Sentence:  For preparative ultracentrifugation , fractions from the column were concentrated by negative pressure dialysis to volumes of 1 ml or less , transferred to cellulose <WSD>tubes</WSD> and diluted to 12 ml with isotonic saline . 
Meaning of the word marked with <WSD> tags:  conduit consisting of a long hollow object (usually cylindrical) used to hold and conduct objects or liquids or gases