SUMMARY 169 



cilium against distortion by bending. In addition to their function 

 of morphogenesis, the basal bodies of ciHa are the site of origin 

 of waves of contraction which pass up the cilia; they are also 

 responsible for anchorage since they connect the ciliary shaft to 

 the root structure. 



The beating of a cilium involves three distinct processes : 

 excitation of contraction, contraction and propagation of the 

 contraction. x\lthough shortening of the fibrils in contraction 

 could occur by the sliding of longitudinal filaments alongside one 

 another, it seems more likely that the peripheral fibrils may be 

 composed of filaments arranged in helices, and that the fibrils 

 shorten and thicken by the sliding of one gyre of the helix relative 

 to the next. The fibrillar shortening required is small compared 

 with that in muscle, and the proposed contraction system would 

 be adequate. The mechanism of the propagation of contraction 

 waves seems likely to be mechanical rather than electrical, and to 

 involve only the peripheral fibrils of the cilium; the suggestion 

 that contraction follows shortening of the fibrils seems to be more 

 plausible than the theory that stretching must precede contraction. 

 Very little is known of the mechanism of excitation; some 

 connection with chemical, particularly ionic, changes is likely, 

 but mechanical excitation is also believed to occur. 



Some calculations of the requirement of energy to overcome 

 the external resistance to movement of several types of cilia and 

 sperm tails have been made; these show reasonable agreement 

 with each other, and some also show agreement with the calculated 

 rate of utilization of ATP by the structures concerned. The 

 availability of energy may aflPect the rate of beat of a cilium, or 

 the rate of beat may be limited by the rate of excitation or the 

 external resistance to bending. 



