GERM CELLS OF ANURANS 291 
SUMMARY OF CONCLUSIONS 
A . The origin and fate of the primordial germ cells 
1. The primordial germ cells of the embryo bullfrog are first 
distinguishable from other entodermal elements in embryos of 7 
mm. total length. They arise from the entoderm as a median 
ridge of yolk-laden cells just dorsal to the roof of the archenteron, 
ventral to the aorta, and separating the two lateral mesodermal 
plates from each other. 
2. In embryos of 8 mm. total length, the germ-cell ridge be- 
comes separated from the underlying entoderm forming the roof 
of the archenteron, partly by the median growth of the two 
lateral plates which pinch off the ridge and also by active 
migration of the germinal elements themselves. In cross-section 
at this stage, the germ cells are found at the root of the forming 
mesentery as an unpaired ridge, consisting of two or three large 
yolk-laden cells. 
3. As development progresses, this median ridge of germ cells 
splits longitudinally and the cells of the two halves migrate lat- 
erally on either side to form two independent ridges, invested 
with peritoneum. This stage is represented in embryos of 9.5 
mm. total length. 
4. The two germinal ridges project into the coelomic cavity 
and enlarge considerably by increase in number of their cellular 
elements. The primitive sex cells actively divide and there is 
also a migration of mesenchymal cells into the ridges from the 
mesonephros and peritoneum. These conditions are found in 
14 to 15 mm. tadpoles. The germ cells have lost their yolk in 
the meantime. 
5. The gonads greatly increase in size. Large cavities are 
formed, the secondary genital spaces, lined by small non-sexual 
cells which have migrated into the gonads from the mesonephros 
by way of the mesentery suspending the gland. When the tad- 
pole has attained a length of 30 mm., the gonads are hollow sacs 
surrounded by a single layer of peritoneum and one or two layers 
of germ cells. 
