AMEBOID MOVEMENT 13 



stimulated ameba be projected at an angle of approximately 60 

 to the parent pseudopod? It might seem at first sight as if the 

 merely physical aspect of the streaming would be a sufficient ex- 

 planation, in that less resistance would be met with in sending a 

 stream off at a small angle than at a large. But it is probable 

 that inertia plays no part in maintaining the direction of stream- 

 ing (see p. 123, footnote, for further discussion). It requires 

 perhaps more energy for a pseudopod to flow off from the main 

 stream at an angle of 120 than at an angle of 30. But it is 

 plain that as many pseudopods are withdrawn as are thrown out, 

 and they are withdrawn at an angle against the main stream of 

 endoplasm in the ameba that is the complement of the angle at 

 which they were projected. Whatever energy might be saved 

 therefore in the projection of a new pseudopod at a small angle 

 with the main stream is lost in withdrawing the pseudopod against 

 the stream at a correspondingly large angle. It is clear therefore 

 that the physics of moving viscous fluids cannot solve the prob- 

 lem. It is probable that the mechanism which controls the direc- 

 tion of locomotion as exemplified in the wavy path of the ameba 

 (see p. 109) is also involved in the direction in which pseudopods 

 are projected. 



Some very interesting special cases of endoplasmic streaming 

 are observed during the process of feeding. As is well known, 

 amebas capture their food by the protoplasm flowing around it 

 and engulfing it. If the object is large the protoplasm may flow 

 around it, in contact with it, so that the shape of the object de- 

 termines the direction in which the enveloping protoplasm flows. 

 If the object is small, particularly if it is a live organism, the be- 

 havior of the ameba is quite different (Kepner and Taliaferro, '13, 

 Schaeffer, '16). To capture such a food object a cup of proto- 

 plasm is gradually formed over it so as to imprison it (Figure 2). 

 If the food organism lies against some flat object, the food cup 

 is brought down to the surface of the object all around, thus 

 making escape impossible, before the protoplasm comes into con- 

 tact with the food organism. Schaeffer ('16, '18) by experimental 

 methods has shown that the stimulus calling forth the formation 

 of food cups as just described, is the mechanical vibration of the 

 water. At least the same response was produced on the part 



