l88 THE BEHAVIOR OF LOWER ORGANISMS. 



posterior end ; the two turn to one side, and a pseudopodium starts out 

 in a new place. If the stimulation took place at the anterior end and 

 was limited to a small area, the new pseudopodium starts out at one 

 side of the original anterior end ; the new course followed, therefore, 

 forms only a slight angle with the former one (Fig. 71, a). But if the 

 stimulus affects all of one side of the body, or a still greater portion of 

 its area, pseudopodia are sent out on the opposite side ; the Amoeba 

 then creeps directly away from the source of diffusion of the chemical. 

 This gives a typical case of negative " chemotaxis," the longitudinal 

 axis being in the line of diffusion of the ions or molecules, with the 

 anterior end directed away from the source of diffusion (Fig. 70) . It 

 will be noted that the reaction is exactly the same as that produced by 

 mechanical stimuli ; in " chemotaxis," where the animal is " oriented," 

 we have the same process as in " driving" the Amoeba in a definite 

 direction by means of mechanical stimuli. All movement toward the 

 chemical is inhibited, because this brings the protoplasm into a region 

 where it is stimulated. Pseudopodia can be sent out, 

 therefore, only on the side away from the chemical, 

 and movement can occur only in that direction. 



The surface currents are changed exactly as are the 

 internal currents ; the facts here are identical with 

 those described for mechanical stimuli (p. 185). The 

 surface currents are thus away from a chemical which 

 causes a negative reaction (see Fig. 42, p. 144). 

 A number of variations in the reactions to chemicals are shown in 

 Fig. 71, all of them taken from actual experiments. As the figure 

 shows, after stimulation frequently two pseudopodia start out in oppo- 

 site directions, one finally prevailing over the other (Fig. 71, d). 



The contraction due to the chemical is often very marked, the ecto- 

 sarc against which the chemical impinges shrinking sharply together 

 and becoming covered with folds (Fig. 71 > ^)- With methyl green as 

 the stimulus, the surface touched by the chemical is sometimes stained, 

 so that the shrinkage in area is very precisely definable. With a solu- 

 tion of NaCl the shrinkage is extreme, while the opposite side spreads 

 out widely, compensating, or more than compensating, for the decrease 

 in surface caused by the shrinkage (Fig. 71, 3). 



The effect of substances not in the form of powder was tried in the 

 following manner : A glass tube was drawn out to a very fine point, 

 and into it was introduced some of the solution to be tested. The fine 

 point of the tube was then brought close to the Amoeba. Some of the 



* FIG. 70. Diagram of " negative chemotaxis " in Amoeba. A chemical diffuses 

 from a center, as indicated by the radii; the Amoeba reacts in such a way as to 

 creep directly away from the source of diffusion, in a line with the radii of diffusion. 



