Final answer:
The major portion of the diencephalon is the thalamus, which together with the hypothalamus, forms the walls of the third ventricle in the brain.
Step-by-step explanation:
The major portion of the diencephalon is the thalamus. This structure, along with the hypothalamus, defines the walls of the third ventricle, which is the space bounded by the medial walls of the hypothalamus and thalamus. The two thalami touch in the center in most brains as the massa intermedia, surrounded by the third ventricle.
Other structures of the diencephalon include the epithalamus, containing the pineal gland, and the subthalamus, which houses the subthalamic nucleus that is part of the basal nuclei. The midbrain, another brain region, is situated between the thalamus and pons and is not part of the diencephalon.