The density of grown-in voids produced in silicon is controlled by a diffusion-limited flux of vacancies to the voids. This flux was computed for both single- and double-octahedral voids. It was found that if a void is initially formed as a double octahedron it would evolve into stable shape which is different from that which is observed experimentally. It Is concluded that voids are initially of single-octahedral shape, but can become double-octahedral at a later stage of evolution. This morphological change is most likely caused by an oxygen monolayer at the void facets. Initially this monolayer covers only a fraction of the void surface but gradually spreads over the whole surface. The remaining spot of non-covered surface is the source of growth steps produced by 2D nucleation. When the non-covered spot becomes small, the steps may pile-up at the boundary between the non-covered and covered parts of a facet, thus giving rise to a new void. The criterion for the single-to-double transition includes oxygen concentration, cooling rate and vacancy concentration.