22 STRUCTURE 



The structure of the tip of the shaft of cilia is usually difficult 

 to study. In micrographs of frayed whole cilia it was found that 

 the longitudinal fibrils are not all of the same length, so that the 

 tapered tip of the cilium could be caused by the fibrils coming to 

 an end at different distances from the tip (e.g. Bradfield, 1953). 

 The series of transverse sections at different distances from the tip 

 oi Pseudotrichonympha flagella reproduced in Plate Ilia shows that 

 fibrils first lose the arms, then one subfibril of each doublet is lost; 

 at this level the fibril spacing is a little less regular and the peri- 

 pheral ring has a rather smaller diameter (Gibbons and Grimstone, 

 1960). The single fibrils then terminate one by one, but it is 

 uncertain in Pseudotrichonympha whether the central fibrils run 

 to the tip or not. 



Rather similar changes occur at the tips of cilia of Anodonta 

 (Gibbons, 1961b), where the arms, secondary filaments and radial 

 links of any fibril all disappear at a particular level, although the 

 levels for different fibrils may be slightly different. When the 

 radial links and arms disappear the peripheral fibrils lose their 

 regular arrangement. 



In Euplotes, Roth (1956) found that the central fibrils were the 

 longest, while Manton and Clark (1951) found that the central 

 fibrils were the shortest in cilia of the spermatozoid of the fern 

 Dryopteris. Several arrangements were found at the tip of sea 

 urchin sperm tails by Afzelius (1959); sometimes the central 

 fibrils disappeared and the peripheral fibrils were reduced in 

 number while still double, and in other cases the peripherals were 

 reduced to single fibrils before the central fibrils disappeared. 

 No support has been forthcoming for the finding in rat tracheal 

 cilia by Rhodin and Dalhamn (1956) that the peripheral fibrils 

 meet and fuse at the tip. In all cases the diameter of the cilium 

 gradually decreases towards the tip until transverse sections are 

 obtained which appear solid, so that the end of the cilium is closed. 



The Transition from Shaft to Basal Body 



In the ciliary basal body the central fibrils are found to be lacking 

 while the walls of the cylindrical structure contain continuations 

 of the nine peripheral fibrils, usually in triplets rather than 

 doublets (see Fig. 4c). 



The central fibrils come to an end at the level of the cell surface 



