In the adult the peritoneal surface just described appears as the cephalic layer of the transverse mesocolon and its continuation dorsad. From the facts previously considered it will be at once apparent that we are really dealing here with a part of the third layer of the primitive omentum. We do not see the original cephalic layer of the transverse mesocolon. This membrane has become fused with the fourth omental layer, and its free serous surface obliterated in the stretch between the vertebral column and the transverse colon. Hence the human adult transverse mesocolon is apparently composed of _two_ layers; the cephalic of these layers appears as peritoneum of the "lesser sac," in conformity with its derivation from the original third omental layer lining the interior of the omental bursa. The caudal layer, on the other hand, is a part of the general or "greater" peritoneal membrane. The entire adult transverse mesocolon, hence, comprises _four_ peritoneal layers, of which only two remain as permanently free serous surfaces. These differ in their derivation, the cephalic layer being a part of the primitive dorsal mesogastrium (third omental layer), while the caudal layer is part of the primitive mesocolon. Between these two layers of the adult transverse mesocolon are included the two obliterated embryonic membranes, _viz._, the fourth omental layer and the original dorsal layer of the transverse mesocolon. Caudad the two layers of the adult transverse mesocolon surround the transverse colon and are continuous along the ventral margin of the intestine with the layers of the great omentum. Toward the vertebral column these layers again diverge. The cephalic layer, lining the "lesser peritoneal cavity" invests the ventral surface of the pancreas. The caudal layer continues over the caudal surface of the body of the gland and transverse portion of the duodenum into the parietal peritoneum and the free mesentery of the jejuno-ileum. Consequently the returning layers of the great omentum are said to surround the transverse colon and unite along the dorsal border of the intestine to form the transverse mesocolon, which membrane is continued dorsad toward the vertebral column as two layers. At the "root" of the transverse mesocolon these layers are then described as diverging, the cephalic passing up to line the ventral surface of the pancreas, while the caudal continues over the caudal surface of the pancreas and third portion of the duodenum into the parietal peritoneum and mesentery.