848 EMBRYONIC IMEMBRANES AND YOLK-SAC. [CHAP. 



During the changes above described as taking place 

 in the amnion, the allantois grows out from the hind- 

 gut as a vesicle lined by hypoblast, but covered ex- 

 ternally by a layer of splanchnic mesoblast (Fig. 114, 3 

 and 4, aZ)\ It soon becomes a flat sac, projecting into 

 the now largely developed space between the subzonal 

 membrane and the amnion, on the dorsal side of the 

 embryo (Fig. 115, ALC). In some cases it extends so 

 as to cover the whole inner surface of the subzonal 

 membrane ; in other cases again its extension is much 

 more limited. Its lumen may be retained or may be- 

 come nearly or wholly aborted. A fusion takes place 

 between the subzonal membrane and the adjoining 

 mesoblastic wall of the allantois, and the two together 

 give rise to a secondary membrane round the oviim 

 known as the chorion. Since however the allantois 

 does not always come in contact with the whole inner 

 surface of the subzonal membrane the term chorion is 

 apt to be somewhat vague ; in the rabbit, for instance, 

 a considerable part of the so-called chorion is formed 

 by a fusion of the wall of the yolk-sac wdth the sub- 

 zonal membrane (Fig. 116). The region of the chorion 

 which gives rise to the placenta may in such cases be 

 distino^uished as the true chorion from the remaining 

 part which will be called the false chorion. 



The mesoblast of the allantois, especially that part 

 of it which assists in forming the chorion, becomes 

 highly vascular ; the blood being brought to it by two 

 allantoic arteries continued from the terminal bifur- 



1 The hypoblastic element in the allantois is sometimes very much 

 reduced, so that the allantois maybe mainly formed of a vascular layer 

 of mesoblast. 



