FORMATION AND PATE OF THE PRIMITIVE STREAK. 473 



or anus of Rusconi. To this structure and to the groove upon 

 it we have applied the terms primitive streak and primitive 

 groove respectively. 



If one of our sections figured^ e. g. fig. 15, is compared with 

 the figure of a transverse section of the primitive streak of a 

 chick on p. 155 of Balfour^s 'Comparative Embryology/ 

 vol. ii (second edition), the resemblance between the two 

 sections is very marked. 



In each case there is an intimate fusion between epiblast and 

 mesoblast, and a more uncertain fusion between mesoblast and 

 hypoblast. In each case the surface is marked by a groove. 



Nor is it at all impossible that the loose pigmented cells 

 noticed above, and lettered ME" in figs. 14 and 15, may be 

 compared with Balfour^s 'Hayer of stellate cells^' shown in 

 the figure in the 'Comparative Embryology^ referred to. In 

 both cases they seem to arise from the hypoblast rather than 

 the epiblast. In the frog, however, they are unrecognisable a 

 short distance from the streak. 



In comparing the two streaks with regard to their relations 

 to the rest of the embryo, we find that we must use the term 

 primitive streak in a rather wider sense than it is usually 

 used. In the chick the anterior limit of the primitive streak 

 may be said to be marked by the posterior end of the noto- 

 chord with which the streak is fused. 



The structure we have so far called primitive streak runs 

 from the ventral lip of the blastopore. It is, however, 

 obvious that had the concrescence continued a little further, so 

 that the whole anus of Rusconi had been obliterated by the 

 fusion of the blastoporic lips, the primitive streak would 

 have then commenced at the posterior end of the notochord 

 as in the chick ; for the notochord and neural folds in the frog 

 are continuous with — that is to say, fused with —the dorsal lip 

 of the blastopore. It is, therefore, clear that the homologue 

 of the primitive streak ofthe chick is in the frog the 

 whole ofthe blastoporic lip, whether fused or not. 



