354 CLASSICS OF MODERN SCIENCE 



What can be the explanation of the fact that nuclear division fol- 

 lows immediately after fertilization, but that without fertiUzation it 

 does not occur in most cases ? There is only one possible explanation, 

 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 explanation, even though 

 the nature of this difference is entirely unknown, because polar re- 

 pulsion 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 pres- 

 ent. 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 quantitative deficiency exists in an tgg which, after the expulsion 

 of the ovogenetic nucleoplasm in the polar bodies, requires fertiliza- 

 tion 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 megalocephala the nuclear substance of the female pronucleus 

 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 formation of four loops, — 

 it follows that an ^ggy which can only form two or three loops from 

 its nuclear reticulum, would not be able to develop parthenogenetically, 

 and that not even the first division would take place. If we further 



