THE EXTRABULBAR OLFACTORY PATHWAY - PRIMARY OLFACTORY FIBERS BYPASSING THE OLFACTORY-BULB IN BONY FISHES

Recent evidence has revealed that some primary olfactory fibers bypass the olfactory bulb and terminate in tel- and/or diencephalic areas (e xtrabulbar olfactory pathway, EBOP). We investigated the projections o f this system in different fishes by means of soybean agglutinin bindi ng studies. In all species in which primary olfactory fibers were labe lled, fiber bundles can be traced beyond the olfactory bulb. These run with the medical forebrain bundle and terminate at different targets, depending on the species. In the teleosts Macrognathus, Mogurnda, Hem ichromis, EBOP fibers can be traced into the ventral telencephalon, pa rs ventralis, pars supracommissuralis and/or into the preoptic nucleus . In most nonteleosts studied (Polypterus, Chalamoichthys, Amia), the EBOP also innervates diencephalic targets. An exception is Acipenser, which displays an innervation pattern similar to that in teleosts. Com parison with results obtained by other techniques suggests that the EB OP consists of primary olfactory fibers, which project not only to the olfactory bulb but also to various other targets in the prosencephalo n of anamniotic vertebrates.