110 THE STORY OF THE LIVING MACHINE. 



mri). The remains of the egg nucleus, after hav- 

 ing discharged the polar cells, form \X\zfemale nu- 

 cleus (Fig. 40, fn). The chromatin material, in 

 both the male and female pronucleus, soon breaks 

 up into a network in which it is no longer possible 

 to see that each contains two chromosomes (Fig. 

 41). Now the centrosome, which is beside the 

 male pronucleus, shows signs of activity. It be- 

 comes surrounded by prominent rays to form an 

 aster (Fig. 41. ce\ and then it begins to move 

 toward the female pronucleus, apparently drag- 

 ging the male pronucleus after it. In this way the 

 centrosome approaches the female pronucleus, and 

 thus finally the two nucleii are brought into close 

 proximity. Meantime the chromatin material in 

 each has once more broken up into short threads 

 or chromosomes, and once more we find that each 

 of the nucleii contains two of these bodies (Fig. 

 42). In the subsequent figures the chromosomes 

 of the male nucleus are lightly shaded, while those 

 of the female are black in order to distinguish 

 them. As these two nucleii finally come together 

 their membranes disappear, and the chromatic ma- 

 terial comes to lie freely in the egg, the male and 

 female chromosomes, side by side, but distinct 

 forming the segmentation nucleus. The egg plainly 

 now contains once more the number of chromo- 

 somes normal for the cells of the animal, but 

 half of them have been derived from each par- 

 ent. It is very suggestive to find further that 

 the chromosomes in this fertilized egg do not fuse 

 with each other, but remain quite distinct, so that 

 it can be seen that the new nucleus contains chro- 

 mosomes derived from each parent (Fig. 42). Nor 

 does there appear to be, in the future history of this 

 egg, any actual fusion of the chromatic material, 



