292 



Basic Structure of Vertebrates 



Fig. 241. Diagram of the fetal structures of a mammal. Sagittal section. The 

 broken lines represent mesoderm. (A) Amnion; (AL) cavity of allantois; (B) brain; 

 (C) chorion; (E) enteron; (EX) extra-embryonic coelom; (H) heart; (NC) noto- 

 chord; (NT) neural tube; (P) placental region of allantois and chorion; (SM) 

 somatopleure; (SP) splanchnopleure; (V) chorionic villi; (YS) cavity of yolk-sac. 

 (Courtesy, Neal and Band: "Chordate Anatomy," Philadelphia, The Blakiston 

 Company.) 



or embryonic body-wall (Figs. 239, 240). The embryo becomes com- 

 pletely surrounded by such a fold, which then grows in centripetally 

 from all directions and finally encloses the embryo. Where opposite 

 edges of the fold meet above the embryo, they coalesce. Reference to 

 Figs. 239 and 241 will serve better than description to make clear the 

 resulting relations of layers and spaces. 



The somatopleural folds which give rise to the amnion and chorion 

 are, at the time of their formation, a living part of the embryo. The 

 statement that the folds eventually enclose the embryo anticipates the 

 fact that the amnion and chorion do not become any part of the adult. 

 Therefore "the embryo" which the folds enclose is the definitive body 

 region of the embryo. Everything else is conveniently referred to as 

 extra-embryonic. 



The allantois, an outgrowth from the hind region of the enteron 

 (Figs. 239, 241), is a product of the splanchnopleure and therefore 

 lined by endoderm. Becoming extensively fused with the chorion, the 

 apposed mesodermal layers of the two membranes develop a rich net- 



