Chap. 19 DEVELOPMENT 379 



ing usually soon gives way and the layers seem never to have been connected. 



The chorion of reptiles and birds is united with the allantois which contains 

 many blood vessels. Together they rest closely against the porous egg shells, 

 and function as a respiratory organ. 



The Allantois. Like the yolk sac, the allantois is an outgrowth of the 

 digestive tract but has a different function (Fig. 19.13). In birds, it fills most 

 of the space between amnion and chorion and fusing with the chorion (cho- 

 rioallantoic membrane) becomes an important respiratory organ. It is also 

 a temporary urinary bladder. 



In mammals, except the guinea pig and some other rodents, and the 

 primates including man its walls may fuse with the chorion and become part 

 of the embryonic section of the placenta. It then functions in the transfer of 

 food, respiratory gases, and waste products between mother and embryo. 



The placenta is discussed in later paragraphs that deal with the human 

 embryo. 



Umbilical Cord. As the embryo grows, the folds of the amnion surround- 

 ing the stalks of the yolk sac and allantois come together in a ventral tube 

 (Fig. 19.15). In the higher mammals this tube is the umbilical cord that 



Myometrium 



Decidua 

 Parietalis 



Decidua 

 Capsularis 



Amnion 



Chorion 

 Frondosum 



■yj Decidua 

 Basalis 



Yolk-sac 



Anterior fornix of vagina 



Fig. 19.15. Outline diagram of human uterus showing the placenta, sacs and 

 embryo. The placenta consists of the chorion where the villi have greatly developed 

 — over most of it, they have disappeared — and of the decidua basalis, a part of the 

 wall of the uterus. Compare with Figure 19.14. Decidua capsularis is the part which 

 covered the embryo when it was first implanted. Placenta and sacs are parts of the 

 afterbirth. (Courtesy, Patten: Human Embryology, ed. 2. New York, The Blakis- 

 ton Co., 1953.) 



