FORMATION OP GERMINAL LAYERS IN CHELONIA. 33 



appears for a short space ( * — c.) ia this section. The 

 entoblast, which is very thick ia front, especially in the 

 head-fold [h. f. marked by a notch in the ectoblast), becomes 

 suddenly reduced at the point c into a thin ventral layer 

 (end.) which stretches posteriorly as far as the point marked 

 with a *, where it seems to unite with the chorda-entoblast. 

 From the point of junction and also continued forward from 

 the chorda-entoblast, the mesoblast sheet stretches forwards as 

 far as c, above the darm-entoblast (end.) and beneath the ecto- 

 blast. Behind the blastoporic passage there is a large mass of 

 cells projecting downward (the Endwulst). On the dorsal 

 surface, directly behind the passage, columnar cells are absent 

 for a short space. This is the longitudinal section of the yolk- 

 plug. Following it, the ectoblast cells appear, but cannot at 

 first be separated from the large mesoblastic mass, for this is the 

 region of the primitive streak where the ectoblast is giving off 

 cells below. Very soon, however, it becomes an independent 

 sheet. The continuation downwards of the yolk-plug forms the 

 whole posterior wall of the blastoporic passage, and is there- 

 fore seen as its floor in cross-sections. The entoblast is con- 

 tinuous with it at the extreme front of the " Endwulst," but 

 becomes a distinct layer on the ventral surface. The meso- 

 blast, utterly indistinguishable from the yolk-plug, stretches 

 away posteriorly. Behind the blastopore the three germinal 

 layers are thus fused. The mesoblast, which is separate from 

 the entoblast on the ventral surface of the '^ Endwulst,'^ is 

 receiving more posteriorly additions from the yolk or germinal 

 wall. 



In the next stage which we figure (figs. 4 a and d), the 

 head-fold has considerably advanced, and the amnion (am.) 

 covers it already so that it is not visible from the dorsal side. 

 The medullary folds have touched each other. At the poste- 

 rior end the yolk plug is included between the diverging 

 medullary folds. 



The sections through the head region of this stage show 

 beautifully, and in an unmistakeable and conclusive manner, the 

 mode of the formation of the mesoblast and of the chorda 



VOL. XXVII, PART 1. WEW SER. C 



