The Melanin Content of the Skin of Rana Temporaria Under Normal Conditions and after Prolonged Light- and Dark-Adaptation. A Photometric Study

Abstract

1. Melanin has been extracted from dorsal and ventral components of the skin of English frogs by a simple method involving the leaching out of water-soluble pigments, the peptic digestion of washed skin, and the centrifugation of unaffected pigment. The melanin was estimated in NaOH solution with the aid of an indian-ink colour standard and by the use of a Duboscq colorimeter calibrated against suitable neutral density filters at various wave-lengths. 2. Melanin varies greatly in amount in dorsal skins, some of which contain ten times as much of the pigment as others. In extreme duskiness and pallor, the difference may be fiftyfold. In spite of such variability, melanin formation seems generally to follow the usual laws of growth. 3. Ventral pigmentation varies more than dorsal, ranging from an almost pigmentless condition to equality with dorsal pigmentation. The mean amount of melanin in ventral skin is about 27% of that in dorsal skin. 4. Melanin occurs in the skin of females at spawning time in large amount. Thus, the pale, red colour of the skin is not due to lack of the pigment. Melanin does not differ significantly in amount in the sexes. 5. Melanin can be determined by careful visual judgement of the tone of the skin to within one-third of the total amount present in dark skin. 6. The melanin content of dorsal skin is increased or decreased when frogs are kept for some weeks in equally illuminated light-absorbing (black) or light-scattering (white) surroundings. The skin of the dark-adapted animal contains 60% more melanin than that of the light-adapted one after 5 weeks of treatment. 7. The melanin content of the skin of a limb is increased under post-mortem conditions following intramuscular injection of post-pituitary extract into the freshly excised living limb. An increase of 27% was determined. No effect is produced by adrenaline under these conditions. 8. Prolonged adaptation to light-absorbing or light-scattering surroundings produces a more fundamental effect than the mere dispersion or aggregation of pre-existing melanin granules in the melanophores, namely, the production of fresh melanin or the degradation of this pigment. The subtractive process, which is evident microscopically, is due to some agent at present unknown. 9. Frog melanin is shown by its absorption characteristics to be chemically very closely similar to, and possibly absolutely identical with, mammalian melanin and the pigment of melanoma.