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Mr. J. Gray. 



During the former the cilium resembles a slack string or fibre, whereas 

 during the effective stroke its rigidity is distinctly greater. 



5. When exposed to an acid solution of appropriate strength the cilia of 

 Mytilus come to rest by a gradual slowing of the whole beat : the amplitude 

 of the beat is not affected. The cilia always come to rest at the end of the 

 effective stroke, i.e., in the position in which the cilium possesses no convertible 

 potential energy. 



The cessation of movement in an acid solution is due to a change which 

 takes place inside the cell, and not at its surface. Evidence is advanced 

 which suggests that the presence of acid prevents the conversion of chemical 

 energy into kinetic energy. 



The effect of acid is entirely reversible by alkalies. The rate of the beat is 

 most simply controlled by controlling the hydrogen ion concentration within 

 the cell ; up to a certain point the higher the internal alkalinity the more 

 rapid is the ciliary beat. 



6. Under normal circumstances the activity of the lateral cilia depends on 

 the presence of potassium ions. This effect is probably due to the general 

 effect of the ion in quickening the beat, which leads in the case of the fronto- 

 lateral cilia to a state of prolonged contraction when potassium is in excess. 



7. Ciliary activity is not sensitive to change in the concentration of 

 magnesium or sodium in the external medium, although these ions play a part 

 in the general equilibrium between the cell and its environment. 



8. The absence of calcium ions may bring about a cessation of ciliary 

 movement, which is antagonised by hydroxyl ions. 



9. The reaction of cilia and of muscles to the various chemical constituents 

 of their environment is essentially the same. The apparent differences are 

 due to : (i) the greater sensitivity of most muscles as compared to cilia ; (ii) the 

 cilia have a much shorter latent period than most muscles ; (iii) a ciliated cell 

 cannot be regarded as directly comparable to a neuro-muscular system. 



10. Cilia are brought to rest if the osmotic pressure of the external 

 medium exceeds a certain value. The stoppage is brought about by a gradual 

 reduction in the amplitude of the beat. These effects are entirely removed on 

 reducing the osmotic pressure. 



11. An hypothesis is put forward that the mechanism of ciliary movement 

 is essentially the same as that of muscular movement. 



The expenses of this research were in part met by a grant from the 

 Government Grant Committee of the Eoyal Society. 



