  Starting from a Zimm model we study selfdiffusion in a solution of
crosslinked monomers. We focus on the effects of the hydrodynamic interaction
on the dynamics and the critical behaviour at the sol-gel-point. Hydrodynamic
interactions cause the clusters' diffusion constant to depend not only on the
cluster's size but also on the cluster's shape -- in contrast to the Rouse
model. This gives rise to a nontrivial scaling of the Kirkwood diffusion
constant averaged over all clusters of fixed size $n$, ${\hat D}_n\sim
n^{-{\hat b}}$ with ${\hat b}=1/d_s-1/2$ given in terms of the spectral
dimension $d_s$ of critical percolation clusters. The long-time decay of the
incoherent scattering function is determined by the diffusive motion of the
largest clusters. This implies the critical vanishing $D_{\rm eff}\sim
\epsilon^a$ of the cluster-averaged effective diffusion constant at the gel
point with exponent $a = (3/2 -\tau +1/d_{s})/\sigma$.
