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Definition:
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Rod-like principal supportive element of the embryo and larva, present in the midline just ventral to the neural tube, and differentiating during the segmentation period to form large vacuolated epithelial cells and a surrounding sheath of fibrous and elastic layers. Layering of the sheath may differ in structure, thickness and development among groups; in cypriniforms there are actually three very thin layers to the sheath. A functional, well developed notochord is present throughout life in certain basal fish groups but not in cypriniforms. (1)
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Request: 
