56 STRUCTURE 



diameter which are connected by filaments to each of the peri- 

 pheral fibrils — like a cartwheel with a large hub and short spokes. 

 The annular filaments were about 100 A thick and were spaced at 

 about 200 A intervals along the long axis of the basal body. The 

 peripheral fibrils appeared as doublets rather than triplets, but 

 were tilted at an angle of about 40° to the tangent to the axial ring 

 as in other basal bodies. At the inner end of the basal body the 

 peripheral fibrils became joined into a fairly dense ring with nine 

 thickenings, and the central region was occupied by a dense peg- 

 like structure, which was about 200 A in diameter and protruded 

 upwards from the proximal end of the basal body for about • 3 to 

 0-5 /x. Radial filaments connected the central peg with the nine 

 thickenings of the peripheral ring. Neither the peg nor the 

 peripheral ring seemed to be firmly connected to the fibrous band 

 which lies underneath, although some rather amorphous material 

 surrounded the proximal end of the basal body. 



It is interesting that Manton (1959b) found a similar peg some 

 0-2 to 0*3 [JL long and 500 A in diameter at the inner end of the 

 basal body of cilia of the fern Pteridium. The peripheral fibrils 

 again became more or less fused at the proximal end of the basal 

 body (which is of normal length) and were surrounded by some 

 dense material associated with a fibrous band. 



Other studies of basal body structure have not shown such 

 detail as this. The central cavity seldom has such an ordered 

 structure; frequently it appears quite structureless or contains 

 amorphous masses, irregular fibres or granules. In some basal 

 bodies the fibril triplets may lose their regular arrangement in 

 the cylinder wall, where they seem to be embedded in a dense 

 matrix without definite filamentous interconnexions. Studies with 

 thinner sections and better techniques may reveal more detail 

 here also. At the inner end of the basal body the fibrils may 

 continue into rootlets of one sort or another, as in some mamma- 

 lian cilia (Rhodin and Dalhamn, 1956); or they may end abruptly. 



The arrangement of centrioles in spermatozoa is not 

 entirely clear or consistent. Fawcett (1958a) mentions the 

 presence of two centrioles in mammalian spermatozoa; these are 

 shown in Fig. 13. A proximal centriole is situated against the 

 posterior face of the nucleus, with its longitudinal axis transverse 

 to the longitudinal axis of the sperm ; this centriole probably has 



