ABSTRACT
Although the mechanical activity of spermatozoa plays an essential role in the process of fertilization there is little precise information concerning the type and speed of movement of individual cells in their normal state of activity. This paper represents an attempt to fill this gap with respect to a sea-urchin whose spermatozoa, being of the typical monoflagellate type, provide valuable material for the study of flagellar propulsion in general.
Persistence of vision occurs whenever the image of an object moves across the retina ; it does not occur if the axis of the eye follows the moving object and thereby yields an image stationary with respect to the retina. The visual impression of the tail as an illuminated envelope and the double image of the head are due to the fact that the axis of the eye cannot follow either a point on the tail or the head along an axis normal to the path of the sperm’s progression, nor can it follow successive wave crests as they move backwards along this path. On the other hand, the axis of the eye readily follows the head along the axis of progression (since this motion is always in the same direction and is relatively slow), and therefore no persistence of vision of the head occurs along this axis. An interesting demonstration of this principle occurs when the stage of the microscope is abruptly moved; the head then yields a brightly illuminated undulatory track superimposed on the less brilliantly illuminated envelope of the tail, because the image of the head is travelling across the stationary retina; the resultant visual impression is of the same fundamental nature as the tracks recorded on a stationary photographic plate by Rothschild & Swann (1949). (See also Plate II figs. 2-6 of this paper.)
The intensity of light required to yield an adequate photographic image of the tail causes extensive scatter from the surface of the head ; the latter appears in these photographs as a relatively large circle of light. This difficulty does not arise in visual observations with lower intensity of illumination.
The asymmetry of the head tracks was interpreted by Rothschild (1951) as an oblique projection of a regular helix resulting from three-dimensional waves passing along the tail. The axis of microscopic vision is, however, so nearly normal to the plane of observation as to render the microscopic projection of a regular helix indistinguishable from a two-dimensional sine wave.
A few observations suggest that the head may be somewhat flattened.
When viewed by a carbon arc fed by alternating current (or inadequately smoothed direct current) the envelope displays an alternation of light and dark bands similar to those described by Uhlela (1911) for the envelopes of the flagella of some Protozoa. When viewed by an arc fed by direct current of steady voltage the envelope is uniformly illuminated ; the banding is therefore an artefact due to pulsations in the intensity of the light source and not, as suggested by Uhlela, to the intrinsic properties of the vibrating flagella. These visual observations have been confirmed photographically. Pl. 1, figs. 10‒13, show the bright lines characteristic of illumination from an inadequately smoothed direct current arc ; Pl. 1, figs. 7‒9, show photographs taken by means of an arc fed by batteries ; the broad bands in the latter case are due to the fact that the duration of the exposure was not an exact multiple of the period required for the passage of one complete wave over the tail.
