NERVOUS SYSTEM AND ORGANS OF SPECIAL SENSE The nervous system consists of the brain and the spinal cord, and of the cranial and spinal nerves. The rudiment of the future brain and spinal cord first appears in the embryos of some Cyclostomes (e.g. _Bdellostoma_), of Elasmobranchs, and of Chondrostei (e.g. _Acipenser_), and of _Neoceratodus_ among the Dipnoi, in the form of a tubular medullary canal pinched off from the epiblast of the dorsal surface of the body. By a somewhat different method, but with the same final result, a medullary canal is formed in other Cyclostomes (e.g. _Petromyzon_), in the Holostei and Teleostei, and in _Lepidosiren_,[438] from a solid ingrowing keel of epiblast which subsequently becomes tubular. Later, the medullary canal in the head enlarges, and becomes divided by two transverse constrictions into three vesicles, the primary fore-, mid-, and hind-brain, leaving the rest of the canal to form the spinal cord. THE SPINAL CORD.—This portion of the medullary canal retains a simpler and more uniform cylindrical structure. Its walls thicken and their component cells become converted into nerve cells and nerve fibres, but a remnant of the original cavity remains in the adult as a minute axial canal, with a ciliated epithelial lining, the central canal of the spinal cord or myelocoele. In most Fishes the spinal cord extends the whole length of the body, but in some Teleosts, especially in certain Plectognathi, it is remarkably short. In a Sun-Fish (_Orthagoriscus_), 2½ metres long, and weighing about a ton and a half, the cord was only 15 mm. in length, or shorter than the brain.