This sea urchin spermatozoon was treated with detergent, Triton X-100, to remove the cell membrane, and then transferred to a solution containing a relatively low concentration of MgATP, to reactivate the bending of its flagellum at a frequency of about 1.5 cycles per second. This spermatozoon was swimming freely, but the photographs have been repositioned to eliminate movement and rotation of the sperm head.
Before reactivation, the spermatozoa were exposed to a suspension of 40 nm gold beads. Some of these beads attach to the outer doublet microtubules that comprise the flagellar axoneme. In this case, two beads were attached to doublet microtubules on opposite sides of the flagellum. As the flagellum bends, the sliding between these doublet microtubules is demonstrated by the movement of the two beads.
Details of this type of experiment were published in Brokaw, C. J. (1991) Microtubule sliding in swimming sperm flagella: direct
and indirect measurements on sea urchin and tunicate spermatozoa. J. Cell Biol.