midbrain (mesencephalon) One of the three sections of the brain of a vertebrate embryo. Unlike the forebrain and the hindbrain, the midbrain does not undergo further subdivision to form additional zones. In mammals it becomes part of the brainstem, but in amphibians, reptiles, and birds the roof of the midbrain becomes enlarged as the tectum, a dominant centre for integration, and may include a pair of optic lobes.

mid·brain / ˈmidˌbrān/ • n. Anat. a small central part of the brainstem, developing from the middle of the primitive or embryonic brain. Also called mesencephalon.

midbrain (mesencephalon) (mid-brayn) n. the small portion of the brainstem, excluding the pons and the medulla, that joins the hindbrain to the forebrain.