Notochord

The notochord is a flexible rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes the nucleus pulposus of the intervertebral disc. The notochord is found ventral to the neural tube.

Notochords were the first "backbones" serving as support structures in chordates such as Haikouichthys. Notochords were advantageous to primitive fish-ancestors because they were a rigid structure for muscle attachment, yet flexible enough to allow more movement than, for example, the exoskeleton of the dominant animals of that time. Embryos of vertebrates have notochords today, as retained a key role in signalling and coordinating development even as it was lost in most adults. In most tetrapods, they eventually develop into the nucleus pulposus of the intervertebral discs.