ORIGIN OF PRIMORDIAL GERM CELLS 



499 



In chick embryos possessing 31 to 33 pairs of somites, the gonad 

 has not commenced to develop, nor has the germinal epithelium 

 been differentiated, as yet, from the coelomic epithelium. At 

 this stage the primordial germ-cells are found in the root of the 

 forming mesentery, in the coelomic epithelium, in the neigh- 

 borhood of the coelomic angle, and, in the medial and posterior 



mm 



wmM 



■^ ^ ;:V'-.-:®<;--v 







^'%s^m 



^r^ 



■:Mr^^^<3. 



Fig. 1 Portion of a transverse section through the twenty-second somite of a 

 33 somite embryo; same embryo as figure 2, for which this figure is the key-plate. 

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

 Wright's stain. X 120. This figure is intended to give an idea as to the usual 

 position of the primordial germ-cells in the 33 somite chick, pr.o., primordial 

 germ-cells; ent., entoderm; sp.m., splanchnic plate of mesoderm; ao., aorta; c.a. 

 coelomic angle. 



region of the splanchnic plate of mesoderm, just where it is pass- 

 ing into the developing mesentery (fig. 1). The vast majority 

 of the germ-cells are found in these regions, but an occasional 

 one is located elsewhere ; far out in the splanchnic mesoderm, and 

 at times in a more dorsal situation in the loose tissue around the 

 aortae. 



