This study was undertaken to examine the relation between the sizes of latent image centers and the rate of development. Information on precise development progress was given by measuring the time-dependent optical densities of layers of primitive, sulfur-sensitized and sulfur-plus-gold-sensitized emulsions at 1090 nm. Information on the size of latent image centers was obtained by applying the simulation on the basis of the nucleation-and-growth model to the measured quantum sensitivities of the emulsion layers during development. The development process of a sulfur-plus-gold-sensitized emulsion consisted of fast and slow components. The fast and slow components corresponded to the development of the grains bearing latent image centers composed of at least 5 atoms and less than 5 atoms, respectively. The fast component could be explained on the basis of electrode reaction theory, and the slow component on the basis of a one-byone reaction mechanism that brought about a long induction period for the development of each grain.