  The formation of normal-state domains in type-I superconducting indium films
is investigated using the high resolution magneto-optical imaging technique.
The observed patterns consist of coexisting circular and lamellar normal-phase
domains surrounded by the superconducting phase. The distribution of domain
surface areas is found to exhibit a threshold, above which only the lamellar
shape is observed. We show that this threshold coincides with the predicted
critical surface area for the elongation instability of the circular shape. The
partition of the normal phase into circular and lamellar domains is determined
by the combined effects of the elongation instability and the penetration of
magnetic flux by bursts at the early stage of pattern formation. It is not
governed by mutual interactions between domains, as usually assumed for
self-organized systems.
