AMEBOID MOVEMENT 123 



some character and orderliness in its locomotion, then for the 

 first time should we be especially interested in what would have 

 to be regarded as a very striking and exceptional characteristic. 



For it is very well known, and it is generally recognized by 

 everybody, that moving organisms usually move in an orderly 

 manner ; it is recognized that organisms tend to move in straight 

 paths excepting where interrupted by the action of some special 

 stimulus. When an organism changes its direction of motion 

 frequently and abruptly, we call it erratic. The mad dashings- 

 about of the hunter-cilkae Didinium and the unceasing gyrations 

 of the whirligig beetleexcite one's curiosity because these organ- 

 isms do not move as other organisms do; they contradict our 

 expectation of movement in a straight line. 



But why should organisms generally tend to move in straight 

 paths? This fundamentally important question has received 

 almost no attention, excepting that rapidly moving animals like 

 birds, flying insects, fishes and other rapidly swimming animals 

 of various kinds and rapidly running animals tend to move in 

 straight paths because of the physical inertia of the mass of the 

 organism. It is easier for a rapidly moving organism to move 

 in a straight line than to change its direction of movement fre- 

 quently and abruptly. 



The ameba however is a very slow moving animal, as animals 

 go, for it (proteus) moves only about 600 microns per minute. 

 Under the microscope, however, which magnifies speed as well 

 as size, the endoplasmic particles rush along rapidly enough to 

 suggest that even here mere physical inertia might be a deter- 

 mining factor in the path of the ameba, which for considerable 

 segments is often very nearly straight. Such suspicion is not 

 justified, however, for the viscosity of the endoplasm taken in 

 connection with the heterogeneous composition of the ameba, 

 makes it improbable that mere physical intertia can affect the 

 path of the ameba. 5 



It is not even necessary that movement of the endoplasmic 



5 According to Ewart ('03) the viscosity of streaming protoplasm in 

 plant cells lies between ^ = .04 and ^ = .2. But the velocity of stream- 

 ing endoplasm in ameba is considerably slower than that in the plant 

 cells which formed the basis for Ewart's calculations. In comparison, we 

 may estimate the viscosity of the endoplasm of ameba as = .1 dynes 



