86 



THE EYE IN EVOLUTION 



small pigmented cell-body into a great disc some twenty times the 

 diameter of the original sphere ; a nerve -fibre supplies each muscular 

 cell and the resulting changes are rapid (Figs. 54 and 55). ^ In all 

 other animals the chromatophores are single cells ; usually they are 

 specialized cells provided with arborizing processes, arranged singly or 

 in a syncytium, and by a process of cytoplasmic streaming the pigment 

 may be concentrated into inconspicuous punctate masses in the centre 

 of the cell or dispersed throughout the branching structure to give a 

 diffuse colour to the animal (Figs. 56 to 59).'^ In Insects, however, the 

 ordinary epidermal cells fulfil this function ; normally a dark brown- 

 black pigment lies beneath an evenly disposed yellow-green pigmented 



Figs. 54 and ."jo. — A Chromatophore of the Cephalopod. 



Fig. 54. 



Fig. 54. — The appearance of the 

 chromatophore with the radiating 

 muscular cells and the small con- 

 centrated clump of central pigment. 



Fig. 55. 



Fig. 55. — The extended mass of pig- 

 ment pulled out by contraction of 

 the muscle cells (after Bozler). 



Hyla arborea 



layer, and on stimulation the former migrates to the surface and 

 disperses itself over the lighter layer thus darkening the animal 

 (Figs. 60 and (U) (Giersberg, 1928-30). 



The coloration resulting from the migration of pigment is often 

 assisted by its new relationship to static pigment. Under the chromato- 

 phores of Cephalopods and Crustaceans, for example, there is an 

 immobile layer of light -reflecting pigment so that considerable varia- 

 tions in colour are possible depending on the amount of light permitted 

 to pass to the deeper tissues (Webb et al., 1952). In Insects, as we have 

 seen, the variegation is enhanced by the migratory brown or red 

 pigment covering over or retreating behind the static green and yellow 

 pigments. In Amphibians such as the tree-frog, Hyla arborea, a colour 

 change from green through lemon-yellow to grey is attained by varia- 

 tions in the dispersion of melanin underneath layers of yellow and white 



1 Sre especially— Phisalix (1894), Hertel (1907), Hofmaim (1907-10), Frohlich 

 (1910), iiozler (1928). 



2 Set^ especially— Spaeth (1913), Perkins (1928), Matthews (1931), Perkins and 

 Snook (1932), Brown (1935), and others. 



