FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES 



909 



AMNION 



ALLANTOIS 



SPLANCHNOPLEURE OF ALLANTOIC SAC 



EXTRA- EMBRYONIC MESODERM 



Fig. 370. Diagrams of extra-embryonic membranes. (A) Transverse section of yolk 

 sac and developing body in teleost and elasmobranch fishes showing relation of body 

 layers to the yolk mass. (B) Transverse section of yolk sac and forming serosa (chorion) 

 in reptiles, birds, and prototherian mammals. (C-E) Diagrams showing extra-embryonic 

 membranes in the pig. (C) Conditions in 16-17 somite pig, age approximately 16 days. 

 The ends of the diagram have been omitted in part, because of length of embryonic vesicle. 

 (D) Conditions in embryo of 5 mm. or about 17-18 days of age. (E) The extra- 

 embryonic membranes in embryo of about 4-5 weeks of age. 



derm and mesoderm, i.e., the splanchnopleure, as the extra-embryonic 

 coelom tends to separate the splanchnopleure from the somatopleure 

 in these forms (fig. 370B). 



b. Amnion 



The amnion is a specialized sac which comes to encompass the embryo in 

 reptiles, birds and mammals (fig. 370B-E). Because of its restriction to these 

 vertebrates, the reptiles, birds and mammals are grouped together as the 

 Amniota, the fishes and amphibia being designated as the Anamniota. 



Eggs which are spawned into the surrounding water, as in fishes and am- 

 phibia, are cradled or cushioned by the surrounding fluid, and the embryo 

 is free to develop without undue pressure from any side. In the Amniota, 

 however, this watery environment must be established artificially and hence 

 the amnion is formed to accommodate and enclose the fluid of this individ- 

 ualized embryonic "swimming" pool. 



