ZOOLOGY: A. C. REDFIELD 
203 
conditions of illumination and temperature; (2) those which result 
in an approximation of the color of the skin to that of the substratum 
on which the lizards live; and (3) those occurring during nervous 
excitement. 
The daily rhythm of melanophore reactions consists in an expansion 
of the melanphore pigment, and a consequent darkening of the skin, 
in the morning and afternoon, and a contraction of the melanophore 
pigment and a paling of the skin at mid-day and at night. These re- 
actions are due to the interaction of illumination and temperature upon 
the pigment cells. At mean temperatures (20°C. to 30°C.) the melan- 
ophore pigment is expanded in the light and contracted in the dark. 
In this way the coloration of the skin in the morning, afternoon, and 
night is explained. At higher temperatures the melanophore pigment 
is contracted irrespective of illumination; the pale coloration of the 
skin at mid-day is thus explained. At lower temperatures the melano- 
phore pigment is expanded irrespective of illumination. 
The responses of the melanophores to illumination and temperature 
are due to the direct action of these stimuli upon the pigment cells or 
some closely associated tissue, for: (1) a local illumination, a local 
shadow, or a local heating of the skin produces a local reaction of the 
melanophores; (2) the reactions to illumination and temperature take 
their normal course in regions of the skin which have been isolated from 
the nervous system. 
The adaptive reactions of the melanophores are initiated by stimuli 
received through the eyes. If horned toads are blindfolded, no adap- 
tive reactions take place. This inhibition of the reaction is not due to 
the mechanical effects of bhndfolding. Upon the adaptive reactions 
are superimposed the daily rhythm of color changes, with the result 
that lizards adapted to a dark substratum become paler at night and 
at mid-day, while lizards adapted to a light-colored substratum be- 
come darker in the morning and afternoon. 
During states of nervous excitement the melanophore pigment of 
the horned toad is so contracted that the color of the skin becomes pale. 
This reaction is brought about by any noxious stimulus, such as pro- 
longed mechanical or faradic stimulation, holding an animal on its 
back, or prying open its mouth. The reaction occurs irrespective of 
illumination, temperature, or the adaptive condition of the skin. The 
coordinative mechanism, by which a local noxious stimulus brings about 
a reaction of the melanophores of the entire body is described in the 
following paper. Probably this mechanism also carries out the adaptive 
reactions of the melanophores. 
