It is useful to review modern treatment of the development mechanism described as an electrode process.
In other words, the ultimate source of the electrons used to develop image comes from Agent 2.
Examples of Agent 1 include: Phenidone, Dimezone S, Metol, p-aminophenol, glycin, Eikonogen.
These agents have nitrogen atoms that give strong affinity to silver halide (argentophilic) as well as hydrophobic end, effectively acting as a surfactant between the developer solution and silver halide crystals.
These agents have stronger reduction potentials than those of Agent 1 group, but they have very poor adsorption on silver halide crystals, due to lack of argentophilic end.