24 



K. MITSUKURI; 



considerably behind the tail-knob, inasmuch as the latter structure is 

 produced by the rising upwards of the ^^ Endwulst.'' The groove 

 extending between the posterior ends of the medullary canal and the 

 yolk-plug, i.e., the primitive groove runs therefore, along the median line 

 of the future tail-hioh and, extending beyond it, reaches the yolk-plvg. 

 The line marking the posterior boundary of the tail-knob crosses the 

 primitive groove making right angles with it. 



Fi"-s. 8 (PI. I.) and 8a (PI. II.) represent the dorsal and ventral 

 views of an embryo in which the tail swelling has become considerably 

 elevated. The amnion has now not only covered the tail end of the 

 embryo but extended far beyond it as the posterior tube of the 

 amnion [described in detail in my Contrib. II and also, recently, by 

 Mehnert (*95)]. The lateral limbs of the posterior horse-shoe shaped 

 opening of the amniotic tube are much longer in this than in Trionyx 

 or Cleinmys — a point characteristic of Chelonia caouana. With this 

 extension backwards of the amnion, the tail-end of the embryo and 

 the yolk-plug are necessarily covered by it but fortunately the yolk- 

 plug is in this specimen seen unusually clearly through the amnion 

 as a white mass, showing its position in the median line closely but dis- 

 tinctly behind the tail-swelling. In the ventral view also (Fig. 8a) the 

 yolk-plug is seen as a median white structure behind the tail-swelling. 



This is tlie last stage of Chelonia caouana in which I was able to 

 detect the yolk-[)lug in the surfiice view, the complications of different 

 structures in this region making surface observations impossible 

 after this. 



Clemmys japon ica . 



Let us turn next to Clemmys japonica. I only introduce here 

 fio-ures enouo-h to establish the fact that an essentially similar course 

 of events takes place in Clemmys as in Chelonia. 



