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The lower part of the brain stem is the medulla oblongata. The pons is a knob on the brainstem. Differentiation of the brain stem from the cerebrum is complex, both anatomically and taxonomically. Some taxonomies describe the brain stem as the pons, medulla and mesencephalon while others include diencephaletic regions.
The adult human brainstem emerges from parts of all three vesicles in the neural tube. The myelencephalon and metencephelon emerge from the rhombencephalon, the mesencephalon matures to become a vital link for nerve pathways related to motivation and the prosencephalon gives rise to the diencephalon and to the cerebral telencephalon.