102 AETHUE EOBINSON, M.D., AND EICHAED ASSHETON, M.A. 



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 notochord 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 Ruscoiri 

 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 of the chick is in the frog the whole of the blastoporic lip, whether 

 fused or not. 



Relation of Anus to Blastopore, 



The relation of the anus to the blastopore has been a much 

 controverted subject. Goette (14) and Balfour (1) described it as an 

 entirely separate perforation of the body-wall ventral to the blastopore ; 

 Spencer (52) stated that the blastopore became directly the anus ; 

 Sidebotham" (51), who gave a most careful and exact account of the 

 facts, agreed with Goette and Balfour on this point, and, failing to 

 recognise the primitive streak in the frog, described it as they had done, 

 as a new opening independent of the blastopore. In Erlanger's (10a) 

 opinion it is a secondary opening in the situation of the posterior part 

 of the original blastopore. Those who have followed our account so far 

 will observe that it agrees most closely with those of Goette, Balfour, 

 and Sidebotham on this point ; but, since the perforation takes place 

 within the primitive streak, the conclusions we draw from the facts are 

 different. We infer, therefore, with Erlanger, that the anus of the frog, 

 although apparently a new perforation, is really a re-opening of a 

 temporarily closed portion of the original blastopore. 



Since the perforation occurs at the base of the diverticulum of the 

 archenteron (which, it will be remembered, was formed by the closing 

 in of the ventral portions of the lateral lips of the blastopore), it may 

 be supposed that it is the most ventral end of the blastopore which, 

 morphologically speaking, persists as the anus. 



