A traveling wave of cortical actin density generates periodic protrusions and large secondary folds. (A) Time lapse of DIC and F-actin confocal fluorescent images of cells undergoing periodic protrusions. Red arrows show the direction of the cortical wave propagation. The stars are two fiduciary markers that show how the cell periphery apparently becomes dilated (21 s) and compressed (52 s) during the periodic protrusive cycle. (B) Time lapse of merged F-actin (green) and PM (red) confocal fluorescent images. Two white bars are fiduciary markers to illustrate the process of producing large accordion-like folds (Video 6). Green arrows point to a partially unfolded region in the middle of the density wave at t = 0 that moves toward the relaxed region (t = 63 s). Yellow arrows point to a large-scale structure on the cell periphery.