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In the vertebrate-dwelling adults of the digenetic trematodes spermatogenesis and oogenesis are not fundamentally different from these processes in other metazoan groups. The fertilized ovum in the egg develops directly into the miracidium which contains in its body cavity a varying number of germinal cells. After entering the molluscan intermediate host the miracidium metamorphoses into the primary germinal sac, the mother sporocyst. In its body cavity the germinal cells multiply and develop into secondary germinal sacs which may be either rediae or daughter sporocysts. There may be one, two, or in a few cases even more generations of rediae, but only one generation of daughter sporocysts is present in those groups that have this type of secondary germinal sacs. In each generation there is multiplication of germinal cells in the body cavity of the germinal sacs. However, the number of embryos produced varies greatly in different trematode groups. Ever since Steenstrup in 1842 presented the theory of alternation of generations for the life cycle of the digenetic trematodes, the method of reproduction in the germinal sacs has been a subject of controversy. Steenstrup's view that this reproduction is an asexual process of internal budding was soon discarded by most workers because of the finding of specific reproductive cells. Grobben in 1882 first proposed the hypothesis that the alternation of generations in the digenetic trematodes is a heterogeny, and that the reproductive cells in the germinal sacs are true parthenogenetic ova. Later, several workers described and figured the for-1 A joint contribution from the

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