126 HEREDITY AND ENVIRONMENT 



the somatic cells as contrasted with the germ 

 cells of Ascaris (Fig. 30). But while the 

 chromosomes invariably divide equally, other 

 portions of. the nucleus may not do so. 

 Achromatin and oxychromatin, like the cyto- 

 plasm, may divide unequally and differenti- 

 ally, and this is probably a prime factor in 

 development. 



On the other hand, the differential division 

 of the cytoplasm is a regular and character- 

 istic feature of ontogeny; indeed, the segrega- 

 tion and isolation of different kinds of 

 cytoplasm in different cells is the most im- 

 portant function of cell division during de- 

 velopment. Thus we find in the division 

 apparatus of the cell a mechanism for the pres- 

 ervation in unaltered form of the species 

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

 the progressive 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 appear- 

 ance. Every sex cell, like every other type of 



