146 



THE PKOG. 



(Fig. 61, TP) is formed, of which the floor and sides, a s well as 

 tIie_ roof, are formed"of a sinde layer of hypoblast c ells. The 

 hinder or intestinal region of the mesenteron ( Fig. 61, Ti), has 

 much the same relations as_bgfore^ts roof being thia, but its 



floor and sides (Fig. 70) of great thickness. The mass of yolk- 

 cells, forming the floor of the intestinal region, becomes more 

 compactly arranged and more definitely restricted ; in front it is 



