MOTOR RESPONSES 341 



locomotion at pH 7.0 in various solutions indicates some fixity of mechanism 

 determining this median minimum. This mechanism might be correlated 

 with the behavior of the membrane in the neighborhood of an ampholyte 

 isoelectric point near neutrality in accord with the view of Robbins (1926). 

 But arguing against such an isoelectric point are the pertinent facts that, 

 (1) the median minimum is lacking in solutions of single salts, though 

 locomotion in the dilute solutions occurs at hydrogen ion concentrations in 

 which it is usually found; (2) the difference between the maximum and 

 the median minimum rate of locomotion depends on the sodium/calcium 

 ratio and only slightly if at all on the total salt concentration; (3) the 

 cations have marked effect on the rate of locomotion on the acid side of 

 the neutral point as well as on the alkaline side; (4) the anions (so far 

 as chloride and phosphate are concerned) have little effect down as far 

 in the acid range as the observations were made. . . . 



We are at present unable to suggest a satisfactory explanation for this 

 median minimum. Whatever the cause of it may be, the relation between 

 the rate of locomotion in Amoeba and the factors in its environment is doubt- 

 less fairly complex, for it is probable that these factors influence locomotion 

 in it by their action on the surface, affecting adhesiveness and other prop- 

 erties of the surface, as well as by their action on permeability of the sur- 

 face membrane (Mast, 1926a). 



B. MARINE AMOEBAE • 



Pantin (1923-31) made observations on the relation between the rate 

 of locomotion in a marine amoeba and various chemicals. He found that 

 as the hydrogen-ion concentration increases, the rate increases rapidly 

 from zero at pH 10 to a maximum at about pH 8, and then decreases 

 rapidly to zero at pH 5.5. He observed no indication of decrease in 

 activity at neutrality. He maintains that the rate is closely correlated with 

 the relative concentration of sodium, potassium, magnesium, and cal- 

 cium salts, that more than one of these salts is necessary for locomotion, 

 and that calcium is required in all combinations. He maintains that cal- 

 cium functions primarily in the contractile mechanism, and the others in 

 the regulation of permeability. The evidence presented in support of 

 these conclusions is, however, not unequivocal. 



Oxygen is necessary for locomotion in Amoeba (Hulpieu, 1930; Pan- 

 tin, 1930) but only in very low concentrations. Pantin maintains that 

 none is immediately necessary, and that it functions in recovery some- 

 what as it does in the contraction of muscles. This, however, has not been 

 demonstrated. 



