278 ALFRED C. REDFIELD 



longer stand out in contrast with the ground-color, but the fus^ 

 cous-black bands now become very prominent both on the back 

 and legs. The lateral scales become white throughout, with the 

 exception of two on each side which are located at the ends of the 

 fuscous-black cross bands of the back. These scales rarely be- 

 come entirely free from pigment. Plate 1 illustrates the extremes 

 of these color changes. ^ 



The color changes of lizards have been shown by Brlicke ('52), 

 Keller ('95), and Carlton ('03) to be due to the migration of pig- 

 ment contained in cells known as chromatophores, which are 

 situated in the dermal layer of the skin. An examination of 

 sections of the skin of the horned toad reveals the presence of 

 chromatophores, filled with black pigment, such as Keller ('95) 

 has called melanophores. When the melanophore pigment 

 expands the ground-color becomes fuscous; when it contracts 

 this portion of the skin becomes drab. The fuscous-black bars 

 across the back and legs contain such an abundance of non- 

 motile pigment that their color does not change. 



In addition to the melanophores there exist in the skin pigment 

 cells containing a yellow pigment. Whether this pigment is 

 motile has not been determined, certainly it causes no prominent 

 color changes. 



2. Daily rhythm of color changes 



At night the melanophore pigment of the horned toad is con- 

 tracted, giving the lizards a pale appearance. In the early 

 morning this pigment expands and the skin becomes uniformly 

 dark. During the heat of midday the melanophore pigment 

 contracts again, but as evening approaches there is a second ex- 

 pansion, followed finally by a contraction once more as night 

 sets in. Thus the pigment is contracted and the animals pale 

 at night and at midday; the pigment is expanded and the animals 

 dark in the morning and afternoon. 



3 Photographs fail to bring out fully the contrast between the dark and the pale 

 condition of the skin. This is because the skin is tinged with yellow, a non-actinic 

 color. Orthochromatic plates and a color screen have been used, but without 

 complete success, in an attempt to overcome this difficulty. 



