Using an antiserum raised against bovine Reissner's fiber a ventricle border of the matrix epithel reacting in the immunohistochemical procedure according to Sternberger et al. (1970) is visible in the very early stage of the embryonic brain of Pleurodeles waltli. In the basal region of the brain the immunoreactive border later develops into the flexural organ a secretory cell formation that is situated in the deuterencephalon and spinal cord. The cells of the flexural organ release a glycoprotein into the cerebrospinal fluid, where, among other things, the Reissner's fiber is formed. Later on, Reissner's fiber is produced no longer by the flexural organ, but the subcommissural organ. It is suggested that in the very early embryonic stage the immunohistochemically reactive ventricle border, too, contains a glycoprotein that might contribute to any events in the ontogeny of the nervous system.