660 MANUAL OF PHYSIOLOGY. 



FORMATION OF THE MEMBRANES. 



(1) The Amnion. The mesoblast around the embryo becomes 

 thickened, and is split into two distinct layers ; this cleavage is at 



FIG. 253 A. 



-2P 



Figs. 253 A, 253 B, and 254 arc diagrammatic views of sections through 

 the developing ovum, showing the formation of the membranes of the chick. 

 (Foster and Balfour.) A, B, C, D, E, and F are vertical sections in the long 

 axis of the embryo at different periods, showing the stages of development 

 of the amnion and of the yolk-sac. I., II., III., and IV. are transverse sec- 

 tions at about the same stages of development, i., ii., and iii. give only the 

 posterior part of the longitudinal section, to show three stages in the forma- 

 tion of the allantois. e. Embyro. y. Yolk. pp. Pleuroperitoneal fissure. 

 vt. Vitelline membrane, a/. Amniotic food. a/, allantois. 



first confined to the neighborhood of the embryo, but gradually 

 spreads over the whole blastoderm. 



The upper of these two layers of the blastoderm receives the 

 name of the somatopleure, and is engaged in the formation of the 

 body walls of the embryo and the amnion. The lower one is 



