374 FRING: [Vou. XIX. 
When the genital ridge is first clearly marked off from the 
endoderm it occupies a median position between the cardinal 
veins and beneath the aorta (Fig. 2), as Bouin and Allen 
have stated is the case in Rana. If a section of the ridge in 
this stage of development is examined under high power one 
finds that it is composed of two distinct types of cells, one 
many times larger than the other (Fig. 3). The large cells, 
which are filled with yolk spherules and have vaguely defined 
boundaries, are the primordial germ-cells. The nuclei of these , 
cells have the “mulberry” shape which La Valette St. George 
(78) discovered to be a characteristic of the nuclei in the 
spermatogonia of Salamandra, and they are usually crowded 
by the yolk spherules into one corner of the cell. The chromatin 
in these nuclei is in the form of minute, faintly staining gran- 
ules which are distributed on linin threads or along the nuclear 
membrane. Each nucleus contains several rounded, deeply 
staining nucleoli of various sizes. Judging from their stain- 
ing reactions most of these nucleoli are plasmosomes, and only 
one or two of the smaller ones are karyosomes. Scattered 
among these germ-cells, and frequently flattened against them, 
are numerous small cells which resemble in all respects the 
cells of the peritoneal epithelium from which they doubtless 
have been derived. These cells are very much smaller than the 
germ-cells; they contain no yolk and they have an elongated, 
deeply staining nucleus which is very large in proportion to » 
the size of the cell. Doubtless these cells migrate into the 
genital ridge after the formation of the mesentery, since there 
are no cells of this type in the genital ridge at the stage of Fig. 
1, and I have seen nothing that would indicate that they are 
derived from the germ-cells. 
Bouin has stated that he finds in Rana temporaria transi- 
tional stages between peritoneal cells and primordial germ- 
cells, and he believes that before the metamorphosis of the 
tadpole new germ-cells are constantly arising from peritoneal 
cells. These observations have not been confirmed by Allen 
(2) in his study of the origin of the germ-cells in Rana pipiens, 
and in Bufo I can find no evidence that the germ-cells are 
