  We analyze the Saffman-Taylor viscous fingering problem in rectangular
geometry. We investigate the onset of nonlinear effects and the basic
symmetries of the mode coupling equations, highlighting the link between
interface asymmetry and viscosity contrast. Symmetry breaking occurs through
enhanced growth of sub-harmonic perturbations. Our results explain the absence
of finger tip-splitting in the early flow stages, and saturation of growth
rates compared with the predictions of linear stability.
