ORAL CHEMORECEPTOR ORGANS OF BULLFROG TADPOLES DURING METAMORPHOSIS

Degeneration of the premetamorphic papillae and development of the fungiform papillae during metamorphosis of bullfrog tadpoles were investigated by electrophysiological and scanning electron microscopic methods. Premetamorphic papillae were observed during the early metamorphic stages, and these degenerated rapidly at about metamorphic stage 20. The anlage of the tongue appeared at about metamorphic stage 10, but the anlage of the fungiform papillae appeared at about metamorphic stage 18. The microvilli at the apex of the fungiform papillae were observed at about metamorphic stage 21. At metamorphic stage 24 the fungiform papillae had a similar structure to that of adult frogs. Taste responses were recorded from the glossopharyngeal nerve of the tadpole. The responses to 1 M sucrose and 0.01 M quinine hydrochloride could be observed at metamorphic stage 6 or later, though during stage 20 the responses were very weak. The response to 0.02 M ammonium chloride appeared at metamorphic stage 6, but disappeared at stage 20 and did not reappear later. These results indicate that the fungiform papillae become functional as chemo-receptor organs at about metamorphic stage 21 and that, before the fungiform papillae function, the premetamorphic papillae serve as chemoreceptor organs in the tadpole.