Conserved in many organisms, the germ plasm is a specialized cytoplasm that contains the mRNAs and proteins required for specification of the germline fate. In many animals, including Drosophila, the germ plasm is localized to a defined spatial domain within oocytes and early embryos, where it directs the formation of germ cells. Once specified, germ cells often must migrate through the embryonic anlage to populate the gonad, where they mature into the eggs and sperm required for sexual reproduction. Therefore, germ cells ensure the passage of genetic material from one generation to the next and are critical for species survival.

Precise regulation of germ plasm localization and its activity is important for germ cell development. In this thesis, we explore the composition of the Drosophila germ plasm and present in depth analysis into how germ cells inherit the germline determinants. We have taken two approaches to uncover novel factors that contribute to germ plasm activity. One is an unbiased, biochemical purification scheme to identify proteins that interact with Oskar, whose localization is both necessary and sufficient to nucleate germ plasm assembly in Drosophila. Secondly, we took a candidate-based approach and surveyed the role of several actin- and microtubule-binding proteins in germ cell development. Among these, we identified the villin-like protein Quail as an important regulator of germ cell formation and migration. Finally, through live imaging analysis, we followed the passage of the germ plasm en route to the germ cells and explored the mechanism of how the germline determinants become incorporated into the very cells they specify.