ORIGIN OF PRIMORDIAL GERM CELLS 507 



to the 18th somite. This appHes to cells both in and without the 

 vessels. In the splanchnic mesoderm region of this embryo two 

 cells are present which are partly within and parth^ without the 

 vessel (fig. 11). 



In regard to the embryos with 25 and 21 somites se^'eral in- 

 teresting facts should be recapitulated. In both, germ-cells are 

 both within and without the blood vessels. In the younger em- 

 bryo the cells in the vessels are more numerous, while in the older 

 embryo this state of affairs is reversed. Also, let it be remem- 

 bered that all investigators hitherto have failed to find the germ- 

 cells in chick embryos with less than 22 somites. 



In one series of an embryo with 19 mesoblastic somites there 

 are present in all 82 primordial germ-cells. The distribution of 

 these is markedly different from anything described hitherto. All, 

 with the exception of 6, are in the blood-vessels. They are found 

 in the large (fig. 13) as well as small channels both of the area 

 vasculosa and embryo proper. They are present in the heart 

 and aortae as well as in the capillaries. They are never found 

 in groups and in only one case could two be seen in a single field. 



The germ-cells in this embryo as well as those still younger are 

 characterized by an immense amount of yolk (figs. 12 and 13). 

 This yolk material consists of spheres, having a diameter of 2-4 

 micra, and is massed principally in that portion of the cell in 

 which is located the attraction-sphere. However, this yolk is 

 not confined only to the pole of the cell occupied by the attrac- 

 tion-sphere, but usually surrounds the nucleus completely (fig. 

 12). This yolk is so abundant, that in preparations fixed with 

 a fluid containing osmic acid, the other cell contents are seriously 

 masked. 



In embiyos of this age the primordial germ-cells possess, in 

 nearly all cases, amoeboid processes. 



The stages of 16, 12 and 10 somites respectively may be passed 

 over rapidly since they present nothing essentialh' different from 

 the 19 somite embryo as far as either the distribution or form of 

 the germ-cells is concerned. Thus, the latter are found in the 

 area vasculosa and in the developing vascular structures of the 

 embryo proper. In the embryo possessing 12 somites, one germ- 



