ORIGIN OF PRIMORDIAL GERM CELLS 505 



repeat, then, this massing of the primordial germ-cells in the 

 vessels is something out of the ordinary for no evidence of it is 

 to be seen in either older or younger embiyos. 



The largest mass (fig. 5) in proximity to the fore brain con- 

 tains 35 germ-cells and extends through a dozen 4 micra sections. 



In the former situation in this embiyo — that is in the splanch- 

 nic mesoderm — there are 56 germ-cells in all. These are distri- 

 buted as follows — 48 in the tissues and 8 in the vessels. The 

 cells in the blood vessels are in some cases larger than average 

 caliber of the vessel so that its walls are pushed out noticeably on 

 either side (fig. 4). 



As regards the number of primordial germ-cells found in the 

 embryos, in which they are present in the blood vessels, it may 

 be said here that the figoires given are only approximately cor- 

 rect. This applies to embryos with 25 somites, and especially to 

 those which are younger, for in the latter, as will be seen, the 

 germ-cells are found in the larger blood vessels, and the loss due 

 to staining and dehydrating is greater than in the case of the 

 older embryos, in which the primordial germ-cells are present 

 in the smaller vessels only. 



That the large cells within the blood vessels are primordial 

 germ-cells, and that they pass out into the tissues, there can be 

 no doubt. In the first place, they are identical in appearance 

 with the germ-cells described in the splanchnic mesoderm of older 

 embryos (compare figs. 2 and 4). This similarity is best ob- 

 served in the embryo of 25 somites and the next younger one 

 studied, which possesses 21 somites. In these two embryos, 

 which have been called transition stages, because the primordial 

 germ-cells are in the vessels as well as in the tissues, in neighbor- 

 ing sections germ-cells may be observed in both situations (figs. 

 7, 8, 9 and 11). Thus, it will be noted that the germ-cells in 

 both situations have the same size and shape (figs. 8 and 9). 

 The mitochondria are similar, nor is there any nuclear difference. 

 Cells within and without the vessels have the same amount of 

 yolk material and the same conspicuous attraction-sphere (figs. 

 8 and 9). 



