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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.
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.
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