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THE POPULAR SCIENCE MONTHLY 



divide unequally and differentially, and this is probably a prime factor 

 in development. 



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

 regular and characteristic feature of ontogeny — indeed the segregation 

 and isolation of different kinds of cytoplasm in different cells is the 

 most important function of cell division in development. Thus we 

 find in the division apparatus of the cell a mechanism for the preserva- 



A 



Fig. 32. Differentiation of Geem Cells and Somatic Cells in the Egg of 

 Ascaris. A and B, second cleavage division showing that the chromosomes 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 the 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.) 



tion in unaltered form of the species plasm, idioplasm or germ-plasm of 

 the nucleus, and for the progressive differentiation of the personal plasm 

 or somatoplasm of the cell body. 



