The stomach becomes visible very early in the development as a fusiform enlargement of the intestinal tube.
The stomach is connected with the dorsal body wall through the dorsal mesogastrium, a duplicate of the peritoneum, and ventrally through the ventral mesogastrium with the transverse septum. The developing liver grows as a ventral budding of the intestinal tube into the transverse septum. The ventral mesogastrium is thus subdivided into a portion connecting the stomach to liver (the hepatogastric ligament or omentum minus) and a second portion that ties the liver and abdominal wall together (the falciform ligament).
The coelomic gap in the region of the midgut (peritoneal cavity) extends towards the left cranially and to the right of the liver and forms the visceral envelope of the liver.
The stomach is shifted to the left and turns 90 degrees, the right wall lying dorsally. This results, though, more from a substantial growth of the left wall (later anterior wall) of the stomach and of the fundus than from an active rotation. The relatively short connective tissue bridge of the stomach to the posterior body wall stretches into a long, extended fold, the dorsal mesogastrium. Later the dorsal pancreas and the spleen anlage grow into it.
Essential that the differing spurts of growth in the various stomach sections not only lead to shape but also to positional changes is the fact that the duodenum is fixed onto the posterior abdominal wall quite early. From its original intraperitoneal location it assumes a secondary retroperitoneal position (stage 20, ca. 49 days).
The duodenum is thus a component of the vessel-pancreas-stalk (see: mesenterial relationships of the intestines).
The caudal shifting of the entire foregut mentioned at the beginning comes to an end with the formation of the diaphragm and the resulting fixation of the esophagus-cardia passage as well as the formation of the vessel-pancreas-stalk at the level of the duodenum. With this fixation of the stomach at the cardia and pylorus the spurts of growth in the stomach wall now lead only to a horizontal movement of the organ to the left with the above mentioned turn of 90 degrees.
As a whole, the duodenum loop lies to the right on the abdominal wall. Its peritoneal layers stick and fuse to the peritoneal layer that lines the abdominal cavity parietally. The duodenum thus becomes "secondarily retroperitoneal". The transition between the foregut and midgut are found on the vertex of the duodenal loop, there where the liver and pancreas anlagen arise. Probably no rotation of the duodenal loop occurs, rather it takes on its C-shape due to the extension of the stomach towards the left and the simultaneous relative fixation by the emerging liver and pancreas anlagen. This results in the inferior horizontal portion of the duodenum being shifted under the superior mesenteric artery (see: blood supply of the intestines).