FOKMATION OK THE GEKM1NAL LAYEKS 113 



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 con 

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

 cells. 



The epiblast shows almost from the first a distinction into 



N 



YP i 



M 



FIG. 24. Longitudinal vertical section through a frog embryo at 

 a later stage in the formation of the mesenteron. 



H, invaginate hypoblast;- MI mesoblast ; MN, mesenteron ; N, noto- 

 chorcl ; 8C, segmentation cavity ; YP, yolk plug, filling up the blasto- 

 pore. 



two layers ; the most superficial cells being somewhat cubical 

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

 continuous and deeply pigmented layer ; while the deeper 

 cells are more spherical, less strongly pigmented, and loosely 

 arranged in a layer two or more cells deep. 



The epiblast cells continue to increase by division, and very 

 early, owing apparently to multiplication of the cells at the 

 margin of the layer, seem to spread over the lower or yolk- 

 cells. Owing to the difference in colour of the two halves of 

 the egg, the various stages of this process can be readily 



ii 



