  We report our observational results of 870 $\mu$m continuum emission and its
linear polarization in the massive star formation site W51 e2/e8. Inferred from
the linear polarization maps, the magnetic field in the plane of sky
(B$_{\bot}$) is traced with an angular resolution of 0$\farcs$7 with the
Submillimeter Array (SMA). Whereas previous BIMA observations with an angular
resolution of 3$\arcsec$ (0.1 pc) showed a uniform B field, our revealed
B$_{\bot}$ morphology is hourglass-like in the collapsing core near the
Ultracompact H II region e2 and also possibly in e8. The decrease in
polarization near the continuum peak seen at lower angular resolution is
apparently due to the more complex structures at smaller scales. In e2, the
pinched direction of the hourglass-like B field morphology is parallel to the
plane of the ionized accretion flow traced by H53$\alpha$, suggesting that the
massive stars are formed via processes similar to the low mass stars, i.e.
accretion through a disk, except that the mass involved is much larger.
Furthermore, our finding that the resolved collapsing cores in e2 and e8 lie
within one subcritical 0.5 pc envelope supports the scenario of
\textit{magnetic fragmentation} via ambipolar diffusion. We therefore suggest
that magnetic fields control the dynamical evolution of the envelope and cores
in W51 e2 and e8.
