THE CELL AND PROTOPLASM. 129 



form the female nucleus (Fig. 40, fri). The chro- 

 matin 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 becomes surrounded 

 by prominent rays to form an aster (Fig. 41, ce), 

 and then it begins to move toward the female 

 pronucleus, apparently dragging the male pro- 

 nucleus 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 dis- 

 tinguish them. As these two nucleii finally 

 come together their membranes disappear, and 

 the chromatic material 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 chromosomes normal for the 

 cells of the animal, but half of them have been 

 derived from each parent. 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 chromosomes 

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

 I 



