In many insect eggs, including those of the Diptera, deeply staining granules, rich in RNA, occur in the posterior polar plasm and during ontogeny become enclosed within the pole cells. The structure and fate of these cells, which generally give rise to the primordial germ cells, and their inclusions have excited interest for over half a century (Hegner, 1908; Huettner, 1923; Rabinowitz, 1941; Poulson, 1947; Counce, 1963; Mahowald, 1962), yet numerous questions concerning them remain unsettled or controversial to this day. For instance, the dual fate of the pole cells in Drosophila, the genus which has been most extensively studied, is still debated (Poulson & Waterhouse, 1960; Hathaway & Selman, 1961).
Recently, Counce (1963), in a light-microscope study, has described the developmental morphology of the polar granules in several species of Drosophila embryos; while Mahowald (1962) has succeeded in identifying them in D. melanogaster at the ultra-structural level.