VI.] THE ALLANTOIS. 183 



the gut the splitting of the mesoblast into somatopleure 

 and splanchnopleure has extended up to the border of 

 the hinder division of the primitive streak. The ventral 

 wall of what we have already termed the postanal 

 section of the alimentary tract is formed by the primi- 

 tive streak. Immediately in front of this is the involu- 

 tion which forms the proctodseum; while the wall of 

 the hindgut in front of the proctodseum owes its origin 

 to a folding in of the splanchnopleure. 



The allantois first appears as a narrow diverticulum 

 formed by a special fold of the splanchnopleure just in 

 front of the proctodseum. This protuberance arises, how- 

 ever, before the splanchnopleure has begun to be tucked 

 in so as to form the ventral wall of the hindgut ; and it 

 then forms a diverticulum (Fig. 63 A, All) the open 

 end of which is directed forward, while its blind end 

 points somewhat dorsalwards and towards the peritoneal 

 space behind the embryo. 



As the hindgut becomes folded in the allantois shifts 

 its position, and forms (Figs. 63 B and 61) a rather wide 

 vesicle lying immediately ventral to the hind end of the 

 digestive canal, with which it communicates freely by a 

 still considerable opening; its blind end projects into 

 the pleuroperitoneal cavity below. 



Still later the allantois grows forward, and becomes 

 a large spherical vesicle, still however remaining con- 

 nected with the cloaca by a narrow canal which* forms 

 its neck or stalk (Fig. 9 G, al). From the first the 

 allantois lies in the pleuroperitoneal cavity. In this 

 cavity it grows forwards till it reaches the front limit of 

 the hindgut, where the splanchnopleure turns back to 

 enclose the yolk-sac. It does not during the third 



