ALLANTOIS. 



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. 



As the hindgut becomes folded in the allantois shifts its 

 position, and forms (figs. 123 B and 124) a rather wide vesicle 



A. 



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 Dobrynin.) 



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

 x'. the roof of x" . the neural canal ; Dd. the hind end of the hindgut ; So. somato- 

 pleure; Spl. splanchnopleure; u. the mesoblast of the splanchnopleure carrying the 

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

 pleuroperitoneal cavity; All. the commencing allantois; 7c*. projection formed by 

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

 the ventral wall of the hindgut ; v. anal imagination ; G. cloaca. 



lying immediately below 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 connected with the cloaca 

 by a narrow canal which forms its neck or stalk (fig. 121 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-sack. It does not during the third day project beyond this 

 point ; but on the fourth day begins to pass out beyond the 

 body of the chick, along the as yet wide space between the 



