Structure evolution of highly oriented polyethylene during cautious melting and crystallization is investigated with both high time resolution and high signal-to-noise ratio by means of small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). The two-dimensional SAXS patterns are transformed to the multidimensional chord distribution function (CDF) in physical space. The results are continuous and smooth movies of the nanostructure, which elucidate the mechanisms of the evolution of semicrystalline structure. We find that in our material crystallization is preceeded by a rather diffuse mesomorphic nanostructure. Based on its variation in relation to other observed features like row nuclei and crystalline lamellae, we propose to associate it to phase separated regions of entangled and disentangled chain segments, respectively. The movies show that the mesophase structure holds the key for the understanding of crystallite orientation and arrangement in the fibre. (c) 2005 Published by Elsevier Ltd.