In this work the motions of chain segments in either the central 50% of a polymer or in the two end sections (25% each) were probed. Motions of the central (or end) section were measured using bilayer welding samples of deuterium-labeled polymers. Symmetric triblock poly(styrene) (HDH) was paired with fully deuterated or fully protonated poly(styrene) of nearly equal molecular weight. Changes in the deuterium profile of a given pair were directly linked to the motions of a given section of the chain. The behavior of the deuterium depth profiles was monitored using specular neutron reflectivity (SNR). Rouse and reptation model predictions for the behavior of chain segments in the end and center sections were developed using computer simulations and minor chain reptation calculations. These dynamics models are representative of two broad classes of dynamics theories: tubeless (Rouse) and tubed (reptation). These experiments were thus designed to discriminate between tubeless and tubed dynamics. Segments in the central sections showed a distinct lag in crossing the interface, while segments in the end sections crossed the interface continuously. This chain centers' lag behavior is a distinctive signature of the reptation model and is not predicted by tubeless models. These experiments offer a simple and direct observation of the highly anisotropic tube motions of entangled polymer melt chains, providing strong support for the use of reptation to describe dynamics of melt polymers at the weld interface.