MESENTEEY OF VERTEBEATA. 5C5 



epithelial layer of tlie rudimentary enteron forms thickenings from 

 wliich. the glandular lobules and their efferent ducts are developed 

 by a process of gemmation, while the pancreatic duct is derived 

 from tlie first rudiment of the gland. This organ, which is never 

 absent except in some divisions of Fishes, is always placed close to 

 the commencement of the mid-gut, or close to the stomach ; 

 its duct is frequently united with that of the liver, or passes into 

 the enteric canal in company with it. Not unfrequently there 

 are two ducts (Chelonii, Crocodilini, Aves [Fig. 320], and some 

 Mammalia), one of which is, as a rule^ connected with the ductus 

 hepato-entericus. 



Mesentery. 

 § 422. 



As the enteric canal is developed the peritoneal fold, which en- 

 closes it, is developed also ; it fastens the canal to the hinder wall 

 of the abdomen. This double lamella, which contains the enteron, 

 forms the mesentery; that portion of it which goes to the stomach 

 is known as the mesogastrium. This does not, however, merely 

 enclose the stomach, like the mesentery of the greater part of the 

 mid-gut, but has its two lamellae continued from the stomach into a 

 double fold, which extends to the anterior wall of the abdomen, 

 where it is again connected with the peritoneum of the abdominal 

 wall. The liver is contained in this continuation of the meso- 

 gastrium to the anterior wall of the abdomen, so that this organ has 

 not only a peritoneal investment, but is also connected with the 

 enteric tube (and especially the stomach and the first part of the 

 mid-gut), and with the ventral wall of the coelom. As long as the 

 enteric tube retains its primitively straight course the relations of the 

 mesentery are simple, and peculiarities in it are only due to the 

 partial absorption of large tracts, as is the case, for example, in 

 Fishes. The increased size also of the liver brings about changes in 

 the chai'acters of the fold which passes from the stomach to the 

 anterior wall of the abdomen ; this fold, where it forms the connection 

 between the latter and the stomach, is known as the small omentum. 

 That portion of it which extends to the wall of the body forms the 

 suspensory ligament of the liver. Other changes are effected in it 

 by its relation to the diaphragm, by the curvature of the stomach, 

 and by the elongation of the mid-gut, which cause the mesentery to 

 be arranged in frill-like folds. These relations are seen as early as 

 Fishes ; they are still simple in the Amphibia, and in the Ophidii and 

 Saurii; in the Chelonii and Crocodilini they are especially modified 

 by changes in the form and position of the stomach. 



The changes of the mesogastrium are most considerable in the 

 Mammalia. As the stomach alters its position this membrane grows 

 out into a wide sac (bursa omentalis), which either hangs down over 



