Mr Gray, The Mechanism of Ciliary Movement 359 



supply of potential energy from chemical energy is fully main- 

 tained*. It is hoped that further work will throw more Hght on this 

 problem. 



The action of Mg- is interesting, although the details of the 

 experiments cannot be given here. It seems probable that the 

 presence of Mg- stabihses the cell— probably the surface of the 

 cell— to the other ions in the medium. It regulates the rate at 

 which other ions can enter the cell, and the rate at which intra- 

 cellular ions leave the cell. In this respect it can usually be re- 

 placed by Ca--. 



During the course of these experiments a fairly close parallel 

 is visible between the effect of ions — both anions and kations — upon 

 muscle fibres and the cihary mechanism. ^ There is, however, one 

 respect in w^hich the ciha of Mijtilus differ from cardiac muscle— yi3. 

 they are remarkably insensitive to the salts of the rare earths. On 

 the other hand spermatozoa and the ciUa on the blastulae of 

 Echinus are just as sensitive to these salts as is cardiac muscle. 

 Possibly the difference depends upon the position of the sensitive 

 surf ace\vithin the cell : if it Hes near the surface, the trivalent ions 

 can reach it, whereas if it hes deeper in the cell they may never 

 penetrate. It is interesting to notice that those ciha which are 

 sensitive to trivalent positive ions are also more sensitive to hydro- 

 gen ions than other types of ciha. 



3. Effect of osmotic pressure. 



When ciha are exposed to any solution whose osmotic pressure 

 is above a certain critical value, all movement ceases: the ciha 

 remain in a position between the beginning and end of the effective 

 beat, and are consequently very obvious. On reducing the osmotic 

 pressure normal movement is at once resumed. 



Although the details in connection with osmotic stoppage of 

 ciha require further investigation, it may be pointed out that the 

 known facts fit in with our initial hypothesis. The tension set up 

 in a fibre by exposure to an acid depends upon the uptake of water: 

 if the amount of water in the cell is reduced below a certain critical 

 amount, it is obvious that this will affect the tension set up m the 

 fibre, and consequently the beat is aft'ected and at a critical point 

 will be abohshed altogether. 



The general conclusion which may now be drawn is that the 

 mechanism of ciliary movement and muscular activity may be of 

 essentially the same nature. 



* The work of other authors does not appear to agree with this conchision. 



