An intervertebral disc or intercentrum is often present between centre of successive vertebrae in embryo. This may fuse with anterior or postorior end of a centrum changing it shape to convex or flat. On the basis of shape of centra, vertebrae may be procoelous, opisthocoelous, amphicoelous, acoelous, biconvex or heterocoelous. Opisthocoelous vertebrae have convexity at anterior end and concavity at posterior end.