492 AETHUR ROBINSON AND ElOHAED ASSHETON. 



of the archenteric cavity into the mesoblast in the manner 

 suggested by O. 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 proceed 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 blasto- 

 pore 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 formation of the anus, any separation of the con- 

 stituent parts of this axial rod into layers in the manner de- 

 scribed 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 the 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. 



