494 ARTHUR ROBINSON AND RICHARD ASSHETON. 



At a later period the mesoblastic plates become entirely 

 separated from the dorsal hypoblast, and afterwards the chorda 

 also is separated ; it then forms a distinct rod, which lies free 

 below the floor of the neural groove and above the dorsal hypo- 

 blast except at its posterior extremity, where it terminates in 

 the mass of cells which form the anterior wall of the neuren- 

 teric canal. 



Before the commencement of the separation of the chorda 

 from the hypoblast there are, here and there, appearances 

 which indicate a projection 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 Ran a fusca O. 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 Gastrula- 

 tion 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 Kana 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, there- 

 fore, 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 epiblast 

 terminates 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 ; there- 

 fore there is no forward extension of the primitive streak in 

 front of the blastopore. 



