Negative stain EM shows actin filaments buckling at the contractomere to generate an actin ball. Negative-stain electron microscopy of an actin assembly reaction (see Methods). Upper middle panel show a membrane sheet containing contractomeres (squared) densely decorated with actin filaments. Lower, upper left, and upper right panels show contractomeres with buckled actin bundles (arrows) rolled into “balls”. Scale bars are 2 μm for the upper middle panel, 500 nm for the upper left panel, and 200 nm for the upper right and lower panels.