22 



HEREDITY AND SEX 



single cell that retains all of the chromatin in its nucleus 

 gives rise to the germ-cells. 



In a marine worm-like form, sagitta, two cells can 

 easily be distinguished from the other cells in the wall of 

 the digestive tract (Fig. 7). They leave their first posi- 

 tion and move into the interior of the body, where they 

 produce the ovary and testes. 



Lepldosteus 



Lepldosleus 



Chrysemys 



SPeriph. End. 



XVil.Cntl 



Fig. 9. — Origin of germ-cells in certain vertebrates, viz. turtle, frog, 

 gar-pike and bow-fin. The germ-cells as darker cells are seen migrating from 

 the digestive tract (endoderm). (After Allen.) 



In several of the insects it has been shown that at a 

 very early stage in the segmentation, one, or a few cells 

 at most, lying at one end of the egg develop almost in- 

 dependently of the rest of the embryo (Fig. 8). Later 

 they are drawn into the interior, and take up their 

 final location, where they give rise to the germ-cells. 



Even in the vertebrates, where, according to the 



