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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/chap19.html b/8700-h/old/chap19.html new file mode 100644 index 0000000..0ec1550 --- /dev/null +++ b/8700-h/old/chap19.html @@ -0,0 +1,941 @@ +<!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> XIX<br> +<br> +<b>OUR PROTIST ANCESTORS</b></center> + +<br> + + +<p class="one">Under the guidance of the biogenetic law, and on the +basis of the evidence we have obtained, we now turn to the +interesting task of determining the series of man’s animal +ancestors. Phylogeny us a whole is an inductive science. From the +totality of the biological processes in the life of plants, +animals, and man we have gathered a confident inductive idea that +the whole organic population of our planet has been moulded on a +harmonious law of evolution. All the interesting phenomena that we +meet in ontogeny and paleontology, comparative anatomy and +dysteleology, the distribution and habits of organisms—all +the important general laws that we abstract from the phenomena of +these sciences, and combine in harmonious unity—are the broad +bases of our great biological induction.</p> + +<p>But when we come to the application of this law, and seek to +determine with its aid the origin of the various species of +organisms, we are compelled to frame</p> + +<br> +<hr> +<p class="page"><a name="page 208">[ 208 ]</a></p> + +<p> </p> + +<p class="one">hypotheses that have essentially a <i>deductive</i> +character, and are inferences from the general law to particular +cases. But these special deductions are just as much justified and +necessitated by the rigorous laws of logic as the inductive +conclusions on which the whole theory of evolution is built. The +doctrine of the animal ancestry of the human race is a special +deduction of this kind, and follows with logical necessity from the +general inductive law of evolution.</p> + +<p>I must point out at once, however, that the certainty of these +evolutionary hypotheses, which rest on clear special deductions, is +not always equally strong. Some of these inferences are now beyond +question; in the case of others it depends on the knowledge and the +competence of the inquirer what degree of certainty he attributes +to them. In any case, we must distinguish between the <i> +absolute</i> certainty of the general (inductive) theory of descent +and the <i>relative</i> certainty of special (deductive) +evolutionary hypotheses. We can never determine the whole ancestral +series of an organism with the same confidence with which we hold +the general theory of evolution as the sole scientific explanation +of organic modifications. The special indication of stem-forms in +detail will always be more or less incomplete and hypothetical. +This is quite natural. The evidence on which we build is imperfect, +and always will be imperfect; just as in comparative philology.</p> + +<p>The first of our documents, paleontology, is exceedingly +incomplete. We know that all the fossils yet discovered are only an +insignificant fraction of the plants and animals that have lived on +our planet. For every single species that has been preserved for us +in the rocks there are probably hundreds, perhaps thousands, of +extinct species that have left no trace behind them. This extreme +and very unfortunate incompleteness of the paleontological +evidence, which cannot be pointed out too often, is easily +explained. It is absolutely inevitable in the circumstances of the +fossilisation of organisms. It is also due in part to the +incompleteness of our knowledge in this branch. It must be borne in +mind that the great majority of the stratified rocks that compose +the crust of the earth have not yet been opened. We have only a few +specimens of the innumerable fossils that are buried in the vast +mountain ranges of Asia and Africa. Only a part of Europe and North +America has been investigated carefully. The whole of the fossils +known to us certainly do not amount to a hundredth part of the +remains that are really buried in the crust of the earth. We may, +therefore, look forward to a rich harvest in the future as regards +this science. However, our paleontological evidence will (for +reasons that I have fully explained in the sixteenth chapter of the +<i>History of Creation</i>) always be defective.</p> + +<p>The second chief source of evidence, ontogeny, is not less +incomplete. It is the most important source of all for special +phylogeny; but it has great defects, and often fails us. We must, +above all, clearly distinguish between palingenetic and cenogenetic +phenomena. We must never forget that the laws of curtailed and +disturbed heredity often make the original course of development +almost unrecognisable. The recapitulation of phylogeny by ontogeny +is only fairly complete in a few cases, and is never wholly +complete. As a rule, it is precisely the earliest and most +important embryonic stages that suffer most from alteration and +condensation. The earlier embryonic forms have had to adapt +themselves to new circumstances, and so have been modified. The +struggle for existence has had just as profound an influence on the +freely moving and still immature young forms as on the adult forms. +Hence in the embryology of the higher animals, especially, +palingenesis is much restricted by cenogenesis; it is to-day, as a +rule, only a faded and much altered picture of the original +evolution of the animal’s ancestors. We can only draw +conclusions from the embryonic forms to the stem-history with the +greatest caution and discrimination. Moreover, the embryonic +development itself has only been fully studied in a few +species.</p> + +<p>Finally, the third and most valuable source of evidence, +comparative anatomy, is also, unfortunately, very imperfect; for +the simple reason that the whole of the living species of animals +are a mere fraction of the vast population that has dwelt on our +planet since the beginning of life. We may confidently put the +total number of these at more than a million species. The number of +animals whose organisation has been studied up to the present in +comparative anatomy is proportionately very small. Here, again, +future research will yield incalculable treasures.</p> + +<br> +<hr> +<p class="page"><a name="page 209">[ 209 ]</a></p> + +<p> </p> + +<p class="one">But, for the present, in view of this patent +incompleteness of our chief sources of evidence, we must naturally +be careful not to lay too much stress in human phylogeny on the +particular animals we have studied, or regard all the various +stages of development with equal confidence as stem-forms.</p> + +<p>In my first efforts to construct the series of man’s +ancestors I drew up a list of, at first ten, afterwards twenty to +thirty, forms that may be regarded more or less certainly as animal +ancestors of the human race, or as stages that in a sense mark off +the chief sections in the long story of evolution from the +unicellular organism to man. Of these twenty to thirty stages, ten +to twelve belong to the older group of the Invertebrates and +eighteen to twenty to the younger division of the Vertebrates.</p> + +<p>In approaching, now, the difficult task of establishing the +evolutionary succession of these thirty ancestors of humanity since +the beginning of life, and in venturing to lift the veil that +covers the earliest secrets of the earth’s history, we must +undoubtedly look for the first living things among the wonderful +organisms that we call the Monera; they are the simplest organisms +known to us—in fact, the simplest we can conceive. Their +whole body consists merely of a simple particle or globule of +structureless plasm or plasson. The discoveries of the last four +decades have led us to believe with increasing certainty that +wherever a natural body exhibits the vital processes of nutrition, +reproduction, voluntary movement, and sensation, we have the action +of a nitrogenous carbon-compound of the chemical group of the +albuminoids; this plasm (or protoplasm) is the material basis of +all vital functions. Whether we regarded the function, in the +monistic sense, as the direct action of the material substratum, or +whether we take matter and force to be distinct things in the +dualistic sense, it is certain that we have not as yet found any +living organism in which the exercise of the vital functions is not +inseparably bound up with plasm.</p> + +<p>The soft slimy plasson of the body of the moneron is generally +called “protoplasm,” and identified with the cellular +matter of the ordinary plant and animal cells. But we must, to be +accurate, distinguish between the plasson of the cytodes and the +protoplasm of the cells. This distinction is of the utmost +importance for the purposes of evolution. As I have often said, we +must recognise two different stages of development in these +“elementary organisms,” or plastids +(“builders”), that represent the ultimate units of +organic individuality. The earlier and lower stage are the +unnucleated cytodes, the body of which consists of only one kind of +albuminous matter—the homogeneous plasson or “formative +matter.” The later and higher stage are the nucleated cells, +in which we find a differentiation of the original plasson into two +different formative substances—the caryoplasm of the nucleus +and the cytoplasm of the body of the cell (cf. pp. <a href= +"chap6.html">37</a> and <a href="chap6.html#page 42">42</a>).</p> + +<br> + + +<table class="capt" summary="Fig. 226. Chroococcus minor."> +<tr> +<td><img src="images3/fig226.GIF" width="282" height="91" alt= +"Chroococcus minor."></td> +<td align="left" valign="bottom"><a name="Fig. 226">Fig. +226</a>—<b>Chroococcus minor</b> (<i>Nägeli</i>), +magnified. A phytomoneron, the globular plastids of which secrete a +gelatinous structureless membrane. The unnucleated globule of plasm +(bluish-green in colour) increases by simple cleavage +(<i>a–d</i>).</td> +</tr> +</table> + +<br> + + +<p>The Monera are permanent cytodes. Their whole body consists of +soft, structureless plasson. However carefully we examine it with +our finest chemical reagents and most powerful microscopes, we can +find no definite parts or no anatomic structure in it. Hence, the +Monera are literally organisms without organs; in fact, from the +philosophic point of view they are not organisms at all, since they +have no organs. They can only be called organisms in the sense that +they are capable of the vital functions of nutrition, reproduction, +sensation, and movement. If we were to try to imagine the simplest +possible organism, we should frame something like the moneron.</p> + +<p>The Monera that we find to-day in various forms fall into two +groups according to the nature of their nutrition—the <i> +Phytomonera</i> and the <i>Zoomonera</i>; from the physiological +point of view, the former are the simplest specimens of the plant +(<i>phyton</i>) kingdom, and the latter of the animal (<i>zoon</i>) +world. The Phytomonera, especially in their simplest form, the +Chromacea (<i>Phycochromacea</i> or <i>Cyanophycea</i>), are the +most primitive and the</p> + +<br> +<hr> +<p class="page"><a name="page 210">[ 210 ]</a></p> + +<p> </p> + +<p class="one">oldest of living organisms. The typical genus <i> +Chroococcus</i> (Fig. 226) is represented by several fresh-water +species, and often forms a very delicate bluish-green deposit on +stones and wood in ponds and ditches. It consists of round, light +green particles, from 1/7000 to 1/2500 of an inch in diameter.</p> + + +<table class="capt" align="left" width="228" summary= +"Fig. 227. Aphanocapsa primordialis."> +<tr> +<td align="justify"><img src="images3/fig227.GIF" width="228" +height="250" alt="Aphanocapsa primordialis."> +<a name="Fig. 227">Fig. 227</a>—<b>Aphanocapsa primordialis</b> +(<i>Nägeli</i>), magnified. A phytomoneron, the round plastids +of which (bluish-green in colour) secrete a shapeless gelatinous +mass; in this the unnucleated cytodes increase continually by +simple cleavage.</td> +</tr> +</table> + +<p class="pic">The whole life of these homogeneous globules of +plasm consists of simple growth and reproduction by cleavage. When +the tiny particle has reached a certain size by the continuous +assimilation of inorganic matter, it divides into two equal halves, +by a constriction in the middle. The two daughter-monera that are +thus formed immediately begin a similar vital process. It is the +same with the brown <i>Procytella primordialis</i> (formerly called +the <i>Protococcus marinus</i>); it forms large masses of floating +matter in the arctic seas. The tiny plasma-globules of this species +are of a greenish-brown colour, and have a diameter of 1/10,000 to +1/5000 of an inch. There is no membrane discoverable in the +simplest <i>Chroococcacea,</i> but we find one in other members of +the same family; in <i>Aphanocapsa</i> (Fig. 227) the enveloping +membranes of the social plastids combine; in <i>Glœcapsa</i> +they are retained through several generations, so that the little +plasma-globules are enfolded in many layers of membrane.</p> + +<p>Next to the Chromacea come the Bacteria, which have been evolved +from them by the remarkable change in nutrition which gives us the +simple explanation of the differentiation of plant and animal in +the protist kingdom. The Chromacea build up their plasm directly +from inorganic matter; the Bacteria feed on organic matter. Hence, +if we logically divide the protist kingdom into plasma-forming +Protophyta and plasma-consuming Protozoa, we must class the +Bacteria with the latter; it is quite illogical to describe +them—as is still often done—as <i>Schizomycetes,</i> +and class them with the true fungi. The Bacteria, like the +Chromacea, have no nucleus. As is well-known, they play an +important part in modern biology as the causes of fermentation and +putrefaction, and of tuberculosis, typhus, cholera, and other +infectious diseases, and as parasites, etc. But we cannot linger +now to deal with these very interesting features; the Bacteria have +no relation to man’s genealogical tree.</p> + +<p>We may now turn to consider the remarkable Protamœba, or +unnucleated Amœba. I have, in the first volume, pointed out +the great importance of the ordinary Amœba in connection with +several weighty questions of general biology. The tiny +Protamœbæ, which are found both in fresh and salt +water, have the same unshapely form and irregular movements of +their simple naked body as the real Amœbæ; but they +differ from them very materially in having no nucleus in their +cell-body. The short, blunt, finger-like processes that are thrust +out at the surface of the creeping Protamœba serve for +getting food as well as for locomotion. They multiply by simple +cleavage (Fig. 228).</p> + +<p>The next stage to the simple cytode-forms of the Monera in the +genealogy of mankind (and all other animals) is the simple cell, or +the most rudimentary form of the cell which we find living +independently to-day as the Amœba. The earliest process of +inorganic differentiation in the structureless body of the Monera +led to its division into two different substances—the +caryoplasm and the cytoplasm. The caryoplasm is the inner and +firmer part of the cell, the substance of the nucleus. The +cytoplasm is the outer and softer part, the substance of the body +of the cell. By this important differentiation of the plasson into +nucleus and cell-body, the</p> + +<br> +<hr> +<p class="page"><a name="page 211">[ 211 ]</a></p> + +<p> </p> + +<p class="one">organised cell was evolved from the structureless +cytode, the nucleated from the unnucleated plastid. That the first +cells to appear on the earth were formed from the Monera by such a +differentiation seems to us the only possible view in the present +condition of science. We have a direct instance of this earliest +process of differentiation to-day in the ontogeny of many of the +lower Protists (such as the Gregarinæ).</p> + +<br> + + +<table class="capt" summary= +"Fig. 228. A moneron (Protamoeba) in the act of reproduction."> +<tr> +<td><img src="images3/fig228.GIF" width="224" height="108" alt= +"A moneron (Protamoeba) in the act of reproduction."></td> +<td align="left" valign="bottom"><a name="Fig. 228">Fig. +228</a>—<b>A moneron (Protamœba)</b> in the act of +reproduction. <i>A</i> The whole moneron, moving like an ordinary +amœba by thrusting out changeable processes. <i>B</i> It +divides into two halves by a constriction in the middle. <i>C</i> +The two halves separate, and each becomes an independent +individual. (Highly magnified.)</td> +</tr> +</table> + +<br> + + +<p>The unicellular form that we have in the ovum has already been +described as the reproduction of a corresponding unicellular +stem-form, and to this we have ascribed the organisation of an +Amœba (cf. Chapter VI). The irregular-shaped Amœba, +which we find living independently to-day in our fresh and salt +water, is the least definite and the most primitive of all the +unicellular Protozoa <a href="chap6.html#Fig. 16">(Fig. 16).</a> As +the unripe ova (the <i>protova</i> that we find in the ovaries of +animals) cannot be distinguished from the common Amœbæ, +we must regard the Amœba as the primitive form that is +reproduced in the embryonic stage of the amœboid ovum to-day, +in accordance with the biogenetic law. I have already pointed out, +in proof of the striking resemblance of the two cells, that the ova +of many of the sponges were formerly regarded as parasitic +Amœbæ (Figure 1.18). Large unicellular organisms like +the Amœbæ were found creeping about inside the body of +the sponge, and were thought to be parasites. It was afterwards +discovered that they were really the ova of the sponge from which +the embryos were developed. As a matter of fact, these sponge-ova +are so much like many of the Amœbæ in size, shape, the +character of their nucleus, and movement of the pseudopodia, that +it is impossible to distinguish them without knowing their +subsequent development.</p> + +<p>Our phylogenetic interpretation of the ovum, and the reduction +of it to some ancient amœboid ancestral form, supply the +answer to the old problem: “Which was first, the egg or the +chick?” We can now give a very plain answer to this riddle, +with which our opponents have often tried to drive us into a +corner. The egg came a long time before the chick. We do not mean, +of course, that the egg existed from the first as a bird’s +egg, but as an indifferent amœboid cell of the simplest +character. The egg lived for thousands of years as an independent +unicellular organism, the Amœba. The egg, in the modern +physiological sense of the word, did not make its appearance until +the descendants of the unicellular Protozoon had developed into +multicellular animals, and these had undergone sexual +differentiation. Even then the egg was first a gastræa-egg, +then a platode-egg, then a vermalia-egg, and chordonia-egg; later +still acrania-egg, then fish-egg, amphibia-egg, reptile-egg, and +finally bird’s egg. The bird’s egg we have experience +of daily is a highly complicated historical product, the result of +countless hereditary processes that have taken place in the course +of millions of years.</p> + +<p>The earliest ancestors of our race were simple Protophyta, and +from these our protozoic ancestors were developed afterwards. From +the morphological point of view both the vegetal and the animal +Protists were simple organisms, individualities of the first order, +or plastids. All our later ancestors are complex organisms, or +individualities of a higher order—social aggregations of a +plurality of cells. The earliest of these, the <i> +Moræada,</i> which represent the third stage in our +genealogy, are very simple associations of homogeneous, indifferent +cells—undifferentiated colonies of social Amœbæ +or Infusoria. To understand the nature and origin of these +protozoa-colonies we need only follow step by step the first +embryonic products of the stem-cell. In all the Metazoa the first +embryonic process is the repeated cleavage of the stem-cell, or +first segmentation-cell (Fig. 229). We have already fully +considered this process, and found that all the different forms of +it may be reduced to one type, the original equal or primordial +segmentation (cf. Chapter VIII). In the genealogical tree</p> + +<br> +<hr> +<p class="page"><a name="page 212">[ 212 ]</a></p> + +<p> </p> + +<p class="one">of the Vertebrates this palingenetic form of +segmentation has been preserved in the Amphioxus alone, all the +other Vertebrates having cenogenetically modified forms of +cleavage. In any case, the latter were developed from the former, +and so the segmentation of the ovum in the Amphioxus has a great +interest for us (cf. Fig. 38). The outcome of this repeated +cleavage is the formation of a round cluster of cells, composed of +homogeneous, indifferent cells of the simplest character (Fig. +230). This is called the <i>morula</i> (= mulberry-embryo) on +account of its resemblance to a mulberry or blackberry.</p> + +<br> + + +<center> +<table class="capt" summary= +"Fig. 229. Original or primordial ovum-cleavage."> +<tr> +<td align="justify" width="344"><img src="images3/fig229.GIF" +width="344" height="107" alt= +"Original or primordial ovum-cleavage."><br> +<br> +<a name="Fig. 229">Fig. 229</a>—<b>Original or primordial +ovum-cleavage.</b> The stem-cell or cytula, formed by fecundation +of the ovum, divides by repeated regular cleavage first into two +(<i>A</i>), then four (<i>B</i>), then eight (<i>C</i>), and +finally a large number of segmentation-cells (<i>D</i>).</td> +</tr> +</table> +</center> + +<br> + + +<p>It is clear that this morula reproduces for us to-day the simple +structure of the multicellular animal that succeeded the +unicellular amœboid form in the early Laurentian period. In +accordance with the biogenetic law, the morula recalls the +ancestral form of the Moræa, or simple colony of Protozoa. +The first cell-communities to be formed, which laid the early +foundation of the higher multicellular body, must have consisted of +homogeneous and simple amœboid cells. The oldest +Amœbæ lived isolated lives, and even the amœboid +cells that were formed by the segmentation of these unicellular +organisms must have continued to live independently for a long +time. But gradually small communities of Amœbæ arose by +the side of these eremitical Protozoa, the sister-cells produced by +cleavage remaining joined together. The advantages in the struggle +for life which these communities had over the isolated cells +favoured their formation and their further development. We find +plenty of these cell-colonies or communities to-day in both fresh +and salt water. They belong to various groups both of the +Protophyta and Protozoa.</p> + +<br> + + +<table class="capt" align="left" summary= +"Fig. 230. Morula, or mulberry-shaped embryo."> +<tr> +<td width="103" align="center"><img src="images3/fig230.GIF" width= +"103" height="101" alt="Morula, or mulberry-shaped embryo."><br> +<br> +<a name="Fig. 230">Fig. 230</a>—<b>Morula,</b> or <b> +mulberry-shaped embryo.</b></td> +</tr> +</table> + +<p>To have some idea of those ancestors of our race that succeeded +phylogenetically to the Moræada, we have only to follow the +further embryonic development of the morula. We then see that the +social cells of the round cluster secrete a sort of jelly or a +watery fluid inside their globular body, and they themselves rise +to the surface of it <a href="chap8.html#Fig. 29">(Fig. 29 <i>F, +G</i>).</a> In this way the solid mulberry-embryo becomes a hollow +sphere, the wall of which is composed of a single layer of cells. +We call this layer the <i>blastoderm,</i> and the sphere itself the +<i>blastula,</i> or embryonic vesicle.</p> + +<p>This interesting blastula is very important. The conversion of +the morula into a hollow ball proceeds on the same lines originally +in the most diverse stems—as, for instance, in many of the +zoophytes and worms, the ascidia, many of the echinoderms and +molluscs, and in the amphioxus. Moreover, in the animals in which +we do not find a real palingenetic blastula the defect is clearly +due to cenogenetic causes, such as the formation of food-yelk and +other embryonic adaptations. We may, therefore, conclude that the +ontogenetic blastula is the reproduction of a very early +phylogenetic ancestral form, and that all the Metazoa are descended +from a common stem-form, which was in the main constructed like the +blastula. In many of the lower animals the blastula is not +developed</p> + +<br> +<hr> +<p class="page"><a name="page 213">[ 213 ]</a></p> + +<p> </p> + +<p class="one">within the fœtal membranes, but in the open +water. In those cases each blastodermic cell begins at an early +stage to thrust out one or more mobile hair-like processes; the +body swims about by the vibratory movement of these lashes or whips +<a href="chap8.html#Fig. 29">(Fig. 29 <i>F</i>).</a></p> + +<p>We still find, both in the sea and in fresh water, various kinds +of primitive multicellular organisms that substantially resemble +the blastula in structure, and may be regarded in a sense as +permanent blastula-forms—hollow vesicles or gelatinous balls, +with a wall composed of a single layer of ciliated homogeneous +cells. There are “blastæads” of this kind even +among the Protophyta—the familiar Volvocina, formerly classed +with the infusoria. The common <i>Volvox globator</i> is found in +the ponds in the spring—a small, green, gelatinous globule, +swimming about by means of the stroke of its lashes, which rise in +pairs from the cells on its surface. In the similar <i> +Halosphæra viridis</i> also, which we find in the marine +plancton (floating matter), a number of green cells form a simple +layer at the surface of the gelatinous ball; but in this case there +are no cilia.</p> + +<p>Some of the infusoria of the flagellata-class (<i>Signura, +Magosphæra,</i> etc.) are similar in structure to these +vegetal clusters, but differ in their animal nutrition; they form +the special group of the <i>Catallacta.</i> In September, 1869, I +studied the development of one of these graceful animals on the +island of Gis-Oe, off the coast of Norway (<i>Magosphæra +planula</i>), Figures 2.231 and 2.232). The fully-formed body is a +gelatinous ball, with its wall composed of thirty-two to sixty-four +ciliated cells; it swims about freely in the sea. After reaching +maturity the community is dissolved. Each cell then lives +independently for some time, grows, and changes into a creeping +amœba. This afterwards contracts, and clothes itself with a +structureless membrane. The cell then looks just like an ordinary +animal ovum. When it has been in this condition for some time the +cell divides into two, four, eight, sixteen, thirty-two, and +sixty-four cells. These arrange themselves in a round vesicle, +thrust out vibratory lashes, burst the capsule, and swim about in +the same magosphæra-form with which we started. This +completes the life-circle of the remarkable and instructive +animal.</p> + +<p>If we compare these permanent blastulæ with the +free-swimming ciliated larvæ or blastulæ, with similar +construction, of many of the lower animals, we can confidently +deduce from them that there was a very early and long-extinct +common stem-form of substantially the same structure as the +blastula. We may call it the <i>Blastæa.</i> Its body +consisted, when fully formed, of a simple hollow ball, filled with +fluid or structureless jelly, with a wall composed of a single +stratum of ciliated cells. There were probably many genera and +species of these blastæads in the Laurentian period, forming +a special class of marine protists.</p> + +<p>It is an interesting fact that in the plant kingdom also the +simple hollow sphere is found to be an elementary form of the +multicellular organism. At the surface and below the surface (down +to a depth of 2000 yards) of the sea there are green globules +swimming about, with a wall composed of a single layer of +chlorophyll-bearing cells. The botanist Schmitz gave them the name +of <i>Halosphæra viridis</i> in 1879.</p> + +<p>The next stage to the <i>Blastæa,</i> and the sixth in our +genealogical tree, is the Gastræa that is developed from it. +As we have already seen, this ancestral form is particularly +important. That it once existed is proved with certainty by the +gastrula, which we find temporarily in the ontogenesis of all the +Metazoa (Fig. 29 <i>J, K</i>). As we saw, the original, +palingenetic form of the gastrula is a round or oval uni-axial +body, the simple cavity of which (the primitive gut) has an +aperture at one pole of its axis (the primitive mouth). The wall of +the gut consists of two strata of cells, and these are the primary +germinal layers, the animal skin-layer (ectoderm) and vegetal +gut-layer (entoderm).</p> + +<p>The actual ontogenetic development of the gastrula from the +blastula furnishes sound evidence as to the phylogenetic origin of +the <i>Gastræa</i> from the <i>Blastæa.</i> A +pit-shaped depression appears at one side of the spherical blastula +(Fig. 29 <i>H</i>). In the end this invagination goes so far that +the outer or invaginated part of the blastoderm lies close on the +inner or non-invaginated part (Fig. 29 <i>J</i>). In explaining the +phylogenetic origin of the gastræa in the light of this +ontogenetic process, we may assume that the one-layered +cell-community of the blastæa began to take in food more +largely at one particular part of its surface. Natural selection +would gradually lead to</p> + +<br> +<hr> +<p class="page"><a name="page 214">[ 214 ]</a></p> + +<p> </p> + +<p class="one">the formation of a depression or pit at this +alimentary spot on the surface of the ball. The depression would +grow deeper and deeper. In time the vegetal function of taking in +and digesting food would be confined to the cells that lined this +hole; the other cells would see to the animal functions of +locomotion, sensation, and protection. This was the first division +of labour among the originally homogeneous cells of the +blastæa.</p> + +<br> + + +<center> +<table class="capt" width="361" summary= +"Fig. 231. The Norwegian Magosphaera planula, swimming about by means of the lashes or cilia at its surface. Fig. 232. Section of same, showing how the pear-shaped cells in the centre of the gelatinous ball are connected by a fibrous process."> +<tr> +<td align="justify" width="361"><img src="images3/fig231.GIF" +width="361" height="204" alt= +"Fig. 231. The Norwegian Magosphaera planula, swimming about by means of the lashes or cilia at its surface. Fig. 232. Section of same, showing how the pear-shaped cells in the centre of the gelatinous ball are connected by a fibrous process."> +<br> +<a name="Fig. 231">Fig. 231</a>—<b>The Norwegian +Magosphæra planula,</b> swimming about by means of the lashes +or cilia at its surface.<br> +Fig. 232—<b>Section of same,</b> showing how the pear-shaped +cells in the centre of the gelatinous ball are connected by a +fibrous process. Each cell has a contractile vacuole as well as a +nucleus.</td> +</tr> +</table> +</center> + +<br> + + +<p>The effect, then, of this earliest histological differentiation +was to produce two different kinds of cells—nutritive cells +in the depression and locomotive cells on the surface outside. But +this involved the severance of the two primary germinal +layers—a most important process. When we remember that even +man’s body, with all its various parts, and the body of all +the other higher animals, are built up originally out of these two +simple layers, we cannot lay too much stress on the phylogenetic +significance of this gastrulation. In the simple primitive gut or +gastric cavity of the gastrula and its rudimentary mouth we have +the first real organ of the animal frame in the morphological +sense; all the other organs were developed afterwards from these. +In reality, the whole body of the gastrula is merely a +“primitive gut.” I have shown already (Chapters VIII +and XIX) that the two-layered embryos of all the Metazoa can be +reduced to this typical gastrula. This important fact justifies us +in concluding, in accordance with the biogenetic law, that their +ancestors also were phylogenetically developed from a similar +stem-form. This ancient stem-form is the gastræa.</p> + +<p>The gastræa probably lived in the sea during the +Laurentian period, swimming about in the water by means of its +ciliary coat much as free ciliated gastrulæ do to-day. +Probably it differed from the existing gastrula only in one +essential point, though extinct millions of years ago. We have +reason, from comparative anatomy and ontogeny, to believe that it +multiplied by sexual generation, not merely asexually (by cleavage, +gemmation, and spores), as was no doubt the case with the earlier +ancestors. Some of the cells of the primary germ-layers probably +became ova and others fertilising sperm. We base these hypotheses +on the fact that we do to-day find the simplest form of sexual +reproduction in some of the living gastræads and other lower +animals, especially the sponges.</p> + +<p>The fact that there are still in existence various kinds of +gastræads, or lower Metazoa with an organisation little +higher than that of the hypothetical gastræa, is a strong +point in favour of our theory. There are not very many species of +these living gastræads; but their morphological and +phylogenetic interest is so great, and their intermediate position +between the Protozoa and Metazoa so instructive, that I proposed +long ago (1876) to make a special class of them. I distinguished +three orders in this class—the Gastremaria, Physemaria, and +Cyemaria (or Dicyemida).</p> + +<br> +<hr> +<p class="page"><a name="page 215">[ 215 ]</a></p> + +<p> </p> + +<p class="one">But we might also regard these three orders as so +many independent classes in a primitive gastræad stem.</p> + +<p>The Gastremaria and Cyemaria, the chief of these living +gastræads, are small Metazoa that live parasitically inside +other Metazoa, and are, as a rule, 1/50 to 1/25 of an inch long, +often much less (Fig. 233, 1–15). Their soft body, devoid of +skeleton, consists of two simple strata of cells, the primary +germinal layers; the outer of these is thickly clothed with long +hair-like lashes, by which the parasites swim about in the various +cavities of their host. The inner germinal layer furnishes the +sexual products. The pure type of the original gastrula (or <i> +archigastrula,</i> <a href="chap8.html#Fig. 29">Fig. 29 <i> +I</i>)</a> is seen in the <i>Pemmatodiscus gastrulaceus,</i> which +Monticelli discovered in the umbrella of a large medusa (<i>Pilema +pulmo</i>) in 1895; the convex surface of this gelatinous umbrella +was covered with numbers of clear vesicles, of 1/25 to 1/8 inch in +diameter, in the fluid contents of which the little parasites were +swimming. The cup-shaped body of the <i>Pemmatodiscus</i> (Fig. +233, <i>1</i>) is sometimes rather flat, and shaped like a hat or +cone, at other times almost curved into a semi-circle. The simple +hollow of the cup, the primitive gut (<i>g</i>), has a narrow +opening (<i>o</i>). The skin layer (<i>e</i>) consists of long +slender cylindrical cells, which bear long vibratory hairs; it is +separated by a thin structureless, gelatinous plate (<i>f</i>) from +the visceral or gut layer (<i>i</i>), the prismatic cells of which +are much smaller and have no cilia. Pemmatodiscus propagates +asexually, by simple longitudinal cleavage; on this account it has +recently been regarded as the representative of a special order of +gastræads (<i>Mesogastria</i>).</p> + +<p>Probably a near relative of the <i>Pemmatodiscus</i> is the <i> +Kunstleria Gruveli</i> (Fig. 233, <i>2</i>). It lives in the +body-cavity of Vermalia (Sipunculida), and differs from the former +in having no lashes either on the large ectodermic cells (<i>e</i>) +or the small entodermic (<i>i</i>); the germinal layers are +separated by a thick, cup-shaped, gelatinous mass, which has been +called the “clear vesicle” (<i>f</i>). The primitive +mouth is surrounded by a dark ring that bears very strong and long +vibratory lashes, and effects the swimming movements.</p> + +<p><i>Pemmatodiscus</i> and <i>Kunstleria</i> may be included in +the family of the Gastremaria. To these gastræads with open +gut are closely related the Orthonectida (<i>Rhopalura,</i> Fig. +233, <i>3–5</i>). They live parasitically in the body-cavity +of echinoderms (Ophiura) and vermalia; they are distinguished by +the fact that their primitive gut-cavity is not empty, but filled +with entodermic cells, from which the sexual cells are developed. +These gastræads are of both sexes, the male (Fig. 3) being +smaller and of a somewhat different shape from the oval female +(Fig. 4).</p> + +<p>The somewhat similar <i>Dicyemida</i> (Fig. 6) are distinguished +from the preceding by the fact that their primitive gut-cavity is +occupied by a single large entodermic cell instead of a crowded +group of sexual cells. This cell does not yield sexual products, +but afterwards divides into a number of cells (spores), each of +which, without being impregnated, grows into a small embryo. The +Dicyemida live parasitically in the body-cavity, especially the +renal cavities, of the cuttle-fishes. They fall in several genera, +some of which are characterised by the possession of special polar +cells; the body is sometimes roundish, oval, or club-shaped, at +other times long and cylindrical. The genus <i>Conocyema</i> (Figs. +7–15) differs from the ordinary <i>Dicyema</i> in having four +polar pimples in the form of a cross, which may be incipient +tentacles.</p> + +<p>The classification of the Cyemaria is much disputed; sometimes +they are held to be parasitic infusoria (like the <i>Opalina</i>), +sometimes platodes or vermalia, related to the suctorial worms or +rotifers, but having degenerated through parasitism. I adhere to +the phylogenetically important theory that I advanced in 1876, that +we have here real gastræads, primitive survivors of the +common stem-group of all the Metazoa. In the struggle for life they +have found shelter in the body-cavity of other animals.</p> + +<p>The small Cœlenteria attached to the floor of the sea that +I have called the Physemaria (<i>Haliphysema</i> and <i> +Gastrophysema</i>) probably form a third order (or class) of the +living gastræads. The genus <i>Haliphysema</i> (Figs. 234, +235) is externally very similar to a large rhizopod (described by +the same name in 1862) of the family of the <i>Rhabdamminida,</i> +which was at first taken for a sponge. In order to avoid confusion +with these, I afterwards gave them the name of Prophysema. The +whole mature body of the <i>Prophysema</i> is a simple cylindrical +or oval tube, with a two-layered wall. The hollow of the tube is +the gastric cavity, and the upper opening of it the mouth (Fig. 235 +<i>m</i>).</p> + +<br> +<hr> +<p class="page"><a name="page 216">[ 216 ]</a></p> + +<p> </p> + +<center> +<table class="capt" width="340" summary= +"Fig. 233. Modern gastræads. Fig. 1. Pemmatodiscus gastrulaceus (Monticelli), in longitudinal section. Fig. 2. Kunstleria gruveli (Delage), in longitudinal section. (From Kunstler and Gruvel.) Figs. 3-5. Rhopalura Giardi (Julin): Fig. 3 male, Fig. 4 female, Fig. 5 planula. Fig. 6. Dicyema macrocephala (Van Beneden). Fig. 7-15. Conocyema polymorpha (Van Beneden): Fig. 7 the mature gastræad, Fig. 8-15 its gastrulation."> +<tr> +<td align="justify"><img src="images3/fig233.GIF" width="340" +height="497" alt= +"Fig. 233. Modern gastræads. Fig. 1. Pemmatodiscus gastrulaceus (Monticelli), in longitudinal section. Fig. 2. Kunstleria gruveli (Delage), in longitudinal section. (From Kunstler and Gruvel.) Figs. 3-5. Rhopalura Giardi (Julin): Fig. 3 male, Fig. 4 female, Fig. 5 planula. Fig. 6. Dicyema macrocephala (Van Beneden). Fig. 7-15. Conocyema polymorpha (Van Beneden): Fig. 7 the mature gastræad, Fig. 8-15 its gastrulation."> +<br> +<a name="Fig. 233">Fig. 233</a>—<b>Modern +gastræads.</b> Fig. 1. <b>Pemmatodiscus gastrulaceus</b> +(<i>Monticelli</i>), in longitudinal section. Fig. 2. <b>Kunstleria +gruveli</b> (<i>Delage</i>), in longitudinal section. (From <i> +Kunstler</i> and <i>Gruvel.</i>) Figs. 3–5. <b>Rhopalura +Giardi</b> (<i>Julin</i>): Fig. 3 male, Fig. 4 female, Fig. 5 +planula. Fig. 6. <b>Dicyema macrocephala</b> (<i>Van Beneden</i>). +Figs. 7–15. <b>Conocyema polymorpha</b> (<i>Van Beneden</i>): +Fig. 7 the mature gastræad, Figs. 8–15 its +gastrulation. <i>d</i> primitive gut, <i>o</i> primitive mouth, <i> +e</i> ectoderm, <i>i</i> entoderm, <i>f</i> gelatinous plate +between <i>e</i> and <i>i</i> (supporting plate, +blastocœl).</td> +</tr> +</table> +</center> + +<br> +<hr> +<p class="page"><a name="page 217">[ 217 ]</a></p> + +<p> </p> + +<p class="one">The two strata of cells that form the wall of the +tube are the primary germinal layers. These rudimentary zoophytes +differ from the swimming gastræads chiefly in being attached +at one end (the end opposite to the mouth) to the floor of the +sea.</p> + +<br> + + +<table class="capt" summary= +"Figs. 234 and 235. Prophysema primordiale, a living gastraead."> +<tr> +<td width="231"><img src="images3/fig234.GIF" width="231" height= +"285" alt="Prophysema primordiale, a living gastraead."></td> +<td align="left" valign="bottom"><br> +<a name="Fig. 234">Figs. 234 and +235</a>—<b>Prophysema primordiale, a living +gastræad.</b> Fig. 234. The whole of the spindle-shaped +animal (attached below to the floor of the sea. Fig. 235. The same +in longitudinal section. The primitive gut (<i>d</i>) opens above +at the primitive mouth (<i>m</i>). Between the ciliated cells +(<i>g</i>) are the amœboid ova (<i>e</i>). The skin-layer +(<i>h</i>) is encrusted with grains of sand below and +sponge-spicules above.</td> +</tr> +</table> + +<br> + + +<p>In <i>Prophysema</i> the primitive gut is a simple oval cavity, +but in the closely related <i>Gastrophysema</i> it is divided into +two chambers by a transverse constriction; the hind and smaller +chamber above furnishes the sexual products, the anterior one being +for digestion.</p> + +<br> + + +<table class="capt" width="187" align="left" summary= +"Figs. 236-237. Ascula of gastrophysema, attached to the floor of the sea."> +<tr> +<td align="justify" width="187"><img src="images3/fig236.GIF" +width="187" height="211" alt= +"Ascula of gastrophysema, attached to the floor of the sea."><br> +<a name="Fig. 236">Figs. 236–237</a>—<b>Ascula of +gastrophysema,</b> attached to the floor of the sea. Fig. 236 +external view, 237 longitudinal section. <i>g</i> primitive gut, +<i>o</i> primitive mouth, <i>i</i> visceral layer, <i>e</i> +cutaneous layer. (Diagram.)</td> +<td width="10"></td> +</tr> +</table> + +<p class="pic">The simplest sponges (<i>Olynthus,</i> Fig. 238) +have the same organisation as the Physemaria. The only material +difference between them is that in the sponge the thin two-layered +body-wall is pierced by numbers of pores. When these are closed +they resemble the Physemaria. Possibly the gastræads that we +call Physemaria are only olynthi with the pores closed. The <i> +Ammoconida,</i> or the simple tubular sand-sponges of the deep-sea +(<i>Ammolynthus,</i> etc.), do not differ from the gastræads +in any important point when the pores are closed. In my <i> +Monograph on the Sponges</i> (with sixty plates) I endeavoured to +prove analytically that all the species of this class can be traced +phylogenetically to a common stem-form (<i>Calcolynthus</i>).</p> + +<p>The lowest form of the Cnidaria is also not far removed from the +gastræads. In the interesting common fresh-water polyp +(<i>Hydra</i>) the whole body is simply an oval tube with a double +wall; only in this case the mouth has a crown of tentacles. Before +these develop the hydra resembles an ascula (Figs. 236, 237). +Afterwards there are slight histological differentiations in its +ectoderm, though the entoderm remains</p> + +<br> +<hr> +<p class="page"><a name="page 218">[ 218 ]</a></p> + +<p> </p> + +<p class="one">a single stratum of cells. We find the first +differentiation of epithelial and stinging cells, or of muscular +and neural cells, in the thick ectoderm of the hydra.</p> + +<table class="capt" width="104" align="left" summary= +"Fig. 238. Olynthus, a very rudimentary sponge."> +<tr> +<td align="center" width="104"><img src="images3/fig238.GIF" width= +"104" height="160" alt="Olynthus, a very rudimentary sponge."> +<br><a name="Fig. 238">Fig. 238</a>—<b>Olynthus,</b> a very +rudimentary sponge. A piece cut away in front.</td> +</tr> +</table> + +<p class="pic">In all these rudimentary living cœlenteria the +sexual cells of both kinds—ova and sperm cells—are +formed by the same individual; it is possible that the oldest +gastræads were hermaphroditic. It is clear from comparative +anatomy that hermaphrodism—the combination of both kinds of +sexual cells in one individual—is the earliest form of sexual +differentiation; the separation of the sexes (gonochorism) was a +much later phenomenon. The sexual cells originally proceeded from +the edge of the primitive mouth of the gastræad.</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="chap18.html">Chapter XVIII</a><br> +<a href="chap20.html">Chapter XX</a><br> +<a href="Title.html#Illustrations">Figs. 1–209</a><br> +<a href="title2.html#Illustrations">Figs. 210–408</a></p> +</body> +</html> + |