FATE OF BLASTOPORE, ETC., IX CHELOXIA. 29 



left behind, so to speak, in the track of the yolk-plug in Chelonia and 

 Clenimijs, while it is absent in Trionijx, showing, as I have said, the 

 less primitive character of the development of this genus. 



In Fig. 20 the medullary canal is entirely closed and the yolk- 

 plug with horse-shoe outline has receded somewhat, although in a 

 surface-view it appears still superposed on the " Endwulst." The 

 yolk-plug in this figure, as well as in some following, is represent- 

 ed a little too boldly. It is very hard to detect, and I discovered it 

 only after searching for it long in not too strong direct sunlight. In 

 a favourable light and position, however, it is distinct and unmis- 

 takable. 



In Fig. 21 the yolk-plug is unnsually clear and has now re- 

 ceded some distance behind the " Endwulst," which appenrs white. 

 There is no sign of the primitive groove. 



Fig, 22 is very much like Fig. 21, but the shape of the yolk-plug 

 is not as anovular as in that fig-ure. 



In Fig. 23, the yolk-plug has receded about as far back as in Fig. 

 21, but here the backward directed tongue of the "Endwulst" is 

 visible, and a faint narrow white streak can be recognised in the 

 median line, passing under the yolk-plng, as a continuation of the 

 tongue. This appearance is due to the cells proliferated along this 

 line. Although the " Endwulst " in this figure thus appears difterent 

 in shape from some which precede it, e.g., that of Fig. 21, I believe 

 it is not safe to conclude from this tliat this structure is different in 

 reality, since the details visible in surfiice views depend so much on 

 the conditions of preservation of the embryos and upon other 

 secondary circumstances. 



Fig. 21 (Pi. II.) is from an embryo which, in general appearance, 

 is very much like that of Fig. 23. In it, however, I was unable 

 to detect any sign of the yolk-plug. Just where the yolk-plug ought 



