THE CHROMATOPHORES OF ANIMALS. 



THE wide distribution of pigments throughout the 

 tissues of animals, the differences in origin, nature, 

 and function which they present, and the variety of forms 

 under which they are manifested, render the exact de- 

 finition of the term chromatophore in the present condition 

 of knowledge a matter of some difficulty. The term has 

 indeed been restricted by Pouchet to the pigment bodies of 

 Cephalopods, and a distinctionhas been drawn by him between 

 these structures and the pigment-cells of Vertebrates, which 

 he has termed chromatoblasts. As it is now, however, in- 

 disputable that the pigment bodies of Cephalopods are true 

 cells, it is no longer possible to restrict the term in this way: 

 chromatophore and chromatoblast are synonymous. 



Chromatophores may safely be defined as pigmented 

 cells ; but all pigmented cells are not chromatophores. We 

 have to distinguish the essentially chromatic cell from other 

 pigmented cells whose colour, as such, is immaterial. The 

 question is one of limitation : which pigmented cells are 

 chromatophores? It would be meaningless, for example, 

 to apply the term chromatophore to pigmented cells of the 

 excretory, or respiratory, or sensory tissues. By following 

 this principle of elimination the physiological answer to the 

 question becomes apparent: chromatophores are pigmented 

 cells whose essential function is chromatic, that is, to bestow 

 particular colours upon an animal. In the case of re- 

 spiratory or nutritive pigmented cells the colour is unessen- 

 tial, and is a mere attribute of the particular respiratory or 

 nutritive bodies, as blue is an attribute of copper sulphate ; 

 but in the case of chromatophores the colour is all in all, 

 and the nature of the pigmented bodies altogether im- 

 material. 



The difficulty is to draw the line. In the cuttle-fish or 

 frog the chromatophores are easily recognisable owing to 

 their highly specialised form and their readily demonstrable 

 function. But what are we to do in those numerous cases 



