324 



EMBRYOLOGY. 



more and more flattened, and in the adult become so thin that 

 formerly the presence of an epithelial covering was wholly denied. 

 Then they assume a condition similar to that of endothelial cells ; 

 as in the case of the latter, their boundaries are demonstrable only 

 after treatment with a weak solution of silver nitrate. 



C. The Glands of the Small Intestine : Liver and Pancreas. 

 (I) The Liver. 



In the section which treats of the liver we must enter upon a dis- 

 cussion not only of the development of the parenchyma of the gland, 

 but also of the various hepatic ligaments the 

 lesser omentum, the ligamentum suspensorium, 

 etc.; in fact, we must begin with the latter 

 because they are developed out of a structure 

 a ventral mesentery which is ontogenetically 

 older than the liver itself. In view of the 

 manner in which the body-cavity arises, as a 

 pair of cavities, such a structure ought to be 

 found along the whole length of the ventral 

 side of the alimentary canal in the same manner 

 as on its dorsal side. Instead of that, it is found 

 only at the anterior region of the alimentary 

 canal, along a tract which extends from the 

 throat to the end of the duodenum. 



This ventral mesentery acquires a special 

 significance, because several important organs 

 take their origin in it ; in front, the heart, 



back to it the terminal parts of the venae 

 omphalomesentericae and of the vena umbili- 

 calis; immediately behind the latter, the liver with its outlet and 

 its blood-vessels. 



The part which, during an early stage of development, encloses the 

 heart is called mesocardium anterius and posterius ; we shall return 

 to it later in considering the development of that organ. The 

 portion (fig. 184) which joins this behind [caudad] has been hitherto 

 less regarded by embryologists. Since it stretches from the lesser 

 curvature of the stomach and the duodenum (du) to. the anterior 

 [ventral] wall of the trunk, it may be especially designated as the 

 ventral gastric and duodenal mesentery, or, under a more compre- 



fig. 184 Diagram (view 

 of a cross section) to 

 how the original re- 

 lations of duodenum, 

 pancreas, and liver, 

 and of the ligamentous 

 structures belonging to 

 them. 



HR, Posterior wall of the 

 trunk ; du, duodenum ; 

 p, pancreas ; I, liver ; 

 ding, dorsal mesentery ; 

 Vttl, ligamentum hepa- 



