23 K. MITSUKURI; 



The above account establishes conclusively, I think, that the 

 changes occurring in the posterior part of the embryo run essentially 

 the same course in Clemvujs as in Chelonia. 



Trionijx japonicus. 



This species differs from the two foregoing species in that 

 it does not show in any specimen the groove which connects the 

 medullary canal with the yolk-plug. This I take to be evidence of 

 its less primitive character. The histological details are however alike 

 in all the essential characters, as will become evident when studying 

 the sections. 



Figs. 1(), 17, and 18 have already been described, when show- 

 ing how the posterior portions of the medullary folds are raised and 

 how they come together and compress laterally the yolk-jjlug. 



FÏO-. 19 is the stao-e next to Fiof. 1(S. The medullary folds have 

 met each other in the median line although not yet fused together in 

 its posterior half. The yolk-phig rests on the posterior ends of the 

 folds, and presents roughly speaking a pentagonal figure, distinct 

 on all sides except behind, one of its angles coinciding with the 

 line of meeting of the medullar}^ folds. Through the yolk-plug the 

 posterior end of that line of meeting can just be seen. A con- 

 dition of the yolk-plug similar to that in this embryo, I was able to 

 discover in several of the Triomjx embryos, but after diligent search 

 nothing like it can be detected in any embryo of Chelonia or Clemmys. 

 I am inclined to think that in these species, the yolk-plug gradually 

 recedes after the staoe resemblins; that in Fi"-. 18, without much 

 changing its level in regard to the medullary folds, while in Trionijx, 

 it is entirely lifted up above that level, attaining the condition given 

 in Fig. ly before it begins to move backwards. These circumstances 

 have probably some relation to the fact that a primitive groove is 



