FORMATION AND FATE OF THE PRIMITIVE STREAK. 491 



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 meso- 

 blast and chorda. It is separated from the epiblast, except at 

 the margins of the blastopore, and it never again, in Ran a 

 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 

 blastopore is closed, the mesoblast, in front of the blastopore, 

 begins to separate from the true hypoblast along the dorso- 

 lateral aspects of the archenteron 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). 



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 hypoblast, at the sides of the chorda, slight depressions are 

 noticeable (fig. 6, Z>), which appear to indicate a continuation 



