The pigmentary effector system. VI. The dual character of endocrine co-ordination in amphibian colour change

Abstract

In foregoing contributions to this series an attempt has been made to analyse the process of co-ordination in a characteristic mode of behaviour encountered in Amphibia and Reptiles. It is now clearly established that the pituitary gland is an essential agency in determining Amphibian colour change. Accepting this conclusion, it is still possible to entertain the hypothesis that the secretion of the pituitary gland is merely a condition of the response of the melanophores to nervous impulse in efferent fibres supplying the skin. The present investigation is an attempt to subject this possibility to more rigorous examination. The direct innervation of Amphibian melanophores has not been established by histological examination, and the time relations of pigmentary effector activity in Amphibia do not suggest the intervention of a peripheral nervous mechanism. As shown in the last communication of this series, the melanophores of Reptiles are directly innervated. Experiments on stimulation and section of peripheral nerve trunks have led investigators on Amphibian colour change to ambiguous results, which have been extensively criticised elsewhere. the chief source of misunderstanding arises from the fact that peripheral nerves in general contain vasomotor components. In work on the chameleon it has been possible to demonstrate segmental effects of nervous origin, by employing as a stimulus to generalised pallor an agency for which the receptor surface is localised. For further analysis of co-ordination in Amphibian colour change it now becomes necessary to undertake a more precise definition of the receptive field. A visual response to undertake naturally suggests itself as the most convenient type for this purpose. In the European frog ( Rana temporaria ) various factors, including humidity and temperature, significantly co-operate with illumination to determine pigmentary effector activity. The South African clawed toad is an aquatic Anuran whose chromatic responses are almost entirely determined by photic stimuli. This circumstance has made it possible to explore the problem of co-ordination more thoroughly than in previous researches recorded in this series of communications. In Xenopus as in other Anura there are three types of pigmentary effector organs. These are ( a ) the epidermal melanophores, ( b ) the dermal melanophores, and ( c ) the xantholeucophores. The dermal melanophores are the most important agents in the visible reaction. Their condition can easily be followed by microscopic examination of the web. In what ensues attention is directed to the dermal melanophores, unless otherwise stated.