HISTOLOGICAL BASIS OF ADAPTIVE COLORS IN PARALICHTHYS ALBIGUTTUS. II 



Even after pigment has been present in all parts of the cell it may become concentrated in the central 

 region leaving the peripheral region clear. In many instances as the pigment becomes concentrated 

 isolated granules or groups of granules remain far out in the protoplasmic processes. The concentration 

 and redistribution of pigment granules are obviously not due to ameboid movements of the cell, 

 but to movements of the pigment granules in the cytoplasm. « 



This description applies both to the cells containing yellow and those containing 

 black pigment. It is in full accord with the findings of Franz (1908) in larvae of 

 Pleuronectes platessa. 



While pigment cells in fishes are distinctly ameboid during embryonic and larval 

 development, changes in the distribution of the pigment within them are probably not 

 due to the form changes of the cells alone. If this were the only factor involved, we 

 should expect that after pigment is once present in all parts of the cell the entire 

 cell would remain pigmented regardless of its changes in form. The outlines of these 

 cells are easily observed in early larva and, as indicated above, pigment-free areas arise 

 after pigment has been present in all parts of the cell. On the other hand, changes 

 in the distribution of the pigment granules frequently occur when no changes in the 

 form of the cell are taking place. Therefore, the conclusion that changes in the dis- 

 tribution of the pigment granules in the chromatophores of larval fishes are due to 

 ameboid form changes in the cells alone is unwarranted. 



The functional activities of chromatophores were studied in considerable detail 

 by Dr. S. O. Mast and the author in living specimens of Paralichthys albigultusfi This 

 species lends itself admirably to work of this kind. Specimens 5 to 10 cm. in length 

 were used which had been kept in aquaria for some time and had been repeatedly 

 transferred from one background to another of a different shade or color. They were 

 consequently accustomed to handling and responded very promptly to changes in the 

 shade or color of the background. A compound microscope was submerged in an 

 aquarium containing water deep enough to cover a small flounder resting on a glass 

 plate on the stage. The flounder was orientated so that the microscope could be focused 

 on chromatophores in the caudal fin. When a white cardboard was placed under 

 the glass plate and beneath the head of the fish the latter assumed a whitish shade. 

 When the white cardboard was replaced by a black one the fish very promptly assumed 

 a very much darker shade. These changes in shade took place so rapidly that the 

 movements of the pigment granules in the melanophores in the field of the micro- 

 scope could be observed quite satisfactorily even under high magnification. 



As the fish assumed a light shade in response to the white background, the pigment 

 granules in many of the melanophores in the field of the microscope could be seen 

 retreating from the peripheral portions of the radial processes along more or less definite 

 radial lines and becoming concentrated in the central area. As the fish assumed a 

 darker shade in response to the black background, the pigment granules in the same 

 melanophores could be seen advancing peripherally apparently along the same radial 

 lines. Occasionally as the pigment granules advanced peripherally or retreated toward 

 the center the radial lines would break, leaving spaces free from pigment granules. 

 These spaces remained open for a short time and then gradually became filled by ad- 

 vancing or retreating granules. No evidence of the retraction and extension of the 



o Bulletin Bureau of Fisheries, vol. xxxiv, 1914, p. 419. 



& These observations were made at the United States fisheries biological station. Beaufort, N. C, during the summer 

 of 1914. 



