FORMATION AND FATE OF THE PRIMITIVE STREAK. 85 



allowed by all, namely, that the superficial extension of the epiblast 

 terminates at the margin of a large circular opening, called the 

 blastopore or the anus of Rusconi, at the margin of which the surface 

 epiblast becomes continuous with the cells lining the archenteric 

 cavity, which, on account of their position, are spoken of as 

 hypoblast, although those situated in the dorsal wall of the cavity are 

 said to be invaginated epiblast in the Newt (48), or partly epiblast 

 and partly differentiated yolk-cells in the Anura (1, p. 102), whilst 

 the lateral walls and the floor of the archenteron are formed by 

 modified yolk-cells. 



Houssay (26) and Moquin-Tandon (38) are, so far as we are aware, 

 the only authors who have combated this supposititious mode of 

 formation of the archenteron. Houssay examined the Axolotl and 

 Moquin-Tandon several Anura. They both state that the archenteron 

 is formed by splitting amidst the yolk-cells, from which it follows that 

 both in the Urodela and Anura there is no invagination of epiblast 

 into the yolk, and that the archenteron is surrounded by modified 

 yolk-cells which eventually form a distinct hypoblastic layer, which is 

 continuous with the epiblast at the margin of the blastopore. 



The blastopore is unanimously defined in Amphibians as an opening, 

 round the margins of which the epiblast and hypoblast are continuous 

 aud which forms the passage of communication between the archen 

 teron and the exterior. But concerning the formation and surround 

 ings of the archenteron there are two opposed opinions, which are — 



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

 Ainphioxus by invagination, and is lined partly by modified yolk-cells 

 and partly by invaginated epiblast. 



2. That the archentei'on 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 



