MOTION 33 



quale to account for the observed phe- 

 nomena. 



A very interesting type of movement 

 is seen in the contractile vacuole, which 

 begins as a tiny drop of clear liquid in 

 the living protoplasm, grows to a def- 

 inite size and suddenly disappears. The 

 writer has found that precisely the same 

 phenomenon can be produced in artificial 

 cells under proper experimental condi- 

 tions and this rhythmic process may con- 

 tinue for days. 



The movements of living protoplasm 

 appear to depend upon the surface forces 

 which act upon all liquids, as well as on 

 changes in the consistency of the proto- 

 plasm. They do not seem to involve 

 anything sujfficiently different from mo- 

 tion in non-living systems to be capable 

 of being considered adequate tests of 

 life. 



The idea of motion due to contraction is 

 in no way indispensable to the conception 

 of life. A cell which is capable of con- 

 traction does not cease to live when it 



