PROTOZOA AS CELLS 41 



double peripheral fibrils are about 25 by 37 m/x in cross-section 

 and average about 55 m/x apart, center to center. Each one is so 

 placed that one end of the figure 8 (one of the subfibrils composing 

 it) is very slightly closer to the center of the flagellum than the 

 other. From the closer subfibril, a pair of short arms, about 

 5 by 14 m/x in cross-section and less dense than the outline of the 

 fibril itself, extends toward the next adjacent fibril. These always 

 point in the same direction on all fibrils in a flagellum, and in three 

 zooflagellates, a ciliate, and several metazoan epithelia (Gibbons, 

 1961) this appears to be clockwise when viewed from the basal 

 end of the flagellum. In longitudinal sections the arms are 

 indistinct, but apparently they are discontinuous projections. Both 

 central and peripheral fibrils are straight; they do not spiral or 

 twist except as the whole flagellum does, and they maintain a 

 strikingly constant spatial relationship within the bundle. 



Nine very slender secondary filaments occur midway between 

 the outer fibrils and the central pair. These are about 5 m/x in 

 diameter and rather sinuous on a small scale; they usually are 

 inconspicuous and rarely are detectable in longitudinal sections. 

 The whole complex of fibrils forms a cylinder somewhat under 

 0-2 /x in diameter; this measurement may vary in different species. 

 The fibrils are embedded in a matrix of low density and surrounded 

 by a unit membrane that is continuous with the plasma membrane 

 of the cell. Often this bears irregular, minute, villous projections. 



At their tips, most nagella taper more or less abruptly. In 

 Trichonympha the diameter of the fiber bundle decreases first while 

 still retaining the 9+2 pattern (see also Roth, 1956); the lateral 

 arms on the peripheral fibrils and the nine secondary filaments 

 disappear at this level, which suggests that their positions are in 



in which flagellum lies; these are connected by minute bridges to 

 smaller particles under the membrane of the flagellum; bp, plate or 

 septum across top of kinetosome; cb,- crescent-shaped, asymmetric 

 axial granule; cf, central fibril of flagellum; cm, cell membrane; 

 cw, cartwheel structure occupying basal part of kinetosome; cy, 

 cylinders in lumen of distal part of kinetosome; d, distal part of 

 kinetosome; fm, flagellar membrane; of, peripheral fibril of 

 flagellum; p, proximal region of kinetosome; s, sheath surrounding 

 central fibrils; sC, distal end of third subfibril of kinetosomal fibril; 

 sf, secondary fibril; t, fine filament extending laterally from apex of 

 kinetosome. From Gibbons and Grimstone, 1960. 



