The formation of the saccus begins while the pollen grains are still within the tetrad, surrounded by the callose wall. Material giving a clear periodic acid-Schiff reaction accumulates between the early sexine and the plasma membrane at the sites of the future sacci. This material is subsequently isolated from the protoplast by the nexine. After dissolution of the callose wall and separation of the grains, the PAS-positive material expands to many times its initial volume, widely separating the nexine and sexine, and so forming the saccus. The saccus thus lies within the exine. Electron micrographs indicate that the PAS-positive material involved in saccus formation comes from golgi vesicles. The reason for the expansion of this material after the separation of the grains remains unknown. Expansion of the sacci is accompanied by increase in thickness of the sexine elsewhere. It is therefore concluded, since some PAS-positive material is present throughout the primexine in addition to the concentrations at the sites of the sacci, that these should be interpreted as local modifications of normal exine structure, and not as entirely new features of the wall.