146 



THE FKOG. 



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

 the roof, are formed of a single layer of hypoblast cells. The 

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

 much the same relations as before, its roof being thin, 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 



