During early development, the nervous system consists of a hollow tube running along the back of the embryo, and the inner lining of this neural tube is packed with stem cells, which divide to produce immature neurons that migrate through the thickness of the tube. At the front end of the tube, successive waves of migrating cells jostle past each other to form the layers of the cerebral cortex, one after the other, from the inside out. Further back, smaller numbers of cells migrate outward to form the spinal cord.
