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 witli amnion closing ami 

 developin.i; .illantoU; d, embryo wit 'i \-ilhnis Mib/onal membrane, and with 

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

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

 reduced, amniotic cavity increasing. ,1, embryonic thickening ol external 

 layer; Ah, amniotic cavity; A I, allantoic stalk; Am, amnion; ('I:, dioriou; 

 Chz, chorionir villi; I). D', zona radiata; Dg, umbilical stalk; Dh, intestinal 

 cavitv: /K, cavity f embryonii (blastodermii vesicle), later of the yolk- 

 sac ('umbilkj.! veiicki; , embryo; /, embryon" thickening ' innei layer; 

 M, of middle laj-er; -Sh, subzonal membrane (serous envelop-:): Sz, villi of 

 gubzonal membrane. (From Sedgwick's Zoology, after KolHker.) 



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