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 Materials and methods). Upper panel shows a membrane sheet containing contractomeres (squared) densely decorated with actin filaments. Lower panel shows an enlarge contractomere with buckled actin bundles (arrows) rolled into a ball. An enlarged contractomere from the top panel is shown in Fig. S9 where the top panel is shown again illustrating the area of enlargement. Scale bars are 2 μm for the upper panel and 200 nm for the lower panel.