Diencephalon

In the 7 week old embryo, an evagination of the base of the diencephalon occurs, forming the infundibulum. This borders the ectodermal protrusion of the stomodeum (Rathke's pouch). The adenohypophysis develops from this pouch. The neurohypophysis and the hypophyseal stalk originate from the infundibulum. Towards the end of the embryonic period, the optic fibres start to cross (chiasma opticum) in the ventral wall of the diencephalon, below the lamina terminalis. At S16, the protrusion of the epiphysis cerebri (pineal gland) on the dorsal aspect can be observed.

Mesencephalon

The mesencephalon has a considerable bend in it. It narrows in the area of the transition to the rhombencephalon to become the isthmus. At S15, the oculomotor nerve can be distinguished in this ventral concavity. In the area of the isthmus, the trochlear nerve appears dorsally.