Equatorial organization of actin filaments is found primarily on the bottom cortex, where ingression is constrained by cell-substrate adhesions. The top cortex, where ingression takes place actively, shows only a limited organization of actin filaments. No "purse string" around the equator can be found, while many actin filament bundles in the region flanking the equator are oriented along the long axis of the cell. This organization suggests that the purse string model of cleavage may not apply to cultured mammalian cells and that ripping forces along opposite directions of the long axis may play a role. The movies show rotating views of two different cells |