360 LESLIE B. AREY 



chemically stiinulating certain types of isolated pigment cells 

 has been shown in a striking manner (Spaeth, '13^; 'IS**). 



All investigations relating to the physiology of pigment 

 cells may be divided into two general categories, depending 

 on whether body chromatophores or retinal pigment cells serve 

 as the material for experimentation. The general tendency, 

 however, to keep these fields of research sharply separated is 

 wrong, inasmuch as the behavior of the two types of cells has 

 much in common, and it is only by summating our knowledge, 

 after the comparative method, that a thorough understanding 

 of either may be gained. 



Light is the commonest and most potent of the natural stimu- 

 lating agents concerned in pigment migration. In general, it 

 may be said that light induces an expansion, and the absence 

 of light a contraction, of pigment cells. 



Photomechanical changes have been demonstrated in the 

 body chromatophores of crustaceans, cephalopods, fishes, am- 

 phibians, and lizards. The earliest experimentation showing 

 the effect of light upon the retinal pigment of vertebrates was 

 performed by Kiihne (77) and by Boll (78), who worked upon 

 the frog. Their results have since been extended upon repre- 

 sentatives of the remaining vertebrate classes, although strik- 

 ing movements of the retinal pigment of reptiles and mammals 

 have not as yet been demonstrated. Among invertebrates, 

 photomechanical changes have been found in the compound 

 eyes of insects (Exner, '89), crustaceans (Exner, '91), arachnids 

 (Szczawinska, '91), and in the eye of cephalopods (Rawitz, '91). 



Of the molluscs, the cephalopods constitute the only group 

 in which a response of the retinal pigment to photic stimula- 

 tion has hitherto been demonstrated. Hensen ('65), upon 

 theoretical grounds, hazarded the guess that the pigment in 

 these animals possessed a certain mobility, for Babuchin ('64) 

 had previously referred to finding the visual rods entirely free 

 from pigment in some specimens of Sepia and Octopus. It was 

 left for Rawitz ('91) to show that in the light the visual rods 

 of Sepia officinalis are pigmented along their whole length, with 

 an especial accumulation at the lens border, whereas in the 



