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.
As a young warrior, Stonefur fights in the battle against ThunderClan over Sunningrocks, and is pinned down by Redtail, but is freed by Oakheart. Oakheart warns Redtail that no ThunderClan cat shall ever harm that warrior before Oakheart dies.