A contribution to the study of fore‐limb eruption in metamorphosing Anura.

Abstract

Summary. The fore‐limbs of Xenopus laevis are freed from their brachial sacs at the onset of metamorphosis in response to an endocrine stimulus. The disappearance of the tissues overlying the young limbs is not a result of mechanical pressure exercised by limb growth, nor is it aided by stresses created by changes in head shape during metamorphosis. Mechanical pressure resulting from movements of the limbs hastens their liberation, but the thinning and perforation of the overlying tissues can take place in the experimentally contrived absence of the limb. Metamorphic changes in the branchial chamber, in particular the involution of larval structures, can play no part in initiating the perforation process in this species. It is suggested that the rôle of substances released by degenerating gills in inducing opercular perforation in the normal metamorphosis of other Anuran species has not been satisfactorily established. The process of fore‐limb liberation in the precociously induced metamorphosis of young tadpoles is regarded as essentially similar to the equivalent process in normal metamorphosis. This consideration, taken with the results of some earlier workers, leads to a rejection of the claim that Anurau fore‐limbs are liberated as a consequence of the histolytic properties of the secretion of glands in their skin. The only factor that may be regarded as universally effective in the perforation process is the specificity of the tissues concerned to atrophy in response to endocrine conditions at the onset of metamorphosis. It is suggested that this factor alone is responsible for initiating the process in Xenopus, and probably in other species as well. There is no justification for regarding the process of perforation as an example of double assurance.