MET'ACHRO'SIS (from Gk. peraxpwvviTat, metachronnynai, to change color, from perri, meta, after xmovivat, ebrannynui, xp6Cetv, ehrozein, to color, from xpc5cae, ehrosis, color, from xpoui, ehroia„rp6a, chroa, skin. color). Color-change. as that of the chameleon, in adaptation to surround ings, and due to changes in the size of the pigment cells of the inner layer of the skin. These special ized pigment-cells are called 'ehromatophores,' and the remarkable changes in the color of the skin of the chameleon. of the tree-toad, the squid. etc., depend on the distribution of these pigment-cells, which dilate (becoming highly ramified) and contract under certain kinds of irritation. The pigment (q.v.) varies in color in different species and in different parts of the body, being black, brown, yellow, and sometimes even red or green. In the goby Heincke found that the chromato phores which are yellow or greenish-yellow when distended become orange-colored when contracted. while the orange or red ones when shrunk become brown or even black; and he detected in the goby a special kind of chromatophores which were filled with iridescent crystals of marvelous deli eacy, appearing when dilated as specks of metallic sheen.
These changes may be due to the direct in fluence of the stimulus of light, or indirectly through the eyesight. Scraper says that Lister demonstrated as long ago as 1858 that the activity of the ehromato phores depends solely on the healthy condition of the eye. As, soon as the eyes are de stroyed or the optic nerve is divided, the chromatophores do not function. Ile was confirmed in this view by l'ouehet's experi ments on fishes and crabs, showing that the chromatophores lost their power of contraction if the two sympathetic were destroyed at the root.
Heincke's observations in some respects contra dict those of Biedemann (1892), on the other hand, has claimed that the color-cells change their shape owing to the direct action of the light and of temperature. Ile shows that the slightest change of teznperature affects the inn tual disposition of the pigment-cells, and conse quently the color, of the frog. It is enough to keep the animal in the hand to provoke a con traction of its black cells. The amount of blood supply also has a definite effect, Steinach also (1891) has proved that light acts as a direct stimulus. Biedemann therefore appears to have proved that the chief agency of changes of color is not in the sensations derived from the eyes, but in those derived from the skin.
It is well known that the under side of flatfish is white or colorless. This is due to absence there of pigment-cells. These, however, are present in very young flounders, hut as they grow 'unsymmetrical the fish turns the left side up ward, and the ehromatophores disappear from the right or under side. Cunningham experi mented with young flounders, by placing a mir ror below the aquarium at an angle of and cut off the light from alcove. In the larger num her of specinnms thus treated, after several months, more or less of the skin of the lower side was pigmented. Ile thus proved that the absence of pigment on that side in the normal fish is due to its position in shadow. It thus appears that the absence of the pigment or of color is due to the absence of light, a mechanical or physical cause.