At 18 days gestation, the notochord (mesoderm) induces overlying ectodermal tissue into the neural plate. The notochord eventually becomes part of intervertebral discs. The neural plate invaginates into the neural tube during the 3-5th weeks of gestation. The neural tube develops into the neurons seen in the brain, brainstem, and spinal cord. The neural crest cells that border the neural tube will develop into the peripheral nervous system.