268 HUMAN PHYSIOLOGY. 



Formation of Fetal Membranes. The amnion appears shortly 

 after the embryo begins to develop, and is formed by folds of the 

 epiblast and external layer of the mesoblast, rising up in front and 

 behind and on each side ; these amniotic folds gradually extend 

 over the back of the embryo to a certain point, where they coalesce 

 and inclose a cavity the amniotic cavity. The membranous parti- 

 tion between the folds disappears, and the outer layer recedes and 

 becomes blended with the vitelline membrane, constituting the chorion 

 the external covering of the embryo. 



The Allantois. As the amnion develops, there grows out from 

 the posterior portion of the alimentary canal a pouch, or diverticulum 

 (the allantois}, which carries blood-vessels derived from the in- 

 testinal circulation. As it gradually enlarges it becomes more vas- 

 cular, and inserts itself between the two layers of the amnion, com- 

 ing into intimate contact with the external layer. Finally, from 

 increased growth, it completely surrounds the embryo, and its edges 

 become fused together. 



In the bird the allantois is a respiratory organ, absorbing oxygen 

 and exhaling carbonic acid; it also absorbs nutritive matter from the 

 interior of the egg. 



Amniotic Fluid. The amnion, when first formed, is in close 

 contact with the surface of the ovum ; but it soon enlarges, and be- 

 comes filled with a clear, transparent fluid, containing albumin, 

 glucose, fatty matters, urea, and inorganic salts. It increases in 

 amount up to the latter period of gestation, when it amounts to 

 about two pints. In the space between the amnion and allantois 

 is a gelatinous material, which is encroached upon and finally dis- 

 appears as the amnion and allantois come in contact, at about the 

 fifth month. 



The chorion, the external investment of the embryo, is formed by 

 a fusion of the original vitelline membrane, the external layer of 

 the amnion, and the allantois. The external surface now becomes 

 covered with villous processes, which increase in number and size 

 by the continual budding and growth of club-shaped processes from 

 the main stem, and give to the chorion a shaggy appearance. They 

 consist of a homogeneous granular matter, and are penetrated by 

 branches of the blood-vessels derived from the aorta. 



The presence of villous processes in the uterine cavity is proof 



