560 . E. A. SPAETH 



the fact that here the pressure effect is practically irreversible. 

 A complete recovery never occurs as it does in the NaCl solution. 



Finally experiments were made with the scales immersed in 

 sea-water. Here again the pressure stimulus overcame the ex- 

 panding tendency of the combined hypertonic medium and the 

 specific effect of NaCl (see p. 533 for the effects of sea-water). 



In all these experiments the scales were placed in open Syracuse 

 glasses and the temperature remained constant. The stimulus 

 was applied instantaneously in order to avoid any possibility 

 of reducing the normal oxygen supply. Hence the three fac- 

 tors which are ordinarily so intimately associated with the 

 effect of pressure, that is, access of oxygen, temperature changes, 

 and central nervous inhibition, were all eliminated, Under 

 these conditions pressure causes a contraction of the melanophores 

 and an expansion of the xanthophores in a 0.1 M solution of 

 NaCl. A contraction of the melanophores was also observed in 

 sea-water and olive oil following a pressure stimulus. 



4. TEMPERATURE EFFECTS 



Until quite recently there has been a remarkable uniformity 

 of opinion among investigators concerning the influence of tem- 

 perature changes upon the chromatophores. Elevation of tem- 

 perature was 'followed by a contraction of the chromatophores 

 while a lowering was claimed to bring about their expansion. 

 Since most of the experiments have been performed upon living 

 animals the usual difficulties of possible psychic stimuli, central 

 nervous influences, etc., have detracted somewhat from the reli- 

 ability of the conclusions. 



The question of a direct stimulation of the chromatophores 

 and the nature of such a reaction is by no means decided. 



V. Frisch ('11 b) lays particular stress upon the smaller quan- 

 tity of absorbed oxygen in a fluid at a high than at a low temper- 

 ature. Upon this basis he has expalined his results with the 

 dead Phoxinus laevis in which the higher temperature caused a 

 rapid contraction of the melanophores. The possibility of an 

 additional contracting stimulus due to the heat per se has been 



