54 HUMAN PHYSIOLOGY 



irregular changes in outline, the border at certain places 

 drawing in toward the center and at others pushing out 

 in blunt protrusions. When one first watches an ameba 

 the impression is that extension rather than contraction 

 is the characteristic of the movement displayed. Closer 

 study shows that the positive phase is really contraction. 

 When a process runs out briskly while the remainder 

 of the cell appears to be at rest the natural inference 

 is that the process itself is active. But the fact seems 

 rather to be that the main mass of the cell is exerting a 



FIG. 7. Ameba. (From Calkins' "Biology," Courtesy of Henry 

 Holt & Co. Publishers.) 



pressure which drives or spurts out the process. The 

 reader may have seen small rubber balls with faces 

 painted on them and tongues of thin rubber which dart 

 out when the balls are squeezed. The protrusion in this 

 case is probably the same in principle as that which 

 obtains with the ameba; pressure in one part results in 

 extension in another, the moving part being passive and 

 the real source of the power not being apparent. 



Uniform conditions of contraction in a cell like the 

 ameba tend to bring it into a spheric form for this is 



