Bose-Einstein correlations in pairs of identical charged pions produced in
a sample of 4.3 million Z(0) hadronic decays are studied as a function of t
he three components of the momentum difference, transverse ("out" and "side
") and longitudinal with respect to the thrust direction of the event. A si
gnificant difference between the transverse. r(tside), and longitudinal, r(
iota), dimensions is observed, indicating that the emitting source of ident
ical pions, as observed in the Longitudinally CoMoving System, hs an elonga
ted shape. This is observed with a variety of selection techniques. Specifi
cally, the values of che parameters obtained by fitting the extended Goldha
ber parametrisation to the correlation function C' = C-DATA/C-MC for two-je
t events, selected with the Durham algorithm and resolution parameter y(cut
) = 0.04, are r(tside) = (0.809 +/- 0.000 (stat)(-0.032)(+0.019) (syst)) fm
, r(iota) = (0.989 +/- 0.011 (stat)(-0.015)(+0.030) (syst)) fm and r(iota)/
r(tside) = 1.222 +/- 0.027 (stat)(-0.012)(+0.075) (syst). The results are d
iscussed in the context of a recent model of Bose-Einstein correlations bas
ed on string fragmentation. The results of a unidimensional analysis are al
so presented.