MOTION 31 



simpler phenomenon than in muscle and 

 we therefore have a better prospect of 

 analyzing it and discovering the mider- 

 lying mechanism. 



Motion in these cases and in plant 

 cells may show great diversity. In plants 

 protoplasmic streaming may range from 

 the most irregular sort of motion to a 

 regular flow which passes up one side of 

 the cell and down the other in a continual 

 cycle. In animal cells the type of irreg- 

 ular motion most studied is that found 

 in the ameba, and hence called ameboid. 

 An ameba consists of a single cell which 

 at first sight looks like a structureless 

 mass of protoplasm but is in reality not 

 so simple as it appears. It creeps about 

 by pushing out irregular projections into 

 which part of its substance flows, while 

 other portions of its body are simultane- 

 ously retracted. Currents of protoplasm 

 (and corresponding countercurrents) are 

 seen throughout the cell. The interior 

 of the mass is liquid while the exterior is 

 firmer, resembling a jelly. The change 



