1. The maximum extent to which an element of the tail of a bull's spermatozoon bends during its contractile cycle is not the same for all elements; the nearer the element lies towards the tip of the tail the greater is the amount of bending.

2. The phase difference between successive elements varies along the length of the tail; and consequently the speed of propagation of the bending wave decreases as the latter moves backwards.

3. The amplitude of transverse movement relative to the head increases progressively along the tail towards the distal end.

4. Distal elements execute figure-of-eight movements relative to the head.

5. The frequency of the bending cycles and the propulsive velocity of the whole cell vary considerably. The average frequency for thirty-one cells was 9.1/sec., and the average propulsive speed for 235 cells was 94 µsec.

6. Cells moving freely in water ‘flashed’ with a frequency similar to that of the bending waves. The rotation of the head about its longitudinal axis appears to be due to the fact that all elements of the tail are not executing their transverse movements in exactly the same plane during the whole of their contractile cycles.

7. The rate at which an element can propel itself forward cannot be greater than about one-third to one-quarter of its average transverse velocity.

8. The distal elements of the tail exert their propulsive effort against the fulcrum provided by the proximal elements.

9. It is impracticable to relate the speed of propulsion to the form and speed of propagation of the waves passing along the tail.

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