The union of the pancreatic and biliary ducts to form the recess of the diverticulum Vateri, which then opens by a single common orifice into the duodenum, is better marked in some of the lower vertebrates than in man.
[Fig. 195] shows the proximal portion of the duodenum of the cassowary (Casuarius casuarius) with the biliary and pancreatic ducts and the diverticulum at their confluence in section.
The development of these two main digestive glands as diverticula from the intestinal canal also explains the direct continuity of the mucous membrane of their ducts with that lining the duodenum, a fact which is of considerable importance in the pathological extension of mucous inflammations from the intestine to the duct system of the glands.
Development of the Pancreas in Lower Vertebrates.—In the embryo of the sheep two pancreatic buds are found, but the duct of the dorsal (proximal) outgrowth (duct of Santorini) subsequently fuses entirely with the main duct.
In the cat there are likewise two pancreatic outgrowths.
In the chick three pancreatic buds are visible about the fourth day.
Amphibia likewise present three embryonic pancreas buds.
The ventral (distal) outgrowth is double, the two portions proceeding symmetrically from each side of the hepatic duct. The single dorsal outgrowth is derived directly from the duodenal epithelium. Later on all these outgrowths fuse to form the single adult gland.
Fish also possess several (up to four) embryonic pancreatic outgrowths.
Recently in human embryos of 4.9 mm. cervico-coccygeal measure three pancreatic outgrowths have been observed, all entirely distinct from each other, one dorsal, budding from the epithelium of the primitive duodenum and two ventral, proceeding from the grooved gutter which represents the primitive ductus choledochus at this period. In embryos of from 6 to 10 mm. the two ventral outgrowths have already fused, hence only two buds, a single ventral and a dorsal, are now encountered.[4]