  It is shown that the concept of a fireball with a definite filamentary
structure naturally emerges from the analysis of the spectra of Gamma-Ray
Bursts (GRBs). These results, made possible by the recently obtained analytic
expressions of the equitemporal surfaces in the GRB afterglow, depend crucially
on the single parameter R describing the effective area of the fireball
emitting the X- and gamma ray radiation. The X- and gamma ray components of the
afterglow radiation are shown to have a thermal spectrum in the co-moving frame
of the fireball and originate from a stable shock front described
self-consistently by the Rankine-Hugoniot equations. Precise predictions are
presented on a correlations between spectral changes and intensity variations
in the prompt radiation verifiable, e.g., by the Swift and future missions. The
highly variable optical and radio emission depends instead on the parameters of
the surrounding medium. The GRB 991216 is used as a prototype for this model.
