FORMATION OP THE GERMINAL LAYERS 



111 



very remote ancestor, i.e., as indicating that the higher animals 

 are descended from forms which, like the Protozoa nowadays, 

 remained throughout their lives single cells. 



£. The Crermiiial Layers. 



At the close of segmentation we have seen that the egg con- 

 sists of cells of two kinds ; firstly, those of the upper half of 

 the egg, which are smaller, pigmented, miore regularly arranged, 

 and comparatively free from food-yolk ; secondly, those of the 

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



Fig. 24. — Longitudinal vertical section through a frog embryo at a 

 - later stage in the formation of the mesenteron. 



H, invaginate hypoblast ; "M, mesoblcist ; MN, mesenteron ; N. noto- 

 chord ; SC, segmentation cavity ; YP, yolk plug, filling up the blasto- 

 pore. 



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 

 two layers ; the most superficial cells being somewhat cubical 

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

 tinuous and deeply pigmented layer ; while the deeper cells are 



