Darwin, and After Darwin Volume I Part 5

In thus saying that the ova of all animals are, so far as microscopes can reveal, _substantially_ similar, I am of course speaking of the egg-cell proper, and not of what is popularly known as the egg. The egg of a bird, for example, is the egg-cell, _plus_ an enormous aggregation of nutritive material, an egg-sh.e.l.l, and sundry other structures suited to the subsequent development of the egg-cell when separated from the parent's body. But all these accessories are, from our present point of view, accidental or advent.i.tious. What we have now to understand by the ovum, the egg, or the egg-cell, is the microscopical germ which I have just described. So far then as this germ is concerned, we find that all multicellular organisms begin their existence in the same kind of structure, and that this structure is anatomically indistinguishable from that of the permanent form presented by the lowest, or unicellular organisms. But although anatomically indistinguishable, physiologically they present the sundry peculiarities already mentioned.

Now I have endeavoured to show that none of these peculiarities are such as to exclude--or even so much as to invalidate--the supposition of developmental continuity between the lowest egg-cells and the highest protozoal cells. It remains to show in this place, and on the other hand, that there is no breach of continuity between the lowest and the highest egg-cells; but, on the contrary, that the remarkable uniformity of the complex processes whereby their peculiar characters are exhibited to the histologist, is such as of itself to sustain the doctrine of continuity in a singularly forcible manner. On this account, therefore, and also because the facts will again have to be considered in another connexion when we come to deal with Weismann's theory of heredity, I will here briefly describe the processes in question.

We have already seen that the young egg-cell multiplies itself by simple binary division, after the manner of unicellular organisms in general--thereby indicating, as also by its amoebiform movements, its fundamental ident.i.ty with such organisms in kind. But, as we have likewise seen, when the ovum ceases to resemble these organisms, by taking on its higher degree of functional capacity, it is no longer able to multiply itself in this manner. On the contrary, its cell-divisions are now of an endogenous character, and result in the formation of many different kinds of cells, in the order required for constructing the multicellular organism to which the whole series of processes eventually give rise. We have now to consider these processes _seriatim_.

[Ill.u.s.tration: FIG. 33.--Stages in the formation of the polar bodies in the ovum of a star-fish. (After Hertwig.) _g.v._, germinal vesicle transformed into a spindle-shaped system of fibres; _p.'_, the first polar body becoming extruded; _p._, _p._, both polar bodies fully extruded; _f. pn._, female p.r.o.nucleus, or residue of the germinal vesicle.]

First of all the nucleus discharges its polar bodies, as previously mentioned, and in the manner here depicted on the previous page. (Fig.

33.) It will be observed that the nucleus of the ovum, or the germinal vesicle as it is called, gets rid first of one and afterwards of the other polar body by an "indirect," or karyokinetic, process of division.

(Fig. 33.) Extrusion of these bodies from the ovum (or it may be only from the nucleus) having been accomplished, what remains of the nucleus retires from the circ.u.mference of the ovum, and is called the female p.r.o.nucleus. (Fig. 33. _f. pn._) The ovum is now ready for fertilization.

A similar emission of nuclear substance is said by some good observers to take place also from the male germ-cell, or spermatozoon, at or about the close of _its_ development. The theories to which these facts have given rise will be considered in future chapters on Heredity.

Turning now to the mechanism of fertilization, the diagrams (Figs. 34, 35) represent what happens in the case of star-fish.

[Ill.u.s.tration: FIG. 34.--Fertilization of the ovum of an echinoderm.

(From _Quain's Anatomy_, after Selenka.) S, spermatozoon; _m. pr._, male p.r.o.nucleus; _f. pr._, female p.r.o.nucleus. 1 to 4 correspond to D to G in the next figure.]

[Ill.u.s.tration: FIG. 35.--Fertilization of the ovum of a star-fish.

(From the _Encycl. Brit._ after Fol.) A, spermatozoa in the mucilaginous coat of the ovum; a prominence is rising from the surface of the ovum towards a spermatozoon; B, they have almost met; C, they have met; D, the spermatozoon enters the ovum through a distinct opening; H, the entire ovum, showing extruded polar bodies on its upper surface, and the moving together of the male and female p.r.o.nuclei; E, F, G, meeting and coalescence of the p.r.o.nuclei.]

The sperm-cell, or spermatozoon, is seen in the act of penetrating the ovum. In the first figure it has already pierced the mucilaginous coat of the ovum, the limit of which is represented by a line through which the tail of the spermatozoon is pa.s.sing: the head of the spermatozoon is just entering the ovum proper. It may be noted that, in the case of many animals, the general protoplasm of the ovum becomes aware, so to speak, of the approach of a spermatozoon, and sends up a process to meet it.

(Fig. 35, A, B, C.) Several--or even many--spermatozoa may thus enter the coat of the ovum; but normally only one proceeds further, or right into the substance of the ovum, for the purpose of effecting fertilization. This spermatozoon, as soon as it enters the periphery of the yolk, or cell-substance proper, sets up a series of remarkable phenomena. First, its own head rapidly increases in size, and takes on the appearance of a cell-nucleus: this is called the male p.r.o.nucleus. At the same time its tail begins to disappear, and the enlarged head proceeds to make its way directly towards the nucleus of the ovum which, as before stated, is now called the female p.r.o.nucleus. The latter in its turn moves towards the former, and when the two meet they fuse into one ma.s.s, forming a new nucleus. Before the two actually meet, the spermatozoon has lost its tail altogether; and it is noteworthy that during its pa.s.sage through the protoplasmic cell-contents of the ovum, it appears to exercise upon this protoplasm an attractive influence; for the granules of the latter in its vicinity dispose themselves around it in radiating lines. All these various phenomena are depicted in the above wood-cuts. (Figs. 34, 35.)

Fertilization having been thus effected by fusion of the male and female p.r.o.nuclei into a single (or new) nucleus, this latter body proceeds to exhibit complicated processes of karyokinesis, which, as before shown, are preliminary to nuclear division in the case of egg-cells. Indeed the karyokinetic process may begin in both the p.r.o.nuclei before their junction is effected; and, even when their junction is effected, it does not appear that complete fusion of the so-called chromatin elements of the two p.r.o.nuclei takes place. For the purpose of explaining what this means, and still more for the purpose of giving a general idea of the karyokinetic processes as a whole, I will quote the following description of them, because, for terseness combined with lucidity, it is unsurpa.s.sable.

[Ill.u.s.tration: FIG. 36.--Karyokinesis of a typical tissue-cell (epithelium of Salamander). (After Flemming and Klein.) The series from A to I represents the successive stages in the movement of the chromatin fibres during division, excepting G, which represents the "nucleus-spindle" of an egg-cell. A, resting nucleus; D, wreath-form; E, single star, the loops of the wreath being broken; F, separation of the star into two groups of U-shaped fibres; H, diaster or double star; I, completion of the cell-division and formation of two resting nuclei. In G the chromatin fibres are marked _a_, and correspond to the "equatorial plate"; _b_, achromatin fibres forming the nucleus-spindle; _c_, granules of the cell-protoplasm forming a "polar star." Such a polar star is seen at each end of the nucleus-spindle, and is not to be confused with the diaster H, the two ends of which are composed of _chromatin_.]

Researches, chiefly due to Flemming, have shown that the nucleus in very many tissues of higher plants and animals consists of a capsule containing a plasma of "achromatin," not deeply stained by re-agents, ramifying in which is a reticulum of "chromatin"

consisting of fibres which readily take a deep stain. (Fig. 36, A).

Further it is demonstrated that, when the cell is about to divide into two, definite and very remarkable movements take place in the nucleus, resulting in the disappearance of the capsule and in the arrangement of its fibres first in the form of a wreath (D), and subsequently (by the breaking of the loops formed by the fibres) in the form of a star (E). A further movement within the nucleus leads to an arrangement of the broken loops in two groups (F), the position of the open ends of the broken loops being reversed as compared with what previously obtained. Now the two groups diverge, and in many cases a striated appearance of the achromatin substance between the two groups of chromatin loops is observable (H). In some cases (especially egg-cells) this striated arrangement of the achromatin is then termed a "nucleus-spindle," and the group of chromatin loops (G, _a_) is known as "the equatorial plate." At each end of the nucleus-spindle in these cases there is often seen a star consisting of granules belonging to the general protoplasm of the cell (G, _c_). These are known as "polar stars." After the separation of the two sets of loops (H) the protoplasm of the general substance of the cell becomes constricted, and division occurs, so as to include a group of chromatin loops in each of the two fission products. Each of these then rearranges itself together with the a.s.sociated chromatin into a nucleus such as was present in the mother cell to commence with (I)[13].

[13] Ray Lankester, _Encyclop. Brit._, 9th ed., Vol. XIX, pp. 832-3.

Since the above was published, however, further progress has been made.

In particular it has been found that the chromatin fibres pa.s.s from phase D to phase F by a process of longitudinal splitting (Fig. 37 _g_, _h_; Fig. 38, VI, VII)--which is a point of great importance for Weismann's theory of heredity,--and that the protoplasm outside the nucleus seems to take as important a part in the karyokinetic process as does the nuclear substance. For the so-called "attraction-spheres" (Fig.

38 II _a_, III, III _a_, VIII to XII), which were at first supposed to be of subordinate importance in the process as a whole, are now known to take an exceedingly active part in it (see especially IX to XI). Lastly, it may be added that there is a growing consensus of authoritative opinion, that the chromatin fibres are the seats of the material of heredity, or, in other words, that they contain those essential elements of the cell which endow the daughter-cells with their distinctive characters. Therefore, where the parent-cell is an ovum, it follows from this view that all hereditary qualities of the future organism are potentially present in the ultra-microscopical structure of the chromatin fibres.

[Ill.u.s.tration: FIG. 37.--Study of successive changes taking place in the nucleus of an epithelium cell, preparatory to division of the cell. (From _Quain's Anatomy_, after Flemming.) _a_, resting cell, showing the nuclear network; _b_, first stage of division, the chromatoplasm transformed into a skein of closely contorted filaments; _c_ to _f_, further stages in the growth and looping arrangement of the filaments; _g_, stellate phase, or aster; _h_, completion of the splitting of the filaments, already begun in _f_ and _g_; _i_, _j_, _k_, successive stages in separation of the filaments into two groups; _l_, the final result of this (diaster); _m_ to _q_, stages in the division of the whole cell into two, showing increasing contortion of the filaments, until they reach the resting stage at _q_].

[Ill.u.s.tration: FIG. 38.--Formation and conjugation of the p.r.o.nuclei in _Ascaris megalocephala_. (From _Quain's Anatomy_, after E. von Beneden.) _f_, female p.r.o.nucleus; _m_, male p.r.o.nucleus; _p_, one of the polar bodies.

I. The second polar body has just been extruded; both male and female p.r.o.nuclei contain two chromatin particles; those of the male p.r.o.nucleus are becoming transformed into a skein. II. The chromatin in both p.r.o.nuclei now forms into a skein.

II _a_. The skeins are more distinct. Two attraction (or protoplasmic) spheres, each with a central particle united with a small spindle of achromatic fibres, have made their appearance in the general substance of the egg close to the mutually approaching p.r.o.nuclei. The male p.r.o.nucleus has the remains of the body of the spermatozoon adhering to it.

III. Only the female p.r.o.nucleus is shown in this figure. The skein is contracted and thickened. The attraction-spheres are near one side of the ovum, and are connected with its periphery by a cone of fibres forming a polar circle, _p.c._; _e.c._, equatorial circle.

III _a_. The p.r.o.nuclei have come into contact, and the spindle-system is now arranged across their common axis.

IV. Contraction of the skein, and formation of two U-or V-shaped chromatin fibres in each p.r.o.nucleus.

V. The V-shaped chromatin filaments are now quite distinct: the male and female p.r.o.nuclei are in close contact.]

[Ill.u.s.tration: (38 continued)

VI., VII. The V-shaped filaments are splitting longitudinally; their structure of fine granules of chromatin is apparent in VII., which is more highly magnified. The conjugation of the p.r.o.nuclei is apparently complete in VII. The attraction-spheres and achromatic spindle, although present, are not depicted in IV., V., VI., and VII.

VIII. Equatorial arrangement of the four chromatin loops in the middle of the now segmenting ovum: the achromatic substance forming a spindle-shaped system of granules with fibres radiating from the poles of the spindle (attraction-spheres); the chromatin forms an equatorial plate. (Compare Fig. 36 G.)

IX. Shows diagrammatically the commencing separation of the chromatin fibres of the conjugated nuclei, and the system of fibres radiating from the attraction-spheres. (Compare again Fig. 36 G.) _p.c._, polar circle; _e.c._, equatorial circle; _c.c._, central particle.

X. Further separation of the chromatin filaments. Each of the central particles of the attraction-spheres has divided into two.

XI. The chromatin fibres are becoming developed into the skeins of the two daughter-nuclei. These are still united by fibres of achromatin. The general protoplasm of the ovum is becoming divided.

XII. The two daughter-nuclei exhibit a chromatin network. Each of the attraction-spheres has divided into two, which are joined by fibres of achromatin, and connected with the periphery of the cell in the same way as in the original or parent sphere, III.]

As I shall have more to say about these processes in the next volume, when we shall see the important part which they bear in Weismann's theory of heredity, it is with a double purpose that I here introduce these yet further ill.u.s.trations of them upon a somewhat larger scale.

The present purpose is merely that of showing, more clearly than hitherto, the great complexity of these processes on the one hand, and, on the other, the general similarity which they display in egg-cells and in tissue-cells. But as in relation to this purpose the ill.u.s.trations speak for themselves, I may now pa.s.s on at once to the history of embryonic development, which follows fertilization of the ovum.

We have seen that when the new nucleus of the fertilized ovum (which is formed by a coalescence of the male p.r.o.nucleus with the female) has completed its karyokinetic processes, it is divided into two equal parts; that these are disposed at opposite poles of the ovum; and that the whole contents of the ovum are thereupon likewise divided into two equal parts, with the result that there are now two nucleated cells within the spherical wall of the ovum where before there had only been one. Moreover, we have also seen that a precisely similar series of events repeat themselves in each of these two cells, thus giving rise to four cells (see Fig. 29). It must now be added that such duplication is continued time after time, as shown in the accompanying ill.u.s.trations (Figs. 39, 40).

[Ill.u.s.tration: FIG. 39.--Segmentation of ovum. (After Hackel.) Successive stages are marked by the letters A, B, C. D represents several stages in advance of C.]

[Ill.u.s.tration: FIG. 40.--The contents of an ovum in an advanced stage of segmentation, drawn in perspective. (After Hackel.)]

All this, it will be noticed, is a case of cell-multiplication, which differs from that which takes place in the unicellular organisms only in its being _invariably_ preceded (as far as we know) by karyokinesis, and in the resulting cells being all confined within a common envelope, and so in not being free to separate. Nevertheless, from what has already been said, it will also be noticed that this feature makes all the difference between a Metazoon and a Protozoon; so that already the ovum presents the distinguishing character of a Metazoon.

I have dealt thus at considerable length upon the processes whereby the originally unicellular ovum and spermatozoon become converted into the multicellular germ, because I do not know of any other exposition of the argument from Embryology where this, the first stage of the argument, has been adequately treated. Yet it is evident that the fact of all the processes above described being so similar in the case of s.e.xual (or metazoal) reproduction among the innumerable organisms where it occurs, const.i.tutes in itself a strong argument in favour of evolution. For the mechanism of fertilization, and all the processes which even thus far we have seen to follow therefrom, are hereby shown to be not only highly complex, but likewise highly specialized. Therefore, the remarkable similarity which they present throughout the whole animal kingdom--not to speak of the vegetable--is expressive of organic continuity, rather than of absolute discontinuity in every case, as the theory of special creation must necessarily suppose. And it is evident that this argument is strong in proportion to the uniformity, the specialization, and the complexity of the processes in question.

Having occupied so much s.p.a.ce with supplying what appear to me the deficiencies in previous expositions of the argument from Embryology, I can now afford to take only a very general view of the more important features of this argument as they are successively furnished by all the later stages of individual development. But this is of little consequence, seeing that from the point at which we have now arrived previous expositions of the argument are both good and numerous. The following then is to be regarded as a mere sketch of the evidences of phyletic (or ancestral) evolution, which are so abundantly furnished by all the subsequent phases of ontogenetic (or individual) evolution.

The multicellular body which is formed by the series of segmentations above described is at first a sphere of cells (Fig. 40). Soon, however, a watery fluid gathers in the centre, and progressively pushes the cells towards the circ.u.mference, until they there const.i.tute a single layer.

The ovum, therefore, is now in the form of a hollow sphere containing fluid, confined within a continuous wall of cells (Fig. 41 A). The next thing that happens is a pitting in of one portion of the sphere (B). The pit becomes deeper and deeper, until there is a complete inv.a.g.i.n.ation of this part of the sphere--the cells which const.i.tute it being progressively pushed inwards until they come into contact with those at the opposite pole of the ovum. Consequently, instead of a hollow sphere of cells, the ovum now becomes an open sac, the walls of which are composed of a double layer of cells (C). The ovum is now what has been called a gastrula; and it is of importance to observe that probably all the Metazoa pa.s.s through this stage. At any rate it has been found to occur in all the main divisions of the animal kingdom, as a glance at the accompanying figures will serve to show (Fig. 42)[14]. Moreover many of the lower kinds of Metazoa never pa.s.s beyond it; but are all their lives nothing else than gastrulae, wherein the orifice becomes the mouth of the animal, the internal or inv.a.g.i.n.ated layer of cells the stomach, and the outer layer the skin. So that if we take a child's india-rubber ball, of the hollow kind with a hole in it, and push in one side with our fingers till internal contact is established all round, by then holding the indented side downwards we should get a very fair anatomical model of a gastraea form, such as is presented by the adult condition of many of the most primitive Metazoa--especially the lower _Coelenterata_. The preceding figures represent two other such forms in nature, the first locomotive and transitory, the second fixed and permanent (Figs. 43, 44).

[14] In most vertebrated animals this process of gastrulation has been more or less superseded by another, which is called delamination; but it scarcely seems necessary for our present purposes to describe the latter. For not only does it eventually lead to the same result as gastrulation--i. e. the converting of the ovum into a double-walled sac,--but there is good evidence among the lower Vertebrata of its being preceded by gastrulation; so that, even as to the higher Vertebrata, embryologists are pretty well agreed that delamination has been but a later development of, or possibly improvement upon, gastrulation.

[Ill.u.s.tration: FIG. 41.--Formation of the gastrula of _Amphioxus_.

(After Kowalevsky.) A, wall of the ovum, composed of a single layer of cells; B, a stage in the process of gastrulation; C, completion of the process; S, original or segmentation cavity of ovum; _al_, alimentary cavity of gastrula; _ect_, outer layer of cells; _ent_, inner layer of cells; _b_, orifice, const.i.tuting the mouth in permanent forms.]

[Ill.u.s.tration: FIG. 42.--Gastrulation. A, Gastrula of a Zoophyte (_Gastrophysema_). (After Hackel.) B, Gastrula of a Worm (_Sagitta_). (After Kowalevsky.) C, Gastrula of an Echinoderm (_Uraster_). (After A. Aga.s.siz.) D, Gastrula of an Arthropod (_Nauplius_). (After Hackel.) E, Gastrula of a Mollusk (_Limnaeus_).

(After Rabl.) F, Gastrula of a Vertebrate (_Amphioxus_). (After Kowalevsky.) In all, _d_, indicates the intestinal cavity; _o_, the primitive mouth; _s_, the cleavage-cavity; _i_, the endoderm, or intestinal layer; _e_, the ectoderm or skin-layer.]

[Ill.u.s.tration: FIG. 43.--Gastrula of a Chalk Sponge. (After Hackel.) A, External view. B, Longitudinal section. _g_, digestive cavities; _o_, mouth; _i_, endoderm; _e_, ectoderm.]

[Ill.u.s.tration: FIG. 44.--_Prophysema primordiale_, an extant gastraea-form. (After Hackel.) (A). External view of the whole animal, attached by its foot to seaweed. (B). Longitudinal section of the same. The digestive cavity (_d_) opens at its upper end in the mouth (_m_). Among the cells of the endoderm (_g_) lie amoeboid egg-cells of large size (_e_). The ectoderm (_h_) is encrusted with grains of sand, above the sponge spicules.]