124 COMPARATIVE PHYSIOLOGY 



so-called homed toad, Phrynosoma, the only reptile which 

 has been made the subject of recent experimental treatment 

 in this field. In the horned toad bodily colour varies 

 between a fuscous shade (associated with the outward migration 

 of pigment granules into the cell processes of the melanophores, 

 which ramify among the yellow interference cells, which lie 

 mmediately below the epidermis) and a pale cinnamon bluff 

 tint (resulting from concentration of the pigment granules in 

 the cell-body of the melanophore). When it is kept upon a 

 neutral background, there is seen to be a daily rhythm of 

 colour-change in Phrynosoma. At night the melanophores 

 are contracted, giving the skin the appearance of pallor. In 

 the early morning the skin becomes uniformly dark through 

 ** expansion " of the melanophores. But during the heat of 

 the day — in its warm natural surroundings — the melanophore 

 pigment contracts, and as evening approaches a second 

 expansion supervenes, until night descends. The condition 

 of pallor in natural surroundings is, therefore, seen at night 

 and at midday. In the cooler parts of the day the skin is 

 fuscous. 



The exact part which light and temperature respectively 

 play in promoting this sequence of reactions has been made the 

 subject of investigation by Parker (1906) and Redfield (1918). 

 From their work it appears that the melanophores of this 

 Uzard respond to light and darkness, warmth and cold, in the 

 manner generally characteristic of reptiles, i.e. bright illumina- 

 tion and low temperature promote darkening of the skin, 

 while warmth and darkness bring about pallor. Light and 

 heat interact so that the effect of the latter predominates at 

 extremes of temperature, and it is thus that, in natural condi- 

 tions, living as these creatures do in a warm climate, pallor 

 intervenes during that part of the day when the temperature 

 rises to a maximum. 



But in addition to this response to direct illumination, 

 the horned toad reacts in bodily coloration to the character 

 of the substratum and to mechanical irritation or disturbance. 

 Any nocuous stimulus, such as electrical excitation of the roof 

 of the mouth or the cloaca, evokes pallor in fuscous individuals 



