50 ^- MITSUKURI; 



the planes corresponding to the transverse sections marked with the 

 same letters in the above series. 



Series VIÏ. 



(PL VI.) 



Trioni/x japonicus. 



A Trionyx emhnjo, taken out 7^12 àcufs after deposition, represented 

 in Fiij. 24 (P/. 77.), in general appearance very much like Figs. 22 or 23. 



(155 D)- 



This, it may be remembered, is one of those embryos in which a 

 most diligent search failed to reveal a yolk-plng (see p. 29). Only 

 one section of this series is introduced here. 



a is from the region where the yolk-plng ought to be present. 

 It will be seen to be essentially like Series YI., i ; except 

 that the groove separating the yolk-plug from the epiblast 

 on each side is not so distinct as in that section. This is 

 undoubtedly the reason why I was unable to recognise the 

 yolk- plug in the surface-view. We ma}^, therefore say, I 

 think, without much straining, that the yolk-plug or 

 what corresponds to it is in reality present in this embryo 

 as in those in which the yolk-plug is plainly visible on 

 the surface. 



A study of the wdiole series shows that in that part which corres- 

 ponds to Series YI, g, the epiblast has become already independent of 

 the "Endwulst" mass below and that the primitive streak is found 

 only in the hinder part beginning at the posterior limit of the 

 " Endwulst." In this embryo, the rising of the tail s\Yelling is also 

 begun. Of this process, however more will be said in the sequel. 



