ORIGIN OF THE GERM-CELLS 



147 



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

 cells dijfers from the somatic cells not 0)ily in the greater size and rich- y 

 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 chromatir in the somatic 

 cells of Ascaris. [BOVERI.] 



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

 the side, later in the second cleavage, showing the two types of mitosis and tbe casting out of 

 chromatin {c) in the somatic cell. C. Resulting 4-cell stage; the eliminated <».riromatin 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 jerm-cells is the sum-total of the ciiromatin retained. In 

 Ascaris megalocephala nnivalcjis the process is as follows (Fig. 73): 

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



