A. "Standard" cell cycle of an enteromonad cell; cell divides after karyokinesis. B. Model of evolutionary change from single karyomastigont morphology to double karyomastigont morphology by arrest of cytokinesis. The cell does not divide after the first karyokinesis and secondary karyokinesis results in a cell with four karyomastigonts. This cell then divides into two cells, each with a double karyomastigont. C. Model of evolutionary change from double karyomastigont morphology to single karyomastigont morphology, either by cytokinesis without karyokinesis, or by fusion of nuclei. (modified from Siddall, Hong and Desser 1992).