The spinal column in the simplest cases consists of an unsegmented rod, the notochord, surrounded by the =skeletogenous layer=, a sheath of mesoblastic origin, which also envelops the nerve cord. Several intermediate stages connect this simple spinal column with the vertebral column characteristic of higher vertebrates. A typical vertebral column may be said to consist of (1) a series of cartilaginous or bony blocks, the vertebral =centra=, which arise in the sheath surrounding the notochord. They cause the notochord to become constricted and to atrophy to a varying extent, though a remnant of it persists, either permanently or for a long period, within each centrum or between successive centra. (2) From the dorsal surface of each centrum arise a pair of processes which grow round the spinal cord and unite above it, forming a =dorsal= or =neural arch=. (3) A similar pair of processes arising from the ventral surface of the centrum form the =ventral= or =haemal arch=. To the ventral arch the ribs strictly belong, and it tends to surround the ventral blood-vessels and the body cavity with the alimentary canal and other viscera.
A =neural spine= or spinous process commonly projects upwards from the dorsal surface of the neural arch, and a pair of =transverse processes= project outwards from its sides. When, as is commonly the case, the two halves of the haemal arch do not meet, the ventral surface of the centrum often bears a downwardly-projecting =hypapophysis=.
The character of the surfaces by which vertebral centra articulate with one another varies much. Sometimes both surfaces are concave, and the vertebra is then said to be =amphicoelous=; sometimes a centrum is convex in front and concave behind, the vertebra is then =opisthocoelous=, sometimes concave in front and convex behind, when the vertebra is =procoelous=. Again, in many vertebrae both faces of the centra are flat, while in others they are saddle-shaped, as in the neck vertebrae of living birds, or biconvex, as in the case of the first caudal vertebra of crocodiles.
In the higher vertebrates pads of fibrocartilage--the =intervertebral discs=--are commonly interposed between successive centra, these or parts of them often ossify, especially in the trunk and tail, and are then known as =inter centra=.