148 Heredity and Environment 



On the other hand, the differential division of the cytoplasm is 

 a regular and characteristic feature of ontogeny ; indeed, the seg- 

 regation and isolation of different kinds of cytoplasm in differ- 

 ent cells is one of the most important functions of cell division 

 during development. Thus we find in the division apparatus of 

 the cell a mechanism for the preservation in unaltered form of the 

 species plasm or germ-plasm of the nucleus, and for the progres- 

 sive differentiation of the personal plasm or somatoplasm of the 

 cell body. 



3. The Origin of the Sex Cells. The sex cells are the latest 

 of all cells of a developing organism to reach maturity, and yet 

 they may be among the earliest to make their appearance. Every 

 sex cell, like every other type of cell, is a lineal descendant of 

 the fertilized egg (Fig. 41), 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 development 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 differentiation 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 cell or gonia divide repeatedly, 

 but the oogonia grow more rapidly and divide less frequently 

 than the spermatogonia. As a result 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. 41). 



(b) The Growth Period. Oocytes and Spermatocytes. This 

 period of rapid cell division is followed by a period of growth 

 without division during which the developing sex cells are called 



