The Cellular Basis 



149 



primary oocytes or spermatocytes. This growth period may he 

 very long in the case of the oocytes, lasting, for example, in the 

 human female from the time of birth to the end of the reproduc- 



A R 



Fig. 49. Differentiation of Germ Cells and Somatic Cells in the 

 Egg of Ascaris. A and B, Second cleavage division showing that the chro- 

 mosomes remain entire in the lower cell, which is in the line of descent of 

 the sex cells ("germ track"), but that they throw off their ends and break 

 up into small granules in the upper cells, which become somatic cells. 

 C, 4-cell stage, the nuclei in the upper (somatic) cells being small and the 

 ends of the chromosomes remaining as chromatic masses in the cell body 

 outside of the nuclei, while the nuclei in the lower cells are much larger 

 and contain all of their chromatin. D, Third nuclear division, showing the 

 somatic differentiation of the chromosomes in all the cells except the lower 

 right one, which alone is in the germ track and will ultimately give rise to 

 sex cells. (After Boveri.) 



