One of a series of segments which develop along the notochord and are separated by myosepta. Septa occur in two planes. Dorsal and ventral septa are plate-like processes which grow dorsally and ventrally to the notochord; horizontal septa bisect this plane perpendicularly. Mesenchymal cells migrate to the skeletagenous septa during the gastrula stage of embryogenesis to form sclerotomic tissue (see somite). At the junction between the myoseptum and skeletagenous septum vertebrae develop; they are consequently intersegmental structures, as are ribs.