Fig. 4. Sliding of a thicker bundle over a thinner bundle in an elastase-treated flagellar axoneme with a head. Dark-field micrographs of a sperm flagellum before (top) and after (bottom) induced sliding. A flagellum was cut in two with a glass needle (arrowhead, top) and treated with elastase. Application of 1 mM ATP at 10^–3 M Ca²⁺ induced lengthwise splitting into two bundles with the thicker bundle sliding to the right (bottom, arrow) (i.e. away from the head), leaving a thinner bundle attached to the glass surface.