GERM CELLS IN TATUSIA NOVEMCINCTA 343 



aud also in the yolk at the meeting point of the germ layers. 

 Thence they migrate to thie splanchnopleure and into the epi- 

 thelimn around the intestine, from which point they find their 

 way to the germinal anlage. 



Allen ('06) discovered in Chrysenis that the germ cells arise 

 in the entoblast in the region between the area opaca and the 

 area pellucida, posterior and lateral to the embryo and wholly 

 without the zone of gastrulation. These migrate between the 

 entoderm cells to the mesentery and finally to the peritoneum 

 on either side of the latter. 



Rubaschkin ('07, '09, '10, '12) investigated conditions in the 

 chick, cat, rabbit, and guinea-pig again tracing back germ cells 

 to the entoderm with further migration through the mesentery 

 to the sex anlage. 



Among those working on mammals may be mentioned Fuss 

 ('12) who studied the rabbit, pig and man. He concludes that the 

 germ cells first become apparent in the region of the primitive 

 streak where no segmentation has yet taken place in the embryo. 

 Scattered cells also may be seen out on the yolk, whence they 

 migrate to the intestinal endoderm and mesoderm en route to 

 the genital region. 



Especially interesting are the conclusions reached by Swift 

 ('14) in his work on the chick. He finds that the germ cells 

 arise anterior and antero-lateral to the embryo in a specialized 

 region of the germ wall entoderm at the margin of the area 

 pellucida. They first appear during the primitive streak stage 

 and continue to arise up until the three somite stage. Although 

 first found between the entoderm and ectoderm, they later enter 

 the mesoderm and the forming blood vessels of the mesoderm. 

 By the blood, they are carried to all parts of the embryo, re- 

 maining distributed till about the twenty somite stage. At 

 this time, the germ cells begin to concentrate in the vessels of 

 the splanchnic mesoderm. After the twenty-five somite stage 

 they appear to have left the blood having passed out through 

 the vessel walls into the splanchnic mesoderm near the mesen- 

 tery. In the thirty and thirty-three somite stages, the germ 

 cells are seen in the root of the mesentery and in the coelomic 



