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.
Later, Yellowfang tells Raggedpelt that instead of being his deputy, she'll be his medicine cat, but they both want more than that. They agree to meet in secret in parts of the territory, though Yellowfang knows is breaking the medicine cat code.