In terms of percolation theory, the glass structure can be described as an arbitrarily formed network with rigid and floppy regions. It is already known, that the maximum size of a floppy region can be calculated from the concentration of network formers and network modifiers. By contrast to previous studies, the crystallization mechanism in subsequently annealed glasses is described. It was assumed that nucleation in highly viscous liquids, i.e. slightly above the glass transition temperature, takes place if the size of a floppy region is at least as large as the size of the critical nucleus. Once nucleation has occurred, the crystals start to grow. Considering a non-isochemical system, certain components are enriched near the growing crystal during crystallization. It is stated, that the liquid near the crystal will first have a floppy structure enabling instantaneous crystal growth to some extent. During crystal growth, the network connectivity will increase until rigidity is reached in the shell and the residual components are shoved away by the growing crystal.