POSTEMBRYONIC SEGREGATION OF THE GERM-LINE IN SEA-URCHINS IN RELATIONTO INDIRECT DEVELOPMENT

The four small micromeres of the sea urchin embryo contribute only to the coelomic sacs, which produce major components of the adult body pl an during postembryonic development. To test the proposition that the small micromeres are the definitive primordial germ cell lineage of th e sea urchin, we deleted their 4th cleavage parents, and raised the de leted embryos through larval life and metamorphosis to sexual maturity , Almost all of the experimental animals produced functional gametes, excluding the possibility that the germ cell lineage arises exclusivel y and obligatorily from descendants of the small micromeres; rather, t he germ cell lineage arises during the postembryonic development of th e rudiment, A survey of the literature indicates that there is no know n case of an embryonic primordial germ cell lineage in a bilaterian sp ecies that displays maximal indirect development.