LECTURE XIV

REPRODUCTION BY GERM-CELLS.

Historical—Differentiation of germ-cells into male and female—Pandorina—Volvox—Sperm-cells and ova in Algæ—Zoosperm form of the male germ-cells—Zoosperms of the Barnacles—Adaptation of the sperm-cells to the conditions of fertilization—Daphnids—Spermatozoa in different animal groups—Their minute structure—Form and structure of the egg-cell—Adaptation of the ovum to given conditions—Dimorphic ova in the same species—Nutritive cells associated with egg-cells—Complex structure of the bird's egg.

Ifwe now turn to the reproduction of the Metazoa and Metaphyta by means of germ-cells we find that a great number of lowly plants produce germ-cells which require nothing more for the development of a new plant beyond certain favourable external conditions, above all, moisture and warmth. Such, for instance, are the 'spores' of the ferns, which are formed on the under surface of the fronds in little clusters of a brown or yellow colour, easily visible to the naked eye. These spores are individually very small, so that thousands go to form one spore-cluster or sporangium, and millions of spores are given off annually by a single fern. Each spore is a germ-cell enclosed in a protective capsule, and may, if carried by the wind to a spot favourable to germination, become a young plant, the so-called prothallium, from which the fern-plant proper subsequently develops.

This reproduction by spores has been regarded as a form of 'asexual reproduction' so-called, and has been classed along with budding and fission under this head. But it has nothing in common with these forms of multiplication except the negative character that the act of fertilization, which we shall inquire into later on, does not in this case occur. This mode of classification has no longer any more justification than the division of the animal kingdom into backboned and backboneless animals, in which the negative character of the absence of vertebræ has led to the slumping of quite heterogeneous forms in one group. I do not mean to dispute that both these classifications were fully justified in their own time; indeed they expressed a step of progress. Nowadays, however, the division 'Invertebrata' or 'backboneless animals' as a scientific conception has been abandoned, and the same should be done with the category'asexual reproduction,' since it groups together quite different things, such as multiplication by single-celled and many-celled 'germs,' and is moreover based on a quite erroneous idea of what 'fertilization' really is. Both terms may very well be retained as a mere matter of convenience, but it is much to be desired that the two apt designations proposed by Haeckel—Monogony for asexual, and Amphigony for sexual reproduction—should come into general use.

Meanwhile it is enough to say that reproduction by 'spores' occurs normally in Algæ, fungi, mosses, and fern-like plants, and that there are also animals in which the germ-cells possess the power of giving rise of themselves to a new individual. But the cases which I am chiefly thinking of are those of so-called virgin birth or parthenogenesis, which are not to be compared with multiplication by spores in regard to their mode of origin; there is a peculiarity in the origin of this mode of multiplication which I can only make clear after we have studied the normal forms of what is called 'sexual reproduction.'

We shall therefore pass on to this mode of reproduction. It is well known that, in all higher animals, just as in Man, an individual cannot reproduce by itself; the co-operation of two individuals is necessary, and these—the male and the female—differ essentially from each other in many particulars. Their union in the act of procreation induces the development of a new individual, whether this matures within the mother in a special receptacle, or whether it is deposited as a 'fertilized egg,' as in birds, the lower vertebrates, and most 'invertebrates.'

As long as Man has lived he has regarded this process of procreation as the essential factor in the origin of new individuals, and as he had no insight into the essence of the process he had necessarily to regard reproduction as something entirely mysterious, and the co-operation of the two sexes as aconditio sine qua nonof reproduction in general; thus copulation and reproduction seemed identical.

This was in the main the state of opinion at the time of the discovery of innumerable minute filaments, the so-called 'spermatozoa' in the 'fertilizing' spermatic fluid of the male. The discovery was made in 1677 by Leeuwenhoek in the case of birds, mammals, and many other animals. Albrecht von Haller (1708-77) was at first inclined to regard these spermatozoa as the rudiments of the embryo, but later on, in the course of his long life, he withdrew this theory, and declared them to be a kind of parasite in the spermatic fluid without anything to do with fertilization. The same opinion was expressed in 1835 by K. E. von Baer, in opposition to the opinion of Prevost and Dumas, who had rightly interpreted the spermatozoaas the essential elements of the spermatic fluid. When one follows the matter out in detail, one finds it almost incredible that such a number of mistakes should have been made, and so many circuitous paths traversed, before even the limited knowledge current in the middle of the nineteenth century was attained—that is to say, enough to give ground for the assertion that fertilization depends upon the contact of the spermatozoon with the body of the egg. In 1843 Martin Barry had found the spermatozoa within the egg-envelope of the rabbit ovum, but it was some time later (1852) that the investigations of Meissner, Bischoff, and Newport established the fact that the zoosperm penetrates through the egg-envelope. All else remained quite obscure, and could not be cleared up as long as it was believed, on the strength of observations which were in themselves correct enough, thatseveralzoosperms were always necessary to fertilize one ovum.

To an understanding of the process even in its most general outlines there was lacking, apart from technical methods, an appreciation of the morphological value of the ovum and the spermatozoon. It was necessary to recognize both ovum and spermatozoon ascellsbefore their union in fertilization could be regarded as the fusion of two cells, as a copulation or conjugation of two minute elementary organisms. But this knowledge only gained ground very gradually, and even in the sixties opinions on the subject were very much divided. Moreover, there was an entire absence of knowledge in regard to 'sexual' reproduction among the lower plants, the Algæ, Fungi, Mosses, and Ferns, as well as of any detailed acquaintance with the processes of fertilization among flowering plants. All this had to be elucidated by the labours of many distinguished observers before it was possible to say so much even as this, that the process of fertilization depends in general on the union of two cells.

I need not discuss the whole of this long process of scientific development, and have only touched upon it because I wished to emphasize that the conception of the process of fertilization was for a long time quite erroneous, and has only attained to clearness in recent times. Pairing as it is seen in the higher animals was for long regarded as the essential part of the process, and a mysterious life-awakening influence was assumed in regard to it; and even when it was understood that not the copulation, but the union of two living units which was always brought about thereby—the union of the male and the female germ-cells—was the essence of 'fertilization,' this was still regarded as a life-awakening process, and the way to a true understanding of the facts was thus once more blocked.

The simplest form of sexual reproduction in many-celled animalsis found, among others, in the Volvocineæ, those green, spherical, freshwater cell-colonies which we have already studied in relation to reproduction by asexual germ-cells. Among them it is the rule that, after a long series of generations producing only 'asexual' germ-cells, colonies occur in which each germ-cell is no longer able to develop a new colony alone, but can do so only after it has united with another germ-cell.

Now, as we have seen, there are Volvocineæ in which the differentiation of cells into those of the body (soma) and those concerned with reproduction has not been established, and all the cells are therefore alike. In these, as for instance in the genusPandorina(Fig. 62, p. 257), when sexual reproduction is to occur the whole colony breaks up into sixteen cells; these burst forth from the gelatinous matrix in which they have been hitherto enclosed, swim about in the water with the help of their two flagella, meet other similar free-swimming cells and conjugate with these. The two swimming cells come close to each other, draw in their flagella, sink to the ground in consequence, and fuse completely both as to the cell-body and the nucleus. They assume a spherical form, lose the eye-spot, become surrounded with a tough cell-skin or cyst, and so remain for a longer or shorter time as so-called 'zygotes' or lasting spores. Then they develop by repeated cell-division into one of the sixteen-celledPandorinacolonies with which we are already familiar; this bursts forth from the capsule and swims freely about in the water again.

Here, therefore, the so-called sexual reproduction depends on the fusion of two cells similar in appearance, and when this phenomenon was first known it was regarded as something quite different from the corresponding reproduction in other multicellular organisms. But we now know that quite nearly related Volvocineæ belonging to the genusVolvoxand to other genera, which exhibit a differentiation into body-cells and reproductive cells, may reproduce sexually by means of twodifferentkinds of germ-cells; and we have also learned through Goebel and others that even genera likePandorina, which consist of only one kind of cells, may yet produce male and female reproductive cells differing essentially in form from one another. InEudorina, for instance, a gelatinous sphere containing sixteen or thirty-two individual cells, asexual reproduction occurs in exactly the same way as inPandorina, that is, each of these cells divides four or five times in rapid succession, and thus forms a new colony, which then bursts forth; but when the time for sexual reproduction comes the colonies behave differently, for some become female and somemale. In the former the cells remain as they were before, but in the male colonies the sixteen or thirty-two cells undergo a peculiar process of division, which ends in each becoming a mass (16-32) of so-called 'zoosperms,' that is, minute, narrow, longitudinally elongated cells with two flagella (Fig. 63 atDshows those ofVolvox). InEudorinathey differ from the female germ-cells or ova externally in form and size, as well as by being much more actively motile, and they contain green and subsequently yellow colouring matter, and a red eye-spot. We here find, for the first time among multicellular organisms, the differentiation of male and female germ-cells; and we learn from this that the essence of fertilization does not lie in thisdifferentiation, since it may be absent, but that this distinction of female and male cells is only of secondary moment. From the fact that the egg-cells are larger and less active, the 'sperm-cells' or zoosperms smaller and livelier, we can already anticipate what will be more definitely established as our knowledge of the facts increases—that a differentiation according to the principle of division of labour has taken place even in the germ-cells, and that the first effect of this is to render the meeting of the cells destined for conjugation easier and more certain. The much smaller and more slender zoosperms swim about in the water in clusters until they come in contact with a female colony; then they separate from each other, bore their way into the soft jelly of the female colony, and 'fertilize' the egg-cell, that is to say, each male cell fuses with a female cell and forms a 'lasting spore,' exactly as inPandorina.

Fig. 63.Volvox aureus, after Klein and Schenck.A, besides the small flagellate somatic cells of the colony there are five large egg-cells (t) which are capable of parthenogenetic development, three recently fertilized egg-cells (o) and a number of male germ-cells (a) in process of multiplication. From each of these, by continued division, a bundle of spermatozoa arises.B, a bundle of thirty-two sperm-cells in process of development, seen from above.C, the same seen from the side. Magnified 687 times.Dindividual spermatozoa, magnified 824 times.

InVolvoxthe state of matters is similar to that inEudorina; here again, in addition to the 'asexual' reproduction through the 'Parthenogonidia' which are like egg-cells in appearance (Fig. 63,A,t), there are also male and female germ-cells which are usually produced alternately with the former, but sometimes at the same time, as in Fig. 63. The egg-cells are large and have no flagella, the sperm-cells lie together in clusters, and after they are mature (D) they swim freely in the water and then bore into another colony, where each unites with an egg-cell. The difference between the two kinds of cells consists therefore in the much greater number, the much smaller size, and the greater activity of the male cells, and in the smaller number but much larger size of the female cells—a differentiation in accordance with the principle of division of labour, depending on the fact that the two kinds of cells must reach each other, and yet must contain a certain mass of living protoplasm. While the small size but large number of male cells, combined with their motility, gives them an advantage in seeking out and boring into the female cells, the large size of the latter, on the other hand, makes up for the loss in mass which the fertilized egg would otherwise suffer from the diminution in size of the male cell. This difference in size may be greatly accentuated; thus in one of the brown sea-wracks, for instance, the spermatozoa are only 5 micro-millimetres in length, while the ova are spherical and have a diameter of 80-100 micro-millimetres, thus containing a mass 30-60,000 times greater (Möbius). Fig. 64 shows an ovum of this species surrounded by spermatozoa

In the course of the evolution of species this contrast between female and male germ-cells became more and more marked, not always in the same direction, however, but in one or another accordingto the conditions of fertilization. It would be erroneous to suppose that, with the higher differentiation of the organism as a whole, the differentiation of the germ-cells became increasingly complex. On the contrary we find even among Algæ, as the case ofFucusshows, a marked difference between the sex-cells, which rather decreases than increases among many of the higher plants. It is not on the more or less complex structure of the organism itself that the nature and degree of the dimorphism of the germ-cells depends, but on the special conditions which are involved in each case, both in the union of the two kinds of sex-cells and in the subsequent development of the product of this union, the 'fertilized ovum.'

Fig. 64.Fucus platycarpus, brown sea-wrack.Ei, ovum, surrounded by swarmingsperm-cells (sp). After Schenck.

Thus it comes about that the male or 'sperm-cells' of the lower plants, of the lower animals, and, again, of the highest animals are similar in structure. In all these organisms the male germ-cells exhibit the minuteness, the form, and the activity of the so-called 'zoosperms' or 'spermatozoa,' that is to say, they are thread-like, very minute corpuscles, which move rapidly forwards in water or other fluid with undulatory movements, and penetrate into the ovum with similar boring movements when they have been fortunate enough to reach their goal. At the anterior end they possess a more or less conspicuous thickening, the so-called 'head' in which the nucleus lies, and this is followed by the 'tail,' a thread-like structure consisting of cytoplasm which effects undulatory movements comparable to those of the flagella of Infusorians and Volvocineæ. The whole spermatozoon is thus a specialized 'flagellate cell.'

When these 'zoosperms' were recognized as the 'fertilizing elements' in higher animals, and when 'sperm-threads' had been found, not only in all mammals and birds, reptiles, amphibians, and fishes, but even in many 'invertebrates,' the conclusion was suggested that the function of fertilization might be discharged by this lively motile substance; for until the eighth decade of the nineteenth century fertilization was still regarded by many as an 'awakening of life' in the egg. Since life depends on movement, in truth on infinitely fine molecular movements, of which the movement of the whole germ-cellfrom place to place is only a visible outcome, fertilization was pictured, by a not very luminous process of reasoning, as the awakening of life in the ovum—in itself incapable of further life—through the transference to it of movement through the agency of the zoosperm. Some investigators even went the length of regarding the ovum as 'dead organic material.'

I mention this at this point, though I do not propose in the meantime to inquire further into the significance of the conjugation of the sex-cells. But the view just referred to is so completely refuted even by the external form of the male germ-cells in many groups of plants and animals, that I cannot discuss these differences in form without at the same time indicating the conclusions which they directly suggest.

The great majority of plants and animals exhibit the zoosperm form of male germ-cells, and this must obviously be interpreted in the light of the fact that the ova to be fertilized are not generally to be found in direct proximity to the sperms shed by the male organism, but are at some distance from them. Among Medusæ and Polyps both male and female germ-cells are liberated into the water, simultaneously it may be, but separated from each other by distances of some feet or yards. The spermatozoa then swim about seeking the ova, which are also floating freely in the sea, guided by a power of attraction on the part of the latter—an attraction of whose nature we know nothing, though in the case of certain fern-ova it has been traced to the secretion of malic acid (Pfeffer).

The same conditions obtain among Sponges. Here, again, the persons or stocks are either male or female; the latter produce large delicate ova, which lie in the interior of the sponge and there await the fertilizing sperms; the former give off the ripe sperms into the water in such abundance that thousands and millions of zoosperms burst forth simultaneously in all directions; these seek about for a female sponge, penetrate into its canal system, and so ultimately reach the ova. Of course only a very few of them reach their goal; the greater number are lost in the water and become the prey of Infusorians, Radiolarians, or other lowly animals. The fact that enormous numbers thus miss their true aim shows us why these zoosperms must be produced in such quantities; it is simply an adaptation to the extraordinarily high ratio of elimination in these cells, just as the number of young annually produced by an animal, or of seeds by a plant, is regulated by natural selection according to the ratio of elimination of the particular species. The more numerous the descendants which succumb each time to unfavourable circumstances, to enemies, or to lack of food, the more prolific must the species be. The same holds true of the regulation of the number of male germ-cells to be produced by an individual; there must be so many developed that, in spite of the unavoidable enormous loss, on an average the number of mature ova necessary to the maintenance of the species always receive spermatozoa.

Also associated with the prodigal production of zoosperms is their minuteness, for the more zoosperms that can be developed out of a given mass of organic substance the smaller they are. Each species is restricted within definite limits of production by its size and the volume of its body, and there is thus an advantage in having the zoosperms of the smallest possible size whenever the chance of the individual sperm successfully reaching an ovum is very small. In all such cases nature has abstained from burdening the male germ-cell with an appreciable contribution of material to the result of conjugation, that is, to the foundation of the new organism; the passive ovum contains in itself alone almost all that is necessary to the building up of the embryo. Fertilization of the ovum by the liberation of the sperm-cells into the water occurs not only in animals of low degree, such as Sponges, Medusæ, Star-fishes, Sea-urchins and their relatives, but also in much higher animals, such as many Fishes and Amphibians, and in these the male cells have the form of motile threads. But the spermatozoon-form of male cell does not occur only in animals and plants which live in the water, or in those which, like mosses and many vascular plants, are at least occasionally covered by a thin layer of rain or dew, in which the zoosperms can swim to the ova, it occurs also in a very large number of animals in which the sperms pass directly into the body of the female, in those, therefore, in which copulation takes place.

But even where copulation occurs we find that in most cases, as, for instance, in Vertebrates, Molluscs, and Insects, the zoosperm-form is retained. The reason for this is obviously twofold: in the first place, in many cases the sperms do not directly reach the ovum as a consequence of copulation, but may have to go a long way within the body of the female, as in mammals; or even when the way is short and certain, the ovum may be encased in a firm covering or shell difficult to penetrate, and the thread-like zoosperm has to face the task of boring its way through this shell, or slipping in through a minute opening, the so-called micropyle. In either case it would be difficult to imagine a form of sperm-cell better adapted to the fulfilment of this task than that of a thread with a thin, pointed head-portion and a long motile tail, which enables the zoosperm to twistitself like a screw through a narrow opening in the egg-envelope, whether the opening was previously present or not.

We can thus understand why, among insects for instance, the male cells should always occur in the form of zoosperms, although in this case they reach a special receptacle in the female reproductive organs, the 'receptaculum seminis,' and are stored up in this. When a mature ovum gliding downwards through the oviduct comes to the place where this receptacle opens into it, the liberation of a few sperm-cells suffices to fertilize it with certainty, provided that they possess the thread-like form, which allows them to slip in through the very minute opening in the egg-envelope. It might be inferred from the certainty with which the ovum must in this case be found by the spermatozoon that only a small number of the latter would require to be produced, and yet even here the number is very large, though not so enormous as in the sea-urchins and other marine animals, which simply allow the sperm-cells to escape into the water. The large number in insects is due to the fact that many of the sperms may miss the micropyle; and also that in many insects a very large number of eggs have to be fertilized in succession. In the course of a life lasting three or four years the queen bee lays many thousand of eggs, most of which are fertilized, and that from a seminal receptacle which has been filled only once.

There are, however, other sperm-cells of thread-like form which are not produced in such enormous multitudes, but in a much more moderate number, perhaps a few hundreds in the testicle. This is so in the little Crustaceans, known as Ostracods, all the freshwater species of which possess zoosperms only moderately numerous and of quite unusual size.

The comparatively small number is explained by the certainty with which each of them reaches the ovum, and the large size may be accounted for in part by the small number which suffices, and which, therefore, admits of the male cell also carrying a considerable portion of the material for the building up of the embryo. Probably, however, the thickness and firmness of the covering of the ovum has something to do with it, for it has no opening for the entrance of the male cell, and it is fully hardened by the time fertilization takes place. Perhaps nowhere can we see more clearly how every little detail of the structure of the organism is dominated by the principle of adaptation than in the arrangements for fertilization, and notably in those which obtain in the Ostracods. I pass by the complicated apparatus for copulation, since we do not yet understand it fully in all particulars. According to my own investigations and those of myformer students, Dr. Stuhlmann and Dr. Schwarz, the essential point seems to be that the colossally large zoosperms, which show no activity within the body of the male, leave it one at a time, so to speak, in single file. In copulation they are pressed out singly, one after the other, through a very fine tube, and then they enter, still singly, through the reproductive aperture of the female into an equally fine passage with spiral windings, through which they ultimately reach a roomy pear-shaped receptacle, the 'receptaculum seminis' of the female. There they lie in a long band composed of several hundreds, and only now attain their full maturity by throwing off an outer cuticle—moulting, so to speak. It is only when they get into a fluid medium that they show the power of undulatory movement, feeble at first, but gradually more energetic and more violent. And these movements enable them to penetrate like gimlets into the calcareous egg-shell. In the normal course it happens that when a mature ovum is deposited from the opening of the oviduct, one of the giant zoosperms at the same time, or shortly afterwards, leaves the 'receptaculum seminis' of the female by way of the spiral passage, and reaches the exterior just behind the ovum. The actual process of penetration has not been observed as yet, but the zoosperm has been seen at a slightly later stage spirally coiled inside the ovum.

Fig. 65.Copulation in a Daphnid (Lyncæid).Emptying of the sperm (sp) into the brood-chamberof the female (♀).abd♂, the abdomen of the male.Magnified 100 times.

In these Ostracods the sperms are often visible with the naked eye, and in some species they are twice the length of the animal; they are thus emphatically giant cells, which can develop a very considerable boring power.

In respect to the various adaptations of the sperm-cells to the conditions of fertilization there is hardly any group more interesting than the water-fleas or Daphnids.

It is amazing how greatly the size of the sperms varies among the Daphnids, and how it stands in inverse proportion to theirnumber, and how both are obviously regulated in relation to the difficulties which stand in the way of each sperm-cell before it can reach the ovum. In some species the sperm-cells are very large, in others extremely small. In the generaDaphnia,Lynceus, and others, copulation occurs as shown in Fig. 65; the sperm-cells (sp) are liberated by the male into the capacious brood-cavity of the female, which at the moment is closed to some extent by the abdomen of the male, in reality closed only partially at the posterior end and at the sides. It seems inevitable that a large proportion of the male elements should stream out again and be lost because of the violent movements of both animals. Accordingly, we find that the sperm-cells are only about the hundredth part of a millimetre in length and of round or rod-like form, and are voided in multitudes into the brood-cavity. Fig. 66,f,g, andh, show such cells in different species, as they occur in the testes to the number of many thousands. But in all the species in which the brood-cavity isclosed, and in which therefore there is not such a serious loss of sperm-cells, the elements are much larger, and at the same time less numerous. They are largest and least numerous in species of genera likeDaphnella,Polyphemus, andBythotrephes, in which the males have a copulatory organ, so that the possibility of loss of the male cells is excluded. Thus the round, delicate, and viscid sperm-cells ofBythotrephes(Fig. 66,b) are more than a tenth of a millimetre in length, but they are developed in proportionately smaller numbers, so that more than twenty are never found in the testis, and often only six or eight, while in copulation only from three to five are ejected. But as there are only two eggs to be fertilized at a time, and as the male cells are expressed into the brood-cavity directly upon the eggs, so that they immediately adhere to them, this small number is amply sufficient.

Fig. 66.Spermatozoa of various Daphnids.a, Sida.b, Bythotrephes.c, Daphnella.d, Moina paradoxa.e,Moina rectirostris.f, Eurycercus lamellatus.g, Alonellapygmæa.h, Peracantha truncata.All magnified 300times.

It is remarkable how different the sperm-cells sometimes are in quite nearly related species of Daphnids, as a glance at Fig. 66 will show; and, on the other hand, how similar they may be in two species which belong to different families, likeBythotrephes longimanus(b), andDaphnella hyalina(c). The last fact may be explained as an adaptation to similar conditions of fertilization. Both species have effective copulatory organs, and their large delicate sperm-cells must immediately adhere when they come into contact with the shell-less ovum, and penetrate into it by means of amœboid processes. Conversely, the difference between sperm-cells of allied species likeSida crystallina(a),Moina rectirostris(e) andM. paradoxa(d) is related to different adaptations to nearly the same conditions of fertilization. InSida(Fig. 66a) the large flat sperm-cells, with their fringed ends and their large soft surface, adhere easily to the ova, and the same end is attained inMoina rectirostrisby means of stiff radiating processes, while in the nearly related species,Moina paradoxa, the male cell (d) resembles an Australian boomerang and presses in like a wedge between the ova and the wall of the brood-sac.

Fig. 67.Spermatozoa of various animals, after Ballowitz, Kölliker, and vom Rath. 1, man. 2, bat (Vesperugo). 3, pig. 4, rat. 5, bullfinch. 6, newt. 7, skate (Raja). 8, beetle. 9, mole-cricket (Gryllotalpa). 10, freshwater snail (Paludina). 11, sea-urchin. Much magnified.

In Fig. 67 a small selection of animal male cells is figured, all ofwhich take the form of sperm-threads or spermatozoa, and yet they differ very much from one another in detail. It would undoubtedly be of great interest to follow out these minute adaptations of the sperm-cells to the conditions of fertilization, and to demonstrate that their size, and especially their form, in the different species of animals are adjusted to the special constitution of the ovum, its envelope, and its micropyles, and to the ease or difficulty with which it can be reached; but much information must be forthcoming before we can even suggest, for instance, why the sperm-cell of the salamander is so enormously long, large, and pointed at the head, while that of Man (Fig. 67, 1) is comparatively short, with broad, flat head and a recently discovered minute apex; or why those of Man and many fishes (such asCobitis) should be so much alike, and so on. From many sides, however, we are led to conclude that even down to the minutest details nothing is in vain, and that everything depends on adaptation.

In general, even the peculiarities of form already indicate this; thus the spirally coiled structure of the head, which is especially well developed in the spermatozoa of birds (Fig. 67, 5), in those of the skate (7), and of the freshwater snail (Paludina) (10), works like a corkscrew, and makes it possible for the spermatozoon to pierce through the resistant envelope of the ovum. Similarly, the sharply pointed head of the insect spermatozoon (Fig. 67, 8 & 9) seems adapted for slipping through the minute pre-formed micropyle in the hard egg-shell.

Of the detailed and complicated structure of spermatozoa we have only recently been made aware through the increasing perfection of the microscope and of technical methods of investigation. Fig. 68 shows one after a diagrammatic figure by Wilson. We see the apical point (sp) for boring into the ovum, the nucleus (n) surrounded by a thin layer of protoplasm, which together form the head, then the middle portion (m) which contains the 'centrosome' (c), and the 'tail' or 'flagellum' which effects the movement of the whole and which itself possesses a complex structure with an 'axial filament' (ax) and an enveloping layer, the latter often drawn out into a spirally twisted, undulating membrane of the most extreme delicacy, as is most clearly seen in the newt (Fig. 67, 6).

Fig. 68.Diagram of aspermatozoon, after Wilson.sp, apical point.n, nucleus.c, centrospere.m, middle piece.ax, axial filament.e, terminal filament.

Not in the Daphnids alone, but in other groups of Crustaceans aswell, sperm-cells of quite peculiar form occur, as, for instance, in the crayfish and its marine relatives, the crabs and the long-tailed Decapods. In these cases the spermatozoa bear long and stiff thorn-like processes, which, as in the sperm-cells ofMoina, make them adhesive, and, according to Brandes, render it possible for them to cling among the bristles on the abdomen of the female until one of the many eggs leaving the oviduct comes within reach. For among these Crustacea there is no true copulation, but the masses of sperm-cells are packed together into sperm-packets or 'spermatophores,' and are affixed by the male near the opening of the oviduct, where they burst and pour forth their contents between the appendages of the female.

All these remarkable and widely divergent structures and arrangements depend not upon chance or on the fantastic expression of a 'formative power,' as an earlier generation was wont to phrase it; they are undoubtedly without exception adaptations to the most intimate conditions of fertilization in each individual case. I lay particular stress upon a recognition of this, because it permits us to infer with certainty that even the variations of the single cell, if they are sufficiently important for the species, may be controlled by natural selection. It is obvious that the adaptations of the sex-cells must depend not on histonal selection, but only upon personal selection, since it is indifferent for the individual sperm-cells whether fertilization is accomplished successfully or not, while it is by no means indifferent for the species. The organism dies without descendants if its sperm-cells do not fertilize, and the carrying on of the species must be left to those of its fellows which produced sperm-cells which fertilize with more certainty; thus it is not the sperm-cells themselves, but the individual organisms which are selected, and that in relation to the quality of the sex-cells they produce.

In contrast with the great diversity of form exhibited by the spermatozoa, the differentiation of the ovum appears very uniform, at least in regard to form and activity. The main form is spherical, but it is subject to many variations in the way of elongation or flattening. In the lower forms of life, as, for instance, among the sponges, and also in the polyps and Medusæ the egg-cells possess, until they are mature, the locomotor capacity of unicellular organisms; they creep about after the manner of amœbæ, and indeed, as I showed years ago, this movement from place to place in many polyps is exactly regulated; thus at a definite time they may leave the place where they originated and may, for instance, creep from the outer layer of cells (ectoderm) of the animal into the inner layer (endoderm) by boring through the so-called 'supporting lamella,' then they may creep further in theendoderm, and finally return to quite definite and often remote spots in the ectoderm (Eudendrium,Fig. 95). In another hydroid polyp (Corydendrium parasiticum) the mature egg-cells leave their former position within the endoderm and creep entirely outside of the animal which produced them, establishing themselves in a definite spot on its external surface, where they await the fertilizing zoosperms. Many ova can accomplish slight amœboid movements, but in most animals these do not suffice for movement from place to place, and the ova remain quietly in the spot where they were developed, or are passively pushed to another. Cases such as that of the polyp I have cited, in which the ovum actually comes to meet the male element, are quite exceptional, for in general the ovum is the passive and the spermatozoon the active or exploring element in fertilization. The female cell is entrusted with procuring and storing the material necessary to the building up of the embryo; and its peculiarities depend chiefly on this.

Fig. 69.Ovum of the Sea-urchin,Toxopneustes lividus,after Wilson.zk, cell-body.k, nucleus or so-called'germinal vesicle,'n, nucleolus or so-called 'germinalspot.' Below there is a spermatozoon of thesame animal, with the same magnification (750times).

It is true that in plants this stored material is seldom considerable, and that is because the ovum so frequently remains even after fertilization within the living tissues of the plant, and is thence supplied, often very abundantly, with food-stuffs; and, moreover, because the young plant that springs from the fertilized ovum maybe very small and simple, and yet capable of immediately procuring its own nourishment. But there are exceptions to this; thus the ova of the brown sea-wracks, or Fucaceæ, for instance, are quite twenty times as large as the ordinary cells of the algæ (Fig. 64), and contain a quantity of food-stuff within themselves. In this case the ova are liberated into the water even before fertilization, and the nutrition of the embryo from the mother-plant is excluded.

In these Algæ we meet, for the first time, with a special organ in which the ova arise. In animals this is much more generally the case, and from sponges upwards there are always quite definite parts and tissues of the body which are alone able to develop eggs, and these are usually well-defined organs of special structure, the ovaries. Similarly, in male animals the spermatozoa arise in special places, and usually in special organs, the spermaries or testes.

Animal ova often consist of more than the simple cell-body, the protoplasm and its nucleus; they almost always contain in the cell-body a so-called 'Deutoplasm,' as Van Beneden has fittingly named the yolk-substance. This consists of fats, carbohydrates, or albuminoids, which often lie in the cell-body in the form of spherules, flakes, or grains—a nutritive material that is often surrounded and enclosed by a small quantity of living matter or formative protoplasm. Apart from these stores of yolk it would be impossible for a young animal to develop from the ovum of a snake or a bird, for such highly differentiated animals could not be formed from an egg of microscopic dimensions if this remained without some supply of food from outside of itself during the period of development. There is obviously need for a considerable amount of building material, so that all the organs and parts, which are composed of thousands and millions of cells, may be developed.

Thus the size of the animal-ovum depends essentially on the quantity of yolk that has to be supplied to the egg, and this depends in the main on whether the egg is still drawing nourishment from the mother during the development of the young animal. Therefore, as a general rule, eggs which are laid, and are surrounded and protected by a shell, are usually much larger than the eggs of animals which go through their development within the body of the mother. The best known illustration of this proposition is offered by mammals and birds, animals of equally high organization and comparable in bodily size. While the eggs of birds may be as much as 15 centimetres in length, and may weigh 1½ kilogrammes, those of most mammals remain microscopically minute, and scarcely exceed a length of 0.3 millimetres. The same principle is often illustrated within one and the same small group of animals, and even in the same species. Here, again, the Daphnids or water-fleas may serve as an example.

Among these there are two kinds of eggs, summer and winter eggs, of which the former go through their development into a young animal within a brood-cavity on the back of the female, while the others are liberated into the water, and are surrounded by a hard shell. The summer eggs receive more or less nourishment from the mother bythe extravasation of the nutritive constituents of the blood into the brood-cavity, and they thus require a smaller provision of yolk than the winter eggs, which are thrown entirely upon their own resources. Accordingly we find that in all Daphnids the summer eggs are at least a little smaller and have less yolk than the winter eggs, as in the genusDaphnella(Fig. 70,AandB), while in some species, e.g. ofBythotrephes, this difference increases so much that the summer eggs are almost without yolk, and therefore very minute (Fig. 71,B). The reason of this lies in the fact that in this case the brood-sac is filled with a nutritive fluid rich in albuminoid substances, so that the embryo during its development is continually supplied with concentrated nourishment. This is not the case with the winter eggs, because these are liberated into the water, and we therefore find that they are of enormous size and quite filled with yolk (Fig. 71,A).

Fig. 70.Daphnella.A, summeregg.B, winter egg.Oe,'oil-globules' of the summeregg.

Fig. 71.Bythotrephes longimanus.A, the brood-sac(Br) of the female containing two winter-ova(Wei), on which five large sperm-cells (sp) are lying.R, dorsal surface of the animal.Dr, glandular layerwhich secretes the shell-substance.BK, copulatorycanal.B, the brood-sac (Br) containing two summer-ova(Sei). Both figures under the same magnification(100).

In this instance, as in all the simpler eggs, the yolk constituents are secretions of the cell-body of the ovum; but nature employs many devices, if I may so speak, to bring up the mass of the egg, and especially of the yolk, to the highest attainable point. Thus in many orders of Crustaceans, for instance in the water-fleas just mentioned, there are special egg-nourishing cells, that is, young ovum-cells whichdo not differ from the rest either in origin or in appearance, only they do not become mature eggs, but at a definite time cease to make progress, and then slowly break up, so that their substance may be absorbed as food by the true ova. Thus there is a much greater and at the same time more rapid growth than could be attained through nourishment from the blood alone. In the Daphnids the ovaries consist of groups of four cells each, only one of which becomes an ovum (Fig. 72,Ei), while the other three (1, 2, and 4) form nutritive cells which break up. This is so in all summer eggs; but in the winter eggs a much larger number of nutritive cells may take part in equipping a single ovum, and in the genusMoinaover forty do so. But here the difference in size between the two kinds of eggs is very marked, the winter eggs being twice the diameter of the summer eggs.

Fig. 72.Sida crystallina, a Daphnid: a fragment of the ovary showing one of the groups of four cells, of which 1, 2, and 4 are nutritive cells, and only 3 becomes an ovum. Magnified 300 times.

In many insects also, e.g. in beetles and bees, similar nutritive cells occur, but there is in these forms a different arrangement which serves at the same time for the formation of the shell, and the supplying to the ovum of the necessary yolk-stuffs—the ovum is surrounded with a dense layer of epithelial cells, a so-called 'follicle.' In mammals and birds also these 'follicle cells' certainly play an important part in the nutrition of the ovum, though it is not yet quite clear how they act—whether they produce within themselves grains of yolk and other nutritive substances and convey these to the ovum by means of fine radiating processes, or whether they themselves ultimately migrate into the ovum and there break up. In any case it is worthy of note that all these follicular cells in insects and vertebrates have the same origin as the egg-cells, that is, they are modified germ-cells. The case is therefore essentially the same as in the nutritive cells of the Daphnids; nature sacrifices the greater number of the germ-cells in order to be able to provide more abundantly for the minority. She thus succeeds in raising the eggbeyond itself, so to speak, and provides the means for a growth which could obviously not be attained by means of the ordinary nourishment supplied by the blood.

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