A series of phase-contrast micrographs showing the reaction to the addition at 0 sec of broth containing 0.1 µg CA/ml. The dark part of the Spitzenkörper (S) began to retract at 60 sec leaving a bright spot (arrows) derived from the original central core. The apical dome became hemispherical and began to grow in a nonpolar manner between 45 and 240 sec The typical longitudinal orientation of the mitochondria (m) was lost and they began to enter the area vacated by the Spitzenkörper after 45 sec. Nucleus (n). ANIMATION. Scale bar = 10 µm.