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Updated: May 25, 2025
It follows the long axis of the body in the shape of a cylindrical axial rod of elastic but firm composition, equally pointed at both ends. This chordalemma is often called the "inner chorda-sheath," and must not be confused with the real external sheath, the mesoblastic perichorda. Human skull. Skull of a new-born child.
The primary axial skeleton remains a simple chorda throughout life in the Acrania, the Cyclostomes, and the lowest fishes. In most of the other Vertebrates the chorda is more or less replaced by the cartilaginous tissue of the secondary perichorda that grows round it. In the mammals it disappears for the most part.
The fin-fringe is supported by a number of square elastic fin-plates. The chorda develops no further, but retains its original simplicity throughout life. It is enclosed by a firm membrane, the chorda-sheath or perichorda. The perichorda forms a cylindrical tube immediately over the chorda, and the central nervous system, the medullary tube, is enclosed in it.
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