In order to collectively study the factors influencing the developability of centers, the in situ measurement and analysis of the rate of development was made for various latent image centers and fog centers on AgBr grains with a variation of the kind of chemical sensitizations. The developability of these centers was analyzed from the viewpoints that the electron transfer from a developer to the electron-accepting level of a center should initiate the development, and that the height of the electron-accepting level of a center should be related to its size, chemical composition, band structure, the parity of the number of valence electrons in it, and the site for its formation. It was confirmed that the smallest developable center in AgBr emulsions was a fog center composed of two Au atoms while the smallest latent image centers in the emulsions with and without gold sensitization were Ag₂Au and Ag₄, respectively. The developabilities of these smallest centers were much smaller than those of larger ones. The developability of a center with a forbidden band as exemplified by Ag₂S (i.e., a fog center in a sulfur sensitized emulsion) was so small that several hundreds of nanoclusters per grain were needed to initiate the development of the grain.