ORIGIN OF PRIMORDIAL GERM CELLS 



501 



The next embryo to be studied possesses 25 pairs of somites 

 and an approximate age of forty-four hours. 



Down to this point the situation in which the primordial germ- 

 cells are found agrees with that given by all who have done any 

 investigation on this subject. Hoffman ('93) and, in fact, all 

 who have worked upon the subject of primordial germ-cell his- 

 tory in the chick, have described the indifferent gonad and the 

 presence of the sex-cells in the splanchnic mesoderm. No one, 





^pr.o. 



rrii&i/ai;f^/ ^'4r> 



Fig. 2 Portion of a transverse section through the twenty-second somite of a 

 33 somite embryo; this figure is drawTi from the same embryo as figure 1. Bensley's 

 acetic-osmic-bichromate fixation and Bensley's anilin acid fuchsin-Wright's 

 stain. X 787. This figure shows the position, form and grouping of the primor- 

 dial germ-cells in the 33 somite chick embryo, -pr.o., primordial germ-cells; ent., 

 entoderm; sp.ni., splanchnic plate of mesoderm; c.a., coelomic angle. 



however, has been able to trace them back to a stage earlier than 

 that of 22 somites. This inability to find them in younger em- 

 bry'os is surprising but nevertheless can be explained. The stu- 

 dents of germ-cell histoiy and migration in the chick have, no 

 doubt, had constantly in mind the work of Beard, Allen, Woods 

 and Jarvis, who investigated this question in forms, in which a 

 definite migration occurred through the entoderm or splanchnic 

 mesoderm. They ha^-e expected this same kind of migration to 



