THE GERM-PLASM THEORY 377 



of the differences between two daughter-cells — on differences which 

 proceed from within and are definitely pre-established. Here, again, 

 the facts do not justify us in making a dogma of the proposition that 

 it is a ' fundamental power ' of every living being to maintain its 

 species by producing replicas of itself. If we look at two directly 

 successive cell-generations, we can hardly, it is true, in most cases, 

 perceive any difference between them, just as in the generations 

 of species ; but if we compare the end of a long cell-lineage with the 

 beginning, then the difference is marked, and we recognize that the 

 difference is due to a gradual summing up of minute, invisible 

 deviations. In my opinion these steps of difference cannot possibly 

 depend merely on direct external influences ; they proceed rather from 

 the hereditary substance the cell receives from the ovum, which, 

 therefore, in order to attain to such many-sided and far-reaching 

 differentiation, must have undergone a frequently repeated splitting 

 up of its qualities. That this splitting is not merely a variation 

 to which the whole of the hereditary substance of the daughter-cells 

 is uniformly subject, according to the influences dependent on their 

 position in relation to other cells of the embryo, will be made clear 

 from the case of the Ctenophora referred to in the next lecture. 

 A scarcely less striking example is that of those animals in which 

 the ova contain the primary constituents for only ,one sex, in which, 

 in other words, there are ' male ' ova and ' female ' ova. This is the 

 case, for instance, among Rotifers, and in plant-lice such as the vine- 

 pest. Phylloxera. Here the eggs from which males develop are 

 smaller than those which produce females. The primary constituents 

 for both male and female are not, as in most animals, contained in the 

 same ovum, to be liberated on one side or the other by influences 

 unknown to us, but in each ovum there is only one of the two 

 sets of primary constituents present, and in this case, therefore, the 

 development of hermaphrodites, which not infrequently occur in 

 other animals, would be impossible. But all these ova have been 

 produced by one primitive reproductive cell, and consequently, at one 

 of the divisions implied in the multiplication of this first cell, a sepa- 

 ration of the male from the female primary constituents must have 

 taken place, that is, a differential division of hereditary substance, for 

 which no external and no intercellular influences can possibly account. 

 If there is, then, a differential division of the ids and with them 

 of the whole idioplasm, the germ-plasm of the fertilized ovum must 

 be broken up in the course of ontogeny into ever smaller groups 

 of determinants. I conceive of this as happening in the following 

 manner. 



