GERM CELLS AND THEIR FORMATION 



115 



As said above, the chief events concern the nuclear structure 

 and we can only point out here that during the period of matu- 

 ration two more cell divisions occur. 



In the testis each primary spermatocyte divides once, form- 

 ing two cells called the secondary spermatocytes, and then each 

 of these divides again, forming altogether four cells called the 



Primordial Germ 

 Cell. 



Period of Multiplica- 

 tion, s chromo- 

 somes. (The number 

 of cell generations 

 much gi eater than is 



indicated here.) _ _ 



Spermatogonia. 



Period of Growth. 

 s chromosomes. 



Period of Matura- 



tion. chromo- 



Period of Metamor- 

 phosis, ^chromo- 



Primary Spermato- 

 cyte. 



Secondary Sper- 

 matocytes. 



Spermatids. 



Spermatozoa. 



FIG. 61. Diagram of the chief events of spermatogenesis. Adapted from 



Boveri. 



spermatids. These are all alike and each becomes metamor- 

 phosed into a spermatozoon without further division. Thus 

 are formed, from each primary spermatocyte, four similar 

 spermatozoa. In the ovary, or sometimes after the primary 

 oocyte has left the ovary, this too divides, but here the division 

 is very unequal, resulting in the formation of one large cell, 

 the secondary oocyte, and one very small cell, called the first 

 polar body. Typically each of these then divides again. The 

 secondary oocyte divides unequally as before, forming a large 



