474 ARTHUR ROBINSON AND RICHARD ASSHETON. 



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 (62) 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 poste- 

 rior 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 diflferent. 

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

 although apparently a new perforation, is really a reopening 

 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 speak- 

 ing, persists as the anus. 



The Primitive Streak of the Frog and other 

 Vertebrates. 



The term primitive streak appears to have been first applied 

 to the dark line which appears in the avian blastoderm at the 

 posterior part of the area pellucida. This Hue is generally 

 looked upon as the optical expression of a linear thickening 

 and fusion of the blastodermic layers, though KoUiker (31, 

 p. 134) maintains that it is due to proliferation of the epiblast- 

 cells alone. It seems certain, however, that it is impossible, 



