168 MOVEMENT OF CILIA AND FLAGELLA 



appears from some observations e.g. the beat of Sabellaria cilia, 

 Fig. 40a, that a second beat may commence before the first 

 bending wave has reached the tip. Even if this is general, the 

 rate of contraction could still limit the rate of beat. Much more 

 information about the ciliary mechanism must be obtained before 

 such ideas can be extended or confirmed. 



7. Summary 



The beating patterns of flagella appear to be simpler than those 

 of cilia, although the same mechanism can account for the move- 

 ments shown by all organelles. In the ** ideal " flagellar beat, 

 sinusoidal waves of bending take place in a single plane and are 

 propagated along the length of the flagellum from base to tip. 

 This symmetrical bending is the result of equal and alternate 

 contraction waves passing along the two sides of the flagellum. 

 In most cases the flagellar beat departs from this ideal in that 

 the rate of propagation of the bending waves along the flagellum 

 changes as the waves pass along the flagellum, and frequently the 

 bending on the two sides of the flagellum is unequal. Inequality 

 of bending is thought to result from inequality of time intervals 

 between the bending movements on the two sides of the flagellum 

 (i.e. the intervals betM^een ' right ' and ' left ', and between 

 * left ' and * right '); in the more extreme cases of asymmetrical 

 bending, the movements that result are typical of the beat of 

 cilia. It is not always clear whether a structure is behaving as 

 a flagellum or a cilium, for all intermediates between a truly 

 symmetrical and a very asymmetrical beat may be found. 

 Symmetrical and asymmetrical patterns of movement have been 

 described in this chapter. 



There is some support for the belief that the movements of 

 cilia and flagella are the result of localized contractions of the 

 internal fibrils, and that these contractions are propagated 

 throughout the length of these organelles. It seems most likely 

 that shortening of the nine peripheral fibrils is responsible for 

 bending of the ciliary shaft. Rigidity of the cilium could not 

 be maintained by the two central fibrils alone, and it is suggested 

 that either internal turgor pressure acting against the elastic 

 tension of the ciliary membrane, or the cross-linked scaffolding 

 of central and peripheral fibrils, could maintain the shape of a 



