ANIMAL TINCTUMUTANTS. 389 



lip 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 distri- 

 bution of these pigment cells, and the power they have of shrink- 

 ing 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 occa- 

 sion them to contract, or, as is frequently the case, when con- 

 tracted, will occasion them to dilate, and the phenomena of tinctu- 

 mutation 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 pre- 

 dominate ; when they are contracted, the skin becomes lighter in 

 color. Besides the pigment cells just described, Heincke discov- 

 ered another kind of chromatophores which were filled with iri- 

 descent 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 Ruthens2Jarri. I have seen 

 this kind of color cells in the skin of the gilt catfish, which be- 

 longs 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 liv- 

 ing animal, and the chromatophores will respond to artificial 

 stimulation for quite a while. In making my observations, how- 

 ever, 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 chloro- 

 form injected hypodermatically rendered the animal anaesthetic, 

 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. Sev- 

 eral 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 incor- 

 rect. 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 axo- 

 lotl could not form pigment when reared in a yellow light. Prof. 

 Semper, on the contrary, declares Bert's axolotls to be albinos. 



