6 BULLETIN OF THE BUREAU OF FISHERIES. 



with a single blow. Fish killed in this manner usually undergo no observable changes 

 in shade or color for some little time. Preparations of the skin studied immediately 

 after this operation show essentially the same distribution of the pigment granules in 

 the chromatophores lying immediately beneath the epidermis as do the scales removed 

 from the Uving fish. There is, therefore, probably no immediate change in any of the 

 chromatophores in the skin. The chromatophores and guanophores in the deeper 

 layers of the skin were studied in sections and also directly in pieces of skin from which 

 some of the scales had been removed. 



Skin desired for sectioning was fixed in Zenker's fluid, dehydrated as rapidly as 

 possible, and immediately embedded in paraffin. The melanin pigment being very 

 stable is not dissolved nor does it undergo any observable change in distribution during 

 this process. The xanthine pigment being readily soluble in alcohol is completely dis- 

 solved. In the deeper layers of the skin the xanthophores can still be observed in 

 sections; in the superficial layers they can not be distinguished from the guanophores 

 which are closely associated with thern. The guanophores remain intact and can be 

 observed quite satisfactorily. 



Skin desired for the study of the nerve supply to the chromatophores was prepared 

 by the pyridine-silver method. 



STRUCTURE AND DISTRIBUTION OF CHROMATOPHORES AND GUANOPHORES. 



The skin of Paralichthys alhiguttus contains chromatophores of two distinct types, 

 viz, melanophores and xanthophores. The former contain melanin granules which vary 

 in color from dark brown to black ; the latter contain xanthine granules which vary in 

 color from yellow to orange. Chromatophores of both of these types occur as isolated 

 chromatic organs. Xanthophores are in many instances partly covered by overlying 

 melanophores; however, chromatophore combinations involving a xanthophore and a 

 melanophore, as described by Ballowitz (1913) in several species of teleosts, probably 

 do not occur in this species. The melanophores are more numerous than the xantho- 

 phores. The averages of a large number of actual counts of the melanophores and 

 xanthophores, respectively, in a given unit area in the superficial layer of the skin in 

 nine specimens indicate a numerical ratio of melanophores to xanthophores in the layer 

 immediately beneath the epidermis of 32 to 19. 



A chromatophore consists essentially of a group comprising several or many pig- 

 ment cells so arranged that the entire body presents a central area from which elongated 

 processes radiate. In Paralichthys the melanophores are more nearly symmetrical 

 and show a larger number of radial processes than the xanthophores. 



A chromatophore is said to be "contracted" when the pigment granules are con- 

 centrated in the central area and "expanded" when the pigment granules are widely 

 distributed. The terms "contracted" and "expanded" will be used in this paper in 

 the sense here indicated and not with reference to the state of retraction or extension 

 of the radial processes of the chromatophores. 



A fully expanded melanophore (fig. i , pi. i ) usually presents a relatively pigment- 

 free central area bounded by a pigmented zone from which elongated pigmented proc- 

 esses radiate. Within the radial processes the pigment granules are usually disposed 

 along more or less definite radial lines. Not infrequently radial processes show a den- 

 dritic structure; in other instances they are distinctly broader distally than proximally. 



