  We suggest that the bulge-to-disc ratios of spiral galaxies are primarily
determined by the angular momenta of their host haloes predicted in current
hierarchical clustering models for structure formation. Gas with low specific
angular momentum becomes self-gravitating and presumably forms stars before it
can settle into a rotationally supported disc. We assume this part of the gas
in a dark halo to form a bulge, while the rest is assumed to settle into a
rotationally supported disc. With these assumptions the predicted bulge-to-disc
ratios in mass and in size, and other correlations between the bulge and disc
components can match current observational results. This model predicts the
existence of a population of low-surface-brightness galaxies which are
bulgeless. The model also predicts that the bulge component has many properties
in common with the disc component, because both form through similar processes.
In particular, many bulges should be supported (at least partially) by
rotation.
