Final answer:
The statement is true. The diencephalon includes the hypothalamus, which, along with the thalamus, helps define the walls of the third ventricle and plays a vital role in regulating homeostasis.
Step-by-step explanation:
The statement that part of the diencephalon structure is formed by the hypothalamus is true. The diencephalon is a region of the brain located deep beneath the cerebrum, and it constitutes the walls of the third ventricle. The two major components of the diencephalon are the thalamus and the hypothalamus. The thalamus acts as a relay station between the cerebrum and the rest of the nervous system, while the hypothalamus is responsible for regulating homeostasis through the autonomic and endocrine systems. Additionally, the hypothalamus is involved in emotional and memory functions as part of the limbic system and also physiologically connected to the pituitary gland, which it regulates.