The long-standing issue, central to polymer science, of the relevance of cellulation to the formation of spherulites has been resolved by the first demonstration of this phenomenon, in spherulites of a highly branched polyethylene copolymer. The two principal alternative mechanisms proposed for the formation of spherulites are shown to be complementary, with cellulation a secondary process occurring only when segregation reduces growth rate sufficiently. A unified context for polymeric crystallization is thereby achieved in which segregation generally plays a passive rather than an active role in the development of spherulites.