FATE OF BLASTOPORE, ETC., IN CHELONIA. 27 



" Endwulst," is straight in this specimen and distinctly reaches the 

 groove surrounding the yolk-plug. This latter structure appears quite 

 elongated and club-shaped. Careful examination shows that only the 

 head of the club is surrounded by the groove and probably ought alone 

 to be looked on as the yolk-plug. The part that corresponds to the 

 stem of the club has only acquired the appearance of being continuation 

 of the yolk-plug through being bounded on each side by a dark area 

 which indicates an empty space within. 



Fig. 14 represents an advanced stage in which the tail-swelling 

 has already been considerably raised. Tliere are two faint longitudinal 

 streaks visible behind the tail-swelling and a third wedge-shaped one 

 in between the two, some distance behind. The sections show" that 

 the primitive groove is continued some distance behind the tail-swel- 

 ing, and the two parallel streaks are probably surface expressions 

 of the lips of the primitive groove. Whether the third wedge-shaped 

 streak represents the yolk-plug, I can not determine, as it is very faint 

 and the sections unfortunately do not give a decided answer. This 

 figure however is enough to show that the primitive groove is conti- 

 nued behind the tail-swelling. 



Fig. 15 represents a much more advanced stage in which the 

 amnion has already extended itself as a long posterior tu1je. The tail- 

 swelling is also very much developed. Different from Fig. 8 of 

 Clielonia, the yolk-pliig is not visible l)ehind the tail. There is, how- 

 ever, a long white streak present all along the median line of the floor 

 of the posterior amniotic tube and extending even beyond its horse- 

 shoe shaped ])Ostorior opening. This streak is often visible in 

 Clemiinjs embryos, and 1 figured it in Contrih. II., V\. I., Figs. 5 & 6. 

 It has also been iK^ticed l)y Mehnert in Eini/s lutaria taurica. ( 95, 

 PI. X., Fig. 2f)). The discussion of its nature is postponed until 

 we come to the description of the sections. 



