  Deep inelastic polarized and unpolarized structure functions for a free
nucleon are obtained in a modified Center-of-Mass bag model, which includes the
symmetry breaking effects from spin-dependent interactions. The quark
distribution functions, calculated at $Q_0^2\simeq$ (0.9GeV/c)$^2$, are evolved
to higher $Q^2$ region and compared with the data and other models. The model
gives a reasonable description for the valence part of the structure functions
at $x>0.3$. For small-$x$ region, the contributions from the sea are necessary.
The spin-dependent effects are important in describing the existing data.
