I now come to inquire whether the expulsion of the second polar body is in reality, as I have already maintained, a reduction in the number of ancestral germ-plasms present in the nucleus of the egg. The view itself is sufficiently obvious, and it would supply an explanation of the meaning of the process which is still greatly wanted; but it will nevertheless be not entirely useless to consider other possible theories.
It would be quite conceivable to suppose that the youngest egg-cells, which multiply by division, may undergo one ‘reducing division’ in addition to the ordinary process. Of course this should occur once only, for if repeated, the number of ancestral idioplasms in the nucleus of the germ-cell would undergo a decrease greater than could be afterwards compensated by the increase due to fertilization. Thus the number of ancestral germ-plasms would continually decrease in the course of generations,—a process which would necessarily end with their complete reduction to a single kind, viz. to the paternal or the maternal germ-plasm. But the occurrence of such a result is disproved by the facts of heredity. Although such an early occurrence of the ‘reducing division’ would offer advantages in that nothing would be lost, for both daughter-nuclei would become eggs, instead of one of them being lost as a polar body, nevertheless I do not believe that it really occurs: weighty reasons can be alleged against it.
Above all, the facts of parthenogenesis are against it. If the number of ancestral germ-plasms received from the parents were reduced to half in the ovary of the young animal, how then could parthenogenetic development ever take place? It is true that we cannot at once assert the impossibility of an early ‘reducing division’ on this account, for as I have shown above, the power to develope parthenogenetically depends upon the quantity of germ-plasm contained in the mature egg; the necessary amount might be produced by growth, quite independently of the number of different kinds of ancestral germ-plasms which form its constituents. The size of a heap of grains may depend upon the number of grains, and not upon the number of different kinds of grains. But in another respect such a supposition would lead to an unthinkable conclusion. In the first place, the number of ancestral germ-plasms in the germ-cells would be diminished by one half in each new generation arising by the parthenogenetic method; thus after ten generations only 1/1024 of the original number of ancestral germ-plasms would be present.
Now, it might be supposed that the ‘reducing division’ of the young egg-cells was lost at the time when the parthenogenetic mode of reproduction was assumed by a species; but this suggestion cannot hold, because there are certain species in which the same eggs can develope either sexually or parthenogenetically (e.g. the bee). It seems to me that such cases distinctly point to the fact that the reduction in the number of ancestral germ-plasms must take place immediately before the commencement of embryonic development, or, in other words, at the time of maturation of the egg. It is only decided at this time whether the egg of the bee is to develope into an embryo by the parthenogenetic or the sexual method; such decision being brought about, as was shown above, by the fact that only one polar body is expelled in the first case, while two are expelled in the second. But if we are obliged to assume that reproduction by means of fertilization, necessarily implies a reduction to one half of the number of ancestral germ-plasms inherited from the parents,—the further conclusion is obvious, that the second division of the egg-nucleus and the expulsion of the second polar body represent such a reduction, and that this second division of the egg-nucleus is unequal in the sense mentioned above, viz. one half of the ancestral germ-plasms remains in the egg-nucleus, the original number being subsequently restored by conjugation with a sperm-nucleus; while the other half is expelled in the polar body and perishes.
I may add that observations, so far as they have extended to such minute processes, do indeed prove that the number of loops is reduced to one half. It has been already mentioned that, according to Carnoy, such reduction occurs in Ascaris megalocephala, but the same author also describes the process of the formation of polar bodies in a large number of other Nematodes[[263]], and his descriptions show that the process occurs in such a way that the number of ancestral germ-plasms must be reduced by half. Sometimes half the number of primary loops pass into the nucleus of the polar body, while the other half remains in the egg. In other cases, as in Ophiostomum mucronatum, the primary nuclear rods divide transversely,—a process which must produce the same effect. It is true that these observations require confirmation, and since, with unfavourable objects, the difficulties of observation are extremely great, there may have been errors of detail; but I do not think that there is any reason for doubting the accuracy of the essential point. And this essential point is the fact that the number of primary loops is divided into half by the formation of the polar body.
But even if we could not admit that such a conclusion is securely founded, it cannot be doubted that the formation of the second polar body reduces to one half the quantity of the nucleus which would have become the segmentation-nucleus in the parthenogenetic development of the egg. This is a simple logical conclusion from the two following facts: first, parthenogenetic eggs expel only one polar body; secondly, there are eggs (such as those of the bee) in which it is absolutely certain that the same half of the nucleus—which is expelled as the second polar body in the egg requiring fertilization—remains in the egg when it is to develope parthenogenetically, and acts as half of the segmentation-nucleus. But this proves that the expelled half of the nucleus must consist of true germ-plasm, and thus a secure foundation is laid for the assumption that the formation of the nucleus of the second polar body must be considered as a ‘reducing division.’
I was long ago convinced that sexual reproduction must be connected with a reduction in the number of ancestral germ-plasms to one half, and that such reduction was repeated in each generation. When, in 1885, I brought forward my theory of the continuity of the germ-plasm, I had long before that time considered whether the formation and expulsion of polar bodies must not be interpreted in this sense. But the two divisions of the egg-nucleus caused me to hesitate. The two divisions did not seem to admit of such an interpretation, for by it the quantity of the nucleus is not divided into halves, but into quarters. But a division of the number of ancestral germ-plasms into quarters would have caused, as was shown above, a continuous decrease, leading to their complete disappearance; and such a conclusion is contradicted by the facts of heredity. For this reason I was led at that time to oppose Strasburger’s view that the expulsion of the polar bodies means a reduction of the quantity of nuclear substance by only half. My objection to such a view was valid when I said that the quantity of idioplasm contained in the egg-nucleus is not, as a matter of fact, reduced to one half, but to one quarter, inasmuch as two successive divisions take place. I may add that I had also considered whether the two successive divisions might not possess an entirely different meaning,—whether one of them led to the removal of ovogenetic nucleoplasm, while the other resulted in a reduction in the number of ancestral germ-plasms. But at that time there were no ascertained facts which supported the supposition of such a difference, and I did not wish to bring forward the idea, even as a suggestion, when there was no secure foundation for it. The morphological aspects of the formation of the first and second polar bodies are so extremely similar that such a supposition might have been considered as a mere effort of the imagination.
Hensen[[264]] also rejected the second part of the supposition that reduction must take place in the number of the hereditary elements of the egg, and that such reduction is caused by the expulsion of polar bodies, because he believed it to be incompatible with the fact, which had just been discovered, that polar bodies are formed by parthenogenetic eggs. He concludes with these words: ‘If this striking fact be confirmed, the hypothesis which assumes that the egg must be divided into half before maturation, is refuted, and there only remains the rather vague explanation that a process of purification must precede the development of the embryo.’ Nevertheless Hensen is the only writer who has hitherto taken into consideration the idea that sexual reproduction causes a regularly occurring ‘diminution in the hereditary elements of the egg.’
III. The Foregoing Considerations Applied To the Male Germ-Cells.
If the result of the previous considerations be correct, and if the number of ancestral germ-plasms contained in the nucleus of the egg-cell destined for fertilization must be reduced by one half, there can be no doubt that a similar reduction must also take place, at some time and by some means, in the germ-plasms of the male germ-cells. This must be so if we are correct in maintaining that the young germ-cells of a new individual contain the same nuclear substance, the same germ-plasm, which was contained in the fertilized egg-cell from which the individual has been developed. The young germ-cells of the offspring must contain this substance if my theory of the continuity of the germ-plasm be well founded, for this theory supposes that, during the development of a fertilized egg, the whole quantity of germ-plasm does not pass through the various stages of ontogenetic development, but that a small part remains unchanged, and at a later period forms the germ-cells of the young organism, after having undergone an increase in quantity. According to this supposition therefore the germ-plasm of the parents must be found unchanged in the germ-cells of the offspring. If this theory were false, if the germ-plasm of the germ-cells were formed anew by the organism, perhaps from Darwin’s ‘gemmules’ which pour into the germ-cells from all sides, it would be impossible to understand why it has not been long ago arranged that each germ-cell should receive only half the number of the ancestral gemmules present in the body of the parent. Hence the expulsion of the second polar body—assuming the validity of my interpretation—is an indirect proof of the soundness of the theory of the continuity of the germ-plasm, when contrasted with the theory of pangenesis. If furthermore, a kind of cyclical development of the idioplasm took place, as supposed by Strasburger, and if its final ontogenetic stage resulted in the re-appearance of the initial condition of the germ-plasm, we should fail to understand how any of the ancestral germ-plasms could be lost during such a course of development.