840 



DEVELOPMENT 



[CH. LIX. 



does in man, and the time and mode of its disappearance differ in 

 different orders of mammals. 



At an early stage, and whilst the changes to which reference has 

 been made are proceeding, three important structures, the amnion, 

 the chorion, and the allantois, are developed. 



Amnion. As the embryo is differentiated, the surface of the 

 ovum beyond its margins, formed by somatopleur, is gradually 

 raised as a circular fold which is looked upon as consisting of head, 

 tail, and lateral portions. The various parts of the fold rise quickly 



2 



FIG. 686. Diagram of a longitudinal section of an ovum showing mode of formation of amnion, allantois, 

 and the primitive blood-vessels. 1, Amnion cavity ; 2, villi on placental part of chorion ; 3, allan- 

 tois; 4, epiblast of chorion ; 5, somatic mesoblast; 0, splanchnic mesoblast; 7, yolk-sac; 8, coelom ; 

 9, vascular area on yolk-sac ; 10, pericardium ; 11, heart ; 12, allantois diverticulum from cloaca ; 

 18, chorion. 



and converge over the embryo, which, at the same time, passes 

 towards the interior of the ovum. Finally the folds meet and fuse 

 together at a point which is called the amnion navel. As soon as the 

 folds fuse, the inner parts separate from the outer and form a closed sac 

 (figs. 630 to 639). The inner wall of the sac is formed by epiblast, the 

 outer by mesoblast, and both are continuous with the same layers of 

 the embryo at the umbilical orifice. At first the amnion closely 

 invests the embryo, but soon the space between the two, the amniotic 

 cavity, becomes filled with fluid, and this increases in amount, till at 

 the end of pregnancy it is present in very considerable quantity. 



