Studies on Amphibian Metamorphosis Xvii. Influence of Non-living Annular Tympanic Cartilage on Tympanic Membrane Formation

INTRODUCTION THE tympanic membrane of the adult frog is divisible into three histologically distinct parts, viz. an inner epithelial layer, continuous with the mucous membrane of the tympanic cavity, the former being closely applied to a thin lamina propria conforming in shape and area to that of the annular tympanic cartilage to which it is attached at its peripheral borders. The third and outer portion of the membrane consists of modified integument which, externally, appears as a conspicuous brownish coloured area almost circular in shape and slightly indented. The histology of the various layers has been described for only a few anurans. In this regard the reader is referred to Moldenhauer's (1878) work on R. esculenta and R. temporaria, to that of Tokura (1925) on R. nigromaculata, and of the writer's (1928, 1934 a) on R. pipiens, R. sylvatica and R. clamitans. The writer (1928) has previously shown that the formation of the integumentary portion of the membrane, during larval metamorphosis, involves a definite sequence of histological changes in the skin covering the presumptive ear region. In general, this consists of two distinct phases in which certain degenerative changes first occur in the various integumentary layers, particularly the stratum compactum, followed by a developmental stage during which the characteristic structure of the adult membrane is gradually formed. Thus, the entire process may be considered as a dedifferentiation of normal integument into that type peculiar to the tympanic membrane. The skin of the presumptive ear region cannot be considered as determined to develop into tympanic membrane as contrasted with other integument of the larva, for when it is transplanted, heterotopically, it fails to undergo membrane transformation during metamorphosis. Conversely, however, any integument transplanted to the ear region will transform into tympanic membrane at the proper metamorphic stage. It was finally found that the dedifferentiation of larval skin into that of the tympanic membrane is the result of inductive influences emanating from the developing and underlying annular tympanic cartilage. This conclusion