  We investigate cosmological structure formation seeded by topological defects
which may form during a phase transition in the early universe. First we derive
a partially new, local and gauge invariant system of perturbation equations to
treat microwave background and dark matter fluctuations induced by topological
defects or any other type of seeds. We then show that this system is well
suited for numerical analysis of structure formation by applying it to seeds
induced by fluctuations of a global scalar field. Our numerical results are
complementary to previous investigations since we use substantially different
methods. The resulting microwave background fluctuations are compatible with
older simulations. We also obtain a scale invariant spectrum of fluctuations
with about the same amplitude. However, our dark matter results yield a smaller
bias parameter compatible with $b\sim 2$ on a scale of $20 Mpc$ in contrast to
previous work which yielded to large bias factors. Our conclusions are thus
more positive. According to the aspects analyzed in this work, global
topological defect induced fluctuations yield viable scenarios of structure
formation and do better than standard CDM on large scales.
