104 THE 



the cells differ more markedly from one another in shape and 

 size ; and the segmentation cavity is much smaller relatively to 

 the entire ovum, and is excentric instead of central in position. 

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

 differences may be attributed to the greater amount of food-yolk 

 present in the frog's egg. 



2. The Epiblast. 



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

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

 upper half are the epiblast cells, while the larger unpigmented 

 cells of the lower half, in which the yolk-granules are mainly 

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



BP __ 



v 



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



The distinction between the two is not an absolute one, the cells 

 at the equator of the egg being 1 intermediate 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 



