Between the second and third lines of peritoneal reflection appears the portion of the mesal surface of the spleen in contact with and invested by the extreme left end of the lesser peritoneal sac. Fig. 184, taken from an adult human subject with the viscera hardened in situ, shows the left or splenic extension of the lesser peritoneal cavity. [Illustration: FIG. 184.--Upper abdominal viscera of adult human subject, hardened _in situ_, with liver and colon removed and stomach turned up. (Columbia University, Study Collection.)] =2. Development of the Pancreas.=--The pancreatic gland is derived from the hypoblast of the enteric tube. The secreting epithelium and that lining the ducts of the adult gland is formed by budding and proliferation of the intestinal epithelium. The gland develops primarily from two outgrowths which are at first separate and distinct from each other. 1. The proximal and dorsal bud grows directly from the hypoblast lining the duodenum immediately beyond the pyloric junction. In embryos of 8 mm. (four weeks) (Fig. 185) it appears as a small spherical outgrowth connected by a slightly narrower stalk with the epithelial intestinal tube. [Illustration: FIG. 185.--Pancreatic and hepatic buds of human embryo of four weeks. (Kollmann.)] 2. The distal and ventral outgrowth is separated from the preceding and is from the beginning closely connected with the similar embryonic outgrowth from the enteric tube which is to form the liver. This portion of the pancreas is, strictly speaking, derived primarily from the epithelium of the primitive hepatic duct and not directly from the duodenum. This primary arrangement of the gland, being formed of two main collections of budding hypoblastic cells, corresponds to the adult system of the pancreatic excretory ducts. The proximal or dorsal outgrowth furnishes that portion of the head of the gland whose excretory system terminates in the _secondary pancreatic duct_ or _duct of Santorini_, while the distal (ventral) outgrowth includes within its area the termination of the principal pancreatic duct or _canal of Wirsung_, which is closely connected with the end of the common bile-duct at the intestinal opening common to both (Figs. 186-187). The method of union of the two pancreatic outgrowths and their respective share in building up the adult gland explains the usual adult arrangement of the excretory system and its variations. [Illustration: FIG. 186.--Pancreatic buds of human embryo of five weeks. (Kollmann, after Hamburger.)] [Illustration: FIG. 187.--Pancreatic buds of human embryo of six weeks. (Kollmann, after Hamburger.)] In the embryo of five weeks (Fig. 186) the two portions have grown in length. The dorsal or proximal outgrowth, developing between the layers of the mesoduodenum, is at this time the larger of the two, composed of a number of glandular vesicles clustered around the stalk represented by the parent duct.