  A geometric construction of Sullivan's Stiefel-Whitney homology classes of a
real analytic variety $X$ is given by means of the conormal cycle of an
embedding of $X$ in a smooth variety. We prove that the Stiefel-Whitney classes
define additive natural transformations from certain constructible functions to
homology. We also show that, for a complex analytic variety, these classes are
the mod 2 reductions of the Chern-MacPherson classes.
