EMBRYO OF TWENTY-SEVEN SEGMENTS 67 



Amnion and Chorion. -These two membranes are developed in all 

 amniote vertebrates (Reptiles, Birds, and Mammals). They are derived 

 from the extra-embryonic somatopleure. The amnion is purely a protec- 

 tive structure, but the chorion of mammals has a trophic function, as 

 through it the embryo derives its nourishment from the uterine wall. 

 Fig. 70 A shows the amnion and chorion developing. The head fold of 

 the somatopleure forms first and envelops the head, the tail fold makes its 

 appearance later. The two folds extend lateral, meet and fuse (Fig. 70 

 B, C}. The inner leaf of the folds forms the amnion, the remainder of 

 the extra-embryonic somatopleure becomes the chorion. The actual 



X 



FIG. 70. Diagrams showing the development of the ananion, chorion and allantois in 

 longitudinal section (Gegenbaur in McMurrich). Ectoderm, mesoderm, and entoderm repre- 

 sented by heavy, light, and dotted lines respectively. Af., Amnion folds; Al., allantois; 

 Am., amniotic cavity; Ch., chorion; Ys., yolk sac. 



appearance of these structures and their relation to the embryo have been 

 seen in Figs. 63 and 64. The amnion, with its eetodermal layer inside, 

 completely surrounds the embryo at the end of the third day, enclosing a 

 cavity filled with amniotic fluid (Fig. 71). In this the embryo floats and 

 is thus protected from injury. The chorion is of little importance to the 

 chick. It is at first incomplete, but eventually entirely surrounds the 

 embryo and its other appendages. 



Yolk Sac and Yolk Stalk. While the amnion and chorion are develop- 

 ing during the second and third day, the embryo grows rapidly. The head 

 and tail folds elongate and the trunk expands laterally until only a rela- 

 tively narrow stalk of the splanchnopleure connects the embryo with the 



