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Development of the human spinal cord. (A-D) The neural tube is functionally divided into dorsal (alar) and ventral (basal) regions, separated by the sulcus limitans. As cells from the adjacent somites form the spinal vertebrae, the neural tube differentiates into the ependymal, mantle, marginal zones, and the roof and floor plates. (E) A segment of the spinal cord with its sensory (alar) and motor (basal) roots. (After Larsen 1993.)