THE HYPOBLAST AND MESOBLAST. 109 



of the mesenteron, which accompanies the elongation of the 

 embryo and the enlargement of the mesenteric cavity. The 

 mesenteron and the segmentation cavity may, as shown in 

 Figs. 52 and 54, communicate with each other for a time during 

 these changes. 



CM 



Y 



FIG. 55. Sagittal section of a Frog Embryo after the disappearance of the 

 segmentation cavity and completion of the mesenteron. x 25. 



BP, blastopore. CH, notochord. E, epiblast : the cell outlines and the distinction 

 between the epidermic and nervous layers are not shown. H, hypoblast. M, mesoblast. 

 T, meseuteron. Y, yolk-cells. 



4. Formation of the Hypoblast, the Notochord, and the Mesoblast. 



During the formation of the mesenteron, the cells forming 

 its walls (Figs. 54 and 56) become arranged in two concentric 

 layers : an inner layer, the hypoblast, which forms the true wall 

 of the mesenteron ; and an outer layer, the mesoblast (Fig. 56, M), 

 which lies between the hypoblast and the epiblast. 



The splitting off of the mesoblast commences in the dorso- 

 lateral walls of the mesenteron, and spreads towards the median 

 plane, both dorsally and ventrally. Before this splitting reaches 

 the mid-dorsal plane, a pair of longitudinal clefts appear along 

 the dorsal surface, by which a median longitudinal rod of cells 

 (Fig. 56, CH) is cut off from the two laterally placed mesoblast 

 sheets, M. This rod, CH, remains attached to the hypoblast for a 

 short time after the mesoblast sheets are completely separated ; 

 but very shortly afterwards the rod in its turn splits off from 

 the hypoblast, and becomes the notochord. 



