1. Department of Biology, University of Antwerp U.I.A., Universiteitsplein 1, B–2610 Wilrijk, Belgium (S.K., J.-P.V.); and
2. Department of Physics, Laboratory of Biomedical Physics, University of Antwerp RUCA, Groenenborgerlaan 171, B–2020 Antwerp, Belgium (W.F.D.)