EXPERIMENTS ON MEDUSA 



small quantity of CaCl 2 or KC1, or both, be added, 

 the centre stops beating. The centre would beat in 

 sea-water were it not for the presence there of Ca, K, 

 and possibly other ions (3). 

 The centre contains some 

 scattered ganglion-cells. It 

 might be argued that the 

 presence of these cells makes 

 the rhythmical contractions 

 in a pure NaCl solution pos- 

 sible. It is easy to prove 

 that such is not the case. 

 The striped skeletal muscles 

 of a froor do not contract 



^> 



rhythmically in blood or 

 serum. I have shown that 

 this is due to the presence 

 of Ca ions in these liquids. 

 If the muscle be put into a pure NaCl or NaBr solution 

 of the same osmotic pressure as the blood, the muscles 

 contract rhythmically (4). Yet these muscles contain 

 no ganglion-cells. Hence it is not the presence or absence 

 of ganglion - cells which determines the spontaneous 

 rhythmical contractions, but the presence or absence of 

 certain ions. Na ions start or increase the rate of 

 spontaneous rhythmical contractions ; Ca ions diminish 

 the rate or inhibit such contractions altogether. How 

 can these ions have such an influence? In order to 

 explain this we must go back to the fundamental 

 character of protoplasmic motion. Protoplasmic 



FIG. 3. EXPERIMENT IN DIVID- 

 ING A HYDROMEDUSA. 



The amputated margin continues 

 to contract rhythmically, while 

 the bell no longer contracts. 



