RESUMEN
It was made possible to study the mode of nuclear division during all the stage of mitotic division of the free-living amoebae belonging to the families Schizopyrenidae and Hartmannellidae, when the massive number of them were fixed and stained. While the four phases of nuclear division vary in different genera, in the amoebae belonging to the same genus the mitotic division was similar. In the family Schizopyrenidae, the nucleus contains a central Feulgen-negative nucleolus which during mitosis form "polar masses". In the family Hartmannellidae, the resting nucleus has either a single Feulgen-negative nucleolus or several nucleoli. During mitosis the nucleolus or nucleoli disappear and a spindle with chromosomes arranged as an equatorial plate develops. The mitotic apparatus is an essential criterion in the differentiation of the genera Hartmannella and Naegleria.