226 



GENERAL ZOOLOGY 



cells and, as a corollary, that once the somatic cells have become 

 differentiated as such, they never form germ cells at any sub- 

 sequent period. 



Gametogenesis is the production of gametes, or eggs and 

 spermatozoa, from primordial germ cells. Oogenesis includes all 



Spermatogenesis GAMETOGENESIS Oogenesis 



Primordial Germ Cell 

 (Diploid chromosome number) 



Division Period 



Grow+h Period, 

 Synapsis 



.st.Mat.Div.. 

 Chromosomes re 

 duced+ohaploid 

 number 



Spermatocyte II 



Spermatid 



#) (B (B C3 



2nd. Mah Division 

 (quantitative) 



V 



I 



/ 



06cy+e]I 

 1st. Polar Body 



3rd.4th.RB 



2nd. PB. 

 Ootid 



Cleavage 



5permato2oa 

 Fig. 139. — Diagram of gametogenesis, fertilization and cleavage. 



stages involved in the development of eggs, and spermatogenesis 

 is the process of producing spermatozoa from primordial germ 

 cells. Oogenesis and spermatogenesis are essentially alike but 

 differ in certain details. 



Oogenesis. — In the female rapid cell division of the primordial 

 germ cells leads to the production of a large number of oogonia, 

 only a few of which are shown in Fig. 139. This period of cell 

 multiplication is known as the division period, during which the 



