The notochord degenerates and disappears entirely in the region of the vertebral bodies.However,it persists in the central part of each developing in-tervertebral disc. Here it undergoes mucoid degeneration to form the gelatinous center of the intervertebral disc, which is called nucleus pulposus. The nucleus pulposus becomes surrounded by a ring of fibrocartilage, called annu-lus fibrosus, which is formed by the mesenchyme derived from the denser portions of the vertebral primordia. The nucleus pulposus and annulus fibrosus together constitute the intervertebral disc.