232 THE CONTINUITY OF THE GERM-PLASM AS THE 



it does not occur in most cases ? There is only one possible ex- 

 planation, viz. the fact that the quantity of the nucleus has been 

 suddenly doubled, as the result of conjugation. The difference 

 between the male and female pronuclei cannot serve as an explana- 

 tion, even though the nature of this difference is entirely unknown, 

 because polar repulsion is not developed between the male and 

 female halves of the nucleus, but within each male and each female 

 half. We are thus forced to conclude that increase in the quantity 

 of the nucleus affords an impulse for division, the disposition 

 towards it being already present. It seems to me that this view 

 does not encounter any theoretical difficulties, and that it is an 

 entirely feasible hypothesis to suppose that, besides the internal 

 conditions of the nucleus, its quantitative relation to the cell-body 

 must be taken into especial account. It is imaginable, or perhaps 

 even probable, that the nucleus enters upon division as soon as its 

 idioplasm has attained a certain strength, quite apart from the 

 supposition that certain internal conditions are necessary for this 

 end. As above stated, such conditions may be present, but division 

 may not occur because the right quantitative relation between 

 nucleus and cell-body, or between the different kinds of nuclear 

 idioplasm, has not been established. I imagine that such a quan- 

 titative deficiency exists in an egg, which, after the expulsion of the 

 ovogenetic nucleoplasm in the polar bodies, requires fertilization in 

 order to begin segmentation. The fact that the polar bodies were 

 expelled proves that the quantity of the nucleus was sufficient to 

 cause division, while afterwards it was no longer sufficient to pro- 

 duce such a result. 



This suggestion will be made still clearer by an example. In 

 Ascaris megalocepJiala the nuclear substance of the female pro- 

 nucleus forms two loops, and the male pronucleus does the same ; 

 hence the segmentation nucleus contains four loops, and this is 

 also the case with the first segmentation spheres. If we suppose 

 that in embryonic development, the first nuclear division requires 

 such an amount of nuclear substance as is necessary for the forma- 

 tion of four loops, it follows that an egg, which can only form 

 two or three loops from its nuclear reticulum, would not be able to 

 develope parthenogenetically, and that not even the first division 

 would take place. If we further suppose that, while four loops 

 are sufficient to start nuclear division, these loops must be of a 



