Fine structure and functional role of the coverings of the eggs inMesostoma ehrenbergii (Focke) (Turbellaria,Neorhabdocoela)

Summary European populations of the turbellarianMesostoma ehrenbergii can produce two types of eggs according to a seasonal cycle. Subitaneous eggs (S eggs) are produced throughout the warm season and develop rapidly inside the parental uteri. They are meant to ensure a fast increase in the number of worms. Dormant eggs (D eggs) are produced during the warm season and are usually deposited in water at the death of the parent. They can hatch at the beginning of the next warm season and are, therefore, meant to ensure the survival of the population throughout the cold season, when adult worms are no longer present.S eggs are bounded by a thin, translucent covering which has a loose structure and is mainly proteinaceous in composition. These characteristics account for the functional role played by the S egg covering during embryonic development. The covering has, in fact, to be permeable to parental nutrients, and to keep pace, by stretching, with the enlargement of the embryo. By contrast, D eggs are bounded by a thick shell, presumably composed of a sclerotin, the precursors of which (polyphenols) are synthesized together with yolk in the vitelline cells. The shell has a smooth surface devoid of pores and has the function of isolating the developing D egg from the external environment.Supported by a grant from the Consiglio Nazionale della RicercheThe authors wish to thank Dr. P. Roach for his help in translating the paper