  We complete the proof of the fact that the moduli space of rank two bundles
with trivial determinant embeds into the linear system of divisors on
$Pic^{g-1}C$ which are linearly equivalent to $2\Theta$. The embedded tangent
space at a semi-stable non-stable bundle $\xi\oplus\xi^{-1}$, where $\xi$ is a
degree zero line bundle, is shown to consist of those divisors in $|2\Theta|$
which contain $Sing(\Theta_{\xi})$ where $\Theta_{\xi}$ is the translate of
$\Theta$ by $\xi$. We also obtain geometrical results on the structure of this
tangent space.
