Final answer:
The adult derivative of the diencephalon cavity is the third ventricle, surrounded by structures such as the thalamus and hypothalamus.
Step-by-step explanation:
The Adult Derivative of the Diencephalon Cavity
The adult derivative of the cavity of the diencephalon is known as the third ventricle. During the embryonic development of the brain, the neural tube forms primary vesicles which further develop into secondary vesicles. One of these, the diencephalon, gives rise to several structures in the adult brain, most notably the thalamus and hypothalamus. These two structures define the walls of the third ventricle, which is the cavity in question. The thalamus serves as a relay and processing station for sensory and motor information, whereas the hypothalamus is involved with various functions such as temperature control, hunger, thirst, and the regulation of the circadian rhythm.
Apart from these, other structures such as the retina also originate from the embryonic diencephalon, which is unique as it presents an example of a nervous tissue that starts as part of the central nervous system (CNS) but ends up as a peripheral structure in the adult nervous system.