Final answer:
The mesencephalon is one of the primary vesicles in the major embryological groupings of the brain, and it develops into the adult midbrain, part of the brain stem. The midbrain maintains a small relative size from embryonic stages to the adult brain, in contrast to the significant growth seen in the forebrain.
Step-by-step explanation:
The major embryological grouping of the brain in which the mesencephalon is involved is the primary vesicles that form during early neural development. The mesencephalon, also known as the midbrain, is one of these primary vesicles.
As embryonic development progresses, the mesencephalon gives rise to adult brain structures that are collectively referred to as the brain stem. Specifically, the mesencephalon develops into the adult midbrain. The midbrain is involved in functions such as vision, hearing, motor control, sleep/wake, arousal (alertness), and temperature regulation.
When comparing the early (25th embryonic day) brain to the adult brain, the relative sizes of the forebrain, midbrain, and hindbrain regions show significant changes. The forebrain becomes much larger and more complex, while the midbrain remains relatively small in comparison to other regions in the adult brain. Moreover, the hindbrain also differentiates and expands but not to the extent of the forebrain.