76 



THE STUDY OF CHICK EMBRYOS 



of the allantois, and, as it is an outgrowth of the splanchnopleure, it is lined with 

 entoderm and covered with splanchnic mesoderm (Fig. 67). It develops rapidly 

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

 At the fifth day it is nearly as large as the embryo (Fig. 68). Its wall flat- 

 tens out beneath the chorion and finally it lies close to the secondary egg mem- 

 brane (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. 

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



Fig. 68. — Diagram of a chick embryo of the fifth day showing amnion, chorion and allantois (Mar- 

 shall). AN, inner or true amnion; A V, outer margin of the area vasculosa; AZ, outer or false amnion 

 (chorion); EM, embryo; Sli, shell of egg; SM, shell membrane; SV, air chamber; TA, allantois; 

 YS, yolk-sac. 



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 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 mammals, while in human embryos both yolk - 

 sac and allantois are unimportant when compared to the chorion. 



