Although it is the defining organ of Chordates, the notochord is perhaps the least understood of vertebrate organs because it is usually considered a transient structure only present in early embryonic development. Most tetrapods replace the notochord with cartilaginous or bony vertebral bodies and remnants of the notochord persist as intra- or intervertebral spaces and intervertebral discs. The presence of cartilage in the notochord of some tetrapods raises further important questions on the evolutionary relationships between notochordal cells and cartilage cells. This book explores these patterns of relationships between the notochord and vertebral axial chondrogenesis.