630 THE DIGESTIVE SYSTEM 



sionally, two ducts opening into the intestine are retained, the original dorsal 

 duct, the accessory duct or duct of Santorini, described by Santorini (see 

 Lewis, '12), and the duct of Wirsung or ventral pancreatic duct. The latter 

 condition appears to be normal in the dog. 



4) Histogenesis of the Pancreas. The original pancreatic diverticula branch, 

 rebranch, and form an elaborate duct system. The secretory portions of the 

 pancreas or the acini arise as terminal outgrowths of the distal portions of the 

 duct system. The pancreas thus is a compound alveolar (acinous) gland. The 

 loose connective tissue of the pancreas forms the surrounding mesenchyme, 

 derived from the mesenteric tissue. 



Two types of secretory cells bud off from the developing duct system. The 

 majority form the acini of the pancreatic gland and pour their secretions into 

 the duct system. This constitutes the exocrine aspect of the pancreas. Other 

 cell masses bud off from the duct system and give origin to the islets of 

 Langerhans. The latter form the endocrine portion of the pancreas (fig. 295G). 



e. Morphogenesis and Histogenesis of the Intestine 



1) Morphogenesis of the Intestine in the Fish Group. In the fishes, the 

 intestinal rudiment of the digestive tube does not undergo extensive elonga- 

 tion during development. A relatively short tube is formed as shown in figure 

 279C, although some coiling of the intestine does occur in teleost fishes. A 

 distinct, small and large division of the intestine is not formed; intestinal and 

 rectal areas only are developed. Specialized rectal outgrowths develop in sharks 

 (fig. 279C), while, in teleost fishes, pyloric evaginations or cecae are formed. 



2) Morphogenesis of the Intestine in Amphibia, Reptiles, Birds, and Mam- 

 mals. The development of the intestine in this group of vertebrates involves 

 considerable elongation and coiling (figs. 280, 281, 282). Two general divi- 

 sions of the intestine are formed, a small intestine, developed from the midgut 

 portion of the primitive metenteron, and a hindgut or colon, derived from the 

 hindgut portion of the gut tube. A rectal area is formed at the caudal end of 

 the hindgut. There is a tendency also for enlargements or extensions to occur 

 in the area of junction between the small intestine and colon in the birds 

 and mammals. 



3) Torsion and Rotation of the Intestine During Development. Twisting 

 and rotation of the stomach and intestine is a general feature of alimentary- 

 tract development. In the shark embryo, the stomach is rotated in such a 

 way that its pyloric end is pulled upward toward the liver, forming a J-shaped 

 structure (fig. 296A). Also, the duodenal and valvular areas of the intestine 

 are rotated vertically, and the place of attachment of the dorsal mesentery 

 moves into a ventro-lateral position. 



The developing stomach and intestine of the frog embryo presents a re- 

 markable and precise rotative procedure. In the early stages, the primitive 

 metenteron is a simple tube, continuing from the forming stomodaeum caudad 



