FATE OF BLASTOPORE, ETC., IN CHELONIA. 49 



no longer present. The primitive streak which is con- 

 tinuons from g backwards is now decided and pronounced. 

 On the right (apparent left) above the streak and restino- 

 on the epiblast is the anterior termination of the yolk- 

 plug. It gradually tliins out like the blade of a razor 

 towards the left (apparent right). 



i is two sections behind // and is in the region of the 

 yolk-plug. Resting on a trough-like depression of the 

 epiblast in the median dorsal line is a triangular mass 

 with a very broad base upwards. This is the yolk-plug. 

 It is continuous directly with the mesoblast below and 

 with the epiblast laterally. Although somewhat different 

 in details, we nuist regard this section as corresponding 

 to Series I., /, Series III., /,-, and Series Y., m. 

 j is two sections behind ?', and is nearly out of the region 

 of the primitive streak and the yolk-plug. There is a 

 slice of the latter on the left (apparent right) and there is 

 still a slight connection between the epiblast and the 

 mesoblast. The layers become independent after this. 



This series shows that although there is no primitive groove in 

 Trionyx, the primitive streak is really present, as in Cleiumys and 

 Chelonia. 



Accompanying the above series of transverse sections is given 

 a median longitudical section of the posterior ]iortion of an embryo 

 (Scries VI. long, PI. Y.) taken out at the same time from the 

 same deposit as above. In this, I was able to recognise the yolk- 

 plug, which is apt to elude us in longitudinal sections. The 

 " Endwulst " mass is shown to be tolerably well-marked at least 

 posteriorly. The mesoblast on each side of the " Endwulst" in e, f, 

 and g of the transverse series goes also round the posterior end of the 

 "Endwulst" and is thus visible in the lon<j:itudinal section behind 

 that structure. The lines through this section indicate by the lettering 



