108 



THE FKOG. 



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

 t]ie_gosition of the cells of its floor, the niesenteric_s lit (.b'ig. 52, 

 t) becomes widened onTint o a large casi tv (Fig. 54, t) ; the 

 rQ of or dorsal wall of Wln'cE^ formed by a well-defijiec^ layer n f 

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



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 blastopore. BP', 

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

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

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



contact TTJth thn npih]'mb3 __while _the^oor and sides consist of 

 Y olk-cells (Fig. 54,jy ). ^ 



During this change the segmentation cavity, B, gradually 

 becomes reduced in size, and ultimately disappears altogether. 

 It ca n always be distinguished from t^he Tnesen fernri by the f act 

 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 



