456 ARTHUR ROBINSON AND RICHARD ASSHETON. 



according to the account of Scott and Osborn (48)^ is enclosed 

 by the neural folds. 



Durham (9) describes a neurenteric canal and a blastopore 

 present at the same time in Rana ; and Sidebotham (51) asserts 

 that the neural folds do not enclose the blastopore, which is 

 closed subsequently to the meeting of the neural folds, and 

 that the anus is derived from an independent proctodseal 

 invagination. 



Morgan^s (39) interesting observations tend to a solution 

 of the difficulty caused by the preceding contradictions, inas- 

 much as they show that in Amblystoraa punctatum the 

 mesial portions of the lateral lips of the blastopore are first 

 approximated, the blastopore thus becoming hour-glass-shaped, 

 and then fused, so that the previously single opening is divided 

 into two ; the anterior of the two is enclosed by the medullary 

 folds, and becomes the neurenteric canal ; the posterior remains 

 as the anal orifice. Goette (15) has lately described a some- 

 what similar condition in Bombinator. 



In Rana helecina, according to Morgan (39), there is 

 behind the blastopore a groove communicating posteriorly in 

 a dark spot, which is a later formation than the blastopore. 

 In describing a very similar groove in Bufo lentiginosus 

 he states that it is separated from the archenteron by three 

 embryonic layers, a point of importance in any comparison of 

 this region with the primitive streak of other Vertebrates. 



Erlanger's (10a) observations upon the Anura have led him 

 to the conclusion that the blastopore closes both from before 

 backwards and from behind forwards. The closure from before 

 backwards is associated with the formation of the primitive 

 streak, which lies in front of the neurenteric canal. The 

 closure from behind forwards gives rise, in the first instance, 

 to a fused mass of cells, which rapidly differentiates into layers, 

 after which the anus forms in the situation previously occupied 

 by the posterior portion of the blastopore. Therefore, so far 

 only as the statements of the authors to whom we have 

 referred are concerned, the blastopore of Amphibians may 

 either — 



