50 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



and sufficient reason; it cannot conceivably have been an evolu- 

 tionary accident. Some types of cells accomplish nuclear division 

 without detectable centrioles, but apparently no eucellular 

 organism has achieved flagellar locomotion, or anything like it, 

 without 9+2 fibrils arising from a kinetosome, which usually also 

 is the point of origin of other, intracellular fibers. This means that 

 the 9+2 arrangement must be indispensable to the mechanics of 

 flagellar locomotion — no reasonable approximation will do — 

 and that the cylinder of nine fibrils is one of very few successful 

 designs for a cytoplasmic center of morphogenetic and kinetic 

 activity. Regardless of whether the first centriole was a kineto- 

 some or vice versa, the fiber arrangement is significant to both 

 functions. 



It is usually supposed that flagellum movement results from 

 the passage of waves of localized contraction along some or all 

 of the peripheral fibers (Gray, 1955; Afzelius, 1959; Gibbons and 

 Grimstone, 1960). Inoue (1959), however, has suggested that 

 only the central fibers may be contractile since these are the ones 

 missing from kinetosomes and from non-motile receptor cilia. 

 The nature of such contraction is not understood and cannot be 

 understood until we know a great deal about the chemical and 

 physical structure of the fibrils. Analyses of isolated cilia of 

 Tetrahymena (Child, 1959; Watson, Hopkins and Randall, 1961) 

 and flagella of some sperm and phytoflagellates (Tibbs, 1957, 

 1958; Jones and Lewin, 1960) show that they are largely protein, 

 with varying amounts of loosely bound nucleotide, carbohydrate 

 and lipid. Adenosine triphosphatase (ATP-ase) activity is 

 present, as indicated also by the cytochemical studies of sperm 

 tails by Nelson (1959), while HofTman-Berling (1955, 1958b) and 

 Bishop and HofTman-Berling (1959) report ATP-reactivation of 

 extracted sperm models. Nelson's electron-microscope cyto- 

 chemical studies showed ATP-ase and succinic dehydrogenase 

 concentrated in the nine peripheral fibers throughout their length ; 

 the other studies did not permit differentiation of fiber and matrix 

 composition. Using immunologic techniques, Finck and Holzer 

 (1961) found that proteins similar to actin or myosin were absent 

 in cilia and sperm tails from the chicken. 



Some information on the composition of kinetosomes is 



