48 Vertebrate Embryology 



grows backward from the posterior end of the 

 digestive tract towards this invagination (Fig. 

 12), with which it finally fuses and thus puts 

 the hind gut in communication with the ex- 

 terior (Figs. 12 /, and 14, /). The formation 

 of the proctodaeum may be completed before 

 the closure of the blastopore, so that the hinder 

 end of the digestive tract may have two open- 

 ings to the exterior. 



It will be understood, from the above de- 

 scription, why it is that the oral and anal 

 cavities are lined with ectoblast instead of 

 with entoblast, as is the rest of the digestive 

 canal. 



The liver may be early recognized as a diver- 

 ticulum pushed out from the front end of the 

 digestive tract in a ventro-posterior direction 

 (Figs. 12 and 14, W). The walls of this 

 diverticulum thicken and become folded, and 

 the mesoblast penetrates between these folds. 

 The diverticulum, which is evidently lined 

 with entoblast, persists as the bile duct, and 

 from it an outgrowth arises to form the gall 

 bladder. 



The pancreas arises as a pair of hollow out- 

 growths from the mesenteron back of the 

 liver. 



