AVES. 



159 



The allantois 1 is essentially a diverticulum of the alimentary 

 tract into which it opens immediately in front of the anus. Its 

 walls are formed of splanchnic mesoblast with blood-vessels, within 

 which is a lining of hypoblast. It becomes a conspicuous object on 

 the third day of incubation, but its first development takes place at 

 an earlier period, and is intimately connected with the formation of 

 the posterior section of the gut. 



At the time of the folding in of the hinder end of the mesenteron 

 the splitting of the mesoblast into somatopleure and splanchnopleure 

 has extended up to the border of the hinder division of the primitive 

 streak. As has been already mentioned, the ventral wall of the 

 postanal section of the alimentary tract is formed by the primitive 

 streak. Immediately in front of this is the involution which forms 

 the proctodseum ; while the wall of the hindgut in front of the anus 

 owes its origin to a folding in of the splanchnopleure. 



The allantois first appears as a protuberance of the splanchno- 

 pleure just in front of the anus. This protuberance arises, however, 

 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. 123 A, All] the open end of which is directed forward, while its 

 blind end points somewhat upwards and towards the peritoneal 

 space behind the embryo. 



A. 



B. 



FlG. 123. TWO LONGITUDINAL SECTIONS OF THE TAIL-END OF AN EMBRYO CHICK TO 

 SHEW THE ORIGIN OF THE ALLANTOIS. A AT THE BEGINNING OF THE THIRD DAY; B AT 

 THE MIDDLE OF THE THIRD DAY. (After Dobryilin.) 



t. the tail ; m. the mesoblast of the body, about to form the mesoblastic somites ; 

 x'. the roof of x". the neural canal ; Del. the hind end of the hindgut ; So. sornato- 

 pleure; Spl. splanchnopleure; n. the mesoblast of the splaiichuopleure carrying the 

 vessels of the yolk-sack; pp. pleuroperitoneal cavity ; Df. the epithelium lining the 

 pleuroperitoneal cavity; All. the commencing allantois; . projection formed by 

 anterior and posterior divisions of the primitive streak; y. hypoblast which will form 

 the ventral wall of the hindgut; r. anal iuvagination ; G. cloaca. 



1 For details on the development of the allautois the reader is referred to the works 

 of Kolliker (No. 135), Gasser (No. 127), and for a peculiar view on the subject Kupft'er 

 (No. 136). In addition to these works he may refer to Dobryuin "TJeber die erste 

 Aiilage der Allantois." Sitz. der k. Akad. Wien, Bd. 64, 1871. E. Gasser, Beitrfifje zur 

 Entwicklungsgeschichte d. Allantois, etc. 



