l^ ' K. MITSUKURI; 



embrace it, so to speak, between themselves (Fig. 17). This seems to 

 raise the yolk-])hig above the original level. It sticks out into the 

 sinus rhomhoidalis as a knob which is rounded in the case of Fig. 17. 

 The dorsal opening of the blastoporic passage lies directly in front of 

 it, and, as in Fig. 17, is often overhung by the anterior end of the yolk- 

 plug and so is invisible in a dorsal surface-view. A narrow groove 

 separates the yolk-plug on each side from the posterior end of the 

 medullary fold of its own side and ought, I beHeve, fo be looked on 

 as corresponding to the backward-directed limbs of the horse-shoe 

 shaped blastopore of the earlier stages. 



The posterior portions of the medullary folds now press towards 

 the median line and obliterate the sinns rlionihoidalisj from the surface 

 view at least. It exists within for some time lono-er. In cominof 

 together, the medullary folds compress laterally the yolk-plug, which 

 thereby changes from its previous rounded shape to a triangular 

 or wedge shape (Fig. 18). At this stage, one is sometimes able 

 to see a delicate irreo-ular Avhite strina* or mass stretchinfi' in front of 

 the anterior end of the yolk-plug and lying in the medullary canal. 

 (There is a small detached mass in Fig. 18). This is perhaps the 

 same thing as the " Achsenfaden " of Dursy,* and is probably 

 the anterior end of the wedge-shaped yolk-plug detached and 

 modified. 



The course of the chano-es in Chelonia and Clcmxuis at these stao-es 

 are very much like that just depicted for Trioni/x. Fig. 2 oî Chelonia 

 and Fig. 9 of CleniDtijs are of embryos nearly corresponding to that 

 of Fig. 18 of Trionyx. 



The stages corresponding to Figs. 2, 9, and 18 have been noticed 

 and figured inany times by writers on reptilian development. The 



'* I unfortunately know Durst's work only by a reference to it in Gasser's paper ('78). 



