Developmental analysis of the extrabulbar olfactory projections in the ranid frog with some phylogenetic considerations

This study has examined the presence and distribution of extrabulbar olfactory projections (nerve fibers that originate from cells in the olfactory epithelium, bypass the olfactory bulbs and project caudally into the brain) in larvae and adult stages of a semi-aquatic frog, Rana esculenta. This species was chosen because previous work had suggested that the presence of an extrabulbar olfactory system (EBOS) may correlate with the detection of water-borne odorants. The main result is that this extrabulbar system is present in both larval and adult specimens of this frog species. In stage 26, the earliest one used in our study, the EBOS is well-developed and projects as far caudal as the rhombencephalon. During successive larval development and until the completion of metamorphosis, there occurs a progressive reduction in the caudal extent of the EBOS. This reduction is more dramatic during the metamorphic climax (stages 31-33). A reduction in the caudal extent of the extrabulbar projections in adult stages is similar to other species previously examined. We are inclined to believe that there is no correlation between a reduction in the caudal extent of the EBOS and transition from water to land.