2ß K. MITSUKURI; 



groove — goes backwards along the median line. It is much narrower 

 than the posterior opening of the medullary canal. It is slightly wider 

 and deeper in its hindmost part than in front. Just behind the spot 

 where the primitive groove ends, a semilunar structure is faintly 

 visible. This is the yolk-plug, differing somewhat in shape from 

 that of Chelonia. In this specimen, I can not make out whether the 

 primitive groove quite reaches the groove bounding the yolk-plug 

 anteriorly. The "Endwulst," which is nearly circuhir in outline, is 

 seen sending a small tongue backwards beneath the hindmost part 

 of the primitive groove, which must be looked upon as the outwardly 

 visible expression of the cells proliferated from this part of tlie 

 primitive groove. 



In Fig. 12, the medullary canal is entirely closed, but a slight 

 widening behind is seen faintly through its roof which marks the spot 

 where it was open, as in Fig. 11. and serves to indicate the position of 

 the neurenteric canal. The primitive groove exists over only the 

 posterior part of the " Endwulst ", which sends out now a. much 

 larger and more distinct tongue backwards beneath the groo\'e than 

 in Fio". 11. The groove is wavy. Its hindmost part is slightly 

 deeper, and in the surface view, darker than in front. In this 

 specimen, there is no question whatever as to whether the primitive 

 groove reaches the groove surrounding the yolk-plug. The latter 

 structure is somewhat different in shape from that seen in Fig. 11, 

 being top-shaped, with its pointed end turned forwards and insinuated 

 into the primitive groove. 



Fio-. 13 is of about the same stäche as Fio-. 12. The medullary 

 canal is completely closed, but the position of the neurenteric canal is 

 still faintly visible from above as a slight enlargement of it. The 

 tongue of the " Endwulst" projecting backwards is clear and decided. 

 The primitive groove, which exists over only the posterior part of the 



