">Chapter XVIII</a><br> +<a href="Title.html#Illustrations">Figs. 1–209</a><br> +<a href="title2.html#Illustrations">Figs. 210–408</a></p> +</body> +</html> + |