106 



THE PROG. 



the surface, but by splitting apart of the yolk-cells as described 

 above, this splitting being preceded by the formation of pigment 

 in the adjacent surfaces of the cells between which the split is 

 to appear. 



The mesenteric slit appears first as a slightly crescentic 

 groove on the surface of the egg (Fig. 51, bp), at the margin of 

 the spreading epiblast, and about midway between the equator 

 and the lower pole of the egg. It is very conspicuous, because 

 the pigmented epiblast cells stop sharply at its upper or convex 

 border, so that the boundary between the epiblast and yolk- 



Fig. 52. 



-Sagittal section of a Frog Embryo during the formation of 

 the mesenteron. x 25. 



B, blastocoel or segmentation cavity. BP, upper or dorsal lip of blastopore. BP', 

 lower or ventral lip of blastopore. EE, outer or epidermic layer of epiblast. EN, inner 

 or nervous layer of epiblast. H, hypoblast. T, mesenteron. Y, yolk-plug. 



cells is here an abrupt one, while round the rest of the circum- 

 ference, as shown on the right-hand side of Fig. 51, the transi- 

 tion is more gradual. 



The groove rapidly extends at its extremities, becoming 

 semicircular, then horse-shoe shaped, and finally, by meeting of 

 its limbs, a complete circle. This circular groove separates the 

 epiblast, which now ends sharply against it round its entire 

 margin, from a circular patch of yolk-cells (Fig. 52, Y, and 

 Fig. 58, A), which still remains at the surface of the egg. The 



