272 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



In the 5 th week the pancreas lies between the layers of the 

 dorsal mesogastrium ; thus right and left surfaces are covered by 

 peritoneum. The left surface, which becomes anterior, retains 

 its covering, but the right becomes applied to the posterior 

 abdominal wall in front of the aorta, crura of the diaphragm 

 and left kidney (Fig. 220). Its peritoneal covering gradually 

 disappears, and thus in the adult the pancreas comes to appear as 

 if it lay behind and outside the cavity of the peritoneum. The 

 complete application and fixation of the pancreas to the posterior 

 abdominal wall only occurs in animals adapted to the upright 

 posture. 



The part of the dorsal mesogastrium between the pancreas and 

 aorta (Fig. 219) is also applied to the posterior abdominal wall, 

 and forms the posterior wall of the lesser sac. 



The Lesser Sac is really the bursa of the stomach. In its 

 anterior wall are situated (Fig 220): (1) The gastro-hepatic 

 omentum or ventral mesentery, which is at first vertical and 

 median ; (2) The stomach ; (3) The gastro-splenic omentum, a 

 part of the dorsal mesentery ; (4) The two anterior layers of the 

 great omentum, also parts of the dorsal mesentery. 



In its posterior wall are situated: (1) The lieno-renal liga- 

 ment (dorsal mesentery) ; (2) The dorsal mesentery of pancreas. 



THE PRIMARY DIVISIONS OF THE ALIMENTARY 



CANAL. 



At the commencement of the 3rd week the embryo appears to 

 be situated as a cap on the yolk sac (Fig. 75, p. 96). As the 

 chorion and amnion represent the extra-embryonic part of the 

 somatopleure, so the yolk sac represents the extra-embryonic part 

 of the splanchnopleure (Fig. 72, p. 93). At the end of the 3rd 

 week the alimentary tract shows three divisions (Fig. 221): 



1. A short anterior diverticulum — the fore-gut; it becomes the 

 pharynx, oesophagus, stomach, and that part of the duodenum 

 situated in front of the opening of the common bile duct ; 



2. The hind-gut — a short posterior diverticulum ; it forms the 

 rectum, sigmoid and descending colon ; its terminal part forms 

 the cloaca from which the allantois grows out ; 



