The ultrastructure of the development of the pollen grain is described from the first meiotic division to the binucleate phase. The first meiotic division is synchronous in all the microspore mother cells and cytomictic channels still join their protoplasts together. The tapetal cells, their individual walls now degraded, almost completely surround but do not touch the microspore mother cells. In the latter the cytoplasm can be divided into two zones. The central zone, the division zone, is occupied primarily by ribosomes, smooth ER and dictyosomes. In the outer zone lie all the other usual organelles and small vacuoles. The plastids are partly surrounded by ER. In interphase the cytomictic channels close and the second division is not synchronous. Autophagic vacuoles form, apparently destroying some of the ribosomes. The callose walls form after meiosis and the tapetum degenerates. The primexine, which appears to have a polysaccharide skeleton, begins to form between the plasmalemma and the callose wall. The callose wall then dissolves, the pores begin to form, sporopollenin begin to accumulate and the exine is completed. The spores then pass through a stage of vacuolation and of amylogenesis and loss of starch. At mitosis the new wall separating the vegetative and generative cells is formed, and it is not perforated by plasmodesmata. The pore structure is now completed and autophagic vacuoles destroy further regions both of the vegetative cell's cytoplasm and to a lesser extent that of the generative cell. New cytoplasm is successively formed, together with another wave of amylogenesis and loss of starch; these two processes occur simultaneously with the dehydration of the grain as the pollen structure matures.