DEVELOPMENT. 



185 



yolk-cells. The middle region of the body is more deeply sunk 

 in the yolk than the two ends, and the embryo thus assumes a 

 curved position (fig. 105). 



This curvature of the embryo is closely connected with the 

 formation of the embryonic membranes. On either side of the 

 ventral plate a fold of the blastoderm arises, and these folds 

 grow towards each other beneath the chorion. Ultimately they 

 meet along the middle line of the ventral plate (fig. 106), 

 and thus form a double investment, the outer layer being the 

 serous envelope, the inner the amnion. Between the two the 

 yolk passes in, as has been explained above (fig. 107). 



A 



Fig. 107. Transverse section through young Embryo of B. f/ermanica. 

 E, epiblast ; HI, mesoblast ; F, yolk-cells. 



At the same time that the embryonic membranes are forming 



v O ' 



the embryonic layers make their appearance. The ventral 

 plate, which was originally one-layered, forms the epillast or 

 outer layer of the embryo, and from this are subsequently 

 derived the middle layer (mesoblast} and the deep laver 

 (hypoblast). 



