Notochord

In animal anatomy, the notochord is a flexible rod made out of a material similar to cartilage. If a species has a notochord, it is, by definition, a chordate. The notochord lies along the anteroposterior ("head to tail") axis, is usually closer to the dorsal than the ventral surface of the animal, and is composed of cells derived from the mesoderm. The notochord has been observed to have many functions including developmental functions. The most commonly cited functions are as a site of muscle attachment, vertebral precursor, and as a midline tissue that provides signals to the surrounding tissue during development.

Notochords are thought to be advantageous (both in an evolutionary and developmental context) because they provide(d) rigid structure for muscle attachment, but were still flexible. In some chordates, it persists throughout life as the main axial support of the body, while in most tetrapods it becomes the nucleus pulposus of the intervertebral disc. The notochord plays a key role in signalling and coordinating development. Embryos of vertebrates still form transient notochord structures today during the gastrulation phase of development. The notochord is found ventral to the neural tube.