  Bose-Einstein correlations in pairs of identical charged pions produced in a
sample of 4.3 million Z0 hadronic decays are studied as a function of the three
components of the momentum difference, transverse ("out" and "side") and
longitudinal with respect to the thrust direction of the event. A significant
difference between the transverse, r_t_side, and longitudinal, r_l, dimensions
is observed, indicating that the emitting source of identical pions, as
observed in the Longitudinally CoMoving System, has an elongated shape. This is
observed with a variety of selection techniques. Specifically, the values of
the parameters obtained by fitting the extended Goldhaber parametrisation to
the correlation function C'= C^{DATA}}/C^{MC} for two-jet events, selected with
the Durham algorithm and resolution parameter ycut=0.04, are
r_t_out=(0.647+-0.011(stat})+0.022-0.124(syst)) fm,
r_t_side=(0.809+-0.009(stat)+0.019-0.032}(syst)) fm,
r_l=(0.989+-0.011(stat)+0.030-0.015(syst})) fm and
r_l/r_t_side=1.222+-0.027(stat})+0.075-0.012(syst). The results are discussed
in the context of a recent model of Bose-Einstein correlations based on string
fragmentation.
