The development of the anterior abdominal wall in the rat in the light of a new anatomical description.

The development of the ventral abdominal muscles of the albino rat was studied histologically in 60 embryos and 10 postnatal specimens. The lower thoracic myotomes were seen to migrate ventrally in the lateral body wall, cross the middle line and continue with their contralateral fellows of the opposite side, thus forming a digastric mesodermal primordium. Differentiation of this primordium into fleshy bellies and intermediate aponeuroses occurred later in the prenatal period. Similar to the bilaminar arrangement of each of the human abdominal aponeuroses, recently described, each of the ventral abdominal muscles of the rat embryo was temporarily bilaminar; and similar to the trilaminar (plywood) arrangement of each wall of the human rectus sheath, the three lateral abdominal muscles of the rat embryo were so arranged. The tendinous intersections of the rectus muscle were seen only postnatally. They might be considered less as the remnants of a segmental origin, but rather as the intermediate tendons of a multigastric longitudinal muscle column.