  We consider models for gamma-ray bursts in which a collimated jet expands
either into a homogeneous medium or into a stellar wind environment, and
calculate the expected afterglow temporal behavior. We show that (i) following
a break and a faster decay, afterglows should exhibit a flattening, which may
be detectable in both the radio and optical bands; (ii) Only observations at
times much shorter than a day can clearly distinguish between a fireball
interacting with a homogeneous medium and one interacting with a stellar wind.
  Using our results we demonstrate that constraints can be placed on progenitor
models. In particular, existing data imply that while some long duration bursts
may be produced by collapses of massive stars, it is almost certain that not
all long duration bursts are produced by such progenitors.
