THE EVOLUTION OF SEX 



21 



first sight the most natural interpretation. It might be 

 said, indeed, that they are among the first cells to 

 differentiate, but only in the sense that they specialize, 

 as germ-cells. 



Fig. 7. — Origin of germ-cells in Sagitta. (From 

 Korschelt and Heider.) 



In a parasitic worm, ascaris, one of the first four 

 cells divides differently from the other three cells. As 

 seen in Fig. 6, this cell retains at its division all of its 

 chromatin material, while in the other three cells some 

 of the chromatin is thrown out into the cell-plasm. The 













1'^'. 







\ " 





^ 



■•^:'- 



^^^ ^^ 'Hf 





O 



Fig. 8. — Origin of germ-cells in Miastor. Note small black proto- 

 plasmic area at bottom of egg into which one of the migrating segmentation 

 nuclei moves to produce the germ-cells. (After Kahle.) 



