MOTOR RESPONSES 313 



pods before there is much contraction, and then the plasmasol flows out 

 through these. As the plasmasol flows out, it collects about the amoeba; 

 and the granules and fluid in it do not stream toward the poles as they 

 do in direct current. Cataphoresis and electroendosmosis are thus neutral- 

 ized, owing to the reversal in the direction of flow of the current. 



There is no change in the responses if acid is added to the culture fluid. 

 However, instead of disintegrating as usual, the plasmasol coagulates 

 after the amoeba breaks up. 



The essential phenomena observed in the effect of the alternating 

 current on Amoeba appear to be: (1) mild contraction in extended 

 pseudopods, beginning at the tip; (2) violent contraction on the two 

 surfaces facing the poles, with the formation of blisters in these regions; 

 (3) formation of highly fluid pseudopods between these two surfaces; 

 and, finally, (4) rupture at the surfaces directed toward the poles, fol- 

 lowed by disintegration of the organism. 



Contraction in the pseudopods, as was repeatedly observed, occurs 

 first in those directed toward the poles and last in those in which the 

 longitudinal axis is perpendicular to the direction of the current. In- 

 deed, there is some indication that there is no contraction at all if the 

 axis is actually perpendicular to the direction of the current. Contraction 

 in the pseudopods under these conditions results in retraction which, 

 in all respects, appears to be the same as retraction of pseudopods in 

 normal locomotion. The retraction appears to be due to increase in the 

 elastic strength of the plasmagel, owing to reversible gelation of ad- 

 joining plasmasol. It may be concluded, then, that the first effect of the 

 alternating current is reversible gelation of the plasmasol adjoining the 

 plasmagel at the tip of the pseudopods directed toward the poles. This 

 results in an increase in elastic strength of the plasmagel in this region, 

 and in a retraction of the pseudopods. The contraction at the surfaces 

 facing the poles is, in the beginning, doubtless due to the same phe- 

 nomena; but the facts ( 1 ) that the plasmagel in this region later changes 

 in color, ( 2 ) that it does not become thicker, ( 3 ) that fluid is squeezed 

 out of it, and (4) that it is killed and then breaks, show that contraction 

 here is associated with profound changes in the plasmagel itself. These 

 changes result in such marked decrease in the strength and elasticity of 

 this structure that it breaks readily. These changes are also associated 

 with simultaneous increase in fluidity of the plasmasol, as indicated by 



