The Germ Cell Cycle in the Digenetic Trematodes

TREMATODES IN THE life cycle of the trematodes the vertebrate-dwelling adults have ovaries and testes in which are produced ova and spermatozoa. Spermatogenesis, oogenesis and fertilization are not fundamentally different from these processes in other metazoan groups (for review of the literature see Cable, 1931; Chen, 1937; Rees, 1939). The fertilized ovum is enclosed in a shell, and is surrounded by yolk cells which provide food for the developing embryo. In the digenetic trematodes, which have an alternation of generations, this embryo becomes the miracidium, which after entering the molluscan intermediate host metamorphoses into a primary germinal sac, the mother sporocyst. Inside the mother sporocyst develop secondary germinal sacs, the rediae or daughter sporocysts. The presence of a digestive system distinguishes rediae from daughter sporocysts and these stages have never been found in the same life cycle. There may be either one or two generations of rediae, but when the secondary germinal sacs are daughter sporocysts only one generation has been found. Finally, in the secondary germinal sacs cercariae are produced, which after reaching the definitive host develop into the adults. The cercariae usually escape from the intermediate host and have a brief period of free life. Their method of reaching the definitive host varies greatly in different groups. They may encyst in the open (the sheep liver fluke), penetrate directly into the definitive host (the schistosomes), or encyst in a second intermediate host (the human liver or lung flukes). With such complicated life cycles frequently involving three hosts, the hazards are very great, and there is an enormous loss of individuals. Therefore, digenetic trematodes usually have an extraordinary reproductive activity both in the production of enormous numbers of eggs by the adults and of large numbers of cercariae by the germinal sacs in the intermediate host. There is some evidence that in a few cases cercariae may develop directly in the primary germinal sacs. Therefore, in the different groups of the digenetic trematodes the life cycle may be an alternation of two, three or four generations. The first generation always begins with the fertilized ovum and includes the miracidium and the primary germinal sac, the mother sporocyst; and the last generation always includes the cercariae and adults.