Wherever in this discussion of the transverse mesocolon the transition between the caudal layer of the membrane and the "parietal" peritoneum is referred to it is necessary to remember that this "parietal" peritoneum is the _secondary_ investment of the abdominal background, formed by the surface of the ascending and descending mesocolon which remains free after the opposite surface and the vertical segments of the large intestine have been anchored by adhesion to the _primary_ parietal peritoneum (cf. p. 81, Fig. 158). A summary at this point of the course of the dorsal mesogastrium, in forming the great omentum and its subsequent connections, would show us that the membrane first enlarges and descends towards the transverse colon (Fig. 177). The omental bag is formed by the descending or superficial segment (starting from the greater curvature of the stomach), turned toward the observer in the figure, and by the ascending or deep layer which is attached above to the dorsal abdominal wall, in front of the vertebral column and aorta along the original line of origin of the dorsal mesogastrium. Gradually growing and descending further, the deep segment becomes attached to the transverse colon. It also becomes connected, especially on the left side, with the diaphragmatic peritoneum (phrenicocolic lig.), so that its original starting point is no longer distinct. Finally the development of the spleen and pancreas between the layers of the dorsal segment and their subsequent connections obscure the original conditions. Fig. 297 shows the primitive condition at a time when the connection with the transverse colon and mesocolon has not yet taken place. The omental bag or bursa epiploica develops in the region of the dorsal mesogastrium and the viscera included between its layers, by changes in the position and extent of the membrane which finally result in placing a part of the right half of the primitive coelom cavity behind the stomach. Up to the sixth week the line of origin of the dorsal mesogastrium is from the mid-dorsal line of the abdomen. It deviates from this origin to the left because the great curvature of the stomach to which it is attached turns in this direction. On this account, and because of the rapid growth of this portion of the mesogastrium, a bag or space is formed behind the stomach. The entrance into this space is situated to the right of the lesser curvature, behind the peritoneal layers connecting the same with the liver (lesser or gastro-hepatic omentum and hepato-duodenal ligament). The ventral wall of this space is formed by the dorsal surface of the stomach itself, the dorsal wall by the mesogastrium, turning to the left and presenting its original right surface, now directed ventrad. The caudal limit of the retro-gastric space is given by the turn of the mesogastrium to reach its attachment along the greater curvature of the stomach (rudiment of great omentum).