Some Problems of Ciliary Structure and 

 Ciliary Function 



BjORX A. Afzelius 



The \Veuney-(jyeu Institute for Expeiiuiental Biology, 

 Stockholm, Su-eden 



The purpose of this paper is to review some of the problems of cihary 

 movement in the hope that new findings on cihary and flagellar fine struc- 

 ture mav shed light on the mechanisms responsible for the movement. 

 Several questions are still to be answered : 



1. Bv what mechanism do cilia and flagella work ? 



2. In what respects does the fine structure of a flagellum differ from 

 that of a cilium ? Is it possible to correlate such differences with their 

 different modes of movement ? 



3. What is the significance of the " magical 9 + 2 filament arrangement" 

 in cilia and flagella ? Are there meaningful variations in their 

 arrangements ? 



4. Is the ciliary beat (or the flagellar beat) to be regarded as a con- 

 traction process ? Are there significant similarities between these 

 movements and the contraction of, for instance, a striated muscle ? 



The observations that are presented here have been made with the 

 electron microscope. The different tvpes of cilia and flagella that have been 

 chosen for study have only this in common : they have been subjected 

 previously to a careful analysis with regard to their mo^■ements. The study 

 has thus been intended to be an attack on the second question above. It 

 was hoped that some definite conclusions could be made and that an 

 answer to the second question would at the same time answer the others. 



Before proceeding further it is necessarv to define the words "cilium" 

 and "flagellum". A cilium is a fine vibratile thread projecting with many 

 others from the surface of a cell. Cilia lash in an orderlv beat in a constant 

 direction. The beat consists of an efl'ective stroke and a recovery stroke. 

 In the effective stroke the cilium is stiff and it dri\"es the water ahead of it ; 

 in the recovery stroke the cilium is more flexible and the tip of the cilium 

 follows a lower curve. It is of interest that the difference in flexibility may 

 be retained some time after the cilium has stopped : when moved with a 



