This is in perfect harmony with our whole conception of the impelling forces in the development of the organic world; the ever-increasing functional capacity of the structure arose from the advantage which this afforded in the struggle for existence, in comparison with which the apparent advantage of the endless life of the individual was of no account whatever.
I will not here follow out this idea. I have merely touched on it in order to make clear that the death of individuals in all multicellular organisms gives us no ground for thinking of the unlimited life of the germ-cells as dependent on a special artifice of nature, such as amphimixis is often supposed to be. Let us always remember that there is parthenogenesis, and that there are unicellular germs (spores) which are never fertilized, and that the reproduction of many species of animals and plants occurs in this way without the intervention of amphimixis at all.
Attempts have recently been made to prove that parthenogenesis is a kind of self-fertilization, and these have been based on the observations of Blochmann and Brauer, which showed that in the bee and in the salt-water Crustacean, Artemia salina, the reducing second maturation division of the ovum-nucleus is not suppressed, but is regularly accomplished, and that the two daughter-nuclei which result from this division unite with each other subsequently. I have already noted that these statements do not hold true, at least with regard to the bee. In this case the second maturing division takes place without any subsequent fusion of the two daughter-nuclei. According to the observations of Dr. Petrunkewitsch, which I have already mentioned, and for the exactness of which I can vouch, the second maturation-spindle is unusually long, so that the two daughter-nuclei are pushed very far apart ([Fig. 79], Rsp 2), and only the inner of the two nuclei (K 4) becomes a segmentation nucleus, while the outer undergoes a remarkable fate; it unites with the inner nucleus which results from the division of the first maturation cell (K 2), and from this union the primitive genital cells of the animal appear to arise—an observation the eventual theoretical importance of which can only be estimated later.
Meantime all we can gain from it is a certain mistrust of the interpretation of the processes of maturation in Artemia which have hitherto been given; at least we are tempted to suppose that the copulation of two nuclei which Brauer observed in Artemia may not have led to the formation of the segmentation nucleus there either, but may have had some other significance.
But, even if we leave this point entirely out of account, there remain all the cases of regular parthenogenesis in which this mode of reproduction occurs alone and not in alternation with the sexual mode. In these only one maturing division is undergone, and only one polar body is formed, and thus there can lie no possibility of supposing a self-fertilization of the ovum.
Fig. 79. The two maturation divisions in the
unfertilized (drone-forming) egg of the bee, after
Petrunkewitsch. Rsp 1, first polar-body in division.
K 1 and K 2, the two daughter-nuclei thereof.
Rsp 2, second directive spindle. K 3 and K 4, the
two daughter-nuclei thereof. In the subsequent
stage K 2 and K 3 unite to form the primordial
sex-cell nucleus. Highly magnified.
It is possible that we may yet discover species among unicellular organisms which multiply without limit in the absence of any amphimixis. R. Hertwig has recently observed phenomena in Infusorians which he is inclined to refer to the suppression of an earlier habit of conjugation, and so to a kind of parthenogenesis. But even if it should be shown that amphimixis plays a part regularly and without exception in the life of all unicellular organisms, the facts in regard to multicellular organisms are not affected; and, finally, the process of amphimixis is one which we have not the slightest ground for assuming to be either an awakener or a maintainer of life, and so I return to the most essential part of the whole problem, the meaning of the chromatin structures, the combination of which is the undoubted result of amphimixis. Do they really represent, as we assumed earlier, the hereditary substance, and what do we mean by this term?