  Within Wigner function formalism, the chiral anomaly arises naturally from
the Dirac sea contribution in un-normal-ordered Wigner function. For massless
fermions, the Dirac sea contribution behaves like a 4-dimensional or
3-dimensional Berry monopole in Euclidian momentum space, while for massive
fermions, although Dirac sea still leads to the chiral anomaly but there is no
Berry monopole at infrared momentum region. We discuss these points explicitly
in a simple and concrete example.
