In a program of investigations on the embryology of the digenetic trematodes which has been carried on for several years at the University of Michigan Biological Station, studies have been made of representatives of the strigeids (Cort and Olivier, 1941), the plagiorchiids (Cort and Olivier, 1943a; Cort and Ameel, 1944), and the schistostomes (Cort and Olivier, 1943b; Cort, Ameel, and Olivier, 1944). Multiplication of individuals in the mother and daughter sporocysts of these groups has been found to be both by direct division and separation of the germinal cells (primary polyembryony of the original zygote), and by a secondary polyembryony of germinal masses.2 These germinal masses are peculiar structures, composed both of unicellular and multicellular components. In the strigeid and plagiorchiid sporocysts the germinal masses are not attached to the body wall but float freely in the body cavities mixed with the developing embryos. From them break off the largest multicellular components to continue development into daughter sporocysts or cercariae. Since the germinal masses are constantly giving off embryos throughout the life of the sporocysts, it is evident that some of their unicellular components (germinal cells) must divide to form new germinal cells over a long period of time. These germinal masses in the strigeids and plagiorchiids can be interpreted, therefore, as floating centers of persistent multiplication of germinal cells, which bring about the production of the extraordinarily large numbers of individuals produced in the germinal sacs of these groups. 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 for such a long time. Since all the work reviewed above on the development in the germinal sacs of the digenetic trematodes was done on groups with daughter sporocysts in their life cycles, we decided in the summer of 1947 to devote most of our attention to the development of the germinal material in rediae. Descriptions and figures of rediae in the literature give a very confusing picture of the germinal material. In most cases, embryos in various stages of development are shown inside the rediae with the smallest ones usually near the posterior end of the body cavity. Also, in some cases, either small masses of germinal material or separate germinal cells are shown just at the posterior tip of the body cavity, sometimes attached to the wall. Brooks (1930) described and figured "germ-masses" in rediae belonging to several different trematode groups. His studies were all made on sections of fixed material, mostly of mature rediae. It is a little difficult to understand sQme of his figures and descriptions, and to follow his interpretations. On the other hand, in