688 MANUAL OF PHYSIOLOGY. 



inferior extremity of the cjecum, which, owing to its feeble growth, 

 is of much smaller calibre than the upper part. 



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 

 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. 



The canal of the fore-gut at a certain point becomes laterally 

 constricted, its transverse section presenting an hour-glass shape, 

 consisting of an upper, and a lower dilated portion, united by a 

 central constricted neck. The lower of these cavities becomes 

 subdivided by the outgrowth of the lateral portions and the up- 

 growth of a part of the lower wall which forms a central septum, 

 so that the fore-gut comes to be composed of an upper undivided 

 tube, giving off' two appendages. 



These appendages consist of hypoblastic tissue, and as they 

 grow into the surrounding mesoblast they divide and subdivide, 

 until at last they come to consist of very minute tubules, which 

 terminate in dilated extremities. The undivided canal forms the 

 permanent trachea, the appendages the main bronchi, whilst 



