1. The spermatozoa of Psamtneckinus irnliaris (P. L. S. Müller; Gmelin) propel themselves by projecting transverse bending waves along their tails. All points on the tail normally execute their movements in approximately the same plane, their envelope forming a plane (or slightly twisted) lamina. The radius of maximum curvature is of the order of 4µ

2. In fresh suspensions at about 180° C. the waves are generated at a frequency of 30-40 per sec. and travel along the tail at a velocity of 800-1000µ per sec. The average amplitude of the waves is 4µ and the average wave-length 24µ

3. Elements of the tail situated near the head seldom bend to the same extent on their two sides. The symmetrical bending cycle of the central elements, on the other hand, sometimes imposes on the tail the form of a sine curve.

4. When moving over the surface of a glass slide the passage of each wave along the tail propels the head of the spermatozoon through a distance of 5-6µ, and at the same time causes it to oscillate laterally through a distance of about 4µ. The rate of forward propulsion of the head is seldom constant during all phases of the bending cycle; in extreme cases the head may move backwards during certain phases of the cycle. This asymmetry is probably due to the asymmetry of bending on the two sides of the tail.

5. Spermatozoa swimming over the surface of a slide travel at an average speed of about 190µ per sec, but their path of progression is seldom straight; most cells travel along a curved track whose radius is usually 30-100µ. Cells swimming farther away from the surface of a slide roll about their longitudinal axes with a frequency of 0.5-3.0 per sec., and their axis of progression is straight. The path of a cell which yaws and rolls as it progresses forms a helix whose properties depend on the rate of yaw, frequency of roll and the rate of the cell's forward progression.

6. Eighty per cent of the cells moving over the surface of a slide yaw in a counter-clockwise direction relative to the observer. This phenomenon can be explained on the assumption that most of the cells when moving freely in a bulk of fluid behave as ‘left-handed’ screws rolling counter-clockwise as they advance; proximity to a surface prevents the cells from rolling and impresses on them an appearance of yawing in a counter-clockwise direction irrespective of whether they are yawing towards their right or left sides.

7. Evidence, is presented which supports the view that all regions of the tail are actively contractile although mechanical forces may affect the propagation of bending waves along the filament.

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