Crystallization from melts of polymer blends containing a noncrystallizable component (solute) results in morphologies that are "open", based upon bundles of lamellas separated from one another by noncrystalline regions. We model the formation of an open spherulite initially as growth of a compact sphere. The growing sphere rejects the noncrystallizable component (solute). To allow for better dissipation of the solute, the interface breaks down into protuberances, the result being the transition of a compact sphere to a more open one. The stabilizing effects are the kinetics of attachment and the capillarity effect at the interface. The balance between the breakdown and stabilization determines the critical radius of instability (R(r)) of a growing sphere. We have computed R(r) from a stability analysis which includes the Lauritzen and Hoffman interface kinetics for the crystallizing species. Results for a blend of isotactic polystyrene and atactic polystyrene indicate that there exists a sharp transition from relatively large stable spheres to unstable ones, over a few degree of undercooling. R(r)’s predicted for undercoolings of 25 K and higher are in the submicrometer range. The value of R(r) provides an upper bound for the bundle diameter. The predictions for the i-PS/a-PS blend are consistent with experimentally observed results : the bundle diameter is in the submicrometer range for large undercoolings and changes little with undercooling, the diameter being in the range of 0.06-0.02 mu m (similar to 2-6 lamellas/bundle) for an undercooling of 35-55 K.