1. The portion of the gland adjacent to the duodenum, corresponding to the "head" of the human organ, is included between the two layers of the mesoduodenum. This membrane is free, so that the dorsal surface of this portion of the pancreas is seen to be invested by the dorsal layer of the mesoduodenum (Fig. 223). The duodenum and the mesoduodenum, the latter containing the head of the pancreas between its layers, can be turned toward the median line, so as to expose the entire ventral surface of the post-cava and right kidney. To illustrate the arrangement which is found in the adult human subject the descending duodenum and pancreas should be allowed to fall over to the right so as to cover the vena cava and the mesal part of the ventral surface of right kidney. The adult human condition will now be produced if we assume that the structures are fixed in this position by the obliteration of the apposed serous surfaces, viz., the parietal peritoneum over kidney and vena cava on the one hand and the right layer of the mesoduodenum and the dorsal visceral peritoneum of the duodenum on the other. [Illustration: FIG. 223.--Abdominal viscera of cat, hardened and removed from body, showing relation of pancreas to mesoduodenum and dorsal mesogastrium, respectively. (Columbia University Museum, No. 728.)] 2. In following out the pancreas of the cat in its entire extent, proceeding to the left of the pylorus, it will be seen that the body of the gland has extended between the two dorsal layers of the great omentum (primitive dorsal mesogastrium) over to the spleen (Fig. 223). Consequently the arrangement in the cat corresponds to the stage in the human development shown in Fig. 219 and Fig. 221 in which adhesion of the dorsal surface of the pancreas to the parietal peritoneum has not yet taken place. It will be quite easy to reconstruct from the facts as demonstrated by the arrangement of the parts in the cat, the stage in the development of the lesser peritoneal sac in which the dorsal wall of the space is still formed by the proximal portion of the free dorsal mesogastrium (great omentum) and the structures included between its two layers. It must then become apparent that the entire serous surface which in the adult human subject we regard as "parietal peritoneum of the lesser sac" lining the dorsal wall of the retrogastric space is derived from what originally was the right layer of the primitive sagittal dorsal mesogastrium. II. RELATION OF GREAT OMENTUM TO TRANSVERSE COLON, TRANSVERSE MESOCOLON AND THIRD PART OF DUODENUM. The second purpose to be accomplished by the study of the cat's abdominal cavity at this stage is the correct appreciation of the adult human conditions which are produced by areas of adhesion between the transverse colon, transverse mesocolon and third part of the duodenum on the one hand, and the dorsal mesogastrium, as great omentum, with the structures contained between its layers, on the other.