Rheotaxis of spermatozoa

1. The response of human and bull spermatozoa to vertical velocity gradients has been examined, the fluid flow being in horizontal planes. 2. Dead human and bull spermatozoa point downstream (negative rheotaxis) in the top half of a parabolic velocity gradient and upstream (positive rheotaxis) in the bottom half. Although spermatozoa point preferentially in one or other direction, parallel to the stream ­ lines, in linear velocity gradients, this cannot be described as positive or negative rheotaxis because the direction in which the spermatozoa point is determined by the direction of shear. 3. The response of dead human and bull spermatozoa to velocity gradients is due to the head of the spermatozoon having a higher specific gravity than the tail, which causes the spermatozoon to sink head first in a medium of lower specific gravity than that of the spermatozoon. 4. Live human and bull spermatozoa exhibit positive rheotaxis, i.e. they swim up stream, in both halves of a parabolic velocity gradient. 5. The behaviour of previously flow-orientated spermatozoa in a just-reversed velocity gradient has been photographed and analysed. Though the mechanism whereby dead spermatozoa re-orientate in such conditions is understood, live spermatozoa, which also re-orientate in these conditions, may do so by a different and, at present, unknown mechanism.