Final answer:
The adult derivative of the wall of the mesencephalon is the midbrain portion of the brain stem, which includes the medulla oblongata, the pons, and the midbrain itself. Understanding embryonic development is crucial for grasping the structure and connections within the adult brain.
Step-by-step explanation:
The question seeks to understand what the adult derivative of the wall of the mesencephalon is. The mesencephalon, or midbrain, is one of the secondary vesicles that arise during the embryological development of the brain. This secondary vesicle does not change its name during the different stages of development and becomes part of the brain stem in the adult brain, specifically the midbrain portion. The other parts of the brain stem include the medulla oblongata, which is derived from the myelencephalon, and the pons, which arises from the metencephalon.
The function and structure of the adult brain can be better understood by considering its embryonic development. The mesencephalon adjacent to the diencephalon, distinguishes itself as a distinct region in the brain stem. This knowledge base can explain why certain neural connections are present in the adult brain, such as those linking the optic tract to both the diencephalon and the mesencephalon.