Studies on Amphibian Metamorphosis

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Abstract
Previous work by the writer has shown that normal development of the tympanic membrane in R. pipiens is due to inductive influences of the annular tympanic cartilage. The present work concerns the nature of the developmental influences, specifically as regards whether or not non-living cartilage still possesses inductive properties. The experiments performed and results obtained were as follows: Controls consisted of autoplastic transplantation of annular tympanic cartilage beneath back integument of metamorphosed and involuting R. palustris larvae. In general, integumentary thinning, coupled with partial to total glandular degeneration, occurred. This is typical of partial membrane formation. Similar transplantations were made involving cartilage killed in ethyl alcohol, saturated chloretone solution, and boiling water, or by desiccation in strong sunlight. The results differed considerably depending on the type of killing treatment employed and the degree of subsequent transplant degeneration. Partial membrane formation was usually induced, however, in the overlying back integument. Similar series were run with half-metamorphosed larvae. In general, membrane induction was even more pronounced. The same experiments were performed on R. catesbeiana larvae. Transplantation, however, was made beneath the skin of the upper jaw, posterior-ventral to the external nares. Wide variations in results were observed. No membrane changes occurred in some cases, while in others considerable transformation was induced. It is concluded that dead annular tympanic cartilage still retains inductive influences capable of initiating varying degrees of tympanic membrane formation in integument. In general, the greater the degree of cartilage degeneration, the less the extent of membrane formation. The results are discussed in general and the conclusion reached that the inductive influences arising in the developing annular tympanic cartilage of the anuran are quite probably chemical secretions or excretions. These, when transmitted to the integument of the ear region during metamorphosis, serve to induce a dedifferentiation of the larval skin into that type characteristic of adult tympanic membrane.