up of fibrous tissue, nerves, blood-vessels, and cavities containing glands and cell elements. The glands contain coloring matter, and the changes of color in the frog's skin are due to the distribution of these pigment cells, and the power they have of shrinking or contracting under nerve irritation. The pigment varies in individuals and in different parts of the body. Brown, black, yellow, green, and red are the colors most frequently observed. The color cells are technically known as chromatophores. If the web of a frog's foot be placed on the 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 orangecolored 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 chromatophores which were filled with iridescent crystals. They were only visible, as spots of metallic luster, 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. I have seen this kind of color cells 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 stimulation for quite a while. In making my observations, however, I preferred 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 anæsthetic, and I could then proceed at my leisure, without paining it and without being inconvenienced by its movements.
The causation of tinctumutation is not definitely known. Several ingenious hypotheses have been advanced, none of them, however, being completely tenable. '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 differed 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. Prof. Semper, on the contrary, declares Bert's axolotls to be albinos.
