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Mr. J. Gray. 



quite strong enough to cause complete stoppage, it is noticeable that a 

 reduced movement is maintained for a very long time (more than 24 hours) ; 

 in such a solution the amplitude of the beat is much reduced, whereas the 

 rate of beat is almost unaffected. In a solution which is strong enough 

 to cause complete stoppage, the cilia remain in the position shown in the 

 accompanying diagram (fig. 6), so that they are unusually conspicuous. 



Fig. 6. — Diagram illustrating the appearance of the terminal cilia of Mytilus when 

 brought to rest by high osmotic pressure of the external medium. The arrow shows 

 the direction of the normal effective beat. 



It is important to notice that the stoppage of the cilia in hypertonic 

 solutions is brought about in an entirely different way to the stoppage in an 

 acid solution, and it is therefore not surprising to find that the stoppage in 

 hypertonic solutions is not influenced by the presence of hydroxyl ions, nor 

 is the effect of an acid solution altered by reducing its osmotic pressure. 



The effect of hypertonic solutions on muscular activity has not been 

 extensively studied, but Demoor and Phillipson (7) have shown that the 

 skeletal muscles of a frog lose their excitability to a direct stimulus when 

 immersed in hypertonic Einger solution ; the muscles shorten somewhat, 

 and the response is gradually abolished. These effects are entirely reversed 

 by treatment with normal Ringer solution. Carlson (4) also found that the 

 rate of beat of the auricles of the tortoise is unaffected by perfusion with 

 hypertonic Einger solution ; the amplitude was, however, much reduced — 

 and recovered in normal Einger. 



The fact that the amplitude of the beat is affected by an increase in the 

 osmotic pressure of the external medium seems to indicate that a loss of water 

 from the cell interferes not with the periodic liberation of energy, but with the 

 actual contractile mechanism. 



IV. Summary and Discussion. 



Let us now review the whole of the available facts, and attempt to form a 

 working hypothesis of the nature of the ciliary mechanism. We have seen 

 that the ciliated cells of Mytilus provide an example of an automatically 

 contractile tissue ; they differ from cardiac or smooth muscle in that their 

 latent period is extremely short, and the rate of beat very much quicker than 



