THE MOVEMENTS AND REACTIONS OF AMCEBA. 185 



strong stimuli. In some cases there is for an instant after a strong 

 stimulation at the anterior end a sudden rush of protoplasm toward 

 the region stimulated ; this is immediately followed by the stoppage 

 and contraction above described. Apparently this sudden rush toward 

 the point stimulated is produced as follows : The first effect of the 

 additional contraction caused by the stimulus is to release a certain 

 amount of surface at the posterior edge of the attached area from its 

 attachment to the substratum. This portion was nearly ready to become 

 released in the ordinary course of events, so that probably a very slight 

 shock would release it at once. Now, as I have shown on p. 167, when 

 a portion of the lower surface of the Amoeba is suddenly released from 

 the substratum, it contracts, causing a strong forward current. This 

 is what happens in the case under consideration. Later this current is 

 stopped by the effect of the stimulus in the anterior region. 



The surface currents in the reaction are changed in the same way as 

 are the internal currents, and are throughout congruent with them. 

 Particles moving forward on the upper surface of the Amoeba stop 

 after the stimulus, then move in the direction of the new forward 

 current. This has been illustrated in detail for Amoeba verrucosa 

 (p. 143), so that we need not go into the matter further here. 



The essential features of the negative reaction to a mechanical stim- 

 ulus are, then, a contraction of the region stimulated, with the formation 

 of a new pseudopodium in what may be considered the region of least 

 resistance, followed by a change in the direction of the currents of 

 protoplasm, thus altering the course of the Amoeba. 



By repeated mechanical stimuli it is possible to drive the Amoeba in 

 any desired direction. I have at times made use of this possibility in 

 order to bring into contact two Amoebae or two pieces of an Amoeba 

 whose courses lay in different directions. Such driving of an Amoeba 

 requires considerable skill and a rather high tension on the part of the 

 operator. The new pseudopodium formed is stimulated to withdraw 

 as often as it is formed, until it finally starts out in the desired direction. 

 If it were possible to stimulate all of one side of an Amoeba at once it 

 would, of course, be driven directly toward the opposite side, even 

 though the stimulus were weak. With chemical and some other stimuli, 

 as we shall see, this is possible. With mechanical stimuli it is usually 

 possible only when the stimulus is very strong. By drawing the tip 

 of the glass rod along one side of a moving Amoeba, it is often possible 

 to make it flow directly toward the opposite side, as illustrated in Fig. 

 68. This point is important for an understanding of the effects of such 

 stimuli as chemicals, heat, and light. 



When a single pseudopodium is stimulated, it is merely withdrawn, 

 wrinkling and becoming warty in the usual way ; there may be no 

 other effect on the movement of the animal. 



