68 



THE STUDY OF CHICK EMBRYOS 



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 that envelops the yolk and forms the yolk sac. The 

 process of unequal growth, by which the embryo becomes separated from 

 the blastoderm, has been falsely described as a process of constriction 

 (see p. 80). 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 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). 



Yolk sac 



Allantois Embryo Amnion Chorion 



Shell 



Air chamber 



Shell membrane 



Margin of area vasculosa 



FIG. 71. Diagram of a chick embryo at the end of the fifth day, showing amnion, chorion and 



allantois (Marshall). X 1.5. 



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 splanch- 

 nopleure, 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. 

 In its ^ wall ramify the allantoic vessels, which have been compared to the 

 umbilical arteries and veins of mammalian embryos. 



