actin polymerization at the leading edge of the cell _____ the plasma membrane forward (protrusion) and forms new regions of ___ _____. new points of anchorage are made between the bottom of the cell and the surface (____) on which the cell is crawling (attachment). contraction at the rear of the cell-mediated by ____ motor proteins moving along actin filaments-then draws the body of the cell forward. new anchorage points are established at the front, and old ones are released at the back, as the cell crawls forward.