Part 34

These processes of 'maturation of the ovum' are common to all animal ova which require fertilization, and they follow almost the same course, only that in many cases the second division of the first polar body does not take place, so that only two polar bodies in all are formed. All these processes have nothing directly to do with fertilization, but it is only through them that the ovum becomes capable of fertilization. This does not prevent the spermatozoon from previously making its way into the ovum, for this is usually the case (Fig. 75, _A_, _sp_); there it waits until the second 'directive division' of the ovum has been accomplished, utilizing the time to become transformed in the manner necessary for the conjugation of the two nuclei. Only in a few species, for example in the sea-urchin, does the egg complete its polar divisions within the ovary, therefore before it has come into contact with the sperm at all.

That we may be able to penetrate still more deeply into the processes of fertilization, the best illustration to take seems to me to be, as yet, the ovum of the thread-worm of the horse (_Ascaris_ _megalocephala_), which has become famous through the classical observations of Ed. van Beneden. Many favourable circumstances unite in this case to make the essentials of the process clearly recognizable. Fertilization takes place within the body of the female, in an enlarged portion of the oviduct, within which a number of the remarkable sperm-cells are always found in a mature female. They are remarkable in being not thread-like, but rather spheroidal cells, bearing, however, a small protuberance something like a pointed horn (Fig. 75, _A_, _sp_). When such a sperm-cell comes in contact with the upper surface of an ovum a swelling forms at the place touched, and the sperm-cell attaches itself firmly to this, and is drawn by it into the ovum. Without doubt, amœboid movements on the part of the sperm-cell itself play some part in this, as can be most plainly seen in the large sperm-cells of many Daphnids which we have already discussed. In the egg of the thread-worm the whole sperm-cell with its nucleus can soon be detected within the substance of the ovum, and it then changes rapidly. Its main body fades more and more completely, until at last it disappears altogether, while the nucleus becomes vesicle-like and soon attains a considerable size (Fig. 75, _B_, _spk_). Meanwhile the residue of the germinal vesicle which remained behind in the ovum after the second directive division (_B_, _Eik_) has changed into a large vesicle-like nucleus (_C_, ♀ _k_), which in the ovum of _Ascaris_, as well as in the spermatozoon, at first contains a nuclear reticulum with irregular fragments of chromatin. Later on, these form a spiral coil in the manner we have already described, and finally this breaks up into two large and relatively thick angular loops or chromosomes (Fig. 75, _C_ and _D_, _chr_).

At the same time a nuclear division apparatus has formed in the space between the two nuclei--the so-called male and female 'pronuclei' (♂ _k_, ♀ _k_)--two centrospheres (_csph_) become visible, at first lying close together, but afterwards moving apart (_D_) to form the poles of a nuclear spindle, in the equatorial plane of which the four chromosomes of the male and female pronuclei are now arranged. The nuclear membranes disappear, and the two nuclei now unite to form one, the segmentation nucleus (_D_). A dividing spindle then develops and brings about the first embryonic cell-division (_E_), and thus the beginning of the 'segmentation' of the ovum; each of the four chromatin loops splits longitudinally, and each of the split halves migrates, one to one, the other to the other daughter-nucleus (_F_). As this same method of distribution of the chromatin substance is repeated at every successive cell-division throughout embryogenesis, and indeed through the whole of development, it follows that the result of fertilization is, that all the cells of the body of the new animal which develops from the ovum contain an equal quantity of paternal and of maternal chromatin. If we are right in regarding the chromatin substance as the hereditary substance, it becomes immediately apparent that this equal division is of the most far-reaching importance, for it shows us that the so-called process of fertilization is the union of equal quantities of hereditary substance of paternal and maternal origin.

The process of fertilization is now known in all its details in a great number of animals in the most diverse groups; it is everywhere the same in its essential features; there is always only one sperm-cell which normally enters into conjugation with the ovum-nucleus, and in every case the sperm-cell, however minute it may be to begin with, forms a nucleus nearly or exactly as large as the nucleus of the ovum, and in all cases it contains the same number of chromosomes as the nucleus of the ovum. Of special interest, however, is the fact that this number is always half the number of the chromosomes exhibited by the somatic cells of the particular animal in question, and that the reduction of the number of chromosomes to half the normal is effected in both male and female germ-cells by the last divisions of these cells, which take place before they have attained to a state of maturity. In the ovum the reduction occurs in the directive divisions, to which we must therefore turn our attention once more, with special reference to the number of chromosomes.

We saw that, in the full-grown ovarian egg, the germinal vesicle rises to the surface and there becomes transformed into the first polar spindle. Now this shows, in its equatorial plane, double the number of chromosomes normal to the species. This duplication comes about, not directly before the nuclear division, but much earlier in the young mother-egg-cell; it is only the change in the time of the splitting of the chromosomes that is unusual. The first maturation division takes place nevertheless in accordance with the usual plan of nuclear division; it is, as I have called it, an 'equation division,' that is, both daughter-nuclei again receive the same number of chromosomes as the young mother-egg-cell had to start with, namely, the normal number of the species. Thus, if the young mother-egg-cell had four chromosomes (Fig. 76, _A_), this number would double to eight at an early stage (_B_), but the first maturing division would give each daughter-nucleus four (_C_ and _D_). In the second maturation division the case is different, for here no splitting and duplicating of the number of chromosomes takes place, but the existing number, by being distributed between the two daughter-nuclei, is reduced to half in each (_E_ and _F_). For this reason I have called it a 'reducing division.' In our example, therefore, the ovum, as well as the second polar body, would contain only two chromosomes (Fig. 76, _F_).

I cannot enter into the details of the process here, for we are dealing with essentials and not with isolated and, so to speak, chance details, but I must emphasize the fact that the same process of reduction of the number of chromosomes takes place in this or an analogous manner in all animal ova, and can be demonstrated also in most of the chief groups of plants. Whether it be, as many have maintained, that the reduction is not always first effected by the 'maturation divisions,' but in some cases takes place earlier in the primitive egg-cell[12], so much is certain, that the nuclei which come together for 'fertilization' only contain half the normal number of chromosomes, and this is true not only of the ovum but also of the sperm-nucleus.

[12] See the discussion of this point in chapter xxii.

Arguing from general considerations, but especially from the theory which regards the chromosomes as the bearers of the hereditary substance, I had come to the conclusion, before there was any full knowledge of the phenomena of the maturation of the ovum, that a reduction of the chromosomes by half _must_ take place, and had postulated a similar 'reducing division' for the sperm-cell, and further, for plants as well as animals--indeed, for all sexually reproducing forms of life. The two divisions in the sperm-cell corresponding to the polar divisions of the ovum with their reduction of chromosomes were demonstrated by Oscar Hertwig in the case of the thread-worm of the horse (_Ascaris megalocephala_)--a form which has proved so very important in relation to the whole theory of fertilization. It is true that in this case the course of the phenomena of reduction is less convincing than in some other forms which have been investigated more recently, as, for instance, the mole-cricket and the bugs. In these instances, at any rate, a 'reducing division' in spermatogenesis, quite corresponding to that of the egg-cell, has been demonstrated, and this demonstration is of particular value owing to the fact that the development of the sperm-cell, as we shall presently see, throws an entirely new light on that of the ovum, and especially on the phyletic significance of the polar bodies.

We began our consideration of the processes of reduction with the full-grown egg-cell, but now let us go back to the earliest rudiments of the ovary of the embryo, and we find that it consists of a single primitive egg-cell, from which, by division, all the other egg-cells arise. In the same way the first rudiment in the testis or spermary is formed by a primitive sperm-cell, which does not differ visibly from the primitive egg-cell. Both now multiply by division for a considerable time, and in the ovary this is followed by a period of growth, during which multiplication ceases, and each cell increases considerably in size and lays in a store of yolk. Each cell thus ultimately reaches the condition with which we started previously, that of the full-grown _mother-egg-cell_.

Although the primitive sperm-cells do not exhibit such pronounced growth as the ova, they have likewise their period of growth, during which multiplication by division ceases, and the cells increase only in size (Fig. 77, _A_). When they have attained their maximum of growth the number of chromosomes is seen to have been doubled by longitudinal splitting (as in the diagram, Fig. 77, _B_, from four to eight). From this _mother-sperm-cell_ there now arise by two divisions in rapid succession (_C_-_F_) four sperm-cells, and the same reduction of the number of chromosomes to half is effected as in the polar divisions of the egg-cell. In the first division, four chromosomes go to each daughter-cell (_D_), in the second, two (_F_). The only essential difference between the corresponding processes in the egg-cell and the sperm-cell lies in the fact that the divisions of the so-called 'spermatocyte' or mother-sperm-cell are equal, so that four granddaughter-cells of equal size arise, while in the mother-egg-cell or 'ovocyte' the divisions are very unequal. In the former the result of the divisions is _four_ cells capable of fertilizing, in the latter _one_ cell capable of being fertilized and three minute 'polar cells' which are incapable of conjugating with a sperm-cell and giving rise to a new individual.

There can thus be no doubt that the polar cells, as Mark and Bütschli long ago suggested, are abortive ova, that is, that, at a remote period in the evolution of animal life, each of these four descendants of a mother-egg-cell became a germ-cell capable of development. It is not difficult to infer that the unequal division, which now leads to an insufficient size in three of these descendants, has gone on _pari passu_ with the continually increasing size of the mature ovum, and had its reason in the fact that it was important above all things to store in the ovum as much protoplasm and yolk as possible. We have already seen that even the dissolution of a number of the sister-cells of the ovum is sometimes demanded, so that the ovum may be surrounded by nutritive follicular cells. In short, the greatest possible quantity of nourishment is conveyed to the ovum in every conceivable way, and it is thus stimulated to a growth which no single cell could attain to if it were dependent on the ordinary nutrition supplied by the blood. And we can understand that nature--to speak metaphorically--did not wish to destroy her own work by finally distributing among four ova all the nourishment she had succeeded in heaping up in all sorts of ways within the mother-egg-cell.

But it may be asked, Why have all these unnecessary divisions been maintained up till the present day? Why have they not long ago been given up, since they can and do only lead to the production of three abortive ova, which are foredoomed to perish? Are they mere vestiges, processes which are in themselves meaningless, but have, so to speak, been maintained by the principle of inertia? This principle is certainly operative in some sense and to some extent even in living nature; a process which has been regularly repeated through a long series of generations does not at once cease to be performed when it is no longer of use to the organism concerned. The eyes of animals which have migrated to lightless depths do not disappear all at once and leave no trace; they degenerate very gradually and only in the course of many generations; and it would thus be quite possible to defend the position that these polar or 'maturation divisions' of the ovum are purely _phyletic reminiscences_ without actual significance.

But I cannot agree with this opinion. If it were actually so we should expect that the formation of the polar bodies would not still take place in all cases in almost the same manner, for all rudimentary parts and processes vary greatly; we should expect that in many animal groups the polar divisions would not occur, or perhaps that only half the number would occur. But this is not so; in all multicellular organisms, from the lowest to the highest, two reducing divisions take place, and always in almost the same manner, with the exception of a single category of ova, of which I shall presently have to speak. We shall see later that even in unicellular organisms analogous processes may be observed.

But it is also intelligible that this twice repeated division of the mother-egg-cell is necessary if the reduction in the number of chromosomes to half is only possible in this way, since _this reduction is indispensable_. If each of the two conjugating germ-cells contained the full normal number of chromosomes, the segmentation-nucleus would contain a double number, and if that went on, the number of chromosomes would increase in arithmetical proportion from generation to generation, and would soon become enormous. Even though we were not otherwise certain that these chromosomes are units of a permanent nature, which only apparently break up in the nuclear reticulum, but in reality persist, the fact of reduction would point in this direction. For if they were not permanent structures and distinct from one another, and if their number depended solely on the quantity of chromatin which the nucleus contains, the reduction in number might be secured if the chromosomes in the growing egg and sperm-cells increased in size more slowly than the cell-body and the other parts of the cell. But from the fact that the reduction takes place not in this simple way, but, in sperm-cells and in ova which require to be fertilized, only through cell-division and a specific mode of nuclear division, we may conclude that it cannot happen otherwise, that chromosomes are not mere aggregates of organic substance, but organs whose number can only be reduced by the extrusion of some of them from the cell.

It is true that there are ova in which the process of reduction does not follow the course we have described, but the exceptions only serve to confirm our view of the reducing significance of the polar divisions, and of their persistence because of the necessity for reduction.

As far back as the middle of the nineteenth century it was known that in various animals the eggs develop without fertilization. This reproduction by 'parthenogenesis' was first established with certainty by the German bee-keeper Dzierzon in 1845, and then scientifically corroborated by Rudolph Leuckart and C. Th. von Siebold. Although parthenogenesis was at first observed only in a few groups of the animal kingdom, in bees and some nocturnal Lepidoptera (Psychidæ and Tineidæ), it has become more and more apparent in the course of years that this 'virgin reproduction' is by no means a rare form of reproduction, and that it occurs regularly and normally in many cases, especially in the very diverse groups of the great series of Arthropoda. Thus among insects it is found in certain saw-flies, gall-flies, ichneumon-flies, in the honey bee, and in common wasps, and it is particularly widespread among plant-lice (Aphides) such as the vine-aphis (_Phylloxera_), whose prodigious multiplication in a short time depends partly on the fact that all the generations, with the exception of one, consist only of females with a parthenogenetic mode of reproduction.

Among the lower Crustaceans also parthenogenesis plays a large rôle, and in many species it even occurs as the sole mode of reproduction, but more often--as is also the case among insects--it occurs alternately with bi-sexual reproduction. For parthenogenesis must not be regarded as asexual reproduction, but rather as _unisexual_, that is, as arising from sexually differentiated individuals (females), and from germ-cells (true ova), but brought about by the agency of individuals of only one sex, the female. These parthenogenetic eggs emancipate themselves, so to speak, from the law that was previously regarded as without exception, that all ova require fertilization to enable them to develop. That this law admits of many exceptions is now universally admitted; thus in the small family of water-fleas (Daphnids) there are even two kinds of eggs, the summer and winter eggs we have already mentioned, which are produced by the same female, and yet the former kind develop without fertilization, while the latter require to be fertilized before they can develop.

It was obviously important to learn the state of affairs in regard to reducing divisions in parthenogenetic ova, to find out whether here also, three, or, in some circumstances, two polar bodies were formed, and whether the second polar division reduced the number of chromosomes to half. If the theory previously advanced as to the importance of the chromatin, and especially of the reducing effect of the second maturing division be correct, we should expect the second division to be wanting in parthenogenetic eggs, since otherwise the number of chromosomes would be reduced to half in each generation, and would thus gradually disappear or sink to one.

Having directed my attention to this problem, I succeeded in establishing for a Daphnid, _Polyphemus_, that the second polar division does not occur, and that only one polar body is formed. Blochmann found the same in the parthenogenetic eggs of plant-lice or Aphides, in which, moreover, the eggs requiring fertilization exhibit, like the winter eggs of Daphnids, two polar divisions. It was thus established that at least those eggs of Aphides and Daphnids which are wholly parthenogenetic retain the full number of chromosomes of their species, as is represented in the diagram, Fig. 78. When parthenogenesis set in the polar divisions were limited to one, and that this could happen justifies us in concluding _a posteriori_ that it could have happened also in the case of ova which required fertilization if that had been necessary or even merely indifferent. The polar divisions are thus not mere 'vestigial' processes; they have an immediate significance, and it lies in the reduction of the number of chromosomes.

But I must make a reservation here; it is not universally true of parthenogenetic eggs that maturation takes place without the second polar division. The first exception was observed in the salt-water crustacean, _Artemia salina_. In this case only one polar body is actually extruded and the number of chromosomes remains normal, as I was able to demonstrate with the small number of ova at my disposal; but according to the investigations of Brauer on more abundant material it appears that, while the second polar division is suppressed in the majority of the ova, and the external extrusion of a second polar body never occurs, the second polar division does nevertheless sometimes take place. The two daughter-nuclei arising from this division unite again immediately afterwards to form a single nucleus, and this now functions as a segmentation nucleus. Of course it again contains the full number of chromosomes, namely, twice 84=168.

In _Artemia_, therefore, the adaptation of the ova to parthenogenetic development is not yet fully established, and the complete abandonment of the second polar division seems to be phyletically striven for, since, although the division still takes place, its effect is neutralized immediately afterwards.