II.] THE ALLANTOIS. 47 



flattened sac projecting into the pleuroperitoneal cavity, 

 the walls of the sac being formed of a layer of splanchnic 

 mesoblast lined by hypoblast. 



It grows forwards in the peritoneal cavity until it 

 reaches the stalk connecting the embryo with the yolk- 

 sac, and thence very rapidly pushes its way into the space 

 between the true and false amniotic sacs (Fig. 9, G, K}. 

 Curving over the embryo, it comes to lie above the 

 embryo and the amnion proper, separated from the 

 shell (and vitelline membrane) by nothing more than 

 the thin false amnion. In this position it becomes 

 highly vascular, and performs the functions of a respi- 

 ratory organ. It is evident that though now placed 

 quite outside the embryo, the space in which it lies is a 

 continuation of that peritoneal cavity in which it took 

 its origin. 



It is only necessary to add, that the serous mem- 

 brane, including the false amnion, either coalesces with 

 the vitelline membrane, in contact with which it lies, 

 or else replaces it ; and in the later days of incubation 

 was called by the older embryologists the chorion a 

 name however which we shall not adopt. 



