PROTOZOA AS CELLS 39 



during preparation, presumably had a somewhat different com- 

 position. Manton predicted that the same arrangement would be 

 shown to obtain in other flagella. In 1953, Fawcett and Porter 

 published the first high-quality electron micrographs of thin- 

 sectioned cilia, from a variety of metazoan ciliated epithelia, and 

 confirmed Manton's prediction for this material. Subsequent 

 studies by Manton and by a multitude of other workers enable 

 us to state as a general truth that all motile cilia and flagella (above 

 the bacteria)* are organized on the 9+2 pattern. 



A number of studies of frayed flagella (Manton and Clarke, 

 1952; Pitelka and Schooley, 1955) seemed to show that the 11 

 fibers of the axis were themselves multiple structures, being 

 composed of up to five or even more subfilaments. Examination 

 of sectioned flagella consistently belies this appearance. The nine 

 peripheral fibers are in all cases double, while the central ones are 

 single. Recent high-resolution micrography has revealed a 

 considerably greater structural complexity than the earlier images 

 showed. The most detailed information for protozoa comes from 

 the superb high-resolution study by Gibbons and Grimstone 

 (1960) on several species of hypermastigote flagellates from the 

 termite gut. 



In Trichonympha (and the other organisms studied by Gibbons 

 and Grimstone) the central fibrils of the flagellum are about 24 m/x 

 in diameter and about 30 m/x apart, center to center (Text-fig. 3). 

 They may be enveloped in a common sheath. Like the peripheral 

 fibrils, they appear tubular in section. This does not necessarily 

 mean that they are hollow, but only that their margins are of 

 higher density than their cores. Some pictures suggest a fine 

 periodic banding in the central fibrils, perhaps indicating that 

 each is composed of a two- or three-strand helix. The 8-shaped 



* Unfortunately, there is no term in common English usage that includes 

 cilia and flagella and excludes the much finer, filamentous appendages of 

 bacteria, which certainly are not homologues of the 9+2 flagella. Dougherty 

 (1957b) has proposed the neologisms, proterokont for the bacterial appendage 

 and pecilokont for the flagella and cilia of all higher organisms. These words 

 are eminently useful when both groups of organisms are under discussion. 

 Through most of the present review, proterokonts will be excluded from 

 consideration, and flagellum will be used as an inclusive term; cilia are merely 

 one of several varieties of flagella. 



