DEVELOPMENT OF THE DIGESTIVE TUBE 



629 



GLANDS OF BRUNNER 

 MESENTERY 



large gland 



;as) 



OPENINGS OF 

 NTESTINAL GLANDS 



'<T^^ VALVE OF KERKRING B 



VILLI 



LYMPHATIC NODULE 



Fig. 298. Structural composition of walls of human digestive tract. (A) Diagram- 

 matic representation of digestive tract structure. (B) Portion of wall of small intestine 

 showing folds of mucosa. (A and B redrawn from Maximow and Bloom, 1942, A Text- 

 book of Histology, Saunders, Phila. B after Braus.) 



chick. The dorsal one appears first as an outpushing into the dorsal mesentery 

 at the end of the third and early fourth days of incubation (fig. 295D). The 

 two ventral diverticula arise during the end of the fourth and early fifth days 

 of incubation as two lateral diverticula of the posterior hepatic evaginations 

 close to the latter's origin from the duodenum. The three diverticula fuse into 

 one pancreatic mass, but tend to retain the proximal portions of the original 

 outpushings as pancreatic ducts. Two or even all three may persist in the adult. 



d) Pig Embryo. Two pancreatic diverticula make their appearance in the 

 pig embryo. One, the ventral pancreatic diverticulum, arises from the proximal 

 end of the hepatic evagination, while the other, the dorsal diverticulum, 

 emerges as a separate dorsal outpushing from the duodenal area approximately 

 opposite the hepatic diverticulum (fig. 295E). In the 20-mm. embryo of the 

 pig, these two diverticula proceed in development as shown in figure 295F. 

 At about the 24-mm. stage, the duct of the ventral pancreas is obliterated, 

 the dorsal pancreatic duct (duct of Santorini) remaining ordinarily as the 

 pancreatic duct of the adult (Thyng, '08). 



e) Human Embryo. Dorsal and ventral pancreatic evaginations occur in 

 the human embryo in a manner similar to that in the pig. Both fuse into one 

 mass, although the dorsal pancreas grows much faster and forms much of the 

 bulk of the pancreatic tissue. The ventral pancreas swings dorsally as the 

 stomach and duodenal area of the intestine are rotated toward the right side 

 of the peritoneal cavity. In doing so, the dorsal pancreas appropriates the duct 

 of the ventral pancreas proximally toward the intestine, while distally it retains 

 its own duct. This combined duct, or duct of Wirsung, first observed by 

 Wirsung in 1642 (see Lewis, '12), is the pancreatic duct of the adult. Occa- 



