Accurate jet diameters have been experimentally obtained during electrospinning by the light-scattering technique together with the Mie theory for cylinder scattering. Afterward, the fluid velocity and extension rate in the jet from the apex of the flow-modified Taylor cone to the straight jet end prior to the jet whipping are feasibly derived. It is found that the extension rate <(epsilon)over dot> is position (or time) dependent; its magnitude is the highest at the apex (region I with <(epsilon)over dot>(I)) and rapidly decreases to a relatively constant value in the main jet (region II with <(epsilon)over dot>(II)) before reaching the jet end (region III), where the extension rate is zero. It is of importance to notice that <(epsilon)over dot>(I) could be as high as 4000 s(-1), which is much higher than the chain retraction rate tau(-1)(e) obtained from the rheological measurement. This experimentally measured <(epsilon)over dot>(I) is consistent with that derived theoretically based on a simple energy conservation between the electric work and the drag flow energy, i.e., <(epsilon)over dot>(I) = (kappa/eta(e))(0.5)Phi E-0.5(s), where kappa is the solution conductivity, eta(e) is the elongational viscosity, E-s is the electric field at the cone apex, and a parameter Phi to characterize the viscoelasticity of the flowing jet. Our analyses of the extension rate in the straight jet reveal the general trend of e.I > <(epsilon)over dot>(II) > tau(-1)(e) > tau(-1)(d), suggesting likely a sequential structure evolution leading eventually to the strings within the jet due to, first, the flow-induced large concentration fluctuations in single phase solution (at tau(-1)(d) < <(epsilon)over dot> < <(epsilon)over dot>(inst.) < tau(-1)(e)) and, subsequently, phase separation (at <(epsilon)over dot>(inst.) < <(epsilon)over dot> < tau(-1)(e)) eventually leading to evolution of the strings with increasing <(epsilon)over dot> (at <(epsilon)over dot> > tau(-1)(e)), where tau(-1)(d) is the chain disentanglement rate, and <(epsilon)over dot>(inst). is the critical stretching reate for onset of the flow-induced thermodynamically instability. To validate this hypothesis, we have developed a simple collecting method to receive the fast-flowing jet in a nonsolvent reservoir in an attempt to quickly freeze its internal structures developed, if any, for the off-line optical and electron microscopy observations. Based on the results obtained from six polymer solutions studied, the formation of dissipative structures of "strings" with various widths due to growth or ordering of phase-separated structures developed on a mesoscopic scale seems to be a general phenomenon. In situ observation of the straight jet by a high-speed camera is also performed to reveal the rapid flowing of dissipative structures of "bulges" developed by macroscopic phase separation in the jet. We conclude that the extremely high extension rate at the cone apex plays a dominant role in the subsequent structure evolution, generally producing the flow-induced phase-separated structures.