Crown cells are considered to be typical for the Saccus vasculosus of gnathostome fishes. In Acipenser ruthenus these cells could be found on the entire floor of the diencephalon by SEM. There is one accumulation of crown cells in the frontal Recessus praeopticus, one caudal to the optic chiasma and one on the floor of the Recessus lateralis. The number of apical processes of the crown cells decreases significantly according to their distance from the Saccus vasculosus. In all basal gnathostome fishes examined until now, crown cells were found on the floor of the diencephalon outside the Saccus vasculosus.