The life cycle of a slime mold begins with a haploid spore which is produced inside the fructification by meiosis. The germinated spore can transform into either an amoeba-like myxamoeba cell or a flagellated swarm cell. Both myxamoeba and swarm cells divide into large populations. This stage composed of amoeboid and flagellated cells is why some authorities place the slime molds in the kingdom Protoctista. Two myxamoeba cells or two swarm cells may join together in a cellular fusion (plasmogamy) followed by nuclear fusion (karyogamy). The combining of their cell contents (protoplasts) represents a very primitive form of sexual reproduction. It also provides a source of genetic variability; however, swarm cells always fuse with swarm cells and myxamoeba cells always fuse with myxamoeba cells. The fusion of two haploid cells results in a diploid zygote which transforms into a developing plasmodium. As the diploid nuclei divide, the plasmodium grows larger and larger, slowly moving (creeping) along the forest floor and feeding like a giant amoeba. The plasmodium of some slime fungi (Fuligo) can grow to the size of a large pizza