THE HYPOBLAST AND MESOBLAST. 109 



of the mesenteron, whicli accompanies the elongation of the 

 embryo and the enlargement of the mesenteric cavity. _'rhe 

 me senteron and the segmentati on cavity may, as shown in 

 Fig s. 52 "and b4>, communicate with each other for a tim e during 

 these changes. 



Fig. 55. — Sagittal section of a Ifrog Embryo after the disappearance of the 

 segmentation cavity and completion of the mesenteron. y. 2.5. 

 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, mesenteron. TT, yolk-cells. 



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



D uring the fo rmatio n of the mesenteron, the, cells forming 

 its wall s fKgs. 54 and 56) b ecome arranged in two con centric 

 layers : — an inner layer, the hypoblast, w hich forms the true wall 

 of the mesenteron ; and an o uter layer, the m ^soV^""^- (-^^g 56, m), 

 which lies between the hypoblast and the epiblast. 



Th e splitting off of the mesoblast commences in the dorso- 

 lateral walls o f the mesenteron , an d spreads towards the me dian 

 glanp, both dorsajly and ventra lly. Eefnre thi.s .s plitti rg ■rpa^^\^of! 

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

 the dorsal surface, by wh ich a median longitudinal ro rl nf nftUg 

 (Fig. 66, Ch) is cut off from the two laterally placed mesob last 

 sheets, M . This rod, CH, remains 'attached to the hypoblast fora 

 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. 



