Lesser omentum, greater omentum and omental bursa

At the late somitic stage, the intestinal tube has a thick dorsal mesentery. A recess is formed on its right side at S13 between the lungs and the oesophagus. This widens and establishes continuity with the peritoneal cavity (pneumato-enteric recess). The recess becomes subdivided into a cranial and a caudal part by the development of the diaphragm. The cranial part becomes the infracardiac bursa (bursa subcardiaca), which is a closed mesothelial cavity located in the ligamentum pulmonale between the oesophagus and the diaphragm, not easily visible in the adult. The caudal part (hepato-enteric recess) abuts the liver and it is the origin of the lesser sac. Similar recesses form on the left side of the intestinal tube, but soon disappear.