The Colour Change of the Minnow (Phoxinus Laevis AG.)*

Records were made of the times required for the melanophores of the normal minnow to reach equilibrium when the fish is transferred from one to another of the following conditions : on an illuminated white background; on an illuminated black background; in darkness. These times give further evidence of the parts played by nervous and hormonal mechanisms in the colour change of the minnow. After section of the spinal cord between the 5th and the 12th vertebrae the fish darkens but gradually becomes pale again if kept on an illuminated white background. Such fish can still show a slow colour change: dark on a black background, pale on a white background and intermediate in darkness. Observations of the times required for these colour changes in the spinal minnow show that these no longer resemble those associated with the unoperated fish; rather, they resemble the time intervals associated with amphibian colour change. Further consideration of the times required for colour change in the spinal minnow indicate that there is not only a hormone causing aggregation of the melanophores but also a hormone causing melanophore dispersion. The part played by double innervation of the melanophores is considered.