454 AETHUE EOBINSON AND EICHAED ASSHETON. 



in Bombinator (40), O. Schultze in Rana fusca (45), and 

 by Scott and Osborn in the Newt (48). 



Whilst it is proceeding the enclosure of the yolk has been 

 rapidly taking place. "It is effected by the epiblast growing 

 over the yolk at all points of its circumference" (1, p. 102). We 

 have in this latter statement of Balfour's a very fair summary 

 of the general opinion that the epiblast and yolk are distinct 

 parts of the ovum ; indeed; Balfour speaks of the yolk-cells as 

 "these large cells which are part of the primitive hypoblast" 

 (1, p. 101), with which, in our opinion, they cannot fairly be 

 compared ; and we shall speak of them in the sequel merely as 

 yolk-cells, though probably the more correct term would be 

 germ segments. It is not necessary, however, to enter into a 

 discussion of the mode of extension of the epiblast at the pre- 

 sent moment, and we can proceed, therefore, to a point which 

 is 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 Eusconi, at the margin of 

 which the surface epiblast becomes continuous with the cells 

 lining the archenteric cavity, which, on account of their posi- 

 tion, 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 



