144 HEREDITY AND ENVIRONMENT 



ical grounds that such a reduction must 

 occur, 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 Weismann's is now an estab- 

 lished 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 (Figs. 22, 32 F), by the con- 

 densation of the nucleus into the sperm head 

 and the outgrowth of the centrosome and cyto- 

 plasm 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 (Figs. 22, 

 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. 



