Early development of the kelp fly,Coelopa frigida (Diptera) ultrastructural changes within the polar granules during pole cell formation.

Changes in the fine structure of polar granules and distribution of these granules in the egg and pole cells during the earliest stages of embryonic development have been observed in the eggs ofCoelopa frigida. 1. Polar granules are composed of dense granules 25-35 Å in diameter. These dense granules associate in different patterns during the cleavage stages of theCoelopa egg. 2. From early cleavage until shortly before blastema formation (preblastoderm stage) the dense granules form globular subunits, 200-250 Å in diameter, within the polar granules. These subunits become more distinct in later cleavage stages. 3. With the formation of the blastema the globular subunits are transformed into electron dense rods of varying length and approximately the same diameter as the globular subunits. Groups of parallel rods are arranged in different directions within each polar granule. 4. When pole cells have been formed, all the polar granules within each cell associate to form a complex, shaped like a concave disc, on the distal side of the nucleus or on the side pointing toward neighboring pole cells. Those components contributed by individual polar granules can still be identified within the disc. 5. The possibility of a correlation between blastema formation (i.e., the arrival of nuclei in the pole plasm) and the structural change within polar granules as an indication of nucleocytoplasmic interaction is discussed.