FORMATION AND FATE OP THE PRIMITIVE STREAK. 455 



opening, round the margins of which the epiblast and hypo- 

 blast are continuous, and which forms the passage of com- 

 munication between the archenteron and the exterior. But 

 concerning the formation and surroundings of the archenteron 

 there are two opposed opinions, which are — 



1. That the archenteron of the Amphibia is a cavity, formed 

 as in Amphioxus by invagination, and is lined partly by modi- 

 fied yolk-cells and partly by invaginated epiblast. 



2. That the archenteron is formed in situ by splitting 

 amongst the yolk-cells, and that it is entirely surrounded by 

 modified yolk-cells. 



To these opinions it will be necessary to return, but in the 

 meantime we pass to a consideration of the various accounts 

 of the fate of the Amphibian blastopore. 



This opening, after remaining for a time, is, according to 

 Balfour's account, decreased in size by the approximation of 

 its lips until it forms " a narrow passage, on the dorsal side of 



which the neural tube opens The external opening of 



this passage gradually becomes obliterated, and the passage is 

 left as a narrow diverticulum leading 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, sodass sie Spaltartig wurde und ihr 

 Langsdurchmesser in der Medianebene des sich entwickelenden 

 Embryoualkorper 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 diverti- 

 culum 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 tem- 

 poraria, 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. 



