108 THE 



marking the posterior pole (Figs. 54, 55). By an alteration in 

 the position of the cells of its floor, the mesenteric slit (Fig. 52, 

 T) becomes widened out into a large cavity (Fig. 54, T) ; the 

 roof or dorsal wall of which is formed by a well-defined layer of 

 small cells, arranged three or four deep, and lying in close 



YP 



FIG. 54. Sagittal section of a Frog Embryo just before the disappearance of 

 the segmentation cavity, x 25. 



B, blastocoel or segmentation cavity. BP, upper or dorsal lip of the blasto pore. BP', 

 lower or ventral lip of the blastopore. CH, notochord. EE, outer or epidermic layer 

 of epiblast. EN, inner or nervous layer of epiblast. H, hypoblast forming dorsal wall 

 of meseiiteron. M, mesoblast. T, mesenteron. Y", yolk-cells. YP, yolk-plug. 



contact with the epiblast, while the floor and sides consist of 

 yolk-cells (Fig. 54, Y). 



During this change the segmentation cavity, B, gradually 

 becomes reduced in size, and ultimately disappears altogether. 

 It can always be distinguished from the mesenteron by the fact 

 that it lies between the epiblast and the yolk-cells, and that its 

 wall is therefore formed on one side by epiblast cells only (Figs. 

 52 and 53, B) ; while the mesenteron, T, always has walls formed 

 by both epiblast and lower layer cells. 



Figs. 52, 54, and 55 show that the segmentation cavity 

 becomes reduced and obliterated, partly by the growth forwards 

 of the cells which form the roof of the mesenteron ; and partly 

 by a shifting in the position of the yolk-cells forming the floor 



