84 KINETIC HORMONES — I 



amount of light reflected from the background to the eyes. It 

 helps to afford protective coloration to the animal, so that on a 

 light background the result is a pale appearance; but to achieve 

 this the dark pigments must be concentrated and the white dis- 

 persed. It is a wide-spread phenomenon in both Crustacea and 

 Vertebrata, and more often than not it is controlled by a pair of 

 antagonistic hormones, which between them can maintain the 

 pigment in the chromatophores at any position between full 

 concentration and full dispersion. 



Observation on the behaviour of chromatophores has been 

 facilitated by the introduction of a chromatophore or melanophore 

 index, by which five stages of pigment dispersion are defined, 

 from 1, fully concentrated, to 5, maximally dispersed (Fig. 3-14). 

 Half stages can be used ; but it is important to use chromatophores 

 in the same region of the body for successive measurements, as 

 different groups of cells can show considerable differences (Hogben 

 and Slome, 1931). It must also be remembered that intervals on the 

 chromatophore index scale are not quantitatively equal nor exactly 

 related to the dosage of hormone needed to shift the pattern from 

 one stage of dispersion to the next. Their representation by equal 

 intervals on a graph can, therefore, be somewhat misleading. 



Before considering hormonal control of responses in more 

 detail, it must be emphasized that several extraneous factors, 

 other than "morphological colour change" (§ 3.2), can also affect 

 chromatophores. 



The direct effect of light on chromatophores usually causes 

 dispersion (rarely concentration) in the absence of either nerve 

 connections with a light receptor, or of any hormones in the 

 circulation. Stephenson (1932) showed this clearly in the hermit 

 crab, Eupagurus prideauxi, which has functional red and yellow 

 chromatophores, not only on the exposed limbs, but also on the 

 abdomen, where they are usually hidden within the whelk shell 

 that it carries. The direct effect of bright light causes pigment 

 dispersion, whereas the secondary effect, due to hormones stimu- 

 lated by the eye, is to cause concentration of pigment. On an 

 illuminated light background in a dull light, only this secondary 

 response is elicited, and the animals are pale all over ; but in bright 

 light the pigment on the limbs disperses considerably. That this 



