658 



MANUAL OF PHYSIOLOGY. 



jections of the somatopleural raesoblast and epiblast also occur. 

 Thus folds are developed, consisting of somatopleural mesoblast 

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

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

 each of them presents two surfaces, one looking toward the era- 



FIG. 253 A. 



Figs. 253 A, 253 B, and 254 are 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 yelk sack. I, II, 

 III, and IV, are transverse sections 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 formation of the allantois. e. Embryo, y. Yelk. pp. Pleuroperitoneal fissure. 

 vt. Vitelline membrane, af. Amniotic food. al. Allantois. 



bryo and the other toward the vitelline membrane. As they 

 meet over the back of the embryo the folds become fused, the 

 membranes looking toward the embryo joining to form the amnion 

 proper, whilst those next the vitelline membrane unite to form 

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



