Sentence:  In attempting to improve specificity of staining , the fluorescein labeled antisera used in both direct and indirect methods were treated in one of several ways : ( 1 ) They were passed through Dowex-2-chloride <WSD>twice</WSD> and treated with acetone insoluble powders ( Coons , 1958 ) prepared from mouse liver or from healthy sweet clover stems or crown gall tissue produced by Agrobacterium tumefaciens ( E. F. Smith + Townsend ) Conn , on sweet clover stems . 
Meaning of the word marked with <WSD> tags:  two times