[en] Temporal development of the avalanche process over the pulsar polar cap due to the electron-positron pair production is investigated. The plasma produced from a single seed charge forms a series of thin but high-density ribbons which move toward the magnetic pole. Each ribbon is spatially and temporally modulated, with characteristic scales which correspond to those of radio waves. These modulations are intimately related with the observed radio emission. The small transverse scales of the ribbon may be responsible for the observed microstructure of pulsar signals. In addition, sheets of charges which can emit optical radiation are produced, but for various reasons they are not likely to be related to the observed optical radiation. Several mechanisms which control the plasma density within the ribbon are discussed. Neither the turbulent resistivity nor the charge redistribution within the ribbon is likely to control the density. It is suggested that the accumulation of charges over the neutron star surface and/or over the acceleration zone are responsible in terminating the avalanche process