The chromatic function--and I use this term to designate the faculty of changing color according to surroundings--is possessed by a number of the lower animals. The chameleon is the best known of all the tinctumutants (_tinctus_, color, and _mutare_, to change), though many other animals possess this faculty in a very marked degree. In order to understand the manner in which these changes or modifications of color take place, one must know the anatomy of the skin, in which structure these phenomena have their origin. The frog is a tinctumutant, and a microscopic study of its skin will clearly demonstrate the structural and physiological changes that take place in the act of tinctumutation. The skin of a frog consists of two distinct layers. The epidermis or superficial layer is composed of pavement epithelium and cylindrical cells. The lower layer, or _cutis_, is made 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 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 anæsthetic, and I could then proceed at my leisure, without being