Photoelectric Measurement O F Melano- Phoral Activity O F Frog Sicik Induced I N Vitro

Changes in dispersion or concentration of pigment within melanophores of vertebrates have been studied chiefly by the use of tlie microscope, and as a scale of measurement the melanophore index of Slome and Hogben ('28) has been adopted widely. This system has the disadvantage of being arbitrary and incapable of detecting slight movements of pigment which may well prove to be significant. Of several methods proposed to meet these difEculties, the photoelectric techniques of Hill et al. ('35) and Smith ('36) appear to be most practical and accurate. Hill and his associates measured photoelectrically the amount of light reflected from the dorsal surface of Fundulus as the fish adjusted to changing background. Smith's procedure differs from Hill's in that tabulations were made of changes in intensity of light transmitted through isolated scales of Tautoga. It is the purpose of the present paper to describe in detail a procedure (Wright, '46) for measuring by photoelectric means color changes in pieces of excised frog skin during and following treatment with pituitary suspensions.