174 



EMBRYOLOGY 



FOLDS OF AMNION 



YOLK SAC 



Fig. 102. The folds of the amnion, the allantois, and the circulation in an 

 early chick embryo. 



The amnion: The head fold advances posteriorly and a tail fold forms. It 

 grows forward and finally meets the head fold. The folds are drawn as a single 

 line but are actually a fused layer of ectoderm and mesoderm. 



The allantois: A saclike structure lined with endoderm and covered with 

 mesoderm. The umbilical veins and arteries develop in the mesoderm of the 

 allantois. 



The circulation: Note three distinct parts: an embryonic circulation with its 

 dorsal aorta and cardinal veins ; a yolk-sac circulation with a flow of blood from 

 the vitelline arteries into the vitelline veins; the allantoic circulation from um- 

 bilical arteries to umbilical veins. 



growth of the hindgut. The allantois is covered with mesoderm which carries 

 blood vessels — the umbilical arteries and veins (Fig. 102). During later 

 development the allantois grows in between the chorion and the yolk sac 

 and becomes highly vascular. Oxygen reaches the blood in the allantois after 

 passing through a porous shell, shell membranes, the chorion, and the walls 

 of the blood vessels in the allantois. The fused membrane of the allantois and 

 chorion is called the chorioallantoic membrane. It is an excellent site for the 

 growth and differentiation of isolated parts of the embryo. The isolated limb 

 bud of the chick embryo has been transplanted to the chorioallantoic 

 membrane to test its ability to self -differentiate. 



During the formation of both the yolk sac and the allantois the splanchnic 

 layer of the lateral mesoderm remains in close contact with the endoderm, 

 and for this reason the two layers are called the splanchnopleure. 



