FUNCTIONAL ARCHITECTURE OF THE VOMERONASAL ORGAN OF THE FROG (GENUS RANA)

The vomeronasal organ in the frog, genus Rana, is composed of three in terconnected cavities; superior, middle and inferior, which are separa ted from and anterior to the principal olfactory cavity. The superior cavity is found just underneath the external naris and forms a vestibu le both for the principal olfactory organ and the vomeronasal organ. T he vomeronasal sensory epithelium is located in the medial region of t he inferior cavity and contains ciliated cells and microvillous recept or cells. Inspection of microscopic sections of frogs that had been sw imming in fluorescent colorants revealed fluorescence on the surface o f the vomeronasal organ, but not on that of the olfactory organ. Obser vations in vivo show that water enters via the external naris by two f issures, one on each side of the movable nasal lid, passes the middle cavity to flow via the sensory epithelium of the inferior cavity. The design of the frog nose makes it possible for this amphibious animal t o sample the chemical composition of its environment; above water the frog can inhale air and expose its olfactory organ to volatile substan ces; in water the vomeronasal organ samples water-borne substances. Th ese new findings are discussed in relation to the air/water interface and the position of the amphibians in the evolution of terrestrial ver tebrates.