184 CO-ORDINATION OF BEAT 



over the whole surface of the animal, and soon settle to the usual 

 pattern by coalescence and elimination. It looks as if, once the 

 cilia have started to beat in a particular direction, their activity 

 is modified so as to cause the minimum of mutual interference 

 (the direction of beat is controlled independently, see p. 193), 

 The first cilium to beat may influence those around it, but the 

 overall activity must depend on the average speed with which 

 the cilia can beat, for faster cilia could here be slowed down by 

 others. Cuts across the animal do not modify the passage of 

 waves in either part of the animal, so that the frequency of beat 

 seems to be determined by the cilia forming the waves. It is 

 interesting here that the longer cilia towards the posterior end 

 of the body normally beat more slowly than the anterior ones, 

 but some waves coalesce so that the waves are longer posteriorly. 

 When two waves coalesce, however, the wavelength does not 

 remain at twice the normal length, for there is a rapid mutual 

 adjustment between adjacent waves so that they are all of about 

 the same length. All observations seem to point to some type 

 of mechanical co-ordination process, but further observations on 

 this type of metachronism are urgently required. 



The metachronism of the membranelles of Stentor seems to be 

 of a quite diiferent type at normal viscosities, since the frequency 

 and metachronal wave velocity are diflPerently aflFected by 

 temperature, small viscosity changes, magnesium chloride and 

 digitoxin (Sleigh, 1956a). Two of these factors which aifect the 

 mechanical activity of the cilia do not influence the metachronal 

 co-ordination at all. This metachronism has the appearance of 

 an internal or neuroid conduction process passing through the 

 internal cytoplasm or the surface layer of the cell and triggering 

 off the ciliary contractions. 



It appears that there are two ways in which such a system might 

 operate (Sleigh, 1957). Firstly, a nerve-like impulse might be 

 conducted along the length of the row of cilia in such a way that 

 the cilia are stimulated to beat as the impulses pass, but do not 

 participate in the transmission of the metachronal impulses. 

 Secondly, there could be a step -by-step or " stepwise " process 

 passing across one interciliary space at a time, in which the 

 excitation of one cilium results in the production of a conducted 

 impulse responsible for the stimulation of the next cilium in the 



