A TYPICAL ORGANELLE — SHAFT 21 



certainty, so that these authors were doubtful about the value of 

 such a numbering system at present. Cleland and Rothschild 

 (1959) and Lansing and Lamy (1961a) found that frequently one 

 fibril was situated in the plane joining the two central fibrils, but 

 this has not been widely observed, and once again distortion may 

 be the cause. Alternatively, it is possible that one subfibril of each 

 pair is more important than the other in determining the dis- 

 tribution of fibrils, e.g. the A subfibrils are connected to the central 

 structures by radial strands (see below), and may be symmetrically 

 arranged with respect to the plane through the central fibrils. 

 Many observations, including the arrangement and size distribu- 

 tion of additional fibrils and sheath thickenings in sperm tails (see 

 pp. 43-48), the position of the compartmenting lamellae of 

 ctenophore comb-plate cilia (see Fig. 5b and p. 32) and the bridge 

 structure between the fibrils 5 and 6 in Anodonta cilia (Fig. 5a), can 

 be used as evidence that there is a plane of bilateral symmetry 

 which passes through the fibril number 1 in the majority of ciliary 

 organelles. Superimposed on this is of course the asyiTunetry 

 resulting from the presence of the arms on the subfibrils, as 

 stressed by Gibbons (1961a). 



In earlier studies the matrix enclosed by the membrane and 

 surrounding the fibrils was thought to be structureless and of 

 similar density to the cell cytoplasm. Local regions of increased 

 density have been seen in many recent studies. Most of these are 

 best explained as radial spokes connecting the armed subfibrils of 

 each peripheral doublet with the central fibrils (Figs. 4b and 5). 

 Gibbons and Grimstone (1960) found their observations on 

 Pseudotrichonympha to be consistent with the suggestion that a 

 ring of nine longitudinal secondary filaments about 50 A in 

 diameter was present half-way between the central and peripheral 

 fibrils; these may be connexions between the radial links. In 

 Anodonta cilia Gibbons (1961b) has found that both radial links 

 and longitudinal filaments are again present, although the longit- 

 udinal continuity of the secondary filaments is not established. 

 Radial strands occur at 270 A intervals in Anodonta (i.e. at twice 

 the distance between the arms) and at 400 A in a trichonymphid 

 flagellate (i.e. at three times the distance between the arms) 

 (Gibbons, 1960). Additional structures have been found in the 

 matrix in some special cases and will be mentioned later. 



