Suggested evolution of tonotopic organization in the frog amphibian papilla.

As one progresses from the most primitive to the most derived frogs, one observes remarkable changes in that peculiarly amphibian auditory organ, the amphibian papilla. In all but the most primitive frog, the papilla comprises two patches with separate innervation and apparently corresponding to a spatial separation of frequency sensitivity (i.e. tonotopic organization). The caudal patch is quite variable and in the more derived frogs exhibits an elongation that apparently corresponds to extension of auditory sensitivity to higher frequencies.