THE FUNCTIONS OF THE PARTS OF CILIA 149 



orientated in such a way that the Une through their centres is at 

 right angles to the plane of movement, and that many workers 

 have found cross-Hnks between the central and peripheral fibrils, 

 so that it looks as if the central fibrils may have a mechanical 

 function. If one imagines a cylinder whose rigidity is maintained 

 by internal turgor pressure, and which contains a contractile 

 structure running from end to end without any intermediate 

 attachments, then the shortening of the contractile structure 

 would result in the cylinder being transformed into a spiral, very 

 much like the contractile stalk of a peritrich ciliate. In haptone- 

 mata, where central fibrils and cross -connexions have not been 

 identified, the contraction appears to take this form, but in cilia 

 and flagella it does not. It seems reasonable, then, to suggest 

 that the central fibrils and the radial strands between the central 

 and peripheral fibrils are responsible for localizing the eflFects of 

 contractions to the particular region of the cilium in which they 

 are taking place. Perhaps the arms and other links also play a 

 part here, and it is interesting that at the tips of the cilia the 

 orderly arrangement of peripheral fibrils disappears at the same 

 time as the arms and radial links. Where an additional ring of 

 supposedly contractile fibres is present in sperm tails, there is 

 always some connexion between the outer and inner rings of 

 fibrils; these could again function to localize the effects of 

 contractions. 



Perhaps it would be appropriate at this point to consider the 

 possible reasons why there are 9 outer and two central fibrils. 

 Astbury, Beighton and WeibuU (1955) suggested that the 

 arrangement of the 9-plus-2 fibrils of similar size is such that if 

 there are two central fibrils, then 9 is the smallest number of 

 outer fibrils which can form a ring around them and yet allow 

 the inner pair to spiral relative to the outer fibrils. This is 

 probably not a good enough reason^ since the fibril structure is 

 much more complex than these authors believed, and the inner 

 fibrils are not known to spiral. Other suggestions have followed 

 rather similar lines, but with the increase of our knowledge of 

 ciliary structure and function, it should be possible to put forward 

 some more valid reason. 



It is interesting that centrioles, even in non-ciliated cells, 

 usually possess 9 groups of longitudinal fibrils ; it is by outgrowth 



