Final answer:
The diencephalon is a major embryological grouping of the brain that forms adult structures like the thalamus and hypothalamus.
Step-by-step explanation:
The diencephalon is a major embryological grouping of the brain. It forms several adult structures in the brain, including the thalamus and the hypothalamus. In the embryonic diencephalon, a structure called the eye cup develops, which eventually becomes the retina, the nervous tissue of the eye.