The adult mesencephalon (midbrain) is the least modified of the brainstem structures with regard to basal and alar plates. Neuroblasts of alar plates migrate to form inferior and superior colliculi (tectum). Inferior and superior colliculi are related to the auditory and visual systems respectively. The general somatic afferent (GSA)  portion of CN V, the mesencephalic nucleus, is also found here. Oculomotor neurons (GSE) arise from mesencephalic neuroblasts while trochlear motor (GSE) neurons migrate to this location from the metencephalon.
 

The basal plates and the floor plate expand. They form the tegmentum of the cerebral peduncles in which the motor nuclei of CN III and IV are found in the central gray along with the general visceral efferents (GVE) to the eye i.e., the Edinger-Westphal nucleus. Corticofugal fiber tracts help form ventrolateral bulges of the cerebral peduncles. The embryologic origin of the red nucleus and substantia nigra (from alar or basal plates) in the cerebral peduncles are uncertain.

The cavity of the original neural tube is little modified in the adult midbrain except to be narrowed by growth of the surrounding midbrain structures; it remains as the narrow cerebral aqueduct.

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