A dynamical approach to the neutron-induced fission of 255,257 Fm isotopes based on the time-dependent generator-coordinate method is applied. As the generating functions the eigensolutions of the constrained Hartree-Fock-Bogoliubov method with the effective D1S Gogny force are used. The here presented collective-dynamics calculations in the two-dimensional collective space described by quadrupole and octupole moments allow to determine the fragment mass distributions of those two Fm isotopes.