345 



M.P. 



Fig. 5 shows the sex-cells ^n a tadpole of 8.3 mm. total length. 

 The mesentery has been definitely formed, the body cavity has ap- 

 peared as a result of the separation .of the splanchnopleuric and 

 somatopleuric layers of the lateral plates of mesoderm and the median 

 unpaired rod of sex-cells has become wholly separated from the mass 

 of endoderm cells which are destined to form the lining of the ali- 

 mentary tract. The sex-cells are not yet distinguishable from the 

 latter in any essential character. This stage is apparently the same 

 as that of the 9 mm. tad- 

 pole of R. temporaria with 

 which BouiN commenced 

 in his account of the em- 

 bryonic development of 

 the ovary. He shows that 

 the sex-cells migrate la- 

 terally from this median 

 position, coming eventually 

 to lie in the paired sex- 

 gland anlagen. He then 

 traces the further history 

 of the sex-glands to the 

 adult stage. The fact that 

 the cells in question are 

 actually sex -cells is so 

 conclusively proven by the 

 work of BouiN as to need 

 no further demonstration. 



Nussbaum's studies began with a stage slightly later, apparently after 

 the sex-cells had moved laterally to their final positions. Both of 

 these authors described the sex-cells as being large and filled with 

 yolk spherules long after the latter have disappeared from the other 

 cells of the body, and both called attention to their resemblance to the 

 endodermal cells. 



In considering the origin of the sex-cells in Rana, one is struck 

 with certain points in which this process seems to difter from that 

 observed in the turtle. While the sex-cells of the turtle arise from 

 regions of the endoderm at some distance on each side of the median 

 line, no observations have so far shown this to be the case in Rana. 

 This point is being studied in other amphibians where conditions are 

 more favorable to its solution. It is a question of great interest and 

 is bound up with that of the migration of the sex-cells in the strict 



Fig. 4. Greater part of a section of the same 

 larva fz'om which the section shown in Fig. 2 was 

 taken. S.C. Sex-cells. 



