THE MECHANISM OF METACHRONISM 185 



row, as well as triggering the contraction of the first cilium. 

 One drawback to the first idea, which was pointed out by Gray 

 (1930), is that the rate of transmission is very slow in comparison 

 with other conduction systems, so that at least some other 

 mechanism must be involved. The second suggested mechanism 

 implies that two parts are involved in the wave transmission, an 

 intracellular excitation process and an intercellular conduction 

 process, and the actual conduction could be of the speed found in 

 nerve if the excitation takes up a large part of the total transmission 

 time. It is interesting that the wave velocity of the membranelles 

 of Stentor is increased by digitoxin, which is believed to lower 

 the threshold of excitability in heart muscle. 



Two observations throw some light on this, and both of them 

 point towards the stepwise theory of transmission. When the 

 row of membranelles along the edge of the peristome of Stentor 

 is cut across, the frequency of the distal region almost always 

 changes (by up to 20 per cent) to a new level at which it remains 

 until the cut heals, which is usually within an hour for a 

 reasonably small cut, and then the frequency returns to the previous 

 level. The frequency change is accompanied by a change in 

 wavelength so that the wave velocity in the distal region remains 

 the same as in the proximal region throughout the experiment. 

 It appears that the wave conduction is dependent on the cytoplasm 

 through which the waves are conducted, and again the trans- 

 mission is independent of the frequency. The frequency is the 

 same throughout any row of membranelles that are in physical 

 continuity, but as soon as there is a break in the row, there is a 

 change in the frequency from the break to the end of the row. 

 In these cutting experiments the frequency in the distal region 

 is usually lower, but occasionally higher, than in the proximal 

 region. Restoration of the continuity of waves across the cut 

 and uniform frequency throughout the row normally occurred as 

 soon as the cut edges came together, even when the cut edges 

 did not meet at exactly the same level; because of this last 

 observation it is unlikely that any specialized conducting structures, 

 such as fibres, are concerned with the metachronal transmission. 



Membranelles of the gullet region of the peristomial field of 

 Stentor are interesting in that they are smaller and show a shorter 

 metachronal wavelength than those in the main part of the row ; 



