910 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG 



The amnion arises generally in two ways, as follows, although intermediate 

 forms are found among certain mammals (see Mossman, '37). 



( 1 ) By a dorsal folding of the somatopleure, in which anterior, lateral, 

 and posterior amnionic folds project dorsad and fuse (see figures 23 8B; 

 242C and G; 370B-E). This method is found in reptiles, birds, pro- 

 totherian mammals, the opossum, pig, rabbit, etc. 



(2) The second main method is by cavitation, i.e., a cavity develops within 

 the cells forming the inner cell mass of the early embryo (fig. 372A 

 and B). Found in human, mouse, rat, etc. In the monkey, the forma- 

 tion of the amnion is somewhat intermediate between the folding and 

 cavitation methods. 



c. Chorion (Serosa) 



The formation of the amnion by the folding method also results in the de- 

 velopment of the chorion or serosa, in that it separates the somatopleure from 

 the splanchnopleure of the yolk sac (fig. 370B and C). However, in those 

 forms which utilize the hollowing out or cavitation method of amnion forma- 

 tion as in the human, the chorion forms directly by the attachment of extra- 

 embryonic mesoderm to the inner aspect of the trophectoderm (fig. 372 A 

 and B). 



d. Altantois 



In most fishes and amphibia, external respiration of the developing embryo 

 is possible by a direct interchange of oxygen and carbon dioxide across the 

 perivitelline fluid and primary embryonic membranes into the surrounding 

 watery medium. However, in eggs which are deposited on dry land, such as 

 those of birds, reptiles, and prototherian mammals, a specialized embryonic 

 structure, the allantois, is formed to permit external respiration to occur. The 

 allantoic diverticulum arises as a mid-ventral outpushing of the caudal end 

 of the hindgut (fig. 370C). The allantois is a hollow, sac-like structure com- 

 posed of entoderm on the inside and splanchnopleuric mesoderm externally. 

 As it extends outward, blood vessels develop in the mesoderm. It eventually 

 comes in contact with the chorion with which it fuses to form the chorio- 

 allantoic membrane (fig. 370D and E). The chorio-allantoic membrane in 

 reptiles and birds contacts the surface membranes of the shell (fig. 299E). 



In the higher mammals an allantoic diverticulum also is formed. In this 

 group of vertebrates, the allantois not only serves the function of external 

 respiration but also is the main instrument in nutrition. In the human embryo, 

 the entodermal evagination from the hindgut forming the allantoic diverticulum 

 is small, and blood vessels develop precociously within the mesoderm of the 

 body stalk (see figure 372B). These blood vessels course distad to the de- 

 veloping chorion and its villi where external respiration is accomplished. 



