Likely a sterile medium means an uncontaminated culture (food free of living organisms) that will grow only the new bacteria introduced for analysis. Source(s): http://www.thefreedictionary.com/sterile
Stonepelt was Bluestar's first mentor and a very well-respected senior warrior of ThunderClan. It is revealed in this book that he took her to the Gathering on her second day as an apprentice because she had caught a squirrel very well for a new apprentice, and the squirrel had been as big as she was.
Although initially introduced in Warrior Cats as a Clan defeated and exiled by Brokenstar, WindClan has done more than enough to make up for this early bout of weakness. With their unmatched speed and relentless tactics, WindClan are invaluable allies and horrifying enemies.