128 HEREDITY AND ENVIRONMENT 



cell, is a lineal descendant of the fertilized egg 

 (Fig. 22) , but the period at which the sex cells 

 become visibly different from other cells varies 

 from the first cleavage of the egg in some 

 species to a relatively advanced stage of de- 

 velopment in others. 



(a) The Division Period. Oogonia and 

 Spermatogonia. — When the primitive sex 

 cells are first distinguishable they differ from 

 other cells only in the fact that they are less 

 differentiated; they have relatively larger 

 nuclei and smaller cell bodies, a condition 

 which is indicative of little differentiation of 

 the cell body since the products of differentia- 

 tion such as fibres, secretions, etc., swell the 

 size of the cell body but do not contribute to 

 the growth of the nucleus. These primitive 

 sex cells or gonia divide repeatedly, but the 

 oogonia grow more rapidly and divide less 

 frequently than the spermatogonia. As a re- 

 sult of this difference in the rate of growth 

 and division the spermatogonia become much 

 smaller and immensely more numerous than 

 the oogonia. This period in the genesis of the 

 sex cells is known as the division period 

 (Fig. 22). 



