Polar granules of Drosophila show distinctive variations in structure at various stages of early embryogenesis and in different species. In most species they are attached to mitochondria at fertilization, then they become free and fragment during the cleavage divisions and during the time of pole cell formation. After pole cells have formed they again coalesce. At the time of migration of the pole cells to the embryonic gonad, the polar granules again fragment and become attached to the outer membrane of the nuclear envelope.

Ribosomes are attached to the periphery of polar granules at the time they are fragmented prior to and during pole cell formation. An hypothesis is presented on the basis of published reports of RNA present in the granules and the ultrastructural observations reported here. This hypothesis suggests that the RNA in polar granules is m-RNA that directs the synthesis of those proteins necessary for the determination of the pole cells to become germ cells.