ANALYSIS OF STENTOR 235 



Following incision, the first membranelle distal to the cut 

 established a new frequency of beating, which was then taken up 

 by all the membranelles in the isolated section. The first mem- 

 branelle of a series may therefore be regarded as a pacemaker which 

 determines the frequency of membranelles distal to it. Being 

 separated from proximal membranelles the pacemaker can establish 

 its own intrinsic rhythm, often different in different sections. 

 Usually, its rate was slower than that of the membranelles on the 

 gullet side, but in a few cases it was more rapid, possibly due to 

 excitation through injury. In the intact feeding organelles, the 

 pacemaker would presumably be some membranelle within the 

 gullet. In this region. Sleigh (1957) found that the wave lengths 

 and wave velocity are smaller than in the distal lengths of the 

 membranellar band; but this discrepancy he resolved by the 

 observation that the membranelles are also closer together in the 

 gullet. Therefore the number of membranelles in one wave length 

 is the same throughout the band and hence the number stimulated 

 per second is the same regardless of their density. " The wave 

 velocity thus depends on the number of cilia involved in the trans- 

 mission, and not on the linear distance traveled by the metachronal 

 wave ". This is further evidence that the cilia themselves are 

 involved in transmission of the metachronal wave and not the 

 basal fiber connecting the basal plates. 



Chemical and physical treatments thus indicated that there is an 

 intraciliary excitation which is separable from a second process, 

 the conduction of the impulse from membranelle to membranelle. 

 From these and the cutting experiments. Sleigh proposed the 

 hypothesis diagramed in Fig. 68. Only a single cilium in each 

 membranelle is shown for presumably the closely packed cilia of 

 each membranelle work together. Each cilium would then be 

 capable of spontaneous beating but at a slower frequency than 

 when excited by interciliary transmission. Increasing or retarding 

 frequency of beat would simply alter the rapidity of ciHary contrac- 

 tion or response to the internal state of excitation and therefore 

 need not affect the rate of conduction of the impulse between the 

 motor organelles. On the contrary, digitoxin, by decreasing the 

 threshold of excitability, as it does in heart muscle, might increase 

 the speed of excitation and therefore lead to a more rapid tripping 

 off of the conducted impulse so that wave velocity would be in- 



