MOTOR RESPONSES 311 



comes somewhat rounded. It remains in this condition a few moments, 

 then a pseudopod appears on one of the sides between the two surfaces 

 facing the poles and projects at right angles to a line connecting the two 

 poles. Almost immediately after the first pseudopod begins to form, an- 

 other usually appears on the opposite surface and extends in the opposite 

 direction. Thus the amoeba becomes oriented perpendicularly to the 

 direction of the current, i.e., at right angles to the direction in which 

 orientation occurs in a direct current. These two pseudopods usually con- 

 tinue to stretch out in opposite directions until the amoeba becomes 

 greatly elongated. Then one is withdrawn, and the amoeba continues in 

 the opposite direction, soon moving out of the field (Fig. 114). 



These pseudopods usually contain no hyaline cap and no plasmagel 

 at the tip, and the plasmagel elsewhere is very thin. Sometimes such a 

 large portion of the distal end is without plasmagel that the plasmasol 

 very definitely streams back at the surface. If the circuit is opened shortly 

 after these pseudopods have begun to form, no change in movement is 

 seen; if it is closed again, there is still no response. This indicates that 

 pseudopods in which the plasmagel is very thin, or absent, do not re- 

 spond to electricity; and that the response to electricity consequently is 

 due to its action on the plasmagel. 



If the current is stronger, movement ceases and the pseudopods re- 

 tract just as they do in weak currents. The contraction which follows 

 the retraction of the pseudopods is much more marked, especially on 

 the surfaces directed toward the poles. Here the plasmagel fairly shrinks 

 up and becomes yellowish in color. This is apparently precisely what 

 occurs at the anodal end of an amoeba subjected to the action of a direct 

 current. If the circuit is opened immediately after the violent contraction 

 has occurred, the amoeba soon recovers. But the two masses of plasma- 

 gel that have become yellowish are usually pinched off. An irreversible 

 transformation takes place in them, which results in the death of a por- 

 tion of the cytoplasm. Occasionally, however, these masses, especially if 

 they are relatively small, are taken into the plasmasol and are there di- 

 gested. 



If observations are made under an oil immersion objective on a speci- 

 men in an alternating current, the plasmagel can be seen to contract and 

 become yellowish. Fluid is squeezed out on either side of the organism 

 and the adjoining plasmalemma is thrown into folds and papillae of 



