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.
A few cats in the series are Firestar and Graystripe. There are currently six sub-series, each containing six books. The first, Warriors (later re-titled as Warriors: The Prophecies Begin), was published from 2003 to 2004.