Time- and temperature-dependent SAXS studies carried out on s-polypropylene, various s-poly(propene-co-octene)s, two poly(ethyleneco-octene)s and poly(-caprolactone) indicate that the transition from the entangled melt to the partially crystalline state occurs generally in two steps. At first, an initial form of lower order builds up which then becomes stabilized to end in the final state with lamellar morphology. AFM observations suggest that the initial structure is composed of crystal blocks in planar assemblies, which then fuse into a homogeneous lamella. The edge length of the blocks in chain direction determines also the lamellar thickness. The size of the blocks corresponds to the minimum necessary to be stable. The crystallinities after isothermal crystallization processes remain invariant over larger temperature ranges, thus demonstrating that the potential of the entangled melt to form crystals is a well-defined property.