Chromatophores and Color Change 



685 



the state oF the chromatophore system of animals is hght, and in the great 

 majority of animals the method of action of the light invokes principally 

 the eyes, central nervous system, and \arious types of efferent pathvva\s, ner- 

 \'ous, hormonal, or both. The importance of the eyes is clearly manifested in 

 observing the immediate cessation, or great change in character, of color 

 changes on the blinding of the animal. Color changes which are controlled 

 by way of the eves are known as secondary responses, in contrast with pri- 

 mary responses, which are those proceeding under the influence of light 

 through routes other than the eyes. The latter may involve either a direct 

 action of light on the chromatophores or an influence of light operating re- 

 flexly on the chromatophores through extra-ocular receptor mechanisms. 



Secondary color changes dominate the situation in most adult animals. 

 Through a wide range of light intensities these changes do not result from 

 changes in the total quantity of light striking the eye, but rather they are 

 determined by the values of the ratios of the amount of light directly strik- 

 ing the eye from above to die amount of light reflected from the background 

 on which the organism resides. On an illuminated black background where 



Logarithm of liqhr intensitu 



Incident intensit tj 

 Vetlectid intensity 



A B 



Fig. 260. A, Relationship between the log of the incident Hght intensitj' and the 

 average diameter of the melanophores of the minnow, Ericyinha. B, Relationship between 

 the ratio of incident intensiiy to reflected intensity striking the eye and the average 

 melanophore diameter. From Brown."' 



the ratio is large, the animal becomes dark, and on an illuminated white 

 background where the ratio is small, the animal becomes pale, irrespective of 

 the total illumination. It can be shown that there is a good direct correlation 

 between the value of the ratio of incident light to reflected light reaching 

 the eye of the animal and the degree of black pigment dispersion-^- ^^^ (Fig. 

 260, B) or the amount of melanin formed in melanophores,'*^"' ^^'-^ and, fur- 

 thermore, that neither of these melanophore responses varies significantly 

 with variation in intensity within a wide range of total illumination. 



