SEX DIFFERENTIATION 



305 



abruptly and finally dies. The death cannot be caused by a lack of chromo- 

 somes, since the egg containing half the normal number will develop beyond 

 gastrulation. Therefore death must be caused by an abnormal combination 

 of the chromosomes of the two species. 



With this discussion of the maturation of the germ cells and the fertiliza- 

 tion of the egg we have completed a cycle of development. The newly 

 fertilized eggs develop with the general characteristics of the species, al- 



9 DIFFERENT COMBINATIONS 



Fig. 194. Maturation of the germ cells and the combinations of chromosomes 

 at fertilization 



