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They all have, at some embryonic stage, a notochord , flanked by muscle, and a dorsal neural tube.
In those chordates which lack bone, muscles work against the notochord to move the animal.
Moreover, these ectopic neural tubes were patterned appropriately with respect to the notochord and dorsal ectoderm.
In the evolution of vertebrates, long bodies with continuously flexible notochords and segmental muscles existed before the appearance of segmental vertebrae.
Let us review the fundamental features of the vertebrate body as context for notochords and backbones.
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