A primitive type of skeletal rod of turgid cells, the forerunner of the backbone in chordates.

[Pisces] A cartilaginous rod in the embryo that runs from head to tail and supports the nerve cord; in most fishes the notochord is replaced in the larval stage by the developing vertebrae, but in some primitive fishes (e.g., coelacanth, hagfishes, chimaeras) the notochord persists in the adult and serves as the vertebral column or 'backbone' of these species.