The mesencephalon is the middle portion of the future brain and is therefore also called 'the midbrain'. In contrast to the prosencephalon and the rhombencephalon, the mesencephalon does not undergo further division during neuronal development.
The mesencephalon first appears on E9 and gives rise to the dorsal and ventral midbrain. The dorsal portion of the mesencephalon (tectum) will eventually comprise the superior and inferior colliculus. Its ventral part (tegmentum) will contain, among others, dopaminergic neurons located in the (the substantia nigra [SNC] and the ventral tegemental area [VTA]), the oculomotor nucleus and the red nucleus.