RECENT WORK ON FLAGELLAR MOVEMENT

Abstract

It is only when a flagellate is able to move without restriction in its native medium that it will swim normally; compress it with a cover-slip, put it in a more viscous fluid, increase the hydrogen-ion concentration of its surroundings, and the movements of its flagellum are likely to be abnormal. In the analysis of flagellar movement, as in the study of the structure of the cell, methods which make observations easier may give a distorted picture of the inviolate living material. A viscous fluid may slow down the flagellar frequency to a speed that can more easily be followed, but the possibility that this alien medium is producing radical changes in the nature of the movement cannot be ruled out. And similarly with compression, which tends to restrict the movements of the flagellum to one plane, and low temperatures and an acid medium which induce morbidity. Hence the great value of Lowndes's technique of cine photomicrography (Lowndes, 1935). With his apparatus flagellates can be filmed swimming normally, and since exposures down to 1/50,000 sec. are obtainable, movements of the flagellum that the eye could not possibly follow can be analysed at leisure afterwards. Gray's work (I930) of filming the frontal and abfrontal cilia on the gills of Mytilus has the same merit, but Lowndes is the first to apply this technique to the movement of flagellates, where methods involving possible distortion had previously held the field. Gray (1928, 193I) reviewed the evidence for the opposed theories of Schafer (I904) and Heidenhain (9 i9I) and their followers as to whether a vibratile element was to be regarded as a passive unit mechanically operated by the cell, or an active unit capable of generating its own energy. He argued convincingly in favour of the latter, stressing the point that since a wave was able to pass down its length at a constant speed and without change of amplitude a flagellum or cilium must be producing some of its own energy. He added that if this could be supported by quantitative measurement the evidence would be conclusive. It was this suggestion which inspired Lowndes's work on the movement of eight common flagellates (1936, I941). By plotting the distance from the base of the flagellum to the crest of the wave, against the time interval between the exposures of his film, he was able to draw graphs of the wave velocity. In each case a more or less straight line was obtained, indicating that the velocity of a wave increased as it passed along the flagellum. What is more, an examination of the form taken up by the flagellum revealed that the amplitude of the waves was either constant or increased, but never reduced. Thus on this point Lowndes bears out Gray's conclusions; both agree that the flagellum is an active and not a passive unit. But many of Lowndes's observations challenge opinions, which, although they rest chiefly on theoretical grounds, have been repeated often enough to become orthodox dogma. Take, for example, the idea of the tractellum and the pulsillum. It used to be thought that a flagellate moved either as a result of waves producing a forward component as they passed from the tip to the base of a flagellum attached to the anterior end, or as