Abstract
The reactions of the sub-epidermal melanophores, in intact and in isolated pieces of skin, to various stimuli,—light, temperature, solutions of various salts and drugs and electric currents—were studied.
The melanophores of normal and eyeless larvæ 1 react to light by expanding, and to darkness by contracting. If the normal larvæ are, however, kept in bright diffuse daylight on an indifferent background for more than 3 to 5 days the melanophores secondarily contract. Likewise if they are kept in darkness for more than 5 days the melanophores secondarily expand. These secondary responses are lasting and are not shown by the melanophores of eyeless larvæ.
When larvæ are blindfolded, instead of their eyes being removed, the melanophores react to light like those of blinded individuals, that is, they expand and remain so; but they react to darkness like those of normal seeing larvæ, that is, they at first contract, but after 5 days or more they expand.
The melanophores of isolated pieces of skin do not react to daylight, to the light from a Nernst glower, or to darkness. The light from an arc lamp, however, causes them to contract.
From an additional series of experiments on larvæ in which the central nervous system had been partially or totally destroyed it is apparent that the primary responses of the pigment cells to light and darkness are caused essentially by direct stimulation. The secondary responses of the seeing larvæ are, on the other hand, due to nervous activities set up by the stimulation of the retinæ, the stimulation of sensory nerve endings in the skin playing no part.
A high temperature (above 38°) causes the melanophores to contract, a low temperature (below 12°) causes them to expand, both in the intact skin and in isolated pieces.
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