CHORDATE ANATOMY 



The amnion and chorion are simultaneously produced by an up-rising 

 fold of the somatopleure (ectoderm accompanied by mesoderm) or 

 embryonic body-wall (Fig. 74). The embryo becomes completely sur- 

 rounded by such a fold which then grows in centripetally from all direc- 

 tions and finally encloses the embryo. Where opposite edges of the fold 

 meet above the embryo they coalesce. Reference to Figs. 74 and 75 

 will serve better than description to make clear the resulting relations of 

 layers and spaces. 



Fig. 75. — Diagram of the fetal structures of a mammal. (The broken lines represent 

 mesoderm.) A, amnion; AL, cavity of allantois; B, brain; C, chorion; E, enteron; 

 EX, extra-embryonic coelom; H, heart; NC, notochord; NT, neural tube; P, placental 

 region of allantois and chorion; SM, somatopleure; SP, splanchnopleure; V, chorionic 

 villi; YS, cavity of yolk-sac. 



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. 72Z), 74, 75), is a product of the splanchnopleure and is lined by 

 endoderm. In the region of its fusion with the chorion the apposed meso- 

 dermal layers of the two membranes develop a rich network of fine blood- 

 vessels which are connected bv the allantoic arteries and veins to the main 



