We have carried out an arrested development study on model thermally developed photographic materials (TDPM) based on silver carboxylates. From both TEM and x-ray diffraction analysis of the media at various stages of development, we infer that (1) during initiation of development small, spheroidal silver(0) particles form; (2) over the course of the development reaction, the number of silver(0) spheroids increases, but not their average size; (3) this process requires secondary nucleation of the silver(0) phase-forming reaction by an as-yet unestablished mechanism; and (4) above some threshold concentration, whose establishment corresponds to the onset of the continuation stage of development, these spheroids aggregate to form the light-absorbing particles comprising the TDPM image. Development kinetics of full-soap and half-soap systems were studied experimentally and by computer simulation to confirm the intermediacy of coordination complexes of silver ion and the role of carboxylic acid as the reaction product responsible for positive feedback in the development reaction. Our inferences are consistent with the Klosterboer-Rutledge model of development, but also imply that features of the recently proposed Usanov model are relevant as well.