Diencephalon

Together with the telencephalon, the diencephalon is part of the prosencephalon. At S15/16, four longitudinal zones appear in addition to the two transversally oriented neuromeres (D1 and D2). The most dorsal longitudinal zone is the epithalamus, followed more ventrally by the dorsal thalamus, the ventral thalamus and the hypothalamus. The hypothalamus is clearly delimited from the other zones by the hypothalamic sulcus.
The diencephalon forms the wall of the 3rd ventricle with its longitudinal zones. The 3rd ventricle continues towards the mesencephalon to become the cerebral aqueduct, and connects to each of the lateral ventricles through the interventricular foramen. By S10, a thickening can be seen in the rostral wall of the 3rd ventricle, ventrally of the lamina terminalis of the telencephalon: the chiasmatic plate. It is only towards the end of the embryonic period (from S19 onwards) that fibres of the optic nerve grow into this plate and cross to the opposite side.