COMPARATIVE ANATOMY OF THE UROGENITAL SYSTEM 293 



The amnion and the chorion are formed by a fold of the body wall (somatopleure) which 

 rises up around the embryo. The folds of the two sides meet above the embryo and fuse 

 across. The outer limb of the fold becomes the' chorion, the inner limb the amnion (see Fig. 

 66^4 and B). The amnion forms a sac inclosing the embryo. The chorion is the outermost 



Fig. 66. — Diagrams to illustrate the mode of formation of the embryonic membranes of amniotes 

 and of the placenta. A, cross-section through an early stage of the formation of the amnion and 

 the chorion; a fold of the somatopleure is seen rising up at a; the outer wall of the fold b becomes the 

 chorion, the inner wall c, the amnion. B, later stage after completion of the process of formation of the 

 amnion and chorion; the fold fuses across above the dorsal surface of the embryo forming two mem- 

 branes, an outer chorion b which incloses embryo and yolk sac, and an inner amnion c, which incloses 

 the embryo. C, sagittal section of a later stage of the embryo to show the origin of the allantois k as an 

 evagination from the digestive tract m. D, later stage following C, showing the spreading of the 

 allantois between the chorion and the amnion and yolk sac; note that the outer wall of the allantois 

 is in contact with the chorion, the two together forming the chorio-allantoic membrane q. E, formation 

 of the placenta in mammals by the penetration of the chorio-allantoic membrane q into the wall of the 

 uterus n; the penetration takes the form of treelike ingrowths which are called the chorionic villi 0; note 

 small size of the mammalian yolk sac d; the placenta consists of the inner part of the uterine wall and 

 the chorionic villi; the latter are generally restricted to certain areas of the chorio-allantoic membrane. 

 In C, D, and E the two layers of which the chorion, amnion, allantois, and yolk sac are each composed 

 are omitted for simplicity, a, amniotic fold of the somatopleure; b, chorion; c, amnion; d, yolk sac; 

 e, somatic mesoderm; /, splanchnic mesoderm; g, ectoderm; h, entoderm; i, notochord; j, neural 

 tube; k, allantois; I, body of embryo, head to the left; m, digestive tract of embryo; re, wall of uterus; 

 0, chorionic villi; p, placenta; q, chorio-allantoic membrane. (Suggested by figures in Hertwig.) 



membrane of the embryo. The yolk sac and allantois are between chorion and amnion on 

 the ventral side of the embryo (Fig. 66C). 



The yolk sac and allantois are highly vascular, their blood vessels being named the vitelline 

 and umbilical (allantoic) vessels. Much has already been said of the vitelline veins. The 



