520 



taken as conclusive evidence of the nature and method of ciliary 

 action. Nevertheless, since the hydraulic theory is the only one cap- 

 able of explaining the extraordinary power which these delicate cell- 

 extensions possess to move masses of thick mucus and even solids of 

 considerable weight, and in view further of the evidence that exists 

 for regarding cilia as extensions of the cell-protoplasm, invested, like 

 the rest of the protoplasm, with a special surface film which serves 

 the purpose of a membranous covering, there seems sufficient reason 

 for provisionally accepting it. 



In the communication already referred to, the fact is mentioned 

 that it is possible to construct a model of india-rubber tubing one 

 side of which has been rendered less extensible than the other, which, 

 when the pressure is increased within it. will bend over towards the 

 less extensible side, returning to the straight when the pressure is 

 relaxed. It may be asked why this model, which illustrates the se- 

 cond hypothesis, should not be regarded, rather than those above 

 described, as indicating the true conditions under which the action 

 of cilia takes place. 



In answer to this inquiry the following may be given as the 

 reasons which would lead to the adoption of the first hypothesis (of 

 prefixed curvation) rather than the second (of differential structure): 



I. The hypothesis of pre-existent curvation is the simpler. Cilia 

 occur in the very lowest plant and animal organisms, in which there 

 is no trace of structural differentiation of the protoplasm ; all differ- 

 ences are those of form alone. Hence it is improbable that there will 

 be any structural differentiation in the cilia, which, as the study of 

 their development shows, are mere protrusions of the cell-protoplasm. 



II. The result is physically more easily produced on the hypo- 

 thesis of a pre-existent curve. For although with a composite tube 

 one side of which is less extensible than the other, we can, by forcing 

 fluid into it, cause it to bend over towards the less extensible side, 

 the pressure required is relatively considerable, because the bending 

 over which results from the difference of structure is opposed by the 

 tendency to straighten with increase of pressure which is the result 

 of the assumption of the curved form : hence two opposing forces come 

 into play. Whereas, given a simple curved form to begin with, a very 

 slight increase or decrease of interior pressure produces a marked 

 decrease or increase of curvature; and the same is true, mutatis 

 mutandis, for the spiral form. 



The conclusion, therefore, to which the study of these models leads 

 is that the theory of the action of a cilium which assumes that the 



