Postembryonic development of the olfactory and vomeronasal organs in the frog Rana chensinensis.

We studied by light microscopy the histological development of the olfactory and vomeronasal organ in tadpoles of the Chinese forest frog, Rana chensinensis, from postembryonic periods to the end of metamorphosis. Unlike Bufo americanus, the olfactory epithelium in larval R. chensinensis is not divided into dorsal and ventral branches in the rostral and mid-nasal regions. The olfactory epithelium in the dorsal portion of the buccal cavity in larval R. chensinensis may correspond to the ventral olfactory epithelium of Bufo, which has been argued to provide a chemosensory function in the tadpoles analogous to the role of taste buds in adults. Bowman's glands were present in the olfactory epithelium of R. chensinensis only after the appearance of the forelimbs during metamorphosis. The appearance of Bowman's glands in the olfactory epithelium at this time suggests that the nose first begins to detect odorants in the air, and this is thus also a metamorphic event. The vomeronasal epithelium appeared a little earlier than the vomeronasal gland in R. chensinensis, unlike in toads (bufonids). This study supports Eisthen's hypothesis that the most recent common ancestor to the tetrapods was aquatic and once had a vomeronasal organ, and that this has been lost in various evolutionary lineages.