PHYSIOLOGY OF CHROMATOPHORES OF FISHES 559 



ruptured melanophores. Thus, what was formerly considered a 

 pressure stimulus he proved to be merely a mechanical diffusion 

 of the pigment and not a true expansion of the melanophores. 



V. Frisch does not conclude from these results that the melan- 

 ophores can not react to a mechanical stimulus. He merely 

 states that if they should react it would be by a contraction, not 

 an expansion. 



It was a simple matter to prove, in the case of F. heteroclitus 

 the correctness of the conclusion of v. Frisch ('11 a) regarding 

 the supposed 'pressure-expansion' effects of the older investiga- 

 tors (v. Siebold '63, etc.) A needle drawn lightly over the sur- 

 face of this fish showed an effect similar to the one described by 

 V. Frisch ('11 a) for Phoxinus. Subsequent examination with 

 the microscope disclosed many ruptured melanophores and struc- 

 tureless masses of diffused pigment granules which constituted 

 the dark line upon the surface of the fish. 



Further experiments with a blunt glass needle and varying 

 degrees of pressure showed conclusively that the pressure stimu- 

 lus is a contracting one for the melanophores. Scales were im- 

 mersed in a 0.1 M NaCl solution and the melanophores allowed 

 to expand completely while the xanthophores contracted before 

 the stimulus was applied. With careful manipulation and using 

 scales upon which the melanophores were comparatively far 

 apart (from the lateral portion of the 'fish) the stimulus could 

 even be applied to a single melanophore. The same melanophore 

 responded repeatedly to the pressure stimulus provided it was 

 not too violent. In such cases the xanthophores within the 

 stimulated area always expanded. Similar experunents per- 

 formed upon scales immersed in olive oil showed a prompt con- 

 traction of the melanophores when stimulated but in this case 

 they did not reexpand as in NaCl. Under normal conditions 

 the melanophores remain expanded in olive oil for some hours. 

 This is probably not due to any specific expanding factor in the 

 oil but merely to the absence of a contracting stimulus. When 

 the nervous inhibition is removed the pigment always migrates 

 distally. This is doubtless the cause of the expansion in olive 

 oil. That this is actually the case seems to be further proved by 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 15, NO. 4 



