[96] Vide Dewar, "The Physiological Action of Light," _Nature_, p. 433, 1877; quoted also by Semper, _loc. cit. ante_, Notes, p. 423. I do not think that the absence of the slight amount of color in the animals reared under the yellow light was due to the "optic current" of Dewar. The microscope showed that the chromatophores were just as large and just as numerous, and that they contained as much pigment, as those reared under the red light. The apparent absence of color was due to tinctumutation.--W.
In most viviparous animals the embryo is developed in almost or absolutely total darkness, yet when it is born it has bright colors. Kerbert has found in the cutis of the embryonic chick, about the fifteenth day, certain pigment-cells. These cells have entirely disappeared by the twenty-third day. It is probable that little, if any, light can reach the chick through the shell and membranes, yet pigment-cells develop and disappear again.[97]
A butterfly emerges from the cocoon arrayed in all the colors of the rainbow; yet it was developed, while in the _pupa_ state, in total darkness. It is not necessary to mention further instances; we readily see that pigmentation in animals is not necessarily dependent on light. Neither is tinctumutation the result of the direct influence of light on the chromatophores. Light, however, if not the direct, is the indirect cause of this phenomenon. Lister, in 1858, showed that animals with imperfect eyesight were not good tinctumutants, notwithstanding the fact that they had the chromatophoric function. He showed, by his experiments on frogs, that the activity of the chromatophores depended entirely on the healthy condition of the eyes,--that is, so far as the phenomenon of tinctumutation was concerned. So long as the eyes remained intact and connected with the brain by the optic nerve, the light reflected from the surrounding objects exerted a powerful influence on the chromatophores. As soon as the optic nerve was severed, the chromatophores ceased to respond to the influence of light and color, no matter how bright and varied they were. The deductions drawn from these experiments are not to be controverted or denied. The chromatophores are influenced by light reflected from objects and transmitted _via_ the optic nerve to the brain; from this organ the impression or irritation goes to the nerve governing the contractile fibres of these pigment-holding glands.[98]