i24 AMEBOID MOVEMENT 



stream be interrupted in order that a straight path may be main- 

 tained. An ameba may stop movement for a minute or more 

 and then be much more apt to resume movement in the original 



per sq. cm. The velocity of streaming endoplasm, as ascertained by ob- 

 servations on Amoeba dubia (in which the endoplasm flows usually rap- 

 i 



idly) is cm. per second. 



880 



Now, given a unit mass of endoplasm moving at a given instant with a 



i 



velocity of cm. per second against viscosity of = .1 dynes per sq. 

 ooo 



cm., how far will the unit mass travel before coming to rest? 



Velocity 



.borce = Mass X Acceleration, and Acceleration =i 



Time 



MV 

 q = Viscosity = F = MA = . 



T 



V 

 Now if M = x, = F = A =: . 



T 



V i 

 T = = 880 = . 



QQ 

 1 



The space travelled over in uniformly accelerated motion equals 



1 ill i 



S = AT 2 = X X ( ) 2 = = .00000645 cm. 



2 2 10 154880 



If, therefore, the force moving the central stream of endoplasm should 

 suddenly be discontinued, the resistance offered by the viscosity of the 

 enveloping endoplasm would allow it to move only .0000645 mm. before 

 coming to rest. But the ameba as a whole moves more slowly than the 

 central stream of endoplasm, the average rate of movement being about 



i 



mm. per second. The effect of the streaming endoplasm on the 



300 



forward movement of the whole ameba would therefore be correspond- 

 ingly decreased. Now if the ameba was perfectly homogeneous and per- 

 fectly symmetrical, and free from external stimulation, and moved in a 

 perfectly homogeneous liquid on a perfectly plane surface, the excessively 

 small amount of mechanical inertia would then be sufficient, theoretically, 

 to cause the ameba to move in a straight instead of an irregular path. But 

 these conditions are never realized. The ameba is unsymmetrical in form, 

 heterogeneous in composition and always unsymmetrically stimulated ; 

 hence it is impossible that the excessively small amount of mechanical 

 inertia can be considered a factor in determining the direction of the 

 ameba's path. 



