The Germ Cell Cycle in the Digenetic Trematodes

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Abstract
Most of the recent work on the germ cell cycle of digenetic trematodes supports the germinal lineage hypothesis, according to which reproduction in the germinal sacs (sporo-cysts and rediae) is a multiplication of the germinal line. The process starts with the fertilized ovum inside the egg and terminates with the production of mature ova and spermatozoa in the ovaries and testes of the adult. In this multiplication of cells of the germinal line both germinal and somatic cells are produced. Somatic cells first form the germinal sac and in most groups the secondary germinal sacs. Finally, cercariae are produced, which after entrance into the definitive host develop into adults. According to this hypothesis the multiplication of cells of the germinal line is a polyembryony by which large numbers of germinal cells are produced from the original fertilized ovum. Variations of the germ cell cycle are then reviewed in different trematodes in which it has been studied. The author considers that much work is necessary before the hypothesis of germinal lineage can be ocnsidered established.

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