258 MORRIS M. WELLS 



has been added, must then be due to the fact that this trace of 

 salt lessens the stimulation in the salt end of the gradient. There 

 are three principal factors affecting the degree of stimulation 

 of the gradients referred to in table 2, namely, the original salt 

 (e.g., NaNOs) the antagonising salt (e.g., Ca(N03)2, and the 

 acid. Before the antagonising salt was added the fishes were 

 negative to the original salt, even though this meant spending 

 most of the time in a degree of acidity which was slightly above 

 their optimum. With the addition of the antagonising salt, 

 however, they reversed their reaction and became positive to 

 the salt end. The antagonising salt must have diminished the 

 original stimulation in the salt end or have increased the stimu- 

 lation in the acid end, or both. The work upon the effect of 

 acids and salts upon permeability suggests that both factors 

 were concerned. Lillie ('10) has shown that calcium salts de- 

 crease the permeability of egg membranes while the salts of 

 sodium increase the permeability. Osterhout ('12, a and b) 

 has shown that sodium salts increase the permeability of plant 

 cells while the addition of a trace of calcium salt maintains nor- 

 mal permeability even in the presence of an excess of the sodium 

 salt. Osterhout has also shown that there exists a mutual 

 antagonism between certain acids and salts as for instance be- 

 tween HCl and NaCl, but the salts of calcium and magnesium 

 work with rather than against the acid. 



In the above experiment, therefore, the addition of the cal- 

 cium salt to the end of the gradient which contained a sodium 

 salt in concentration strong enough to cause the fishes to give a 

 negative reaction, resulted in the fishes becoming positive. This 

 reversal in the reaction of the fishes must have been due to the 

 decrease in the stimulating power of the salt end. It has already 

 been shown that increasing the acidity of water will cause fishes 

 to become positive to concentrations of sodium salts to which 

 they are normally negative (table 2) and it was found that the 

 higher the acidity the higher was the concentration of sodium 

 salt selected by the fishes. 



Table 2 also shows that the antagonistic action between cal- 

 cium and sodium salts is detected and reacted to, when the con- 



