482 ARTHUE ROBINSON AND RICHARD ASSHETON, 



positive proof, we are still impressed by the fact that the 

 anus is neither the remains of the blastopore, nor is it formed 

 secondarily by reopening of the fused lips of the blastopore, 

 but it appears as a new formation below and in front of the 

 blastoporic opening ; at least Hatschek figures it in that 

 situation, and his description is as follows : — " Die Unterbre- 

 chung der Communication zwischen Darm, und Medullarrohr 

 erfolgt ungefahr gleichzeitig oder auch etwas spater als der 

 Durcbbruch des Afters. Der After bricht ventralwarts von 

 dieser Communications-offnung, die den letzten Rest des 

 Gastrulamundes reprasentirt " (16, p. 79). 



The anus of the frog and many other Vertebrata bears a 

 similar relation to the neurenteric canal, but in no other Ver- 

 tebrate except Amphioxus is the anus a distinctly new forma- 

 tion ; on the contrary, it is very evidently an aperture formed 

 by the reopening of a temporarily closed orifice, or by perfo- 

 ration of the homologue of that orifice. In view of this fact, 

 it seems very probable that future observations will modify 

 Hatschek's results^ and remove the contradiction which at 

 present exists between Amphioxus and other Vertebrata. If, 

 however, this proves not to be the case, it will then have to be 

 decided whether the condition which is found in Amphioxus, 

 or that which is so general amongst the other Vertebrata, is 

 the more primitive. 



The Fate of the Primitive Streak in the Frog. 

 In discussing the primitive streak of the frog it will be 

 convenient to consider separately — 



(I) The fate of the ventral moiety. 



(II) The fate of the dorsal moiety. 



Goette (15) has pointed out that in Petromyzon the homo- 

 logue of the primitive streak must be the whole rim of the 

 blastopore. Apparently, however, he does not extend the 

 same reasoning to the case of the frog. For in that case, 

 according to his description, and to the figure he gives, in 

 which he indicates the position of the primitive streak by a 

 bracket, he does not include the ventral lip of the anus. 



