stage of a microscope and examined
with an achromatic lens, the chromatophores can readily be made out.
Artificial irritation will immediately occasion them to contract, or, as
is frequently the case, when contracted, will occasion them to dilate,
and the phenomena of tinctumutation may be observed _in facto_. Under
irritation the orange-colored chromatophores, when shrunk, become brown,
and the contracted yellow ones, when dilated, become greenish yellow.
When all the chromatophores are dilated, a dark color will predominate;
when they are contracted, the skin becomes lighter in color. Besides the
pigment-cells just described, Heincke discovered another kind of
chromatophore, which was filled with iridescent crystals. They were only
visible, as spots of metallic lustre, when the cells were in a state of
contraction. He observed these latter chromatophores in a fish belonging
to _Gobius_, the classical name of which is _Gobius ruthensparri_.[94] I
have seen this kind of color-cell in the skin of the gilt catfish, which
belongs to a family akin to _Gobius_. The skin of this fish retains its
vitality for some time after its removal from the body of the living
animal, and the chromatophores will respond to artificial irritation for
quite a while. In making my observations, however, I prefer to dissect
up the skin and leave it attached to the body of the fish by a broad
base. A few minims of chloroform injected hypodermatically rendered the
animal anaesthetic, and I could then proceed at my leisure, without being
inconvenienced by its movements. The causation of tinctumutation is now
definitely known. The theory that light acts directly on the
chromatophoric cells has been proved to be incorrect. Even the theory
that light occasions pigmentation is no longer tenable. I have, time and
again, reared tadpoles from the eggs in total darkness, yet they differ
in no respect from those reared in full daylight. The chromatophores
were as abundant and responded to irritation as promptly in the one as
in the other. The distinguished Paul Bert declared that the young of the
axolotl could not form pigment when reared in a yellow light. Professor
Semper, on the contrary, declares Bert's axolotls to be albinos, and
states that albinism is by no means infrequent in the axolotl; also that
Professor Koelliker, of Wuertzburg, reared a family of white axolotls in a
laboratory where there was an abundance of light, and that he (Semper)
never suc