506 



by a large accumulation of facts which show conclusively not only that 

 sinaple tubular protrusions of a cell composed of an external pellicle 

 and more or less fluid contents, like those observed by Kölsch, are 

 capable, under the influence of variations of fluid tension, alone of ex- 

 hibiting all the varieties of movement which are characteristic of the 

 action of cilia; but also, in those cilia and flagella which are of suf- 

 ficient size for any structure to be made out in them (and we may 

 include in the list the cilia-hke tentacles of the Suctoria), it has been 

 noted that their structure is in fact that of a tubular prolongation 

 from the cell enclosed by a pellicle or cuticula. And, as we have 

 also seen, a study of the development of cilia leads to the same con- 

 clusion, viz., that they are tubular protrusions of the cell - substance 

 covered at first only by an extension of the cell-pellicle. That in most 

 cases this pellicle undergoes further difi"erentiation is further shown by the 

 observation that the cilia lose for the most part as development and 

 evolution proceed their power of independent motion. That the diä"er- 

 entiation takes the form of a structural alteration which involves a 

 lessened extensibility of a certain part or of certain parts of the 

 pellicle is an assumption which is at least justified by the fact that it 

 furnishes the simplest possible explanation of the manner in which the 

 great regularity in the movement of ordinary cilia may be brought 

 about. 



There is however another argument which although an indirect 

 one furnishes, in my judgment, the strongest of reasons for believing 

 that the action of cilia can only be produced by variations of 

 fluid pressure — hydraulic pressure — within hollow structures. For 

 by no other hypothesis is it conceivable — to me at least — how 

 fllaments of such extreme tenuity and of so soft and yielding a nature 

 as cilia can overcome the enormous resistance which they must meet 

 with when working, as many ciHa habitually work, in a fluid so viscous 

 and difficult to move as mucus. Were the action to be regarded as 

 that of a whip the movement of the projecting cilia being produced by 

 rhythmic movements imparted to their prolongations within the cell- 

 protoplasm (and this is of all others perhaps the theory of their action 

 which has been most widely held), it is impossible to conceive that 

 mucus, or even water, could be moved by such immeasurably fine fila- 

 ments even were we to suppose them to possess a rigidity equal to 

 that of steel — whereas we know that cilia are on the contrary more soft 

 and pliable than the pile of velvet. The difficulty is no less if we are 

 to adopt another theory which has obtained considerable support, and 

 which is very nearly allied to that enunciated by Dr. Pütter himself. 



