FERTILIZATION OF THE OVUM "It is conceivable, and indeed probable, that every part of the adult contains molecules derived both from the male and from the female parent; and that, regarded as a mass of molecules, the entire organism may be compared to a web of which the warp is derived from the female and the woof from the male." HUXLEY.' IN mitototic cell-division we have become acquainted with the means by which, in all higher forms at least, not only the continuity of life, but also the maintenance of the species, is effected ; for through this beautiful mechanism the cell hands on to its descendants an exact duplicate of the idioplasm by which its own organization is determined. As far as we can see from an a priori point of view there is no reason why, barring accident, cell-division should not follow cell-division in endless succession in the stream of life. It is possible, indeed probable, that such may be the fact in some of the lower and simpler forms of life where no form of sexual reproduction is known to occur. In the vast majority of living forms, however, the series of cell-divisions tends to run in cycles in each of which the energy of division gradually comes to an end and is only restored by an admixture of living matter derived from another cell. This operation, known as fertilization or fecundation, is the essence of sexual reproduction ; and in it we behold a process by which on the one hand the energy of division is restored, and by which on the other hand two independent lines of descent are blended into one. Why this dual process should take place we are as yet unable to say, nor do we know which of its two elements is to be regarded as the primary and essential one. According to the older and more familiar "dynamic " hypothesis, brought forward by Butschli ('76) and Minot ('77, '79) and afterwards supported by such investigators as Engelmann, Hensen, Hertwig, and Maupas, the essential end of sexuality is rejuvenescence, i.e. the restoration of the growth-energy and the inauguration of a new cycle of cell-division. Maupas's celebrated experiments on the conjugation of Infusoria, although not yet adequately confirmed, have yielded very strong evidence that in these unicellular animals, even under normal conditions, the processes of growth and division sooner or later come to an end, undergoing a process of natural " senescence," which can only be counteracted by conjugation. That conjugation or fertilization actually has such a dynamic effect is disputed by no one. What is not determined is whether this is the primary motive for the process — i.e. whether ' the need of fertilization is a primary attribute of living matter — or whether it has been secondarily acquired in order to ensure a mixture of germ-plasms derived from different sources. The latter view has been urged with great force by Weismann, who rejects the rejuvenescence theory in toto and considers the essential end of fertilization to be a mixture of germ-plasms (" Amphimixis ") as a means for the production, or rather multiplication, of variations which form the material on which selection operates. On the other hand, Hatschek ('87, i) sees in fertilization exactly the converse function of checking variations and holding the species true to the specific type. The present state of knowledge does not, I believe, allow of a decision between these diverse views, and the admission must be made that the essential nature of sexual reproduction must remain undetermined until the subject shall have been far more thoroughly investigated, especially in the unicellular forms, where the key to the ultimate problem is undoubtedly to be sought.