Fig. 9. Model of cystic bulge movement. (A) An actin cone has two domains. The first domain is actin cone itself, which is highly crosslinked and moves forward as a unit. The second domain is cortical actin, which is dynamic and responsible for pushing actin cone forward, as indicated by purple arrows. As the actin cone moves forward (blue arrows), cytoplasm (green) is squeezed out by the thick network in the actin cone. The membrane and axoneme (green bar) become directly connected just behind the actin cone (see the Movie 9 at http://dev.biologists.org/supplemental/). (B) Enlarged diagram of the region of the actin cones and a speculative model of actin dynamics. Actin polymerization occurs near the membrane. The actin filaments elongate and are crosslinked into an actin cone. New filaments branch out from the side the of old filaments due to the activity of Arp2/3 complex. This branching is more active at the front of the actin cone because of the concentration of Arp2/3 complex, and more pushing force should be generated there. Probably because actin depolymerization is faster at the back (and/or there is less assembly), the actin cone becomes thinner toward the back and eventually disappears.