Popular Science Monthly/Volume 46/January 1895/Animal Tinctumutants
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 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. and states that albinism is by no means infrequent in the axolotl; also that Prof. Kölliker, of Würtzburg, reared a whole family of white axolotls in a laboratory where there was an abundance of light, and that he (Semper) never succeeded in rearing an albino, though there was less light in his laboratory than in Kölliker's, and his axolotls came from the same stock. Bert made the mistake of confounding albinism with the phenomena of etiolation as observed in plants. In fact, he gives the name etiolation to the albinism noticed in his axolotls.
There is a marked difference between the functions of the chlorophyll bodies found in plants and the chromatophores found in animals. The former play one of the most prominent parts in the drama of plant life, inasmuch as they subserve a vital function, while the latter act a minor part, because they only serve as an instrument or means of protection. Light is of great importance in its influence on chlorophyll, which is a microscopic elementary body on which the vital strength of the plant depends; while it is not at all necessary to the chromatophores, cell bodies secreting pigmentary matter for the purpose of protection. Many animals live in total darkness, yet have an abundance of pigment. I myself have seen black beetles in Mammoth Cave, Kentucky, in the neighborhood of Gorin's Dome, which is a mile or so from the entrance of the cavern. Beetles rarely range over a hundred yards from their place of birth, consequently these beetles must have been reared in darkness. On this occasion I was in search of other data, so made no critical examination of these insects. I am not prepared, therefore, to give an accurate description of them. When speaking of light, I have reference to diffused daylight, which carries no heat rays. Heat is a prominent factor in the production of color; the discussion of this fact, however, does not belong to the subject under consideration. Some experiments made several years ago on newts show that the absence of light does not influence pigmentation. My animals were kept under observation from the extrusion of the eggs until the full maturity of the animals had been reached. Great care was taken to make the experiments as accurate and as conclusive as possible. Those reared in total darkness or in a red light were always dark-colored; those reared in a yellow light were almost, if not quite, as dark as those reared in the red light; while those reared in white ironstone crocks and in diffused daylight were very much lighter, being light pearl-gray in color. This apparent (for the microscope showed that it was only apparent) absence of color in the last-mentioned individuals was due to tinctumutation. Like experiments were made on frogs with the same results. In most viviparous animals the embryo is developed in almost or 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.
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 chromatic 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 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 color, no matter how bright and varied they were. The deductions drawn from these experiments are not to be controverted or denied. The chromatophores are influenced by light reflected from objects and transmitted via the optic nerve to the brain; from this organ the impression or irritation goes to the nerves governing the contractile fibers of these pigment-holding glands.
Pouchet followed Lister and confirmed his conclusions by experiments on fishes and crabs. He remarked that the plaice, a fish with a white under surface and a particolored back, had the chromatic function highly developed. Among a number of specimens which appeared pale on the white sandy bottom, he met "one single dark-colored fish in which, of course, the chromatophores must have been in a state of relaxation, and this specimen was as distinct from its companions as from the bottom of the aquarium. Closer investigation proved that the creature was totally blind, and thus incapable of assuming the color of the objects around it, the eyes being unable to act as a medium of communication between them and the chromatophores of the skin." Thus far Pouchet had only confirmed Lister's observations, although it is highly probable that he was unaware of Lister's experiments. But he went a step further. There are two ways in which cerebral impressions may be transmitted from the brain to the skin: one, by way of the spinal cord and the pairs of nerves arising from it and known as spinal nerves.; the other, by two nerves running close to the vertebral column—the sympathetic nerves. Pouchet cut the spinal cord close to the brain, yet the chromatophores still responded to light impression, showing that they did not receive the message through the cord and spinal nerves. He then divided the sympathetic nerves, and the chromatophores at once lost their power of contraction. Thus he proved that the sympathetic nerves were the transmitters of the optical message and not the cord. This discovery of Pouchet is, psychologically, of the greatest importance, though he failed to recognize it as such. He was satisfied with its anatomical and physiological importance. When we remember that the action of the sympathetic nerve is almost if not entirely reflex in character, we can see at once the psychological importance of this discovery. This fact makes the phenomenon of tinctumutation an involuntary act on the part of the animal possessing the chromatic function, and thus keeps inviolate the fundamental laws of evolution, which, were the facts otherwise, would be broken. By a series of experiments on newts and frogs I have confirmed the conclusions of Pouchet in toto. I have gone still further in demonstrating the fact that the sympathetic nerves are the conductors of the optical stimulus. Atropia, to a certain extent, paralyzes the sympathetic. Injections of this drug beneath the skin of a frog render the division of the sympathetic unnecessary. The chromatophores will not respond to light impression if the animal be placed under the influence of atropine.
A large number of the lower animals possess the chromatic function. Several years ago I placed in a large cistern several specimens of the gilt catfish. This is a pond fish and is quite abundant throughout the middle States. It is of a beautiful golden yellow color on the belly and sides, shading into a lustrous greenish yellow on the back and head. Several months after these fish had been placed in the cistern it became necessary to clean it, and the fish were taken out. They were of a dirty drab color when taken out, but soon regained their vivid tints when placed in a white vessel containing clear water. They had evidently changed color in order to harmonize with the black walls and bottom of the cistern. Some katydids are marked tinctumutants. I took one from the dark foliage of an elm tree and placed her on the lighter colored foliage of a locust. She could be easily seen when first placed on the locust. In a few moments, however, she had faded to such an extent that she was scarcely noticeable. I have observed that objects is evidently one of Nature's weapons of defense. In some animals it is developed in a wonderful manner. Wherever it is found it becomes to the animal possessing it a powerful means of defense by rendering it inconspicuous, and in some instances wholly unnoticeable.larvæ of certain moths, beetles, and butterflies also possess the chromatic function. The chromatophores in the larva of Vanessa are quite abundant, and this grub is a remarkably successful tinctumutant. The power of changing color so as to resemble, in coloring, surrounding