104 



THE FEOG. 



A 



t he cells differ more markp.dly from one another in sh ape and 

 BJze ; and th e segmentation cavity is rmirh puna.ner relative ly to 

 t he entire o vum, and is excentric instead of central in positio n J 

 From the description given above it will be seen that all these 

 diffp.rpnnppjnay jje attrib iit"*^ ^'" ^^'^ fyfnliftr amount o f food-yol k 

 present in the frog's e^Et. 



2. T he Epiblast . 



Of the two kinds of cells of which the egg consists at the 

 close of segmentation (Fig. 50), ^ e smaller pigmented cells of the 



^BEP^— ^^^^ "'^'0 t 3ie. epiblast c ells, wTiila f'Tia 1g.r<TPT T|ppi"prypQr.fa.^ ■ 



c ejls of the lower half, in which t he yolk-granules are m ainly- 

 contained, mav be spoken of as the lower layer cells or yolk-cells. 



Fig. 51. — Median sagittal section of a Frog Embryo, showing the spreading 

 of the epiblast and the commencing formation of the mesenteron. x 25. 



B, blastocoel or segmentation cavity. BP, lip of blastopore. EE, outer or epidermic 

 layer of epiblast. EBT, inner or nervous layer of epiblast. Y, lower layer or yolk cells. 



T he distinction between the two is not an abs olute o ne, the cells 

 at the equator of the egg being mtermediate in all respects, 

 between the epiblast and the yolk-cells. As seen from the 

 surface, the limit is indicated by the boundary line between the 

 black and the white areas of the egg, and at the close of seg- 

 mentation these two areas are approximately equal in extent. 

 In the succeeding stages the black area increases rapidly at the 

 expense of the white area (Figs. 51, 52, 54), and in a few hours 

 the pigmented epiblast cells have covered the whole of the egg 



