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diff --git a/8700-h/old/chap6.html b/8700-h/old/chap6.html new file mode 100644 index 0000000..61ea6ad --- /dev/null +++ b/8700-h/old/chap6.html @@ -0,0 +1,1251 @@ +<!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> VI<br> +<br> +<b>THE OVUM AND THE AMŒBA</b></center> + +<br> + + +<p class="one">In order to understand clearly the course of human +embryology, we must select the more important of its wonderful and +manifold processes for fuller explanation, and then proceed from +these to the innumerable features of less importance. The most +important feature in this sense, and the best starting-point for +ontogenetic study, is the fact that man is developed from an ovum, +and that this ovum is a simple cell. The human ovum does not +materially differ in form and composition from that of the other +mammals, whereas there is a distinct difference between the +fertilised ovum of the mammal and that of any other animal.</p> + +<br> + + +<table class="capt" align="left" width="209" summary="The human ovum"> +<tr> +<td align="justify"><img src="images/fig1.GIF" width="209" height="86" alt= +"The human ovum"> +<a name="Fig. 1">Fig. 1</a>—<b>The human +ovum.</b> The globular mass of yelk (<i>b</i>) is enclosed by a +transparent membrane (the ovolemma or zona pellucida [<i>a</i>]), +and contains a noncentral nucleus (the germinal vesicle, <i>c</i>). +Cf. <a href="#Fig. 14">Fig. 14.</a></td></tr> +</table> + +<p class="pic">This fact is so important that few should be unaware of its +extreme significance; yet it was quite unknown in the first quarter +of the nineteenth century. As we have seen, the human and mammal +ovum was not discovered until 1827, when Carl Ernst von Baer +detected it. Up to that time the larger vesicles, in which the real +and much smaller ovum is contained, had been wrongly regarded as +ova. The important circumstance that this mammal ovum is a simple +cell, like the ovum of other animals, could not, of course, be +recognised until the cell theory was established. This was not +done, by Schleiden for the plant and Schwann for the animal, until +1838. As we have seen, this cell theory is of the greatest service +in explaining the human frame and its embryonic development. Hence +we must say a few words about the actual condition of the theory +and the significance of the views it has suggested.</p> + +<p>In order properly to appreciate the cellular theory, the most +important element in our science, it is necessary to understand in +the first place that the cell is a <i>unified organism,</i> a +self-contained living being. When we anatomically dissect the +fully-formed animal or plant into its various organs, and then +examine the finer structure of these organs with the microscope, we +are surprised to find that all these different parts are ultimately +made up of the same structural element or unit. This common unit of +structure is the cell. It does not matter whether we thus dissect a +leaf, flower, or fruit, or a bone, muscle, gland, or bit of skin, +etc.; we find in every case the same ultimate constituent, which +has been called the cell since Schleiden’s discovery. There +are many opinions as to its real nature, but the essential point in +our view of the cell is to look upon it as a self-contained or +independent living unit. It is, in the words of Brucke, “an +elementary organism.” We may define it most precisely as the +ultimate organic unit, and, as the cells are the sole active +principles in every vital function, we may call them the +“plastids,” or “formative elements.” This +unity is found in both the anatomic structure and the physiological +function. In the case of the protists, the entire organism usually +consists of a single independent cell throughout life. But in the +tissue-forming animals and plants, which are the great majority, +the organism begins its career as a simple cell, and then grows +into a cell-community, or, more correctly, an organised cell-state. +Our own body is not really the simple unity that it is generally +supposed to be. On the contrary, it is a very elaborate social +system of countless microscopic organisms, a colony or +commonwealth, made up of innumerable independent units, or very +different tissue-cells.</p> + +<br> +<hr> +<p class="page"><a name="page 37">[ 37 ]</a></p> + +<p> </p> + +<br class="one"> +<br> +<p>In reality, the term “cell,” which existed long +before the cell theory was formulated, is not happily chosen. +Schleiden, who first brought it into scientific use in the sense of +the cell theory, gave this name to the elementary organisms +because, when you find them in the dissected plant, they generally +have the appearance of chambers, like the cells in a bee-hive, with +firm walls and a fluid or pulpy content. But some cells, especially +young ones, are entirely without the enveloping membrane, or stiff +wall. Hence we now generally describe the cell as a living, viscous +particle of protoplasm, enclosing a firmer nucleus in its +albuminoid body. There may be an enclosing membrane, as there +actually is in the case of most of the plants; but it may be wholly +lacking, as is the case with most of the animals. There is no +membrane at all in the first stage. The young cells are usually +round, but they vary much in shape later on. Illustrations of this +will be found in the cells of the various parts of the body shown +in <a href="#Fig. 3">Figs. 3–7.</a></p> + +<p>Hence the essential point in the modern idea of the cell is that +it is made up of two different active constituents—an inner +and an outer part. The smaller and inner part is the nucleus (or +<i>caryon</i> or <i>cytoblastus,</i> <a href="#Fig. 1">Fig. +1<i>c</i></a> and <a href="#Fig. 2">Fig. 2<i>k</i></a>). The outer +and larger part, which encloses the other, is the body of the cell +(<i>celleus, cytos,</i> or <i>cytosoma</i>). The soft living +substance of which the two are composed has a peculiar chemical +composition, and belongs to the group of the albuminoid +plasma-substances (“formative matter”), or protoplasm. +The essential and indispensable element of the nucleus is called +nuclein (or caryoplasm); that of the cell body is called plastin +(or cytoplasm). In the most rudimentary cases both substances seem +to be quite simple and homogeneous, without any visible structure. +But, as a rule, when we examine them under a high power of the +microscope, we find a certain structure in the protoplasm. The +chief and most common form of this is the fibrous or net-like +“thready structure” (Frommann) and the frothy +“honeycomb structure” (Bütschli).</p> + +<br> + + +<table class="capt" width="155" align="left" summary="Stem-cell of one of the echinoderms"> +<tr> +<td align="justify"><img src="images/fig2.GIF" width="155" height="138" alt= +"Stem-cell of one of the echinoderms"> +<a name="Fig. 2">Fig. 2</a>—<b>Stem-cell of one +of the echinoderms</b> (cytula, or “first +segmentation-cell” = fertilised ovum), after <i>Hertwig. +k</i> is the nucleus or caryon.</td> +</tr> +</table> + +<p class="pic">The shape or outer form of the cell is infinitely varied, in +accordance with its endless power of adapting itself to the most +diverse activities or environments. In its simplest form the cell +is globular (Fig. 2). This normal round form is especially found in +cells of the simplest construction, and those that are developed in +a free fluid without any external pressure. In such cases the +nucleus also is not infrequently round, and located in the centre +of the cell-body (Fig. 2<i>k</i>). In other cases, the cells have +no definite shape; they are constantly changing their form owing to +their automatic movements. This is the case with the +amœbæ (<a href="images/fig15.GIF">Fig. 15</a> and <a +href="#Fig. 16">16</a>) and the amœboid travelling cells <a +href="#Fig. 11">(Fig. 11),</a> and also with very young ova <a +href="#Fig. 13">(Fig. 13).</a> However, as a rule, the cell assumes +a definite form in the course of its career. In the tissues of the +multicellular organism, in which a number of similar cells are +bound together in virtue of certain laws of heredity, the shape is +determined partly by the form of their connection and partly by +their special functions. Thus, for instance, we find in the mucous +lining of our tongue very thin and delicate flat cells of roundish +shape <a href="#Fig. 3">(Fig. 3).</a> In the outer skin we find +similar, but harder, covering cells, joined together by saw-like +edges <a href="#Fig. 3">(Fig. 4).</a> In the liver and other glands +there are thicker and softer cells, linked together in rows <a +href="#Fig. 3">(Fig. 5).</a></p> + +<p>The last-named tissues (Figs. 3–5) belong to the simplest +and most primitive type, the group of the +“covering-tissues,” or epithelia. In these +“primary tissues” (to which the germinal layers belong) +simple cells of the same kind are arranged in layers. The +arrangement and shape are more complicated in the “secondary +tissues,” which are gradually developed out of the primary, +as in the tissues of the muscles, nerves, bones, etc. In the bones, +for instance, which belong to the group of supporting or connecting +organs,</p> + +<br> +<hr> +<p class="page"><a name="page 38">[ 38 ]</a></p> + +<p> </p> + +<p class="one">the cells <a href="#Fig. 6">(Fig. 6)</a> are +star-shaped, and are joined together by numbers of net-like +interlacing processes; so, also, in the tissues of the teeth <a +href="#Fig. 7">(Fig. 7),</a> and in other forms of +supporting-tissue, in which a soft or hard substance (intercellular +matter, or base) is inserted between the cells.</p> + +<br> + + +<center> +<table class="capt" width="431" cellpadding="0" +cellspacing="0" summary= +"Fig. 3--Three epithelial cells. Fig. 4--Five spiny or grooved cells. Fig. 5--Ten liver-cells."><tr><td> +<img src="images/fig3.GIF" width="431" height="167" alt= +"Fig. 3--Three epithelial cells. Fig. 4--Five spiny or grooved cells. Fig. 5--Ten liver-cells."><br><br> +<a name="Fig. 3">Fig. 3</a>—<b>Three epithelial cells</b> +from the mucous lining of the tongue.<br> +<a name="Fig. 3">Fig. 4</a>—<b>Five spiny or grooved +cells,</b> with edges joined, from the outer skin (epidermis): one +of them (<i>b</i>) is isolated.<br> +<a name="Fig. 3">Fig. 5</a>—<b>Ten liver-cells:</b> one of +them (<i>b</i>) has two nuclei.</td> +</tr> +</table> +</center> + +<br> + + +<p>The cells also differ very much in size. The great majority of +them are invisible to the naked eye, and can be seen only through +the microscope (being as a rule between 1/2500 and 1/250 inch in +diameter). There are many of the smaller plastids—such as the +famous bacteria—which only come into view with a very high +magnifying power. On the other hand, many cells attain a +considerable size, and run occasionally to several inches in +diameter, as do certain kinds of rhizopods among the unicellular +protists (such as the radiolaria and thalamophora). Among the +tissue-cells of the animal body many of the muscular fibres and +nerve fibres are more than four inches, and sometimes more than a +yard, in length. Among the largest cells are the yelk-filled ova; +as, for instance, the yellow “yolk” in the hen’s +egg, which we shall describe later <a href="#Fig. 15">(Fig. +15).</a></p> + +<p>Cells also vary considerably in structure. In this connection we +must first distinguish between the active and passive components of +the cell. It is only the former, or <i>active</i> parts of the +cell, that really live, and effect that marvellous world of +phenomena to which we give the name of “organic life.” +The first of these is the inner nucleus (<i>caryoplasm</i>), and +the second the body of the cell (<i>cytoplasm</i>). The <i> +passive</i> portions come third; these are subsequently formed from +the others, and I have given them the name of +“plasma-products.” They are partly external +(cell-membranes and intercellular matter) and partly internal +(cell-sap and cell-contents).</p> + +<p>The nucleus (or caryon), which is usually of a simple roundish +form, is quite structureless at first (especially in very young +cells), and composed of homogeneous nuclear matter or caryoplasm <a +href="#Fig. 2">(Fig. 2<i>k</i>).</a> But, as a rule, it forms a +sort of vesicle later on, in which we can distinguish a more solid +<i>nuclear base (caryobasis)</i> and a softer or fluid <i>nuclear +sap (caryolymph).</i> In a mesh of the nuclear network (or it may +be on the inner side of the nuclear envelope) there is, as a rule, +a dark, very opaque, solid body, called the <i>nucleolus.</i> Many +of the nuclei contain several of these nucleoli (as, for instance, +the germinal vesicle of the ova of fishes and amphibia). Recently a +very small, but particularly important, part of the nucleus has +been distinguished as the <i>central body</i> (centrosoma)—a +tiny particle that is originally found in the nucleus itself, but +is usually outside it, in the cytoplasm; as a rule, fine threads +stream out from it in the cytoplasm. From the position of the +central body with regard to the other parts it seems probable that +it has a high physiological importance as a centre of movement; but +it is lacking in many cells.</p> + +<p>The cell-body also consists originally, and in its simplest +form, of a homogeneous viscid plasmic matter. But, as a rule,</p> + +<br> +<hr> +<p class="page"><a name="page 39">[ 39 ]</a></p> + +<p> </p> + +<p class="one">only the smaller part of it is formed of the living +active cell-substance (protoplasm); the greater part consists of +dead, passive plasma-products (metaplasm). It is useful to +distinguish between the inner and outer of these. External +plasma-products (which are thrust out from the protoplasm as solid +“structural matter”) are the cell-membranes and the +intercellular matter. The <i>internal</i> plasma-products are +either the fluid cell-sap or hard structures. As a rule, in mature +and differentiated cells these various parts are so arranged that +the protoplasm (like the caryoplasm in the round nucleus) forms a +sort of skeleton or framework. The spaces of this network are +filled partly with the fluid cell-sap and partly by hard structural +products.</p> + +<br> + + +<table class="capt" width="287" align="left" summary= +"Fig. 6.--Nine star-shaped bone-cells."> +<tr> +<td align="center"><img src="images/fig6.GIF" width="287" height="251" alt= +"Fig. 6--Nine star-shaped bone cells."> +<a name="Fig. 6">Fig. 6</a>—<b>Nine +star-shaped bone-cells,</b> with interlaced branches.</td></tr> +</table> + + + +<p class="pic">The simple round ovum, which we take as the starting-point of +our study (Figs. 1 and 2), has in many cases the vague, indifferent +features of the typical primitive cell. As a contrast to it, and as +an instance of a very highly differentiated plastid, we may +consider for a moment a large nerve-cell, or ganglionic cell, from +the brain. The ovum stands potentially for the entire +organism—in other words, it has the faculty of building up +out of itself the whole multicellular body. It is the common parent +of all the countless generations of cells which form the different +tissues of the body; it unites all their powers in itself, though +only potentially or in germ. In complete contrast to this, the +neural cell in the brain <a href="#Fig. 9">(Fig. 9)</a> develops +along one rigid line. It cannot, like the ovum, beget endless +generations of cells, of which some will become skin-cells, others +muscle-cells, and others again bone-cells. But, on the other hand, +the nerve-cell has become fitted to discharge the highest functions +of life; it has the powers of sensation, will, and thought. It is a +real soul-cell, or an elementary organ of the psychic activity. It +has, therefore, a most elaborate and delicate structure.</p> + +<table class="capt" width="203" align="left" summary="Fig. 7.--Eleven star-shaped cells."> +<tr> +<td align="center"><img src="images/fig7.GIF" width="203" height="154" alt= +"Fig. 7--Eleven star-shaped cells."> +<a name="Fig. 7">Fig. 7</a>—<b>Eleven +star-shaped cells</b> from the enamel of a tooth, joined together +by their branchlets. +</td> +</tr> +</table> + +<p class="pic"> +Numbers of extremely fine threads, like the electric wires at a large +telegraphic centre, cross and recross in the delicate protoplasm of +the nerve cell, and pass out in the branching processes which +proceed from it and put it in communication with other nerve-cells +or nerve-fibres (<i>a, b</i>). We can only partly follow their +intricate paths in the fine matter of the body of the cell.</p> +<p>Here we have a most elaborate apparatus, the delicate structure +of which we are just beginning to appreciate through our most +powerful microscopes, but whose significance is rather a matter +of</p> + +<br> +<hr> +<p class="page"><a name="page 40">[ 40 ]</a></p> + +<p> </p> + +<p class="one">conjecture than knowledge. Its intricate structure +corresponds to the very complicated functions of the mind. +Nevertheless, this elementary organ of psychic activity—of +which there are thousands in our brain—is nothing but a +single cell. Our whole mental life is only the joint result of the +combined activity of all these nerve-cells, or soul-cells. In the +centre of each cell there is a large transparent nucleus, +containing a small and dark nuclear body. Here, as elsewhere, it is +the nucleus that determines the individuality of the cell; it +proves that the whole structure, in spite of its intricate +composition, amounts to only a single cell.</p> + + +<table class="capt" width="216" align="left" summary= +"Fig. 8.--Unfertilised ovum of an echinoderm."> +<tr> +<td align="justify"><img src="images/fig8.GIF" width="216" height="145" alt= +"Fig. 8--Unfertilised ovum of an echinoderm."> +<a name="Fig. 8">Fig. 8</a>—<b>Unfertilised +ovum of an echinoderm</b> (from <i>Hertwig</i>). The vesicular +nucleus (or “germinal vesicle”) is globular, half the +size of the round ovum, and encloses a nuclear framework, in the +central knot of which there is a dark nucleolus (the +“germinal spot”). +</td> +</tr> +</table> + + + +<p class="pic">In contrast with this very elaborate and very strictly +differentiated psychic cell <a href="#Fig. 9">(Fig. 9),</a> we have +our ovum (Figs. 1 and 2), which has hardly any structure at all. +But even in the case of the ovum we must infer from its properties +that its protoplasmic body has a very complicated chemical +composition and a fine molecular structure which escapes our +observation. This presumed molecular structure of the plasm is now +generally admitted; but it has never been seen, and, indeed, lies +far beyond the range of microscopic vision. It must not be +confused—as is often done—with the structure of the +plasm (the fibrous network, groups of granules, honey-comb, etc.) +which does come within the range of the microscope.</p> + +<p>But when we speak of the cells as the elementary organisms, or +structural units, or “ultimate individualities,” we +must bear in mind a certain restriction of the phrases. I mean, +that the cells are not, as is often supposed, the very lowest stage +of organic individuality. There are yet more elementary organisms +to which I must refer occasionally. These are what we call the +“cytodes” (<i>cytos</i> = cell), certain living, +independent beings, consisting only of a particle of <i> +plasson</i>—an albuminoid substance, which is not yet +differentiated into caryoplasm and cytoplasm, but combines the +properties of both. Those remarkable beings called the <i> +monera</i>—especially the chromacea and bacteria—are +specimens of these simple cytodes. (Compare Chapter XIX.) To be +quite accurate, then, we must say: the elementary organism, or the +ultimate individual, is found in two different stages. The first +and lower stage is the cytode, which consists merely of a particle +of plasson, or quite simple plasm. The second and higher stage is +the cell, which is already divided or differentiated into nuclear +matter and cellular matter. We comprise both kinds—the +cytodes and the cells—under the name of <i>plastids</i> +(“formative particles”), because they are the real +builders of the organism. However, these cytodes are not found, as +a rule, in the higher animals and plants; here we have only real +cells with a nucleus. Hence, in these tissue-forming organisms +(both plant and animal) the organic unit always consists of two +chemically and anatomically different parts—the outer +cell-body and the inner nucleus.</p> + +<p>In order to convince oneself that this cell is really an +independent organism, we have only to observe the development and +vital phenomena of one of them. We see then that it performs all +the essential functions of life—both vegetal and +animal—which we find in the entire organism. Each of these +tiny beings grows and nourishes itself independently. It takes its +food from the surrounding fluid; sometimes, even, the naked cells +take in solid particles at certain points of their surface—in +other words, “eat” them—without needing any +special mouth and stomach for the purpose (cf. Fig. 19).</p> + +<p>Further, each cell is able to reproduce itself. This +multiplication, in most cases, takes the form of a simple cleavage, +sometimes direct, sometimes indirect; the simple direct (or +“amitotic”) division is less common, and is found, for +instance, in the blood cells <a href="#Fig. 10">(Fig. 10).</a> In +these the nucleus first divides into two equal parts by +constriction. The indirect (or “mitotic”)</p> + +<br> +<hr> +<p class="page"><a name="page 41">[ 41 ]</a></p> + +<p> </p> + +<center> +<table class="capt" border="0" width="328" cellspacing="0" +cellpadding="0" summary= +"A large branching nerve-cell, or soul-cell."> +<tr> +<td><img src="images/fig9.GIF" width="328" height="556" alt= +"Fig. 9--A large branching nerve-cell, or soul-cell."><br><br> +<a name="Fig. 9">Fig. 9</a>—<b>A large branching +nerve-cell, or “soul-cell”,</b> from the brain of an +electric fish (<i>Torpedo</i>). In the middle of the cell is the +large transparent round <i>nucleus,</i> one <i>nucleolus,</i> and, +within the latter again, a <i>nucleolinus.</i> The protoplasm of +the cell is split into innumerable fine threads (or fibrils), which +are embedded in intercellular matter, and are prolonged into the +branching processes of the cell (<i>b</i>). One branch (<i>a</i>) +passes into a nerve-fibre. (From <i>Max Schultze.</i>)</td> +</tr> +</table> +</center> + +<br> + <br> +<hr> +<p class="page"><a name="page 42">[ 42 ]</a></p> + +<p> </p> + +<br class="one"> +<br> +<p class="one">cleavage is much more frequent; in this the +caryoplasm of the nucleus and the cytoplasm of the cell-body act +upon each other in a peculiar way, with a partial dissolution +(<i>caryolysis</i>), the formation of knots and loops +(<i>mitosis</i>), and a movement of the halved plasma-particles +towards two mutually repulsive poles of attraction +(<i>caryokinesis,</i> <a href="#Fig. 11">Fig. 11.</a>)</p> + +<br> + + +<table border="0" cellpadding="0" cellspacing="0" summary= +"Fig. 10.--Blood-cells, multiplying by direct division."> +<tr> +<td><img src="images/fig10.GIF" width="120" height="145" alt= +"Fig. 10--Blood-cells, multiplying by direct division."></td> +<td align="left" valign="bottom"> +<p class="capt"><a name="Fig. 10">Fig. 10</a>—<b>Blood-cells, +multiplying by direct division,</b> from the blood of the embryo of +a stag. Originally, each blood-cell has a nucleus and is round +(<i>a</i>). When it is going to multiply, the nucleus divides into +two (<i>b, c, d</i>). Then the protoplasmic body is constricted +between the two nuclei, and these move away from each other +(<i>e</i>). Finally, the constriction is complete, and the cell +splits into two daughter-cells (<i>f</i>). (From <i>Frey.</i>)</p> +</td> +</tr> +</table> + +<br> + + +<p>The intricate physiological processes which accompany this +“mitosis” have been very closely studied of late years. +The inquiry has led to the detection of certain laws of evolution +which are of extreme importance in connection with heredity. As a +rule, two very different parts of the nucleus play an important +part in these changes. They are: the <i>chromatin,</i> or coloured +nuclear substance, which has a peculiar property of tingeing itself +deeply with certain colouring matters (carmine, hæmatoxylin, +etc.), and the <i>achromin</i> (or <i>linin,</i> or <i> +achromatin</i>), a colourless nuclear substance that lacks this +property. The latter generally forms in the dividing cell a sort of +spindle, at the poles of which there is a very small particle, also +colourless, called the “central body” +(<i>centrosoma</i>). This acts as the centre or focus in a +“sphere of attraction” for the granules of protoplasm +in the surrounding cell-body, and assumes a star-like appearance +(the cell-star, or <i>monaster</i>). The two central bodies, +standing opposed to each other at the poles of the nuclear spindle, +form “the double-star” (or <i>amphiaster,</i> <a href= +"#Fig. 11">Fig. 11</a>, B C). The chromatin often forms a long, +irregularly-wound thread—“the coil” +(<i>spirema,</i> Fig. A). At the commencement of the cleavage it +gathers at the equator of the cell, between the stellar poles, and +forms a crown of U-shaped loops (generally four or eight, or some +other definite number). The loops split lengthwise into two halves +(B), and these back away from each other towards the poles of the +spindle (C). Here each group forms a crown once more, and this, +with the corresponding half of the divided spindle, forms a fresh +nucleus (D). Then the protoplasm of the cell-body begins to +contract in the middle, and gather about the new daughter-nuclei, +and at last the two daughter-cells become independent beings.</p> + +<p>Between this common mitosis, or <i>indirect</i> +cell-division—which is the normal cleavage-process in most +cells of the higher animals and plants—and the simple <i> +direct</i> division (Fig. 10) we find every grade of segmentation; +in some circumstances even one kind of division may be converted +into another.</p> + +<p>The plastid is also endowed with the functions of movement and +sensation. The single cell can move and creep about, when it has +space for free movement and is not prevented by a hard envelope; it +then thrusts out at its surface processes like fingers, and quickly +withdraws them again, and thus changes its shape (<a href= +"#Fig. 12">Fig. 12</a>). Finally, the young cell is sensitive, or +more or less responsive to stimuli; it makes certain movements on +the application of chemical and mechanical irritation. Hence we can +ascribe to the individual cell all the chief functions which we +comprehend under the general heading of +“life”—sensation, movement, nutrition, and +reproduction. All these properties of the multicellular and highly +developed animal are also found in the single animal-cell, at least +in its younger stages. There is no longer any doubt about this, and +so we may regard it as a solid and important base of our +physiological conception of the elementary organism.</p> + +<p>Without going any further here into these very interesting +phenomena of the life of the cell, we will pass on to consider the +application of the cell theory to the ovum. Here comparative +research yields the important result that <i>every ovum is at first +a simple cell.</i> I say this is very important, because our whole +science of embryology now resolves itself into the problem: +“How does the multicellular</p> + +<br> +<hr> +<p class="page"><a name="page 43">[ 43 ]</a></p> + +<p> </p> + +<p class="one">organism arise from the unicellular?” Every +organic individual is at first a simple cell, and as such an +elementary organism, or a unit of individuality. This cell produces +a cluster of cells by segmentation, and from these develops the +multicellular organism, or individual of higher rank.</p> + +<table border="0" cellpadding="0" cellspacing="0" summary= +"Fig. 11.--Indirect or mitotic cell-division."> +<tr> +<td><img src="images/fig11.GIF" width="130" height="460" alt= +"Fig. 11--Indirect or mitotic cell-division."></td> +<td align="left" valign="bottom"> +<p class="capt"><b>A. Mother-cell</b><br> +(Knot, spirema)<br> +1. <b>Nuclear threads</b> (chromosomata) (coloured nuclear matter, +chromatin)<br> +2. Nuclear membrane<br> +3. Nuclear sap<br> +4. Cytosoma<br> +5. Protoplasm of the cell-body<br> +<br> +<br> +<b>B. Mother-star,</b> the loops beginning to split lengthways +(nuclear membrane gone)<br> +1. Star-like appearance in cytoplasm<br> +2. Centrosoma (sphere of attraction)<br> +3. Nuclear spindle (achromin, colourless matter)<br> +4. Nuclear loops (chromatin, coloured matter)<br> +<br> +<br> +<br> +<b>C. The two daughter-stars,</b><br> +produced by the breaking of the loops of the mother-star (moving +away)<br> +1. Upper daughter-crown<br> +2. Connecting threads of the two crowns (achromin)<br> +3. Lower daughter-crown<br> +4. Double-star (amphiaster)<br> +<br> +<br> +<br> +<b>D. The two daughter-cells,</b><br> +produced by the complete division of the two nuclear halves +(cytosomata still connected at the equator) (Double-knot, +Dispirema)<br> +1. Upper daughter-nucleus<br> +2. Equatorial constriction of the cell-body<br> +3. Lower daughter-nucleus.<br> + </p> +</td> +</tr> +</table> + +<p class="capt"><a name="Fig. 11">Fig. 11</a>—<b>Indirect or +mitotic cell-division</b> (with caryolysis and caryokinesis) from +the skin of the larva of a salamander. (From <i>Rabl.</i>).</p> + +<br> + + +<p>When we examine a little closer the original features of the +ovum, we notice the extremely significant fact that in its first +stage the ovum is just the same simple and indefinite structure in +the case of man and all the animals (<a href="#Fig. 13">Fig. +13</a>). We are unable to detect any material difference between +them, either in outer shape or internal constitution. Later, though +the ova remain unicellular, they differ in size and shape, enclose +various kinds of yelk-particles, have different envelopes, and so +on. But when we examine them at their birth, in the ovary of the +female animal, we find them to be always of the same form in the +first stages of their life. In the beginning each ovum is a very +simple, roundish, naked, mobile cell, without a membrane; it +consists merely of a particle of cytoplasm enclosing a nucleus (<a +href="#Fig. 13">Fig. 13</a>). Special names have been given to +these parts of the ovum; the cell-body is called the <i>yelk</i> +(<i>vitellus</i>), and the cell-nucleus the <i>germinal +vesicle.</i> As a rule, the</p> + +<br> +<hr> +<p class="page"><a name="page 44">[ 44 ]</a></p> + +<p> </p> + +<p class="one">nucleus of the ovum is soft, and looks like a small +pimple or vesicle. Inside it, as in many other cells, there is a +nuclear skeleton or frame and a third, hard nuclear body (the <i> +nucleolus</i>). In the ovum this is called the <i>germinal +spot.</i> Finally, we find in many ova (but not in all) a still +further point within the germinal spot, a “nucleolin,” +which goes by the name of the germinal point. The latter parts +(germinal spot and germinal point) have, apparently, a minor +importance, in comparison with the other two (the yelk and germinal +vesicle). In the yelk we must distinguish the active <i>formative +yelk</i> (or protoplasm = first plasm) from the passive <i> +nutritive yelk</i> (or deutoplasm = second plasm).</p> + +<br> + + +<table border="0" cellpadding="0" cellspacing="0" summary= +"Fig. 12.--Mobile cells from the inflamed eye of a frog."> +<tr> +<td><img src="images/fig12.GIF" width="180" height="181" alt= +"Fig. 12--Mobile cells from the inflamed eye of a frog."></td> +<td align="left" valign="bottom"> +<p class="capt"><a name="Fig. 12">Fig. 12</a>—<b>Mobile cells +from the inflamed eye of a frog</b> (from the watery fluid of the +eye, the <i>humor aqueus</i>). The naked cells creep freely about, +by (like the amœba or rhizopods) protruding fine processes +from the uncovered protoplasmic body. These bodies vary continually +in number, shape, and size. The nucleus of these amœboid +lymph-cells (“travelling cells,” or planocytes) is +invisible, because concealed by the numbers of fine granules which +are scattered in the protoplasm. (From <i>Frey.</i>)</p> +</td> +</tr> +</table> + +<br> + + +<p>In many of the lower animals (such as sponges, polyps, and +medusæ) the naked ova retain their original simple appearance +until impregnation. But in most animals they at once begin to +change; the change consists partly in the formation of connections +with the yelk, which serve to nourish the ovum, and partly of +external membranes for their protection (the ovolemma, or +prochorion). A membrane of this sort is formed in all the mammals +in the course of the embryonic process. The little globule is +surrounded by a thick capsule of glass-like transparency, the <i> +zona pellucida,</i> or <i>ovolemma pellucidum</i> <a href= +"#Fig. 14">(Fig. 14).</a> When we examine it closely under the +microscope, we see very fine radial streaks in it, piercing the <i> +zona,</i> which are really very narrow canals. The human ovum, +whether fertilised or not, cannot be distinguished from that of +most of the other mammals. It is nearly the same everywhere in +form, size, and composition. When it is fully formed, it has a +diameter of (on an average) about 1/120 of an inch. When the mammal +ovum has been carefully isolated, and held against the light on a +glass-plate, it may be seen as a fine point even with the naked +eye. The ova of most of the higher mammals are about the same size. +The diameter of the ovum is almost always between 1/250 to 1/125 +inch. It has always the same globular shape; the same +characteristic membrane; the same transparent germinal vesicle with +its dark germinal spot. Even when we use the most powerful +microscope with its highest power, we can detect no material +difference between the ova of man, the ape, the dog, and so on. I +do not mean to say that there are no differences between the ova of +these different mammals. On the contrary, we are bound to assume +that there are such, at least as regards chemical composition. Even +the ova of different men must differ from each other; otherwise we +should not have a different individual from each ovum. It is true +that our crude and imperfect apparatus cannot detect these subtle +individual differences, which are probably in the molecular +structure. However, such a striking resemblance of their ova in +form, so great as to seem to be a complete similarity, is a strong +proof of the common parentage of man and the other mammals. From +the common germ-form we infer a common stem-form. On the other +hand, there are striking peculiarities by which we can easily +distinguish the fertilised ovum of the mammal from the fertilised +ovum of the birds, amphibia, fishes, and other vertebrates (see the +close of Chap. XXIX).</p> + +<p>The fertilised bird-ovum <a href="#Fig. 15">(Fig. 15)</a> is +notably different. It is true that in its earliest stage <a href= +"#Fig. 13">(Fig. 13 E)</a> this ovum also is very like that of the +mammal (Fig. 13 F). But afterwards, while still within the oviduct, +it takes up a quantity of nourishment and works this into the +familiar large yellow yelk. When we examine a very young ovum in +the hen’s oviduct, we</p> + +<br> +<hr> +<p class="page"><a name="page 45">[ 45 ]</a></p> + +<p> </p> + +<center> +<table class="capt" border="0" width="274" cellpadding="0" +cellspacing="0" summary= +"Fig. 13--Ova of various animals, executing amœboid movements."> +<tr> +<td><img src="images/fig13.GIF" width="274" height="363" alt= +"Fig. 13--Ova of various animals, executing amœboid movements."><br><br> +<a name="Fig. 13">Fig. 13</a>—<b>Ova of various animals, +executing amœboid movements,</b> magnified. All the ova are +naked cells of varying shape. In the dark fine-grained protoplasm +(yelk) is a large vesicular nucleus (the germinal vesicle), and in +this is seen a nuclear body (the germinal spot), in which again we +often see a germinal point. Figs. <i>A1–A4</i> represent the +ovum of a sponge (<i>Leuculmis echinus</i>) in four successive +movements. <i>B1–B8</i> are the ovum of a parasitic crab +(<i>Chondracanthus cornutus</i>), in eight successive movements. +(From <i>Edward von Beneden.</i>) <i>C1–C5</i> show the ovum +of the cat in various stages of movement (from <i> +Pflüger</i>); Fig. <i>D</i> the ovum of a trout; <i>E</i> the +ovum of a chicken; <i>F</i> a human ovum.</td> +</tr> +</table> +</center> + +<br> + + +<p class="one">find it to be a simple, small, naked, amœboid +cell, just like the young ova of other animals (Fig. 13). But it +then grows to the size we are familiar with in the round yelk of +the egg. The nucleus of the ovum, or the germinal vesicle, is thus +pressed right to the surface of the globular ovum, and is embedded +there in a small quantity of transparent matter, the so-called +white yelk. This forms a round white spot, which is known as the +“tread” (<i>cicatricula</i>) <a href="#Fig. 15">(Fig. +15 <i>b</i>).</a> From the tread a thin column of the white yelk +penetrates through the yellow yelk to the centre of the globular +cell, where it swells into a small, central globule (wrongly called +the yelk-cavity, or <i>latebra,</i> Fig. 15 <i>d'</i>). The yellow +yelk-matter which surrounds this white yelk has the appearance in +the egg (when boiled hard) of concentric layers (<i>c</i>). The +yellow yelk is also enclosed in a delicate structureless membrane +(the <i>membrana vitellina, a</i>).</p> + +<p>As the large yellow ovum of the bird</p> + +<br> +<hr> +<p class="page"><a name="page 46">[ 46 ]</a></p> + +<p> </p> + +<p class="one">attains a diameter of several inches in the bigger +birds, and encloses round yelk-particles, there was formerly a +reluctance to consider it as a simple cell. This was a mistake. +Every animal that has only one cell-nucleus, every amœba, +every gregarina, every infusorium, is unicellular, and remains +unicellular whatever variety of matter it feeds on. So the ovum +remains a simple cell, however much yellow yelk it afterwards +accumulates within its protoplasm. It is, of course, different, +with the bird’s egg when it has been fertilised. The ovum +then consists of as many cells as there are nuclei in the tread. +Hence, in the fertilised egg which we eat daily, the yellow yelk is +already a multicellular body. Its tread is composed of several +cells, and is now commonly called the <i>germinal disc.</i> We +shall return to this <i>discogastrula</i> in Chap. IX.</p> + +<br> + + +<table border="0" cellpadding="0" cellspacing="0" summary= +"Fig. 14.--The human ovum."> +<tr> +<td><img src="images/fig14.GIF" width="231" height="230" alt= +"Fig. 14--The human ovum."></td> +<td align="left" valign="bottom"> +<p class="capt"><a name="Fig. 14">Fig. 14</a>—<b>The human +ovum,</b> taken from the female ovary, magnified. The whole ovum is +a simple round cell. The chief part of the globular mass is formed +by the nuclear yelk (<i>deutoplasm</i>), which is evenly +distributed in the active protoplasm, and consists of numbers of +fine yelk-granules. In the upper part of the yelk is the +transparent round germinal vesicle, which corresponds to the <i> +nucleus.</i> This encloses a darker granule, the germinal spot, +which shows a <i>nucleolus.</i> The globular yelk is surrounded by +the thick transparent germinal membrane (<i>ovolemma,</i> or <i> +zona pellucida</i>). This is traversed by numbers of lines as fine +as hairs, which are directed radially towards the centre of the +ovum. These are called the pore-canals; it is through these that +the moving spermatozoa penetrate into the yelk at impregnation.</p> +</td> +</tr> +</table> + +<br> + + +<p>When the mature bird-ovum has left the ovary and been fertilised +in the oviduct, it covers itself with various membranes which are +secreted from the wall of the oviduct. First, the large clear +albuminous layer is deposited around the yellow yelk; afterwards, +the hard external shell, with a fine inner skin. All these +gradually forming envelopes and processes are of no importance in +the formation of the embryo; they serve merely for the protection +of the original simple ovum. We sometimes find extraordinarily +large eggs with strong envelopes in the case of other animals, such +as fishes of the shark type. Here, also, the ovum is originally of +the same character as it is in the mammal; it is a perfectly simple +and naked cell. But, as in the case of the bird, a considerable +quantity of nutritive yelk is accumulated inside the original yelk +as food for the developing embryo; and various coverings are formed +round the egg. The ovum of many other animals has the same internal +and external features. They have, however, only a physiological, +not a morphological, importance; they have no direct influence on +the formation of the fœtus. They are partly consumed as food +by the embryo, and partly serve as protective envelopes. Hence we +may leave them out of consideration altogether here, and restrict +ourselves to material points—<i>to the substantial identity +of the original ovum in man and the rest of the animals</i> <a +href="#Fig. 13">(Fig. 13).</a></p> + +<p>Now, let us for the first time make use of our biogenetic law; +and directly apply this fundamental law of evolution to the human +ovum. We reach a very simple, but very important, conclusion. <i> +From</i></p> + +<br> +<hr> +<p class="page"><a name="page 47">[ 47 ]</a></p> + +<p> </p> + +<p class="one"><i>the fact that the human ovum and that of all +other animals consists of a single cell, it follows immediately, +according to the biogenetic law, that all the animals, including +man, descend from a unicellular organism.</i> If our biogenetic law +is true, if the embryonic development is a summary or condensed +recapitulation of the stem-history—and there can be no doubt +about it—we are bound to conclude, from the fact that all the +ova are at first simple cells, that all the multicellular organisms +originally sprang from a unicellular being. And as the original +ovum in man and all the other animals has the same simple and +indefinite appearance, we may assume with some probability that +this unicellular stem-form was the common ancestor of the whole +animal world, including man. However, this last hypothesis does not +seem to me as inevitable and as absolutely certain as our first +conclusion.</p> + +<br> + +<table class="capt" width="236" align="left" summary= +"Fig. 15.--A fertilised ovum from the oviduct of a hen."> +<tr> +<td align="justify"><img src="images/fig15.GIF" width="236" height="134" alt= +"Fig. 15--A fertilised ovum from the oviduct of a hen."> +<a name="Fig. 15">Fig. 15</a>—<b>A fertilised +ovum from the oviduct of a hen.</b> The yellow yelk (<i>c</i>) +consists of several concentric layers (<i>d</i>), and is enclosed +in a thin yelk-membrane (<i>a</i>). The nucleus or germinal vesicle +is seen above in the cicatrix or “tread” (<i>b</i>). +From that point the white yelk penetrates to the central +yelk-cavity (<i>d'</i>). The two kinds of yelk do not differ very +much.</td> +</tr> +</table> + + + +<p class="pic">This inference from the unicellular embryonic form to the +unicellular ancestor is so simple, but so important, that we cannot +sufficiently emphasise it. We must, therefore, turn next to the +question whether there are to-day any unicellular organisms, from +the features of which we may draw some approximate conclusion as to +the unicellular ancestors of the multicellular organisms. The +answer is: Most certainly there are. There are assuredly still +unicellular organisms which are, in their whole nature, really +nothing more than permanent ova. There are independent unicellular +organisms of the simplest character which develop no further, but +reproduce themselves as such, without any further growth. We know +to-day of a great number of these little beings, such as the +gregarinæ, flagellata, acineta, infusoria, etc. However, +there is one of them that has an especial interest for us, because +it at once suggests itself when we raise our question, and it must +be regarded as the unicellular being that approaches nearest to the +real ancestral form. This organism is the <i>Amœba.</i></p> + +<table class="capt" width="221" align="left" summary= +"Fig. 16.--A creeping amœba."> +<tr> +<td align="justify"><img src="images/fig16.GIF" width="221" height="155" alt= +"Fig. 16--A creeping amœba."> +<a name="Fig. 16">Fig. 16</a>—<b>A creeping +amœba</b> (highly magnified). The whole organism is a simple +naked cell, and moves about by means of the changing arms which it +thrusts out of and withdraws into its protoplasmic body. Inside it +is the roundish nucleus with its nucleolus.</td> +</tr> +</table> + + +<p class="pic">For a long time now we have comprised under the general name of +amœbæ a number of microscopic unicellular organisms, +which are very widely distributed, especially in fresh-water, but +also in the ocean; in fact, they have lately been discovered in +damp soil. There are also parasitic amœbæ which live +inside other animals. When we place one of these amœbæ +in a drop of water under the microscope and examine it with a high +power, it generally appears as a roundish particle of a very +irregular and varying shape (Figs. 16 and 17). In its soft, slimy, +semi-fluid substance, which consists of protoplasm, we see only the +solid globular particle it contains, the nucleus. This unicellular +body moves about continually, creeping in every direction on the +glass on which we are examining it. The movement is effected by the +shapeless body thrusting out finger-like processes at various parts +of its surface; and these are slowly but continually changing, and +drawing the rest of the body after them. After a time, perhaps, the +action changes. The amœba suddenly stands still, withdraws +its projections, and assumes a globular shape. In a little while, +however, the round body begins to expand again, thrusts out arms in +another</p> + +<br> +<hr> +<p class="page"><a name="page 48">[ 48 ]</a></p> + +<p> </p> + +<p class="one">direction, and moves on once more. These changeable +processes are called “false feet,” or pseudopodia, +because they act physiologically as feet, yet are not special +organs in the anatomic sense. They disappear as quickly as they +come, and are nothing more than temporary projections of the +semi-fluid and structureless body.</p> + +<br> + + +<table border="0" cellpadding="0" cellspacing="0" summary= +"Fig. 17.--Division of a unicellular amœba."> +<tr> +<td><img src="images/fig17.GIF" width="276" height="303" alt= +"Fig. 17--Division of a unicellular amœba."></td> +<td align="left" valign="bottom"> +<p class="capt"><a name="Fig. 17">Fig. 17</a>—<b>Division of +a unicellular amœba</b> (<i>Amœba polypodia</i>) in six +stages. (From <i>F. E. Schultze.</i>) the dark spot is the nucleus, +the lighter spot a contractile vacuole in the protoplasm. The +latter reforms in one of the daughter-cells.)</p> +</td> +</tr> +</table> + +<p>If you touch one of these creeping amœbæ with a +needle, or put a drop of acid in the water, the whole body at once +contracts in consequence of this mechanical or physical stimulus. +As a rule, the body then resumes its globular shape. In certain +circumstances—for instance, if the impurity of the water +lasts some time—the amœba begins to develop a covering. +It exudes a membrane or capsule, which immediately hardens, and +assumes the appearance of a round cell with a protective membrane. +The amœba either takes its food directly by imbibition of +matter floating in the water, or by pressing into its protoplasmic +body solid particles with which it comes in contact. The latter +process may be observed at any moment by forcing it to eat. If +finely ground colouring matter, such as carmine or indigo, is put +into the water, you can see the body of the amœba pressing +these coloured particles into itself, the substance of the cell +closing round them. The amœba can take in food in this way at +any point on its surface, without having any special organs for +intussusception and digestion, or a real mouth or gut.</p> + +<p>The amœba grows by thus taking in food and dissolving the +particles eaten in its protoplasm. When it reaches a certain size +by this continual feeding, it begins to reproduce. This is done by +the simple process of cleavage (Fig. 17). First, the nucleus +divides into two parts. Then the protoplasm is separated between +the two new nuclei, and the whole cell splits into two +daughter-cells, the protoplasm gathering about each of the nuclei. +The thin bridge of protoplasm which at first connects the +daughter-cells soon breaks. Here we have the simple form of direct +cleavage of the nuclei. Without mitosis, or formation of threads, +the homogeneous nucleus divides into two halves. These move away +from each other, and become centres of attraction for the +enveloping matter, the protoplasm. The same direct cleavage of the +nuclei is also witnessed in the reproduction of many other +protists, while other unicellular organisms show the indirect +division of the cell.</p> + +<p>Hence, although the amœba is nothing but a simple cell, it +is evidently able to accomplish all the functions of the +multicellular organism. It moves, feels, nourishes itself, and +reproduces. Some kinds of these amœbæ can be seen with +the naked eye, but most of them are microscopically small. It is +for the following reasons that we regard the amœbæ as +the unicellular organisms which have</p> + +<br> +<hr> +<p class="page"><a name="page 49">[ 49 ]</a></p> + +<p> </p> + +<p class="one">special phylogenetic (or evolutionary) relations to +the ovum. In many of the lower animals the ovum retains its +original naked form until fertilisation, develops no membranes, and +is then often indistinguishable from the ordinary amœba. Like +the amœbæ, these naked ova may thrust out processes, +and move about as travelling cells. In the sponges these mobile ova +move about freely in the maternal body like independent +amœbæ <a href="#Fig. 17">(Fig. 17).</a> They had been +observed by earlier scientists, but described as foreign +bodies—namely, parasitic amœbæ, living +parasitically on the body of the sponge. Later, however, it was +discovered that they were not parasites, but the ova of the sponge. +We also find this remarkable phenomenon among other animals, such +as the graceful, bell-shaped zoophytes, which we call polyps and +medusæ. Their ova remain naked cells, which thrust out +amœboid projections, nourish themselves, and move about. When +they have been fertilised, the multicellular organism is formed +from them by repeated segmentation.</p> + +<p>It is, therefore, no audacious hypothesis, but a perfectly sound +conclusion, to regard the amœba as the particular unicellular +organism which offers us an approximate illustration of the ancient +common unicellular ancestor of all the metazoa, or multicellular +animals. The simple naked amœba has a less definite and more +original character than any other cell. Moreover, there is the fact +that recent research has discovered such amœba-like cells +everywhere in the mature body of the multicellular animals. They +are found, for instance, in the human blood, side by side with the +red corpuscles, as colourless blood-cells; and it is the same with +all the vertebrates. They are also found in many of the +invertebrates—for instance, in the blood of the snail. I +showed, in 1859, that these colourless blood-cells can, like the +independent amœbæ, take up solid particles, or +“eat” (whence they are called <i>phagocytes</i> = +“eating-cells,” <a href="#Fig. 19">Fig. 19</a>). +Lately, it has been discovered that many different cells may, if +they have room enough, execute the same movements, creeping about +and eating. They behave just like amœbæ <a href= +"#Fig. 12">(Fig. 12).</a> It has also been shown that these +“travelling-cells,” or <i>planocytes,</i> play an +important part in man’s physiology and pathology (as means of +transport for food, infectious matter, bacteria, etc.).</p> + +<p>The power of the naked cell to execute these characteristic +amœba-like movements comes from the contractility (or +automatic mobility) of its protoplasm. This seems to be a universal +property of young cells. When they are not enclosed by a firm +membrane, or confined in a “cellular prison,” they can +always accomplish these amœboid movements. This is true of +the naked ova as well as of any other naked cells, of the +“travelling-cells,” of various kinds in connective +tissue, lymph-cells, mucus-cells, etc.</p> + +<table class="capt" width="203" align="left" summary= +"Fig. 18.--Ovum of a sponge."> +<tr> +<td align="justify"><img src="images/fig18.GIF" width="203" height="129" alt= +"Fig. 18--Ovum of a sponge."> +<a name="Fig. 18">Fig. 18</a>—<b>Ovum of a +sponge</b> (<i>Olynthus</i>). The ovum creeps about in a body of +the sponge by thrusting out ever-changing processes. It is +indistinguishable from the common amœba.)</td> +</tr> +</table> + + +<p class="pic">We have now, by our study of the ovum and the comparison of it +with the amœba, provided a perfectly sound and most valuable +foundation for both the embryology and the evolution of man. We +have learned that the human ovum is a simple cell, that this ovum +is not materially different from that of other mammals, and that we +may infer from it the existence of a primitive unicellular +ancestral form, with a substantial resemblance to the +amœba.</p> + +<p>The statement that the earliest progenitors of the human race +were simple cells of this kind, and led an independent unicellular +life like the amœba, has not only been ridiculed as the dream +of a natural philosopher, but also been violently censured in +theological journals as “shameful and immoral.” But, as +I observed in my essay <i>On the Origin and Ancestral Tree of the +Human Race</i> in 1870, this offended piety must equally protest +against the “shameful and immoral” fact that each human +individual is developed from a simple ovum, and that this human +ovum is indistinguishable from those of the other mammals, and in +its earliest stage is like a naked amœba.</p> + +<br> +<hr> +<p class="page"><a name="page 50">[ 50 ]</a></p> + +<p> </p> + +<p class="one">We can show this to be a fact any day with the +microscope, and it is little use to close one’s eyes to +“immoral” facts of this kind. It is as indisputable as +the momentous conclusions we draw from it and as the vertebrate +character of man (see <a href="chap11.html">Chap. XI).</a></p> + +<br> + + +<center> +<table class="capt" border="0" width="355" cellspacing="0" +cellpadding="0" summary= +"Fig. 19--Blood-cells that eat, or phagocytes, from a naked sea-snail."> +<tr> +<td><img src="images/fig19.GIF" width="355" height="116" alt= +"Fig. 19--Blood-cells that eat, or phagocytes, from a naked sea-snail."> +<br><br> + +<a name="Fig. 19">Fig. 19</a>—<b>Blood-cells that eat, or +phagocytes, from a naked sea-snail</b> (<i>Thetis</i>), greatly +magnified. I was the first to observe in the blood-cells of this +snail the important fact that “the blood-cells of the +invertebrates are unprotected pieces of plasm, and take in food, by +means of their peculiar movements, like the +amœbæ.” I had (in Naples, on May 10th, 1859) +injected into the blood-vessels of one of these snails an infusion +of water and ground indigo, and was greatly astonished to find the +blood-cells themselves more or less filled with the particles of +indigo after a few hours. After repeated injections I succeeded in +“observing the very entrance of the coloured particles in the +blood-cells, which took place just in the same way as with the +amœba.” I have given further particulars about this in +my <i>Monograph on the Radiolaria.</i></td> +</tr> +</table> +</center> + +<br> +<p>We now see very clearly how extremely important the cell theory +has been for our whole conception of organic nature. +“Man’s place in nature” is settled beyond +question by it. Apart from the cell theory, man is an insoluble +enigma to us. Hence philosophers, and especially physiologists, +should be thoroughly conversant with it. The soul of man can only +be really understood in the light of the cell-soul, and we have the +simplest form of this in the amœba. Only those who are +acquainted with the simple psychic functions of the unicellular +organisms and their gradual evolution in the series of lower +animals can understand how the elaborate mind of the higher +vertebrates, and especially of man, was gradually evolved from +them. The academic psychologists who lack this zoological equipment +are unable to do so.</p> + +<p>This naturalistic and realistic conception is a stumbling-block +to our modern idealistic metaphysicians and their theological +colleagues. Fenced about with their transcendental and dualistic +prejudices, they attack not only the monistic system we establish +on our scientific knowledge, but even the plainest facts which go +to form its foundation. An instructive instance of this was seen a +few years ago, in the academic discourse delivered by a +distinguished theologian, Willibald Beyschlag, at Halle, January +12th, 1900, on the occasion of the centenary festival. The +theologian protested violently against the “materialistic +dustmen of the scientific world who offer our people the diploma of +a descent from the ape, and would prove to them that the genius of +a Shakespeare or a Goethe is merely a distillation from a drop of +primitive mucus.” Another well-known theologian protested +against “the horrible idea that the greatest of men, Luther +and Christ, were descended from a mere globule of +protoplasm.” Nevertheless, not a single informed and +impartial scientist doubts the fact that these greatest men were, +like all other men—and all other vertebrates—developed +from an impregnated ovum, and that this simple nucleated globule of +protoplasm has the same chemical constitution in all the +mammals.</p> + +<br> + + +<hr noshade align="left" size="1" width="20%"> +<p class="ref"><a href="Title.html">Title and Contents</a><br> +<a href="glossary.html">Glossary</a><br> +<a href="chap5.html">Chapter V</a><br> +<a href="chap7.html">Chapter VII</a><br> +<a href="Title.html#Illustrations">Figs. 1–209</a><br> +<a href="title2.html#Illustrations">Figs. 210–408</a></p> +</body> +</html> + |