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.
When Bluestar walks beside Stonefur and Mistyfoot, his sister, Fireheart notes how similar the three cats look. It is revealed by Bluestar to Fireheart that Stonefur is one of her kits, along with his sister Mistyfoot, and that his other sister, Mosskit, had died when she took them to RiverClan.