Electron microscope studies of cyst stages of Sarcocystis tenella: the origin of micronemes and rhoptries.

The origin and the development of the micronemes and rhoptries was investigated at the ultrastructural level in cyst stages of Sarcocystis tenella. During endodyogeny the micronemes appear in small vesicles distributed at the periphery of two large granular vacuoles in each daughter cell. Later, these vacuoles were divided into numerous vesicular spiral formation-centers, producing micronemes at the apical pole of young merozoites. Rhoptries were observed to originate from desnifications with in the same large vacuoles which gave rise to the micronemes.