ORIGIN OF THE GERM-CELLS 



147 



two-cell stage ! Moreover, from the outset the progenitor of the g6rm- 

 cells differs from the somatic cells not only in the greater size and rich- 

 ness of chromatin of its nuclei, but also in its mode of mitosis; for in 

 all those blastomeres destined to produce somatic cells a portion of 



Fig. 73. — Origin of the primordial germ-cells and casting out of chromatin in the somatic 

 cells of Ascaris. [BoVERl.] 



A. Two-cell stage dividing; s. stem-cell, from which arise the germ-cells. B. The same from 

 the side, later in the second cleavage, showing the two types of mitosis and t^e casting out of 

 chromatin (c) in the somatic cell. C. Resulting 4-cell stage ; the eliminated ftRromatin at c. 

 D. The third cleavage, repeating the foregoing process in the two upper cells. 



the chromatin is cast out into the cytoplasm, where it degenerates, and 

 only in the germ-cells is the sum-total of the chromatin I'etained. In' 

 Ascaris mcgalocepJuila ?tnivalens the process is as follows (Fig. 73): 

 Each of the first two cells receives two elongated chromosomes. As 



