Final answer:
The midbrain, or mesencephalon, is the middle vesicle in the developing brain and is distinct from the forebrain and hindbrain. It remains a separate region and does not divide further. It constitutes a small proportion of the entire brain.
Step-by-step explanation:
The midbrain, also known as the mesencephalon, is the middle vesicle in the developing brain and is part of the primary vesicle stage of development. It is one of the three major regions of the brain, along with the forebrain (prosencephalon) and hindbrain (rhombencephalon). The midbrain remains a distinct region and does not differentiate into any finer divisions. It constitutes a small proportion of the entire brain compared to the hindbrain and forebrain.