In recent studies of germinal development in rediae (Cort, Ameel, and Van der Woude, 1948) it was found that in a psilostome and several echinostome species well developed germinal masses were present in the posterior end of the body cavity of both mother and daughter rediae. These masses are attached to the wall by strands of tissue and are composed of both unicellular and multicellular components. The unicellular components are interpreted as germinal cells and the multicellular components as very immature embryos that have started their development while still attached to the mass. These germinal masses appear to be persistent centers of multiplication of germinal cells, and the multicellular components which continually break off from them furnish the constant stream of free embryos that develop in the body cavities of the rediae, and escape from them. In mature and sometimes even in old rediae these germinal masses still persist, and free embryos in all stages of development are present, the smallest of which correspond in structure to the largest multicellular components of the germinal mass. Germinal masses of this type provide for the continuous production of embryos throughout the whole life of the rediae, and make possible the production of rather large numbers of individuals. The important thing, then, is not so much the presence in these germinal masses of small embryos (multicellular components), as the ability of some of their germinal cells (unicellular components) to continue dividing so that new embryos can be produced over such a long period of time. This type of germinal mass can be traced back in the development of the redial embryos to a stage in which it is represented only by a morula-like group of germinal cells in the primitive body cavity (Cort, Ameel, and Van der Woude, 1949, figs. 1, 4, 8, 9). Since the psilostomes and echinostomes belong to the order FASCIOLATOIDEA Szidat, 1936, which we consider to be the most primitive group of the digenetic trematodes, it becomes of special interest to study the germinal development in rediae of more advanced and specialized groups. In the summer of 1947 we had a particularly favorable opportunity to study the germinal development of Halipegus eccentricus Thomas, 1939. This species, which belongs to the family HEMIURIDAE, appears to represent a very highly specialized type. Its miracidia have lost their free life and are very complicated in structure (Thomas, 1939). The mother sporocyst grows to large size and produces large numbers of rediae, which grow into large elongate sacs without locomotor appendages, and with a reduced digestive system. Finally, the cercaria of this species and, as far as known, those of the whole family, have extraordinarily modified tails with a variety of peculiar appendages. The bodies of the cercariae are withdrawn into a chamber at the base of the tail, and