THE MECHANISM OF METACHRONISM 187 



threshold, and the conducted impulse accelerates this build-up, 

 then the conducted impulses could control the rate of beating of 

 the cilia . It is noteworthy that in any row of Stentor membranelles 

 the frequency is normally the same throughout the row from the 

 first cilium, so that it appears that the first cilium of any row 

 initiates the rhythm for that row, i.e. it is the pacem.aker. The 

 pacemaker cilium communicates its rhythm to all the cilia that 

 follow it in the row, but in all cases where this communication 

 is prevented, another cilium takes over the pacemaker function. 

 The first active cilium beyond the cut in the experiments quoted 

 above was the pacemaker for the cilia in the distal region, and 

 although it normally shows a frequency lower than it did before 

 the cut was made, a higher frequency, which could have resulted 

 from a local injury to the cilia, was occasionally measured. 

 Occasionally the metachronal transmission from the gullet region 

 to the distal end of the row may be disrupted by natural means, 

 e.g. when a large object is carried into the gullet or when the 

 edge of the peristome is caught up by some object in the 

 environment. When either of these things has occurred, the 

 frequency in the distal region has always been found to be reduced. 

 It is also interesting that the transmission may be prevented by 

 stoppage of the beating of the cilia in these cases, which provides 

 additional evidence for the stepwise mechanism proposed above. 



The action of the pacemaker and the postulated transmission 

 mechanism in Stentor are illustrated in Fig. 50, in which a pace- 

 maker cilium is show^n passing rhythmic transmitted impulses to 

 two cilia which follow it. The rate of beat of the pacemaker 

 depends on factors discussed in Chapter IV (p. 167). It is 

 interesting that this idea of a pacemaker is directly comparable 

 with that found in the heart of vertebrates ; the chambers of the 

 frog heart have intrinsic rhythms decreasing in the order sinus, 

 atria, ventricle and bulbus cordis, yet in the whole heart all parts 

 beat with the rh3^hm originated by the sinus. 



Although this system of metachronal transmission appears to 

 be used under most experimental conditions by these membran- 

 elles, the co-ordination is modified in media of high viscosity 

 (p. 84). In a very viscous medium the metachronal wave 

 velocity is increased, apparently because the excitation of the 

 cilia is caused by viscous drag communicated through the medium. 



