CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 



75 



developing. The head-fold of the somatopleure forms first and envelops the head, 

 the tail-fold makes its appearance later. The two folds extend laterad, meet 

 and fuse (Fig. 67 B). The inner leaf of the folds forms the amnion, the remainder 

 of the extra-embryonic somatopleure becomes the chorion. The actual appear- 

 ance of these structures and their relation to the embryo we have seen in Figs. 60 

 and 61. The amnion, with its ectodermal layer inside, completely surrounds the 

 embryo by the fourth day, enclosing a cavity filled with amniotic fluid (Fig. 68). 

 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 developing 

 during the second and third 



day, the embryo grows rapidly. 4gy- -~^Af. j m 



The head- and tail-folds elon- 

 gate and the trunk expands lat- 

 erally until only a relatively nar- 

 row stalk of the splanchno- 

 pleure connects the embryo with 

 the yolk. This portion of 

 the splanchnopleure has grown 

 more slowly than the body of 

 the embryo and is termed the 

 yolk-stalk. It is continuous 

 with the splanchnopleure which 

 envelops the yolk and forms the 

 yolk-sac. The process of un- 

 equal growth by which the em- 

 bryo becomes separated from 

 the yolk has been described as a 



process of constriction. This, as Minot points out, is an error. The splanchno- 

 pleure at first forms only an oval plate on the surface-of the yolk but eventually 

 encloses it. In Fig. 67, C and D, the relation of the embryo to the yolk-sac is 

 seen at the end of the first week of incubation. The vitelline vessels ramify on 

 the surface of the yolk-sac and through them all the food material of the yolk is 

 conveyed to the chick during the incubation period (about twenty-one days). 



Allantois. — We have seen that in the fifty-hour chick a ventral evagination, 

 the hind-gut, develops near its caudal end (Fig. 66) . From it develops the anlage 



Fig. 67. — Diagrams showing the development of the 

 amnion, chorion and allantois (Gegenbaur in McMurrich's 

 "Human Body") . Af., amnion folds; AL, allantois; Am., 

 amniotic cavity; Ds., yolk-sac. 



