The recessus mesocoelicus was studied in Cebus, Macaca fascicularis and in Pan troglodytes. The shape of this recessus is variable within and across species (funnel-shaped, simple tube, a tube ending with 2 smaller diverticula). The ependyma of the oral wall of the recessus is of the same kind as the ependyma of the subcommissural organ, whereas the ependyma of the caudal wall has changed in Macaca and Pan in the sense, that it has obviously lost its secretory activity. No Reissner’s fibre was found in the recessus in the specimen studie