It is suggested that the nucleation of bubbles may proceed through an intermediate stage of formation and growth of hollow-half-bubbles. The hollow-half-bubble can be formed by a simple, continuous deformation of the magnetization in the film. The inner wall repels the outer wall and facilitates its expansion. The inner wall is not stable, but the contractive driving force is relatively small. Hence, particularly in the presence of coercivity, the inner wall may survive sufficiently long to stabilize the outer wall. A transparent graphical analysis of the stability conditions of each wall is given.