372 KING. [Vou XIX. 
cells (Fig. 1, G) which lies directly beneath the aorta (Ao.) 
and between the lateral mesodermal plates (L. M.). The cells 
of this ridge resemble the cells of endoderm in all respects and 
they are continuous with the endodermal cells which later form 
the lining of the digestive tract: there is no probability that 
these cells are of mesodermal origin, since they are always 
sharply marked off from the lateral mesodermal plates. When 
the tadpole is eight or nine days old this ridge of cells becomes 
separated from the endoderm, and it then forms a median cord 
of cells, the genital ridge, lying between the cardinal veins 
and supported by the mesentery (Fig. 2 G). The cells of 
the genital ridge are very conspicuous in sections at this time, 
as they still contain numerous large, deeply staining yolk 
spherules, although the neighboring cells have already absorbed 
the greater part of their yolk. 
Many investigators who have studied the origin of the 
germ-cells in amphibians have asserted that these cells are 
derived from a germinal epithelium which is a modifica- 
tion of the peritoneal epithelium lining the body-cavity. This 
is the view advocated by Waldeyer (91), Semon (84), Hoff- 
mann (44), Kolessnikow (55), Leydig (60), Spengel (86), 
Iwakawa (45), and more recently by Bouin (11). The last 
named investigator, however, admits the possibility that the 
first germ-cells in Rana are derived from endoderm, since in 
early stages of development they have all of the character- 
istics of the primitive endodermal cells. On the other hand, 
Nussbaum (73) maintains that in amphibians, and also in 
other vertebrates, the germ-cells are not derived from perito- 
neal epithelium, but that they are developed from undifferen- 
tiated embryonic cells which are set apart during early cleav- 
age for this especial purpose. This theory has been supported 
by the researches of Woods (95) on Acanthias, and by Beard 
(7) on Raja batis. The latter investigator states: “The germ- 
cells may be regarded as unicellular organisms which pass 
one part of their life-history within a multicellular sterilized 
stock, the embryo or metazo6n, formed by one of them at a 
definite period in the life-cycle.” 
