At approximately 21 days after implantation, three ridges appear in the embryo and fold to compose the neural tube, from which the nervous system will develop.

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The neural tube, which forms the basis of the nervous system, begins to develop from the neural plate, not from three ridges. The neural plate folds to form the neural tube around 18 to 22 days after fertilization.

At Approximately 21 Days After Implantation, Three Ridges Appear in the Embryo