132 MOVEMENT OF CILIA AND FLAGELLA 



The movement of bull sperm is further modified from the sine 

 wave pattern; in fact, Gray (1958) suggested that while the 

 sperm of the sea urchin move like eels or snakes, the movements 

 of bull sperm resemble those of fish like the trout. This appears 

 to result from the fact that the anterior part of the bull sperm acts 

 as a fulcrum against which the posterior part of the tail can 

 exert a propulsive force; in the sea urchin sperm all parts of 

 the tail show similar lateral movements. The difference between 

 the movement of these two types can no doubt be correlated with 

 the difference between the structure of the two sperm, for the 

 more complex anterior region of the anterior part of the mammal 

 sperm tail may reduce the flexibility of this part of the tail. The 

 bull sperm also differs from the sea urchin sperm in that the head 

 is relatively larger. 



A moving bull sperm (PI. XV f-m) shows waves of bending 

 which resemble those of the sea urchin in that the amplitude of 

 the bending wave progressively increases towards the distal end, 

 while the tail is long enough to accommodate rather more than 

 one complete wave. They differ in that bending waves of the 

 bull sperm show a decreased speed of propagation of the waves 

 and a decreased wavelength as the waves approach the distal end. 

 The proximal part of the tail bends little and the distal part 

 moves much more in broad sweeping movements, so that the 

 *' optical envelope " traced out by the bull sperm in motion is 

 roughly triangular (PI. XVe), while that of the sea urchin is 

 more nearly elliptical (PL XVd). The distal tip of the tail of 

 the bull sperm traces out figure-of-eight movements, so that during 

 part of the beating cycle the tip may even be moving forward 

 relative to the head. These movements are comparable with 

 those of the tail of a trout-like fish. 



During a large part of the transverse movement of the distal 

 region of the tail, it is inclined at a large angle to the axis of 

 progression, and so is able to exert a large propulsive force 

 compared with that of a more proximal region of the tail with a 

 smaller angle of inclination (see Fig. 33). Transverse forces are 

 relatively larger where there is a small angle of inclination, but, 

 provided there is more than one wavelength in the complete tail, 

 the resultant force transverse to the axis of progression will be 

 near zero. It seems that the relative inflexibility of the anterior 



