THE INTESTINAL CANAL. 685 



The epithelial lining of the intestines is derived from the hypo- 

 blast, and the muscular, vascular, connective tissue, and serous 

 coverings are mesoblastic in their origin. 



The liver is developed from two diverticula of the duodenum, 

 in connection with which arise cylinders of cells. The hypoblast 

 develops into the liver cells and the cells lining the ducts, the 

 mesoblast furnishing the vascular and connective tissue parts of 

 the organ. The two diverticula are connected by a transverse 

 piece, and form the right and left lobes of the liver. 



The process connecting the liver to the duodenum forms the 



FIG. 275. 



Diagram of the alimentary canal of a chick at the fourth day. (Foster and Balfour, 

 after GOtte.) Iff- Diverticulum of one lung. St. Stomach. /. Liver, p. Pancreas. 



common bile duct, and from this the gall bladder is developed as 

 an outgrowth. 



The vessels of the embryo which are in relation to the liver 

 will be described under the vascular system. 



The pancreas arises as an outgrowth from the duodenum, its 

 constituent parts originating in a manner similar to those of the 

 liver. 



The spleen is derived from the mesoblast, and is developed in 

 one of the peritoneal processes of the stomach. 



The lungs are developed in connection with the oesophagus, of 

 which they are early outgrowths. 



