AN INTRODUCTION TO THE METAZOA 



105 



of the growth period they are termed primary oocytes (Fig. 51, b). 

 The primary oocytes contain only one half the number of chromo- 

 somes characteristic of the somatic cells and oogonia. As in 

 the primary spermatocytes, these chromosomes are bivalent. 



p.b. 2 



FIG. 51. Diagrams illustrating the maturation, fertilization, and cleavage 

 of an egg. The primordial germ cell is represented as possessing four 

 chromosomes. 



