In the summer of 1947 studies were made of the germinal development in mother sporocysts and rediae of Halipegus eccentricus Thomas, 1939, in experimentally infected snails of the genus Physa. In the miracidium and early mother sporocyst of this species there is a morula-like germinal mass, located at the posterior end, consisting of unicellular components regarded as germinal cells. As the mother sporocyst elongates, this mass of germinal cells increases in size and breaks up into a number of individual masses consisting of both unicellular and multicellular components attached to and distributed along the body wall. All the masses may remain compact or some may take the form of strings extending across the body cavity. By the time the mother sporocysts reach maturity, only a single posteriorly located germinal mass is present, and free embryos fill the body cavity. Germinal masses of very large size and complexity were still present in mother sporocysts 39 days after infection. Within 26 days some redial embryos has escaped from the mother sporocyst. H. eccentricus has but one generation of rediae. The course of development and organization of the germinal material in the redia is similar to that of the mother sporocyst. In very young redial embryos there is a morula-like mass of germinal cells in the primitive body cavity just back of the developing intestine. As the redial embryos elongate the body cavity becomes larger and is practically filled with the rapidly growing germinal material. In older embryos and young free rediae, numerous germinal masses are present all along the walls of the body cavity, or in some cases they may form strings extending across it. Later, all of these break up into free embryos except a single large germinal mass attached at the posterior end which is still present and giving off embryos in mature rediae. The very rapid early multiplication of the germinal material, and the unusually large germinal masses provide for the production of large numbers of individuals. It was estimated that in natural infections as many as 30,000 to 40,000 cercarial embryos were present at the time of examination, and that the total cercarial production in such infections would be many times that number.