The kidneys are formed form three important preceeding stuctures. The first is the pronephros. It is not very clear whether this exists in the human embryo is it does clearly in those of lower animals. But if it does, it stays for just a short while and may occur in the intermediate mesoderm which is opposite the cranial somites. It gives way to the mesonephros which develops tubules and extends fromt the septum transversum to the third lumbar vertebrae. Its  long duct is then called the mesonephric duct or Wolffian duct. The next renal system is the metanephros which forms below the mesonephros at the sacral region. A ureteric bud then grows out of the caudal part of mesonephric duct and grows into the metanephros. It consolidates itself and becomes the definitive kidney complete with secretory and tubular parts.