They undergo structural development inside the RBCs, becoming trophozoites and schizonts. Rather than producing more and more trophozoites, some of the merozoites produce gametocytes. The trophozoites moult into merozoites, which have a tetrad structure coined a Maltese-cross form. Some protozoa have two-phase life cycles, alternating between proliferative stages (e.g., trophozoites) and dormant cysts. Cysts are oval-shaped cells slightly smaller than trophozoites. Living trophozoites and cysts are yellowish or greenish in color. The antigen is expressed only by "P. falciparum" trophozoites. Uninucleated trophozoites convert into cysts in a process called encystation. It forms cysts with four nuclei which excyst in the body and become trophozoites. In trophozoites, the two nuclei are visible. A trophozoites feed on the haemoglobin and forms a granular pigment called haemozoin. The trophozoites will then ingest these dead cells. The mature trophozoite undergoes binary fission and up to 16 immature trophozoites are produced. In their free-living state, trophozoites feed on bacteria. The eggs with the trophozoites are passed out into the environment through the feces.