Photographic development mechanism was studied electron-microscopically. Spherically decorated print-out silver particles which were situated on the surface of octahedral silver iodo-bromide crystals were developed by low-pH Metol-citric acid-silver nitrate physical developer. Then, long filament was formed with the decorated particle on its tip. These results lead to the conclusion that the silver filament growth starts at the base of the filament, i.e., at the silver/silver halide interface. Through the experimental observation the existence of the “root” of developed filament that was not confirmed. Developed silver filaments often showed ruptured appearance after fixation as was indicated by Muller.