I will present an hydrodynamic model for the motility of keratocytes or fibroblasts on substrates in vitro. Cells or fragment of cells have been observed to switch from a stationary round state to a motile and anisotropic crescent-shaped state. Experimentally, a polarization of the actin network occurs in a preferred direction prior to motility and determins the direction of motion. In this talk, I will present first the model for actin flow of Callan-Jones et al for two-dimensionnal cells lying on a substrate with a strong friction. Using Schwarz function techniques, we derive a dynamic equation for the shape contour including the polymerisation-depolymerisation process and show that static circular shapes are stable for enough tension of the lipidic membrane. We extend the model to incorporate the actin cortex whose anisotropy is due to a preferred orientation at the lipidic membrane. To do so, we use the theory of active polar gels of Kruse et al. inspired from the theory of liquid crystals. Since this cortex has a size of order one ten of the cell, we perform a boundary layer analysis. The presence of the cortex is responsible for a modification of the boundary conditions at the cell border. We show that an increase of the motor activity destabilisizes the cell in the tensile case but we also show that a polarization of the whole actin network is necessary to induce a translation motion.