[Illustration: FIGS. 138-140.--Diagrammatic representation of three stages in the development of the mesoduodenum, duodenum, and pancreas leading to the secondary "retroperitoneal" position of these viscera.] [Illustration: FIG. 138.--Free mesoduodenum in sagittal plane, including head of pancreas between right and left layers.] [Illustration: FIG. 139.--Mesoduodenum folded to right; left leaf has become ventral; right dorsal, directed toward primitive prerenal parietal peritoneum.] [Illustration: FIG. 140.--Fixation of head of pancreas and duodenum under cover of secondary parietal peritoneum by adhesion of apposed surfaces of mesoduodenum and primitive parietal peritoneum.] Figs. 138 and 139 shows the arrangement in the cat where a free duodenum and mesoduodenum exists, with the pancreas included between its layers.[2] [2] The student should not be confused by the fact that a considerable portion of the pancreatic gland in the cat will be found included between the layers of the great omentum, extending over to the left side of the abdomen. This circumstance will be found of importance in studying the development of the dorsal mesogastrium and of the structures connected with it. For the present attention should only be given to the right extremity or head of the pancreas, situated close to the duodenum and included between the layers of the mesoduodenum. It will be noticed that the duodenum in the cat can be carried over to the median line (Fig. 138) exposing the entire ventral aspect of the right kidney and the inferior vena cava beneath the primary lumbar parietal peritoneum. This manipulation will also expose the dorsal surface of the head of the pancreas, covered by what originally was the right leaf of the mesoduodenum.