those reared in a yellow light[96] were almost but not
quite as dark; while those reared in white ironstone crocks and in
diffused daylight were very much lighter, being pearl-gray in color.
This apparent (for the microscope showed that it was only apparent)
absence of color in the last-mentioned specimens was due to
tinctumutation.

[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]

[97] Karl Semper, _Animal Life_, p. 422.

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 colo