A cover was kept on the pool during the early stages of cloning. The pool was filled with a dark green liquid, possibly a form of clone feed, which the clone breathed. The pool was connected to a console that held a person as their thoughts were copied into the clone which received them via an umbilical cord. (TV: The Sontaran Stratagem / The Poison Sky) Nearly the same method was used to create gangers from the Flesh. (TV: The Rebel Flesh / The Almost People)