It is a fundamental concept in developmental biology that the fate of embryonic cells is regulated by morphogenetic determinants localized in the ooplasm. In Drosophila, heterotopic transplantation experiments have conclusively demonstrated that cytoplasmic factors localized to the posterior pole plasm of the oocyte and embryo are requisite for the formation of pole cells, the primordial germ cells. However, the molecular nature of these cytoplasmic factors, the mechanism of localization within the ooplasm, and their mode of action in development are unknown. The genetic and developmental analysis of maternal effect mutants that affect pole cell formation in Drosophila melanogaster are intended to allow one to elucidate the mechanism of determination and how the determined state is maintained throughout development. A detailed genetic and developmental analysis of one such grandchildless mutant, tudor, (tud) has been undertaken. The properties of mutations of the recessive maternal effect gene tud indicate that the gene product of the tudor locus is required for the proper determination of germ cells in Drosophila melanogaster. Specifically, the germ plasm of six different alleles of tud has been analyzed at the ultrastructural level, and it is found that different alleles contain different amounts of assembled polar granule material (a cytoplasmic organelle classically thought to be the germ cell determinants). The ability or inability to form germ cells correlates directly to the amount of assembled polar granule material observed in the germ plasm. For example, one allele produces polar granules approximately 1/3 the size of wild type polar granules and this allele produces fertile progeny. On the other hand, alleles that produce no apparent assembled polar granule material in the germ plasm produce no fertile progeny. Therefore, mutations at the tudor locus, which disrupt the normal assembly of the germ plasm, result in the failure to localize the germ plasm determinants to the posterior pole.