PHYSIOLOGY OF CHROMATOPHORES OF FISHES 



535 



(fig. 6, pi. 1). Since this is not the case when the contraction 

 is brought about by certain salt solutions (potassium salts for 

 example), it is possible that the appearance of the empty pro- 

 cesses is due to a selective absorption of water by the melano- 

 phores. Considered as an osmotic phenomenon it is evident that 

 a hypotonic fluid is correlated with a contraction of the melano- 

 phores. 



The changes in the xanthophores are less distinct. The cy- 

 tolytic effect of the distilled water occurs from the half-expanded 

 state and is never preceded by a complete contraction as in the 

 melanophores. 



In attempting to compare the time required for a complete 

 contraction of the melanophores in different individuals it be- 

 came e\'ident that scales from smaller fish react more rapidly. 



February 3, 1912 



To compare the variation in the time of the complete contraction 

 in distilled water, six females of different sizes were selected and from 

 each of these the large scale lying on the dorsal, median line between 

 the eyes was removed to distilled water. 



It is the actual size of the scale however and not that of the 

 fish which determines the time of contraction. This was easily 

 demonstrated by comparing large and small scales from the same 

 individual. In such cases the differences were quite as great 

 as in the above table. 



The previous history of the individual and its physiological 

 condition at the time of the experiment may be additional sources 

 of error which make it impossible to attach any considerable sig- 

 nificance to differences in the time of reaction even when size, 



