SPERM MOVEMENT PROBLEMS AND OBSERVATIONS 21 



vertical direction. This can be achieved by putting a sperm suspen- 

 sion on a glass plate, with a rotating glass disk in contact with the top 

 surface of the suspension. The advantage of this system is that the 

 velocity gradient is constant rwh -1 , where r is the radius, «> is the 

 angular velocity, and h is the distance between the disks. Unfortu- 

 nately, it is difficult to achieve the required velocity gradients, with- 

 out rotating the upper glass disk at inconveniently high speeds. This, 

 coupled with quite serious technical difficulties — the bottom and 

 top glass plates must not be more than about 180 microns apart — led 

 me to abandon this system and examine the behavior of bull and 

 human spermatozoa in vertical parabolic velocity gradients, of the 

 form —2Uz/a' 2 , where U is the velocity at the center of the container, z 

 is the vertical distance from the center at which observations are made, 

 and a is half the depth of the container, the width and length of the 

 container being to all intents and purposes infinite. I thought it 

 convenient to start by examining the behavior of dead spermatozoa. 

 These behave in an interpretable way. In a vertical parabolic ve- 

 locity gradient, dead bull spermatozoa point downstream in the top 

 half of the gradient and upstream in the bottom half. The explana- 

 tion is as follows. Because their average specific gravity is greater 

 than that of the suspending medium, bull spermatozoa slowly sink. 

 But the head is more compact than the tail and has a greater specific 

 gravity (Kihlstrom, 1958), so that the spermatozoa sink head first. 

 This means that in the top half of the velocity gradient, the head 

 gets pushed forward, whereas in the bottom half, the tail gets pushed 

 forward. One may, therefore, ask the question: "As, presumably, 

 the specific gravity of the head of a live bull spermatozoon is also 

 greater than that of the tail and as the average specific gravity of a 

 bull spermatozoon is greater than that of seminal plasma, do bull 

 spermatozoa always swim downward and, if not, how do they stop 

 themselves from doing so?" 



It was of interest to see how a dead spermatozoon turned around 

 when the direction of the velocity gradient was reversed. Figure 2 

 shows how this happens in a just reversed velocity gradient, the 

 spermatozoon having been orientated in the "wrong" direction just 

 beforehand. The rotation, which is partly in the vertical direction, is 

 around the head. 



So much for dead spermatozoa. If we now turn to live ones, it is 



