FIG. 547. Diagrammatic figures illustrating the formation of the foetal 

 membranes of a mammal, a, embryo before appearance of amnion; b, embryo 

 with yolk-sac and developing amnion; c, embryo with amnion closing and 

 developing allantois; d, embryo with, villous subzonai membrane, and with 

 mouth and anus; e, embryo in which vascular layer of allantois is applied to 

 subzonai membrane, and has grown into the villi of the latter, yolk-sac 

 reduced, amniotic cavity increasing. A, embryonic thickening of external 

 layer; Ah, amniotic cavity; Al, allantoic stalk; Am, amnion; Ch, chorion; 

 Chz, chorionic villi; D, D', zona radiata; Dg, umbilical stalk; Dh, intestinal 

 cavity; Ds, cavity of embryonic (blastodermic vesicle), later of the yolk- 

 sac (umbilical vesicle); E, embryo; /, embryonic thickening of inner layer; 

 M, of middle layer; ,5 h, subzonai membrane (serous envelope); Sz, villi of 

 subzonai membrane. (From Sedgwick's Zoology, after Kolliker.) 



681 



