FOEMATION OF GERMINAL LAYERS IN CHELONTA. 229 



blast {end.) and beneath the ectoblast. Behind the blastoporic jiassa^re 

 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 slioet. The continuation 

 downwards of the yolk-plug forms the whole posterior wall of the 

 blastoporic passage, and is therefore seen as its floor in cross-sections. 

 The entoblast is continuous wäth it at the extreme front of the 

 " Endwulst," but becomes a distinct layer on the ventral surface. 

 The mesoblast, 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. 4a and />), the head-fold 

 has considerably advanced, and the amnion {aw.) covers it already so 

 that it is not visible from the dorsal side. The medullary folds have 

 touched each other. At the posterior end the yolk plug is included 

 between the diverging medullary folds. 



The sections through the head region of this stage show beauti- 

 fully, and in an unmistakeable and conclusive manner, the mode of 

 the formation of the mesoblast and of the chorda dorsalis. Figs. 

 25 — 28 are selected to illustrate these points. 



Fig. 25 is the most anterior section represented. It goes through 

 the posterior part of the head. The amnion is closed over it, but the 

 digestive cavity is still widely open below. The darm -entoblast 

 formed by columnar cells does not reach the chorda-entoblast, but is 



