Electrospun polymer nanofibers demonstrate outstanding mechanical and thermodynamic properties as compared to macroscopic-scale structures. Our previous work has demonstrated that these features are attributed to nanofiber microstructure [Nat. Nanotechnol. 2, 59 (2007)]. It is clear that this microstructure is formed during the electrospinning process, characterized by a high stretching rate and rapid evaporation. Thus, when studying microstructure formation, its fast evolution must be taken into account. This study focuses on the dynamics of a highly entangled semidilute polymer solution under extreme longitudinal acceleration. The theoretical modeling predicts substantial longitudinal stretching and transversal contraction of the polymer network caused by the jet hydrodynamic forces, transforming the network to an almost fully stretched state. This prediction was verified by x-ray phase-contrast imaging of electrospinning jets of poly(ethylene oxide) and poly(methyl methacrylate) semidilute solutions, which revealed a noticeable increase in polymer concentration at the jet center, within less than 1 mm from the jet start. Thus, the proposed mechanism is applicable to the initial stage of the microstructure formation.