CELL DIVISION AND GAMETOGENESIS 



229 



Comparing oogenesis with spermatogenesis, the main differ- 

 ences are: (1) a more intensive growth period in oogenesis; (2) a 

 larger total number of spermatozoa than of ova; and (3) the 

 completion of oogenesis at the ootid stage. 



Chromosomes in Gametogenesis. — What is perhaps the most 

 striking feature of gametogenesis, viz., the behavior of the 

 chromosomes, may now be considered. Assuming that the 



Fig. 141. — Four stages in the formation of the first polar body in the egg of 

 Planocera inquilina. d\ sperm nucleus. (From Patterson and Wieman.) 



zygotic number of chromosomes of the fertilized egg is 8, both 

 primordial germ cells and somatic cells exhibit 8 chromosomes 

 at each mitotic division. So far as the somatic cells are concerned 

 this condition prevails in all subsequent mitotic divisions. It 

 also holds for the division period of gametogenesis, as indicated 

 in the diagram (Fig. 139). During the growth period of the 

 nucleus of the developing oocyte or spermatocyte, the chromo- 

 somes lose their identity in the resting nucleus. At the end 

 of the prophases of the first maturation divisions, however, 



