106 



THE FEOG. 



t he surfac e, b ut by splitting apart of the yolk-cells a s describ ed 

 above, thi s splitting bein g preceded b y the for mation of pigm ent 

 i n the adjacent surfaces of the cel ls between which the split is 

 t6 appea r. 



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 midw ay between the equ ator 

 an d the lower pole of the egg. It is very consB icuous, because 

 the pigmented epiblast ceOs^top 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, blastoccel or segmentation cavity. BP, upper or dorsal lip of blastopore. BP', 

 lower or veJitral 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 rapidl y e xtends at its extreini ties. becoming 

 s emicircu lar, 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 



