The membranous lining of the coelomic, especially the abdominal, cavity, composed mainly of flattened epithelial cells that produce a small amount of watery, or serous, fluid. In the embryo the coelomic wall is lined by this membrane, which continues over the developing viscera so that they are suspended and supported by the reflected peritoneum, principally from the dorsal body wall, but also from the ventral body wall in the region of the liver.