The =arch= (Arcus vertebræ) is constructed on the dorsal aspect of the body. It consists originally of two lateral halves, each of which is considered to consist of a pedicle and a lamina. The pedicles form the lateral parts of the arch, and are cut into in front and behind by the =vertebral notches= (Incisura vertebralis cranialis, caudalis). The notches of two adjacent vertebræ form =intervertebral foramina= for the passage of the spinal nerves and vessels; in some vertebræ, however, these are complete foramina instead of notches. The laminæ are plates which complete the arch dorsally, uniting with each other medially at the root of the spinous process. The body and the arch form a bony ring which incloses the =vertebral foramen= (Foramen vertebrale); the series of vertebral rings, together with the ligaments which unite them, inclose the =vertebral canal= (Canalis vertebralis), which contains the spinal cord and its coverings and vessels. The =articular processes=, two =anterior= and two =posterior= (Processus articulares craniales, caudales), project from the borders of the arch on either side. They present joint surfaces adapted to those of adjacent vertebræ, and the remaining surface is roughened for muscular and ligamentous attachment. The =spinous process= (Processus spinosus) is single, and projects dorsally from the middle of the arch. It varies greatly in form, size, and direction in different vertebræ. It furnishes attachment to muscles and ligaments. The =transverse processes= (Processus transversi) are two in number and project laterally from the side of the arch, or from the junction of the arch and body. In the thoracic region each has a facet for articulation with the tubercle of a rib (Fovea costalis transversalis). They also give attachment to muscles and ligaments. Some vertebræ have also a =ventral= or =hæmal spine=. =Mammillary processes= (Processus mammillares) are found in most animals on the last thoracic and anterior lumbar vertebræ between the transverse and anterior articular processes or on the latter. =Accessory processes= (Processus accessorii), when present, are situated between the transverse and posterior articular processes. =Development.=—The vertebræ are developed by ossification in the cartilage which surrounds the notochord and forms the sides of the neural canal. There are =three primary centers of ossification=, one for the body and one for each side of the arch. =Secondary centers= appear later for the summit of the spinous process (except in the cervical region), the extremities of the transverse processes, and the thin epiphyseal plates at the extremities of the body.