solutions are also discussed.]{}

The problem of understanding the quark and lepton masses and mixing angles represents one of the major unsolved questions of the standard model. Recently additional information on the fermion mass spectrum has come from the measurement of the atmospheric neutrino masses and mixing angles by Super-Kamiokande [@SKamiokandeColl]. The most recent data disfavours mixing involving a sterile neutrino, and finds a good fit for $\nu_{\mu} \rightarrow \nu_{\tau}$ mixing with $\sin^22\theta_{23}>0.88$ and a mass square splitting $\Delta m^2_{23}$ in the $1.5-5\times 10^{-3} {\rm\ eV}^2$ range at 90% CL [@HSobel]. Super-Kamiokande has also provided additional support for solar neutrino mixing. The most recent Super-Kamiokande data does not show a significant day-night asymmetry and shows an energy independent neutrino spectrum, thus it also disfavours the sterile neutrino mixing hypothesis, the just-so vacuum oscillation hypothesis, and the small mixing angle (SMA) MSW [@MSWMechanism] solution [@YSuzuki]. The preferred solution at the present time seems to be the large mixing angle (LMA) MSW solution, although a similar solution with a low mass splitting (the LOW) solution is also possible. A typical point in the LMA MSW region is $\sin^22\theta_{12}\approx 0.75$, and $\Delta m^2_{12}\approx 2.5\times
10^{-5} {\rm\ eV}^2$ [@BaKrSm].

If one accepts the recent 