118 ARTHUR ROBINSON, M.D., AND RICHARD ASSHBTON, M.A. 



At this period the chorda is continuous with the hypoblast, and the 

 mesoblastic plates and the hypoblast are still fused just outside the 

 chorda, as well as on the ventral aspect. 



Along the lines of attachment of the mesoblastic plates to the hypo- 

 blast, at the sides of the chorda, slight depressions are noticeable 

 (fig. 6, D), which appear to indicate a continuation of the archenteric 

 cavity into the mesoblast in the manner suggested by 0. Hertwig (21) 



Eventually the mesoblast in front of the blastopore is entirely 

 separated from the hypoblast, first dorsally and then ventrally, the 

 ventral fusion remaining in the region of the liver for a considerable time. 



These changes do not take place simultaneously, but appear to pro- 

 ceed from before backwards, so that the mesoblast is still adherent to 

 the chorda and the hypoblast, in the posterior portion of the embryo, 

 for a short time after the separation has been completed more 

 anteriorly. 



The Primitive Streak Mesoblast. 



The fusion of all the germinal layers in the lips of the blastopore 

 continues after the margins of the orifice have concresced, and there is, 

 therefore, behind the neurenteric canal an axial rod of tissue, which is 

 grooved upon its upper surface, and which is not distinguishable into 

 definite layers (fig. 15). 



We have not been able to find in Rana temporaria, before the forma- 

 tion of the anus, any separation of the constituent parts of this axial 

 rod into layers in the manner described and figured by Erlanger (10a). 

 On the contrary, we find that the anus is formed as a perforation 

 through the fused mass. But, after the separation of mesoblast from 

 hypoblast has extended as far as the margins of the primitive streak, 

 we find that the lower cells of the mesoblast are more loosely 

 arranged, and that they are of more rounded form than the more 

 superficial cells. When these lower more rounded cells are followed 

 towards the streak they are seen to be continuous with the hypoblast 

 of the primitive streak. The upper and denser layer of the mesoblast, 

 traced in the same direction, is found to be continuous with epiblast 

 (fig. 15). 



These appearances suggest the idea that the mesoblast of this region 

 is formed partly from the epiblast and partly from the hypoblast, but 

 there is no definite proof that such a double formation actually occurs. 



