All neurons of higher vertebrate CNS are born in one place and migrate to another. In the spinal cord and parts of the brainstem neurons are born near the ventricular surface. They then migrate in a radial, or vertical, direction away from the ventricular surface, but stop short of the outer pial surface and differentiate in this position. The outermost zone remains free of neuron cell bodies, but instead becomes populated by axons. Hence, in the adult spinal cord and much of the brainstem, gray matter is inside, white matter is outside.
Some neurons migrate vertically and stop short of the pial surface, just as in the spinal cord.
They differentiate in this deep position to form most of the thalamus, hypothalamus, and limbic parts of the forebrain. Some neurons migrate vertically but do not stop short. They travel all the way out to the pial surface, where they form a layer of cells given the name cortical plate because it is located at the brain surface. The hippocampus starts as a simple cortical plate with one layer of cells, and never adds additional cellular layers. The cerebral isocortex begins development like the hippocampus.