Despite many successful applications of non-photographic dielectric track detectors in numerous scientific and technological fields, the basic mechanisms of track formation and development are still rather unknown for these detectors. In this paper an attempt is made to estimate the relative importance for track formation in plastics of direct interactions of heavy charged particles and of events of their secondary electrons. Both classes of events are weighted rather differently in existent track formation models. The discussion is mainly based on results obtained from calculations on the spatial distribution of events along and around the paths of fast ions and of electrons.