86 ARTHUR ROBINSON', M.D., AND RICHARD ASSHETON, M.A. 



from the hind end of the mesenteron into the neural canal (1, p. 108). 

 It forms the post-anal gut, and gradually narrows and finally atrophies." 

 This account is in the main conformable with Goette's first description 

 of Bombinator : " Die verengte sich vorherrschend von beiden Seiten 

 her, so dass sie spaltartig wurde und ihr Laugsdurchmesser in der 

 Medianebene des sich entwickelenden Embryonalkorper lag " (14, p. 

 132). And, further, Balfour (1, p. 108) states that the anus is 

 formed in Rana temporaria at an earlier period than in Bombinator, as 

 described by Goette, but in the same manner, that is, by the fusion of 

 a diverticulum from the mesenteron with a cutaneous invagination. 

 Almost the same change takes place in Rana fusca (46) ; Spencer, 

 however, as a result of his study of Rana temporaria, came to the 

 conclusion that the blastopore was transformed into the permanent 

 anus (52) ; and Alice Johnson and Lilian Sheldon (28) arrived at a 

 similar conclusion concerning the blastopore of Triton cristatus, which 

 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 Bana ; 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, inasmuch as they 

 show that in Ambli/stoma 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 somewhat similar condition in Bombinator. 



In Bana helecina, according to Morgan (39), there is behind the 

 blastopore a groove terminating 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. 



