The appearance of long range density fluctuations in two isotropic semi-rigid polymers (PEN and PEEK), during the induction period prior to crystallization, when heating above the glass transition temperature, T-g, has been demonstrated by simultaneous wide- and small-angle X-ray scattering techniques. The parallel use of dielectric spectroscopy, to characterize the segmental dynamics of the amorphous polymers polymer above T-g, and of X-ray scattering, to estimate the induction period for cold crystallization, reveals that segmental mobility of the supercooled melt is a key factor in controlling the development of the pre-crystalline nanostructures.