The growth dynamics of polycrystalline Ge clusters during annealing of Ge-Ag co-sputtered films was studied by employing in-situ transmission electron microscopy and Auger electron spectroscopy. During crystallization of a-Ge, polycrystalline Ge clusters of up to 2 mum in diameter, surrounded by the Ge-depleted zones of about 100 nm width, grew laterally on the film plane as a result of surface segregation of Ge. Environmental concentration around the Ge clusters and the morphological evolution suggest that the rate-controlling process of the growth of the Ge clusters is surface and/or grain boundary diffusion of Ge atoms across the Ge-depleted zone.