The ventriculus mesencephali is localized in front of the cerebellum and forms a recessus of the dorsal wall of the aquaeductus mesencephali. The ependyma was found one-layered; in some places clusters of cubic or cylindrical ependymal cells occur. Some cells show basal processes. Supra-ependymally there are homogeneous globules and intraventricular nerve fibres. The occurrence of different types of ependymal cells indicates a morphological variability of ependyma even within this small part of the ventricle system of the brain.