The germinal cells in the rediae of Clinostomum marginatum are in groups of two to five free in the body cavity; only occasionally were single germinal cells seen. In small redial embryos only the germinal cell groups are present in the primitive body cavity. In later stages up to mature rediae they are scattered in different parts of the body cavity mixed with the embryos. These groups of germinal cells must be dividing rapidly since large numbers of embryos are produced. This distribution of germinal material is very different from that in all other rediae we have studied, which have included representatives of the order Fasciolatoidea and of the familes Hemiuridae, Lissorchiidae, Allocreadiidae, Troglotrematidae and Heterophyidae. In the rediae belonging to these groups more or less persistent germinal masses are present at the posterior end of the body cavity which serve as centers of multiplication of germial cells. C. marginatum belongs to a different order of the digenetic trematodes from any of these forms, being related to the schistosomes and strigeids. Perhaps the germinal cell groups in the rediae of this species are the prototypes of the complicated floating germinal masses of the strigeids. In fact they are almost exactly like the early stages of the strigeid germinal masses in very small embryos of daughter sporocysts. This mechanism for multiplication in the rediae of C. marginatum is very effective since this species appears to produce more cercariae from a single infection than any other trematode speces we have studied.