44 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



with the center of the kinetosome, or interconnecting adjacent 

 kinetosomes, are described in Trichonympha by Gibbons and 

 Grimstone. 



The distal end of the kinetosome is a zone of particular interest. 

 Here the two central fibrils of the flagellum originate, most 

 frequently at a small dense body that may be spheroidal, flat, or 

 markedly asymmetric. Below the granule, or at the same level if 

 no granule is present, one or more thin septa may stretch partly 

 or completely across the core. The distal terminus of the 

 kinetosome in a typical instance may thus be defined as the point 

 of origin of the central fibrils. It is usually at or near (often 

 slightly below) this level that the flagellum acquires its limiting 

 membrane as an extension of the surrounding cell membrane. A 

 thickened ring in or just beneath the membrane, or fibrous con- 

 nections between it and the kinetosome, commonly mark this 

 point of emergence of the flagellum from the cytoplasm proper. 

 This relationship holds even where the kinetosome is retracted 

 more or less deeply into the cell, as in trypanosome flagellates and 

 many other organisms. In such cases, the cell membrane is deeply 

 invaginated; at the bottom of the invagination the membrane 

 recurves to form the surface membrane of the flagellum, and the 

 central fibrils make their appearance. It would seem that the 

 presence of the central fibrils and of a flagellar membrane are 

 almost always coincident in motile flagella. 



Two sorts of exceptions to this rule of association have been 

 reported. One is the cilia of Eztplotes patella, investigated by Roth 

 (1956). The core of the kinetosome in these cilia is occupied by 

 tubular fibrils of about the same diameter and appearance as the 

 central fibrils, but more sinuous. Whether these are direct 

 continuations of the central fibers is not certain ; a granule occurs 

 at the apex of the kinetosome, and none of Roth's micrographs 

 shows unequivocally that the fibrils traverse the granule. 



Another intriguing exception is the spermatozoid of the brown 

 alga, Dictyota, studied by Manton (1959b). When this cell is first 

 liberated from the parent plant, it is coiled, and the whole length 

 of the single functional flagellum is wrapped about the cell body, 

 under the cell membrane. Sections show clearly that the 1 1 fibrils 

 of the axis are all present, without a limiting membrane of their 

 own. The kinetosome is recognizable by its lack of central fibrils 



