Based on experimental findings, a mechanism of structural formation during the induction period of polymer crystallization is proposed. Recently we have found that, during the induction period of crystallization of poly(ethylene terephthalate) before crystal nuclei are produced, a correlation peak appears in small-angle x-ray scattering, and grows in intensity and position with time while the crystallization temperature is kept constant. The position of this peak is of course different from that of the intercrystallite correlation peak, the so-called long-period peak, in the crystallization stage; the former peak position is considerably lower than the latter. Surprisingly the time evolution of the former peak obeys the spinodal decomposition kinetics. In this study the cause for such a phenomenon is investigated from the viewpoint of the orientation fluctuations of rigid molecular segments using depolarized light scattering as well as small-angle neutron scattering (SANS). The depolarized light-scattering measurements indicate that during the induction period the parallel ordering of the rigid molecular segments occurs and this ordering process agrees with the spinodal decomposition kinetics in the isotropic-nematic transition theory proposed for stiff polymers by Doi et al. Further, the critical concentration of rigid segments at which the isotropic state becomes unstable is discussed using the molecular stiffness or the persistence length estimated from the molecular conformations observed by an isotope-labeling method for SANS.