662 



MANUAL OF PHYSIOLOGY. 



Thus folds are developed, consisting of soraatopleural mesoblast 

 and of epiblast, which tend to grow upwards and meet over the 

 back of the embryo. These folds are the amniotic folds, and each 

 of them presents two surfaces, one looking towards the embryo 

 and the other towards the vitelline membrane. As they meet 

 over the back of the embryo the folds become fused, the mem- 

 branes looking towards the embryo joining to form the amnion 



FIG. 254. 



IV 



Diagrammatic sections of an embryo, showing the destiny of the yolk- 

 sac, ys. vt. Vitelline membrane, pp. Pleuroperitoneal cavity, ac. Cavity 

 of the amnion. a. Amnion. a! '. Alimentary canal, ys. Yolk-sac. 



proper, whilst those next the vitelliue membrane unite to form 

 the false amnion, which, separating from the amnion proper, re- 

 tires towards the vitelline membrane, with which it unites to form 

 the primitive chorion. 



The true arauion then is a sac formed of an outer layer derived 

 from the mesoblast and an inner layer derived from the epiblast. 

 The false amnion likewise consists of mesoblast and epiblast, but 



