152 MOVEMENT OF CILIA AND FLAGELLA 



Theories concerning the mechanism of contraction are best 

 considered first, since we can apply biological and mechanical 

 observations on other structures to the known anatomy of the 

 cilium. Older theories involved suggestions that the cycle of 

 movemicnt of a cilium resulted from such things as the flowing 

 of hyaloplasm into and out of a curved hollow outgrowth (Grant, 

 1835; Schafer, 1891), or of the rhythmic contraction of the 

 ciliary membrane (Heidenhain, 1907). The suggestion of 

 Sharpey (1835) that cilia possess internal contractility is much 

 more in line with current theories, most of which assume that 

 the peripheral fibrils are capable of shortening in some way. 



A large amount of investigation on the mechanism of contraction 

 of muscle, particularly vertebrate striated muscle, has led to the 

 formulation of several hypotheses about the shortening of muscle 

 fibres. It is only reasonable that some attempt should be made 

 to see if the suggested mechanism can be applied to the ciliary 

 fibrils. On the basis of the observed structure and change in 

 striation widths of striated muscle under various conditions, 

 Hanson and Huxley (1955) concluded that there were two types 

 of parallel filament, both rather less than one sarcomere in length, 

 which could be cross-linked and could move longitudinally with 

 respect to one another, both when stretched and in contraction, 

 without change in filament length. That is, contraction is caused 

 by filaments sliding along one another without themselves 

 shortening. Although this hypothesis is attractive and the 

 evidence for it is very strong, not all workers have agreed with it. 

 Sjostrand and Andersson-Cedergren (1957), for example, have 

 found that the filaments, which are continuous through many 

 sarcomeres, shorten and thicken on contraction, and may interlink 

 and have cross-bridges. Very high magnification electron 

 micrographs by these latter authors show a helical organization 

 of the filaments, apparently built up of very small rodlets; the 

 orientation of these rodlets changes during contraction to become 

 more transverse, so that these authors suggest a folding mechanism 

 rather than a sliding one. 



An interesting case of an apparently helical structure which 

 shows similar changes of shape on shortening has been described 

 by Brenner et al. (1959) for the tail sheath of the T-even 

 bacteriophages. In the extended condition this sheath is 800 A 



