EXPERIMENTS ON MEDUSA 



21 



cells, but of the physical qualities of the colloidal sub- 

 stances in the tissues. But must we conclude from 

 this that the Na ions are the cause of the spontaneous 

 rhythmical contractions of the Medusae ? I think 

 not. The ions only bring about a certain labile equi- 



FIG. 4. ARRANGEMENT FOR PRODUCING AUTOMATICALLY PULSATING 



AIR-BUBBLES. (See text.) 



librium in the condition of the colloids of the con- 

 tractile tissue which allows the true cause of the 

 contractions to be effective. But what is this 

 cause ? 



J. Rosenthal seems to have been the first to call 

 attention to the fact that it is in no way essential for 

 a rhythmical phenomenon to have a rhythmical cause, 

 and that constant conditions can lead to rhythmical 

 effects. If a small, constant stream of water flows 

 into a pipette, it will pass out rhythmically in drops. 

 The weight of the drop must be greater than the 



