66 



THE STUDY OF CHICK EMBRYOS 



folds elongate ajidJh£jrunk_ex2aiids^kterallYjmt^ 



of the spranclmopleure connects the embryo with th e_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 unequal growth by which the embryo 

 becomes separated from the yolk has been described as a process of constriction. 

 This, as Minot points out, is an error. The splanchnopleure at first forms only an 

 oval plate on the surface of the yolk, but eventually encloses it. In Fig. 70, C and 

 D, the relation of the embryo to the yolk sac is seen at the end of the first week of 

 incubation. The viteUine vessels ramify on the surface of the yolk sac and 



A B 



Fig. 70. — Diagrams showing tlie development of the amnion, chorion and allantois in longi- 

 tudinal section (Gegenbaur in McMurrich). Ectoderm, mesoderm, and entoderm represented by 

 heavy, light, and dotted lines respectively. Af., amnion folds; AL, allantois; Am., amniotic 

 cavity; Ch., chorion; Vs., yolk sac. 



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. 69). From it develops the anlage 

 of the allantois, which, as an outgrowth of the splanchnopleure, is lined with 

 entoderm and covered with splanchnic mesoderm (Fig. 70) . It develops rapidly 

 into a vesicle connected to the hind-gut by a narrow stalk, the allantoic stalk. 

 At the fifth day the allantois is nearly as large as the embryo (Fig. 71). Its wall 

 flattens out beneath the chorion and finally it lies close to the shell but is attached 

 only to the embryo. The functions of respiration and excretion are ascribed to it. 



