Evolution of Endocrine Regulation of Metamorphosis in Lower Vertebrates

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Abstract
Metamorphosis commonly occurs in lower vertebrates, but its endocrine regulation is not well known outside of the Amphibia. Paedomorphosis (progenesis and neoteny) is a frequent occurrence as well. The only endocrine-related hypothesis is that of Etkin for amphibians. This hypothesis has been modified to accommodate some of the recent findings, but other data have not been incorporated and still other parts remain untested. Presumably, the hypothesis relies entirely on endogeneous factors, and the importance of external factors needs to be examined. Facultative neotenic amphibians may be useful animals for testing certain aspects of the Etkin hypothesis. There are insufficient data for fishes at this time to evaluate the possible universality of the amphibian hypothesis.

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