  Let $X\subset\Bbb C^n$ be an affine variety and $f:X\to\Bbb C^m$ be the
restriction to $X$ of a polynomial map $\Bbb C^n\to\Bbb C^m$. In this paper, we
construct an affine Whitney stratification of $X$. The set $K(f)$ of stratified
generalized critical values of $f$ can be also computed. We show that $K(f)$ is
a nowhere dense subset of $\Bbb C^m$, which contains the set $B(f)$ of
bifurcation values of $f$ by proving a version of the isotopy lemma for
non-proper polynomial maps on singular varieties.
