234 THE BIOLOGY OF STENTOR 



of beat and wave length. Each of these factors is variable. In 

 addition, the membranelles can be oriented to point outward and 

 backward as they do in forward swimming (c). 



It will be recalled that the membranelles are rooted in triangular 

 basal plates all of which are connected by an inner fiber. It was 

 natural for early microscopists to have supposed that the impulse 

 producing metachronal rhythm passed along this fiber, exciting 

 one membranelle after the other; but there are at least two argu- 

 ments against this supposition. The wave velocity (roughly 700 /^ 

 per second) is slower than any known neuroid transmission (Sleigh, 

 vide infra). And second, on resuming their beat the membranelles 

 do not start at once in metachronal rhythm, which is only later 

 established after a brief period of irregular beating. 



Coordination in the membranellar band of S. polymorphus was 

 the subject of astute investigations by Sleigh (1956, 1957). By 

 several approaches he shows that the frequency of beating of the 

 membranelles is dissociable from the wave velocity or rapidity of 

 transmission of the impulse from one membranelle to the next. 

 Both frequency and wave velocity decreased with lower tempera- 

 ture but the decrease was more rapid in the frequency of beating. 

 Increasing viscosity of the medium by addition of methyl cellulose 

 resulted in decreased frequency of beating but no change in the 

 wave velocity. This corresponds to expectations, for external 

 resistance should decrease the frequency of stroke without affecting 

 internal mechanisms of transmission. Magnesium chloride 

 increased the frequency of beating without affecting the wave 

 velocity ; and with aluminum chloride the trivalent ion was several 

 times more effective in producing the same response. If these 

 metal cations may be regarded as reducing the internal viscosity 

 of the protoplasm in cilia, increased frequency would be explained 

 as due to lower internal resistance. Digitoxin greatly increased the 

 wave velocity but only slightly increased the frequency of beat and 

 the shape of the effect- vs. -concentration curves was different. 

 Finally, cutting the membranellar band interrupted the wave 

 conduction but did not prevent the reappearance of metachronal 

 rhythm in separated sections distal to the gullet (Fig. 67D). This 

 experiment at once excluded that metachronal waves originate only 

 in the gullet region and can be stopped by cutting the fiber which 

 connects the basal plates of the membranelles. 



