The movie shows, in real time, the explosive acrosomal reaction of Thyone (sea cucumber) sperm. The field shows several sperm heads (bright contrast) with tails of some in the plane of focus. From the head at lower left, a thin thread, resulting from the rapid polymerization of actin, is extruded. The acrosomal process is thought to contribute to fertilization by penetrating the jelly coat of the egg.