Either of the pair of structures lying on either side of the dorsal aorta in which fluid balance is regulated and waste is excreted out in the form of urine. Amphibians (and fish) have a early kidney, the pronephros/pronephric kidney, which consitsts of a single nephron attached to a pronephric duct, which in turn is linked to the cloaca. The pronephros functions as primary filtration system for the embryo while the mesonephric kidney develops. The pronephros starts to degenerate at NF stage 53. The mesonephros starts to form at NF stage 39 and contuinues to grow into the adult kidney.