Whichever view, the latter or the theory of the continuity of the germ-plasm, be correct, in either case the male germ-cells of the young animal must contain the same germ-plasm as that which existed in the fertilized maternal egg, that is to say, they must contain all the ancestral germ-plasms of the father and the mother. Here therefore a reduction must occur, for otherwise the number of ancestral germ-plasms would be increased by one half at every fertilization. The egg-cell would furnish 1/2, but the sperm-cell 2/2 of the total quantity of germ-plasm present in the germ-cells of the parents. But there is no reason for believing that the reduction of germ-plasm in the sperm-cell must proceed in precisely the same way as in the egg-cell, viz. by the expulsion of a polar body. On the contrary, the processes of spermatogenesis are so remarkably different from those of ovogenesis that we may expect to find that reduction is also brought about in a different manner.
The egg-cell does not expel the superfluous ancestral germ-plasms until the end of its development, and in a form which induces the destruction of the separated portion. This is certainly remarkable, for germ-plasm is a most important substance, and although it seems to be wasted in the production of enormous quantities of sperm- and egg-cells, such waste is only apparent, and is in reality the means which renders the species capable of existence. It may perhaps be possible to prove that in this case also the waste is only apparent. Such proof would be forthcoming if it could be shown that the means by which reduction is brought about in eggs is advantageous, and therefore also, ceteris paribus, necessary. We see that everywhere, as far as our observation extends, the useful is also the actual, unless indeed it is impossible of attainment or can only be attained by the aid of processes which are injurious to the species. And if it be asked why germ-plasm is wasted in the maturation of egg-cells, the following may perhaps be a satisfactory answer.
Let us suppose that the necessary reduction of the germ-plasm does not take place by the separation of the second polar body, but that it happens during the first division of the first primitive-germ-cell which is found in the embryo, so that the two first egg-cells resulting from this division would already contain only half the number of ancestral germ-plasms from the father and the mother, contained in the fertilized egg-cell. In this case the main object, the reduction of the ancestral germ-plasms, would be gained by a single division, and all the succeeding nuclear divisions, causing the multiplication of these two first germ-cells, might take place by the ordinary form of nuclear division, viz. ‘equal division.’ But perhaps nature not only cares for this one main object alone, but also secures certain secondary advantages at the same time. In the case which we have supposed the egg-cells of the mature ovary would only contain two different combinations of germ-plasm, which we may call combinations A and B. Even if millions of egg-cells were formed, every one of them would contain either A or B, and hence (at least as far as the female pronucleus is concerned) only two kinds of individuals could arise from such eggs—viz. offspring A’ and B’. All the offspring A’ would be as similar to one another as identical twins, and the same would be true of offspring B’.
But if the 100th instead of the 1st embryonic germ-cell entered upon the ‘reducing division,’ a hundred cells would undergo this division at the same time, and thus two hundred different combinations of ancestral germ-plasm would arise, and two hundred different kinds of germ-cells would be found in the mature ovary. A still greater number of different combinations of hereditary tendencies would arise if the ‘reducing division’ occurred still later; but undoubtedly the diversity in the composition of the germ-plasm must be greatest of all when the ‘reducing division’ does not take place during the period in which the germ-cells undergo multiplication, but at the end of the entire course of ovarian development, and separately in each full-grown mature egg ready for embryonic development. In such a case there will be as many different combinations of ancestral germ-plasms as there are eggs, for, as I have shown above, it is hardly conceivable that such a complex body as the nuclear substance of the egg-cell—composed of innumerable different units—would ever divide twice in precisely the same manner. Every egg will therefore contain a somewhat different combination of hereditary tendencies, and thus the offspring which arise from the different germ-cells of the same mother can never be identical. Hence by the late occurrence of the ‘reducing division’ the greatest possible variability in the offspring is secured.
If my interpretation of the second polar body be accepted, it is obvious that the late occurrence of the ‘reducing division’ is proved. At the same time we receive an explanation of the advantage gained by the postponement of the reduction of the germ-plasm until the end of the ovarian development of the egg; because the greatest possible number of individual variations in the offspring are produced in this way.
If I am not mistaken, this argument lends additional support to the idea which I have previously propounded,—that the most important duty of sexual reproduction is to preserve and continually call forth individual variability, the foundation upon which the transformation of species is built[[265]].
But if it be asked whether the postponement of the ‘reducing division’ to the end of the ovarian development of the egg is inconsistent with the preservation of the other half of the dividing nucleus, I should be inclined to reply that a ‘reducing division’ of the mature egg, resulting in the production of two eggs, was probably the phyletic precursor of the present condition. I imagine that the division of the mature egg-cell—although it is now so extremely unequal—was equal in very remote times; but that for reasons of utility, connected with the specialization of the eggs of animals, it gradually became more and more unequal. It is now hardly possible to give in detail the various reasons of utility which have brought about this condition, but it may be assumed that the enormous size attained by many animal egg-cells has been especially potent in producing the change.
A careful consideration of this last point seems to me to be demanded by a comparison of the egg-cells with the male germ-cells. Just as the female germ-cells of animals are distinguished by the attainment of a large size, the male germ-cells are generally remarkable for their minute proportions. In most cases it would be physiologically impossible for a large egg-cell, rich in yolk, to attain double its specific size in order to undergo division into two equal halves and yet to remain of the characteristic size. Even without the additional difficulties imposed by the necessity for such a division, all means—such as cells used as food, or the passage of food from follicular cells into the ovum, etc.—are employed in order to bring the egg-cell to the greatest attainable size. Furthermore, the ‘reducing division’ of the nucleus cannot take place before the egg has attained its full size, because the ovogenetic nucleoplasm still controls the egg-cell, and must be removed before the germ-plasm can regulate its development. By arguments such as these I should attempt to render the whole subject intelligible.
But the case is entirely different with the sperm-cells, which are generally minute: here it is quite conceivable that a ‘reducing division’ of the nuclei may take place by an equal division of the sperm-cells, occurring towards the end of the period of their formation; that is to say, in such a way that both products of division remain sperm-cells, and neither of them perishes like the polar bodies. But the other possibility also demands consideration, viz. that the reducing division may occur at an earlier stage in the development of sperm-cells. At all events, the arguments adduced above, which proved that the consequence would be a want of variability in the egg-cells, would not apply to an equal extent in the case of the male germ-cells. Among the egg-cells it may be very important that each one should have its special individual character, produced by a somewhat different composition of its germ-plasm, inasmuch as a considerable proportion of the eggs frequently developes, although this is never the case with all of them. But the production of sperm-cells is in most animals so enormous that only a very small percentage can be used for fertilization. If, therefore, e. g. ten or a hundred spermatozoa contained germ-plasm with exactly the same composition, so that, as far as the paternal influence is concerned, ten or a hundred identical individuals would result if they were all used in fertilization, such an arrangement would be practically harmless, for only one spermatozoon out of an immense number would be employed for this purpose. From this point of view we might expect that the ‘reducing division’ of the sperm-nucleus would not take place at the end of the development of the sperm-cell, but at some earlier period. There is no necessary reason for the assumption that this division must take place at the end of development, and without some cause natural selection cannot operate. It is, of course, conceivable that the causes of other events may also involve the occurrence of this division at the end of development; but we do not at present know of any such causes. I should not consider the influence of the specific histogenetic nucleoplasm, i.e. the spermatogenetic nucleoplasm, to be such a cause, because the quantitative proportions are very different from those which obtain in the formation of egg-cells, and because it is not inconceivable that the small quantity of true germ-plasm which must be present in the nuclei of the sperm-cells at every stage in their formation might enter upon a ‘reducing division’ with the spermatogenetic nucleoplasm, even when the latter preponderated.
As soon as we can recognize with certainty the forms of nuclear division which are ‘reducing divisions,’ the question will be settled as far as spermatogenesis is concerned. It has been already established that various forms of nuclear division occur at different periods of spermatogenesis. I make this assertion, not only from my own observations, but also from observations which have been made and insisted upon by others. Thus, van Beneden and Julin[[266]] stated in 1884 that direct and karyokinetic nuclear divisions alternate with each other in the spermatogenesis of Ascaris megalocephala. Again, Carnoy[[267]] distinctly states that the different cell-generations in the same testis may not uncommonly exhibit considerable differences as regards karyokinesis. ‘This may go so far that direct and indirect division may proceed simultaneously.’ Platner[[268]], in his excellent paper on karyokinesis in Lepidoptera, also points out that the karyokinesis of the spermatocytes is essentially different from that of the spermatogonia. According to his description, the latter form may be very well interpreted as a ‘reducing division,’ for no equatorial plate is formed, and the chromatin rods (or granules, as they are better called in this case) remain from the first on both sides of the equatorial plane, and finally unite at the opposite poles to form the two daughter-nuclei. Furthermore, if Carnoy has correctly observed, the form of karyokinesis which I have previously interpreted as a ‘reducing division’ occurs in the sperm-mother-cells—a karyokinesis in which the chromatin rods either do not divide longitudinally, or else divide in this way after they have left the equatorial plate and are proceeding towards the poles. Carnoy does not himself attach any special importance to these observations, for he only considers them as proofs that the longitudinal splitting of the loops may occur at various periods in different species—either at the equator, or on the way towards the poles, or even at the poles themselves. We cannot conclude from the author’s statements whether this form of nuclear division only occurs in a single cell-generation during spermatogenesis, as it must do if it really represents a ‘reducing division.’ Until this point is settled, we cannot decide with certainty whether the described form of karyokinesis is to be considered as the ‘reducing division’ for which we are seeking. Fresh investigations, undertaken from these points of view, are necessary in order to settle the question. It would be useless to seek further support for the theory by going into further details, and by critically examining the numerous observations upon spermatogenesis which have now been recorded.