The neural tube keeps its original form in the mesencephalon. Its roof is similar to that of the spinal cord. In the basal plate, the red nucleus and the substantia nigra are the most prominent structures, rather than the columns of nuclei. The tectum (superior and inferior colliculus), made up of a number of layers, appears in the alar plate. The ventricle narrows and becomes the cerebral aqueduct (aquaeductus mesencephali).