20 



STRUCTURE 



spacings being wider than others, e.g. see Fig. 6c. Measurements 

 made by the author on the best micrographs of transverse sections 

 of cilia and flagella pubhshed by a number of workers have shown 

 that such an inequahty does occur, but they also show that in all 

 cases the ring of fibrils is not a true circle. This is likely to result 

 from distortion in the preparation of the sections, and an example 

 is shown in Fig. 6b. Peachey (1958) has shown that 30 to 50 per cent 



O 



D 



O 



C) 



O 



o 



o 



o 



o 



C) 



e 



o 



b 



o 



O 9 



o 



o 



o 



o 



(D 



o 



O C) 



o 



o 







o 



o 



C) 



e 



o 



o 



a 



Fig. 6. Diagrams of the distribution of fibril doublets in the 



axial bundle of cilia (see text). 



of " compression " may occur in thin sectioning for the electron 

 microscope, and much of this may remain after flattening of the 

 sections. In all the cases examined the inequality of spacing was 

 regularly distributed around the peripheral ring in a manner that 

 would be expected from such distortion. Until we can be sure of 

 obtaining undistorted sections, or of knowing accurately the effects 

 of compression, this point will not be proved beyond doubt. 



The same distortion makes it difficult to be certain of the 

 symmetry of fibril arrangement. According to Bradfield's hy- 

 pothesis, one fibril of the peripheral ring is bisected by the plane 

 of symmetry (at right angles to the plane through the two central 

 fibrils), and this fibril should therefore be distinguishable from the 

 others. This idea has been used as a basis for numbering the 

 fibrils of the peripheral ring (see Fig. 5), taking the fibril in the 

 plane of symmetry as number 1, and then numbering the remain- 

 der in a clockwise direction, i.e. in the direction in which the arms 

 point (Afzelius, 1959). This numbering system will be used in 

 this book as a convenience, although, as Gibbons and Grimstone 

 (1960) have found, it is not always possible to identify fibril 1 with 



