We have investigated the dynamics of polymers in bimodal polyethylene (PE) melts in the transition region from Rouse- to reptationlike behavior by varying the mass fraction Phi(t) of long tracer chains (N approximate to 3N(e) or 4N(e)) in a short-chain matrix (N approximate to N-e=entanglement segment number) over the full concentration range. At short times (ns) the dynamic structure factor for single-chain relaxation was investigated by neutron-spin-echo (NSE) spectroscopy. To obtain information about the long-time (ms) dynamics the tracer diffusion coefficient (D-NMR) was measured by pulsed-field-gradient (PFG)-NMR. We discuss our NSE data within a mode analysis which includes the relaxation rates W-p of the independent normal modes of the internal chain dynamics and the center-of-mass diffusion coefficient D-NSE as model parameters. Only modes exceeding the Phi(t)-dependent length of a single entanglement strand N-e(Phi(t)) are found to be strongly hindered by topological constraints. The D-NSE are Phi(t)-independent and systematically faster than the strong concentration-dependent D-NMR, suggesting an effective time-dependent diffusion coefficient. The Hess model, which we have generalized for polydisperse melts, provides a time-dependent diffusion coefficient. Taking chain-end effects into account we get an excellent description of the NSE data. The mobility of the chain ends is much higher than the mobility of the inner segments resulting in an entanglement segment number which increases with decreasing tracer concentration. The concentration dependence of N-e(Phi(t)), as obtained from the mode analysis and the Hess model, is in agreement with our calculation within a self-consistent modification of the model by Kavassalis and Noolandi for entanglement formation.