  A hallmark of Weyl semimetal is the existence of surface Fermi arcs
connecting two surface-projected Weyl nodes with opposite chiralities. An
intriguing question is what determines the connectivity of surface Fermi arcs,
when multiple pairs of Weyl nodes are present. To answer this question, we
first show that the locations of surface Fermi arcs are predominantly
determined by the condition that the Zak phase integrated along the normal
direction to the surface is $\pi$. More importantly, the Zak phase can reveal
the peculiar topological structure of Weyl semimetal directly in the bulk.
Here, we show that the non-trivial winding of the Zak phase around each
projected Weyl node manifests itself as a topological defect of the
Wannier-Stark ladder, the energy eigenstates emerging under an electric field.
Remarkably, this structure leads to "bulk Fermi arcs," i.e., open line segments
in the bulk momentum spectra. It is argued that bulk Fermi arcs should exist in
conjunction with the surface counterparts to conserve the Weyl fermion number
under an electric field, which is supported by explicit numerical evidence.
