DEVELOPMENT OF THE ALIMENTARY TRACT. 



1703 



In general, the serous membranes lining the pleural and peritoneal coeloms rep- 

 resent the specialized mesoblastic layer forming the immediate boundary of these 

 cavities. The peritoneum, therefore, covering the lower surface of the diaphragm 

 and certain surfaces of the liver is derived from those portions of the septum trans- 

 versum that constitute the upper and lower walls of the hepatic recesses which are 

 instrumental in freeing the liver from its primary position within the septum. The 

 separation of the liver from the diaphragm is incomplete not only above, as already 

 noted, but also behind ; consequendy the greater part of the posterior surface of the 

 organ remains attached to the posterior body-wall by areolar tissue and is non-peri- 

 toneal, the remains of the peripheral portion of the lower layer of the septum trans- 

 versum, which becomes the peritoneum of the liver, being reflected at the sides 

 backward as the coronary li'o^aments. 



Coincidently with the development of the liver and its liberation from the sep- 

 tum transversum, the stomach undergoes change in its axis, which becomes less vertical 

 and more obliquely transverse, and in consequence its attachment to the liver, 

 the primitive gastro-hepatic omentum, is drawn towards the right and assumes a 



Fig. 1438. 



Neural tube. 



Notochoid 



M>otome 



Wolffian duet. '-"^ 

 Body-cavity i^Ml ' 



Intestine. 



Mesentery 



Lower limb-bud 



Body-wall continuous with amnion '''^'•- 

 Transverse section of rabbit embryo of eleven and a half days, showing primitive mesentery. X 35. 



Umbilical veins 

 Umbilical artery 



transverse position almost at right angles to its former sagittal plane. These altera- 

 tions in the position of the stomach and its anterior mesentery affect the mesogas- 

 trium, which becomes elongated and twisted towards the right to follow the stomach 

 in order to maintain its attachments to the greater curvature. The result of these 

 changes is the production of a pocket behind the stomach, the floor and left wall of 

 which are the mesogastrium, the roof being the under surface of the liver. This 

 pocket, \}i\^ lesser sac of the peritone^im, communicates with the remaining part of 

 the peritoneal cavity on the right by means of a passage behind the displaced lesser 

 or gastro-hepatic omentum, the free border of the latter bounding the opening lead- 

 ing into the passage or vestibule (page i749). The opening, at first large, later 

 diminishes in size and becomes the forameri of Wmslozv, which leads from the 

 greater peritoneal sac into the vestibule of the lesser. 



Beneath the stomach very soon appears an e.x.tension of the pocket, which 

 pushes out between the stomach above and the transverse colon below. This protru- 

 sion, the o?nental sac, continues to grow downward and forms an apron which later, 

 as the greater omentum, covers the loops of the small intestine. On referring to 

 Fig. 1439, it is evident that the greater omentum at first comprises a duplicature the 



