134 MOVEMENT OF CILIA AND FLAGELLA 



region permits it to be used as a fulcrum against which the tail 

 can push, with the result that a transverse force generated there 

 can balance forces due to distal bending waves. 



Pronounced rotational movements are found in bull sperm, 

 but the occurrence of yawing movements seems less frequent than 

 in sea urchin sperm. The absence of yaw may be only apparent, 

 for the rotational movements are rapid, and would tend to 

 obscure the tendency to yaw; indeed this may be the purpose 

 of rotation. It is almost certain that the rotational movements 

 take the form of a rolling rather than a rocking motion, and that 

 although the main part of the tail traces out a lamina in a single 

 plane, the distal part of this lamina may be twisted. It is likely 

 that the twist here is more pronounced than in sea urchin sperm 

 because the frequency of rotation is nearer to the frequency of 

 bending waves. As the head of the sperm rocks and rolls under 

 dark ground illumination, its flattened surface reflects light in 

 flashes; Gray found that the average frequency of flashing was 

 8*2 per sec, while the average frequency of bending waves was 

 9*1 per sec. The twisted beat of the terminal part of the tail 

 may be connected with the fact that the ordered arrangement of 

 fibrils is lost in this region (see p. 43). 



Rather similar bending waves propagated along a flagellum 

 may thus be used in diflPerent w^ays to produce forward progression 

 of these two types of sperm. The reasons for the diflference may 

 be that : (1) the flat head of the bull sperm is nearly 3 times 

 as long and 4 or 5 times as wide as that of the sea urchin sperm, 

 and will increase the drag opposing forward movement; (2) the 

 sperm of mammals may be required to swim for a longer time 

 than those of the sea urchin, hence the larger mitochondrial 

 sheath and the thick mid-piece of the bull sperm. 



The movement of mono- and bi-flagellate protozoa has been 

 studied by Lowndes (1936-45) and Brown (1945). It appears 

 from this work that in almost all cases, as well as in mono- and 

 bi-flagellate zoospores and sperm of plants, the flagella are used 

 in a quite different way from that described for animal sperm 

 tails, probably because the ** head " structure is very large 

 compared with the '* tail " thickness in the protozoan. The 

 flagellum or flagella emerge from that end of the animal which 

 is anterior when it is moving, and their beating causes a rotation 



