The main result of this paper is Theorem 4 on the existence of singularities of Lacunary power series on prescribed open boundary arcs of the circle of convergence. The influence of the gaps on the length of these arcs is expressed in terms of newly introduced integral densities. Theorem 4 contains the known Fabry-Po´lya theorem on gaps, describing the closed arcs having singularities, and suggests its extensions, using additional information on the coefficients of the power series. An essential step in its proof is Theorem 2, providing necessary and sufficient conditions (in terms of the so called ''Coefficient Functions'') for the analytic continuation of power series across fixed open boundary arcs.