138 HEREDITY AND ENVIRONMENT 



otherwise the number of chromosomes would 

 double in every generation, and he held that 

 this reduction must take place in one of the 

 maturation divisions; this hypothesis of Weis- 

 mann's is now an established fact. 



As the result of these two maturation divi- 

 sions four cells are formed from each cell 

 ( spermatocyte or oocyte) of the growth 

 period. In the spermatogenesis each of these 

 four cells is transformed into a functional 

 spermatozoon (Fig. 32 E), by the condensa- 

 tion of the nucleus into the sperm head and the 

 outgrowth of the centrosome and cytoplasm 

 to form the tail. In the oogenesis only one 

 of these four cells becomes a functional egg 

 while the other three are small rudimentary 

 eggs which are called polar bodies and which 

 take no further part in development (Fig. 23, 

 C-F). The fertilization of the egg usually 

 takes place coincidently with the formation of 

 the polar bodies, and so we come back once 

 more to the stage from which we started, thus 

 completing the life cycle. 



4. Sex Determination. In the formation 

 of the sex cells one can frequently distinguish 



