FORMATION AND PATE OP THE PRIMITIVE STREAK. 117 



archcnteron, notochord, and floor of the neural tube are continuous in 

 the most anterior limit of the primitive streak, and is the dorsal or 

 most anterior lip of the blastopore. 



The neurenteric canal, which is bounded anteriorly by the dorsal lip 

 of the blastopore, is therefore the most anterior portion of the 

 blastopore. 



The ventral moiety of the primitive streak shortly after the perfora- 

 tion of the anus ceases to exist, or, as we have preferred to term it, 

 ceases to be " functional," and splits up. 



The dorsal moiety of the primitive streak becomes folded upon itself 

 like, and along with, the neural plate, and becomes separated from 

 the skin, and remaining " functional," gives rise to the whole of the 

 tail with the exception of the greater part of the skin. 



We cannot find at any time any trace of blastopore or primitive 

 streak anterior to any part of the neural plate or tube. 



The Formation and Separation of the Mesoblast. 



After the transformation of the superficial layer of the yolk-cells 

 into epiblast is completed the remainder of the yolk may be looked 

 upon as hypoblast and modified hypoblast, for it eventually becomes 

 transformed, partly into the true hypoblastic lining of the enteric 

 cavity, and partly into mesoblast and chorda. It is separated from 

 the epiblast, except at the margins of the blastopore, and it never 

 again, in Rana temporaria, becomes fused with the epiblast in front of 

 the blastopore, after the manner described by Schultze (45) in Rana 

 fusca. 



During the completion of the archenteron, and before the blastopoi-e 

 is closed, the mesoblast, in front of the blastopore, begins to separate 

 from the true hypoblast along the dorsolateral aspects of the archen 

 teron by a process of delamination. 



A slit-like space, which rapidly distends, appears between the 

 hypoblast and mesoblast in these regions (fig. 12, H), whence it 

 gradually extends both towards the dorsal and towards the ventral 

 middle lines ; but considerably before these clefts reach near the mid- 

 dorsal line two sagittal clefts appear. The latter clefts separate the 

 chorda in the middle line from the mesoblastic plates laterally (fig. 12, 

 S, and fig. 6, S). 



