THE BEHAVIOR OF AMCEBA 



till a pseudopodium is sent out in the direction desired by the experi- 

 menter. The animal may now be compelled to follow a definite straight 

 course, by stimulating any pseudopodium 

 which tends to diverge from this course. 



If the posterior end of a moving 

 Amoeba is stimulated, the animal con- 

 tinues to move forward, usually hastening 

 its course a little. The posterior end is 

 of course already contracted, and the 

 new stimulation merely causes it to con- 

 tract a little more. 



The negative reaction is of course 

 the method by which Amoeba avoids 

 obstacles. If an Amceba in creeping 

 comes against a small solid body, the 



,, , i 11^11 FIG. 10. Negative reaction to a 



reaction is often less sharply defined than mec hanicai stimulus when the entire 



in the Cases which we have thus far anterior end is strongly stimulated, a 

 , i_ i A i i i and b, successive stages. The arrow 



described. A typical example is shown x shows the original direction of 

 in Fig. ii. A progressing Amceba came 

 in contact at the middle of its anterior 



motion; the arrows in a show the 

 currents immediately after stimulation. 

 In b a new tail (f) has been formed 



edge With the end Of a dead alga fila- from the former anterior end, uniting 

 , ., , with the old tail (t). 



ment. Thereupon the protoplasm ceased 



to flow forward at the point of contact c, while on each side of 

 this point the motion continued as before. In a short time, there- 

 fore, the animal had the form and position shown by the broken 

 outline in Fig. 1 1 ; the filament projected deeply into a notch at the 

 anterior edge. Motion continued in this manner would have divided 

 the Amceba into two parts. But soon motion ceased on one side (#), 

 while it continued on the side y. The currents in x became reversed 



A B 



FIG. ii. Method by which Amceba avoids an obstacle. 



and flowed around the end of the filament into y, as shown at B, Fig. ii. 

 Thus the animal had avoided the obstacle by reversing a part of the 

 current and flowing in another direction. 



But not all mechanical stimuli cause a negative reaction. Some- 



