Electron microscopy of dividing fission yeast cells shows establishment of an annular rudiment (AR) of electron-transparent material under the old cell wall as the first sign of elaboration of the cell plate. The AR grows centripetally, finally closing at the mid-point of the cell. During the inward growth of the AR it is thickened by addition of denser material which becomes the scar plug after fission; the electron-transparent material is lost at fission. Lying always between the cytoplasmic membrane and the cell wall is a dark layer of variable thickness. This layer becomes markedly thickened into a fillet at the base of the centripetally growing cell plate. The fission process begins after the cell plate is completely elaborated. One striking feature of fission is the migration of dense material from the fillet at the base of the cell plate outwardly through the matrix of the cell wall to its final resting place as a dark ring, a "fuscannel," adjacent to the fission scar. The inclusion of Golgi bodies in many sections suggests their involvement in cell plate elaboration, presumably through production of the dense bodies which are seen to fuse with the dark layer proximal to the growing cell plate.