The primitive stomach and proximal duodenum derive from the foregut caudal to the developing lung, whereas the duodenum beyond the origin of the liver bud (the future ampullary papilla) derives from the cephalic end of the midgut.20
After the fourth week of intrauterine development, the primitive stomach rotates 90° clockwise around its longitudinal axis, ending with the left side facing anteriorly and the right side posteriorly.22
This accounts for the course of the left vagus along the anterior wall and the right vagus along the posterior wall. The left wall of the stomach grows faster than the right, resulting in the size difference between the curvatures. The dorsal mesentery, which attaches the stomach to the posterior body wall, forms the omental bursa, or lesser peritoneal sac. The ventral mesentery attaches the stomach and duodenum to the liver.
As the stomach grows and rotates, the cephalic end moves leftward and downward, forming the fundus and cardia. The caudal end moves upward and to the right, forming the antrum and pylorus. Hence, the long axis of the stomach runs from above left to below right.
The duodenum also grows rapidly, forms a C-loop projecting ventrally, rotates to the right, and becomes retroperitoneal. Because of rapid epithelial proliferation during the fifth and sixth weeks, the duodenal lumen is temporarily obliterated. The duodenal lumen recanalizes over the ensuing weeks as some cells degenerate.
The epithelium and glands of the stomach and duodenum derive from the embryonic endoderm. The connective tissue, muscle, and serosa derive from the mesoderm.