The notochord's a flexible, rod-shaped body found in embryos of all chordates. It's composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some vertebrates and chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it's replaced by the vertebral column. The notochord's found ventral to the neural tube. Notochords were the first "backbones" serving as support structures in chordates such as Haikouicthys. Noto… (More on Notochord)