Progress of development is classified into parallel development (P type) and granular development (G type). Filamentary silver developed with P type developers grows out from AgBr grain surface but that with G type developers does not. It was suggested that this difference was attributed to the difference of the mixed potential during development with two type developers. The compact silver deposit by G type developer of the more negative potential is formed by the direct reduction of lattice silver ions, accompanied by formation of silver ion vacancies and collapse of AgBr grain.