LIGHT AND PIGMENTATION 



89 



— guanine. This may form a white highly reflecting layer, as in 

 Crustaceans, or, as in many Vertebrates, may be contained in white 

 gtianophores or variegated iridocytes, the iridescent colour changes of 

 which are due to the arrangement, form and movements of plate-like 

 crystals of guanine — a form of coloration akin to that due to the diffrac- 

 tion of light by the scales offish and reptiles or the feathers of birds. The 

 colour changes in these cells are sometimes quite remarkable ; thus in 

 the killifish, Fundulus, a single iridocyte may exhibit blue-green, 

 orange, yellow and red phases in successive moments. 



The factors causing colour changes in animals include extremes of 

 temperature, humidity, contact stimulation, and psychic stimuli, 

 particularly excitement and fear ; but the most general and much the 

 most important is light. 



Light acts upon chromatophores in one of three ways — by a direct, 

 primary effect on the cells themselves, by a secondary reaction through 

 the eye, or by indirect reactions through receptor mechanisms other 

 than the eyes (the central nervous system and the pineal body). 



A further response — the endogenous diurnal variation in coloration, largely 

 controlled by hormones and nervous centres situated in the mid-brain — we have 

 already discussed.^ 



{a) When light acts directly upon the chromatophores themselves 

 the reaction may be called a peimary response. This is the most 

 primitive mechanism and the only one available to unicellular plants 

 (diatoms) or animals {Euglena), but it is frec|uently retained in higher 

 forms, usually as a generalized darkening in the shade and lightening 

 with illumination, a change, however, normally obscured by the more 

 dominant secondary responses through the eyes. The primary 

 response, however, can be observed in young specimens the chromato- 

 phores of which have not yet come under the control of the secondary 

 mechanism, in blinded animals (Osborn, 1940), in denervated regions 

 after nerve section and degeneration, and in isolated fragments of the 

 skin when exposed to illumination, a reaction demonstrated in 

 crustaceans (Keeble and Gamble, 1905) and in some sea-urchins 

 (Kleinholz, 1938 ; Millott, 1954-57) (Figs. 64 to 67). 



The direct motor resjionse of individual ectodennal cells to the stimulus 

 of light survives among the higher animals in the movements of the retinal 

 rods and cones - and in the contraction of the pupillary mviscles, both of which 

 are ectodermal in origin. In the iris of Cephalopods, Fishes and Amphibia a 

 direct contraction to light commonly occurs,^ and although the primitive response 

 in the higher ^Mammals and man has been replaced by a reflex nevu'o-mechanism, 



1 p. 19. " p. "31. 



3 Brown-Sequard (1847-.59), Budge (1855), Miiller (1860), Schur (1868), Steinach 

 (1890-92), Magnus (1899), Guth (1901), Marenghi (1902), Hertel (1907), Young (1933), 

 Weale (1956), and others. 



Fundulus 



Diatom 



