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This paper describes evidence for the formation of secondary fissure structures, in low-density polyethylene (PE), and in cross-linked polyethylene (XLPE). These structures can be observed by optical and by scanning electron microscope techniques. The secondary fissures are not a volume effect like spherulites, but are associated with the fracture surface only. An explanation of this effect is given, as well as its relation to the observed electrical discharges in tree channels.