Cyanobacteria that undergo repeated binary fission in three planes to form more or less cubical aggregates of cells. Multiple fission occurs simultaneously in most cells of the aggregate and is followed by the massive release of motile baeocytes. The baeocyte initiates growth by enlarging symmetrically into a spherical vegetative cell that, just prior to the onset of binary fission, attains a size that is characteristic and constant for any given strain.