120 ARTHUR ROBINSON, M.D., AND RICHARD ASSHETON, M.A. 



Before the commencement of the separation of the chorda from the 

 hypoblast there are, here and there, appearances which indicate a pro- 

 jection of the archenteric cavity into the mass of chorda-cells, in some 

 cases merely as a cleft-like space, in others as a more distinct 

 diverticulum. 



In Rana fusca 0. Schultze found a peculiar secondary fusion of the 

 mesoblast with the epiblast in front of the dorsal lip of the blastopore, 

 " bildet sich gegen Ende der Gastrulation von der Mitte der dorsalen 

 Urmundlippe aus eine nach dem Kopfe hin vorwarts schreitende 

 lineare Verwachsung des ausseren und mittleren Blattes aus " (45). 

 This line of fusion he compares to the primitive streak. 



We have already said that in Rana temporaria there is no true 

 layer of mesoblast along the dorsal axial line in front of the blastopore, 

 and that in this situation the chorda is formed by differentiation of the 

 yolk-cells ; it follows, therefore, that if the fusion occurs in Rana, it 

 will be between the last-mentioned cells and the epiblast. 



We have examined our sections carefully with reference to this 

 point, and we find that the epiblast after its separation from the yolk- 

 cells does not again fuse with the subjacent layer in the dorsal axial 

 line in front of the blastopore. 



It will be remembered that the separation of the blastopore ter- 

 minates at a distance of - 352 mm. (p. 462) in front of the dorsal lip of 

 the blastopore, that is, at the commencement of the primitive streak. 

 In other words, the dorsal lip of the blastopore has an antero- 

 posterior length of "352 mm., and during the early stages this length 

 does not increase ; therefore there is no forward extension of the 

 primitive streak in front of the blastopore. 



The mere fact that in Rana temporaria the distance from the dorsal 

 margin of the blastopore to the point at which the epiblast is separated 

 as a distinct layer remains constant throughout the earlier stages is 

 not a proof that the blastoporic lips undergo no concrescence from 

 before backwards, but it is a proof that the epiblast after its separation 

 does not again fuse with the subjacent layer. 



The Formation of the Coslom. 



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

 blast, at the sides of the chorda, it is possible to find at irregular 



