EMBRYO OF TWENTY-SEVEN SEGMENTS 69 



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, which takes its origin from the splanchno- 

 pleure of the hind-gut and is composed of an inner layer of entoderm and 

 an outer layer of splanchnic mesoderm, functions 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 impor- 

 tant, in some mammals, while in human embryos both yolk sac and allan- 

 tois are unimportant when compared to the chorion. 



