EMBRYO OF TWENTY-SEVEN SEGMENTS 



67 



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

 arteries and veins of mammalian embryos. 



The chick embryo is thus protected by the amnion which develops from the 

 inner, leaf of the folded somatopleure and is composed of an inner ectodermal and 

 an outer mesodermal layer. Nutriment for the growth of the embryo is supplied 

 by the yolk sac and carried to the embryo by the vitelline veins. The allantois, 



Yolk sac 



Allantois Embryo Amnion Chorion 



Shell 



Air chamber 



Shell mrwhi'tuie 



Margin of area vascidosa 



Fig. 71. — Diagram of a chick embryo at the end of the fifth day showing amnion, chorion and allan- 

 tois (Marshall). X 1.5. 



which takes its origin from the splanchnopleure of the hind-gut and is composed 

 of an inner layer of entoderm and an outer layer of splanchnic mesoderm, func- 

 tions as an organ of respiration and serves as a reservoir for the excreta of the 

 embryonic kidneys. As we shall see, the allantois becomes more important, the 

 yolk sac less important in some mam.mals, while in human embryos both yolk 

 sac and allantois are unimportant when compared to the chorion. 



