Rediae of Himasthla elongata were obtained from the periwinkles Littorina littorea and studied using transmission electron microscopy. It demonstrated that the redial germinal mass is a specialized reproductive organ consisting of germinal cells, supporting cells and stem cells. The germinal mass also contains early cercarial embryos emerging as a result of cleavage division of mature germinal cells. Small stem cells, which have a high nuclear/cytoplasmic ratio and heterochromatin-rich nuclei, are the source for new germinal cells. The latter grow and differentiate; the number of organelles in them increases and specific structures such as annulate lamellae and the nuage appear. The nuclear chromatin of differentiating germinal cells becomes progressively less condensed while the nucleolus enlarges. Uneven distribution of the cytoplasm around the nucleus and the concentration of mitochondria in its thickened area result in the polarization of the germinal cells. Supporting cells of the germinal mass, differin