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.
In the world of Warrior Cats: Untold Tales, there are four seasons, each lasting ten game days. (Or 90 minutes) New game files begin in the second day of summer (AKA Greenleaf), then progress through fall/autumn (Leaf-fall), winter (Leaf-bare), spring (Newleaf), then circles back to summer.