FORMATION OF THE GERMINAL LAYERS. 



121 



egg, which are smaller, pigmented, more regularly arranged, 

 and comparatively free from food-yolk : secondly those of the 

 lower half of the egg which are larger, less regular, and almost 

 free from pigment, but much distended by food-yolk, which is 

 present in such quantity as to render them comparatively inert. 



The former are the epiblast cells : the latter may conveniently 

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



The epiblast shows almost from the first a distinction into 

 two layers : the most superficial cells being somewhat cubical 

 in shape and closely applied side by side so as to form a con- 



Fig. 25. Longitudinal vertical section through a frog embryo at a later 

 stage in the formation of the mesenteron. x 30. 



H, invaginate hypoblast : M, mesoblast : M N, mesenteron : N, noto- 

 chord : SC, segmentation cavity : YP, yolk plug, tilling up the blastopore. 



tinuous and deeply pigmented layer ; while the deeper epiblast 

 cells are more spherical, less strongly pigmented, and loosely 

 arranged in a layer two or more cells deep. 



The limit between the epiblast and the lower layer cells is 

 indicated on the surface of the egg by the boundary line between 

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

 ation these two areas are approximately equal in extent. In the 

 succeeding stages the black area increases rapidly at the expense 

 of the white area, and in a few hours the pigmented epiblast 

 cells have covered the whole of the egg with the exception of a 



