236 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



cause of division ; and Roux ^ may be right in referring the 

 opposition to electrical forces. However this may be, it is per- 

 fectly certain that the development of this opposition is based 

 upon internal conditions arising during growth in the nucleus 

 itself. The quantity of nuclear thread cannot by itself determine 

 whether the nucleus can or cannot enter upon division ; if so, 

 it would be impossible for two divisions to follow each other in 

 rapid succession, as is actually the case in the separation of 

 the two polar bodies, and also in their subsequent division. In 

 addition to the effects of quantity, the internal conditions of the 

 nucleus must also play an important part in these phenomena. 

 Quantity alone does not necessarily produce nuclear division, 

 or the nucleus of the ^%% would divide long before maturation 

 is complete, for it contains much more nucleoplasm than the 

 female pronucleus, which remains in the ^gg after the expul- 

 sion of the polar bodies, and which is in most cases incapable 

 of further division. But the fact that segmentation begins im- 

 mediately after the conjugation of male and female pronuclei, 

 also shows that quantity is an essential requisite. The effect of 

 fertilization has been represented as analogous to that of the 

 spark which kindles the gunpowder. In the latter case an 

 explosion ensues, in the former segmentation begins. Even 

 now, many authorities are inclined to refer the polar repulsion 

 manifested in the nuclear division which immediately follows 

 fertilization, to the antagonism between male and female ele- 

 ments. But, according to the important discoveries of Flemming 

 and van Beneden, the polar repulsion in each nuclear division 

 is not based on the antagonism between male and female loops, 

 but depends upon the antagonism and mutual repulsion between 

 the two halves of the same loop. The loops of the father and 

 those of the mother remain together and divide together through- 

 out the whole ontogeny. 



What can be the explanation of the fact that nuclear division 

 follows immediately after fertilization, but that without fertiliza- 

 tion 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 



^ W. Roux, 'Ueber die Bedeutung der Kerntheilungsfiguren,' Leipzig, 

 1883. 



