Sentence:  In the present work whole sera have been fractionated by chromatography on DEAE-cellulose using single <WSD>gradients</WSD> similar to those described by Sober and Peterson , and certain chemical and serological properties of the fractions containing antibodies of the ABO and Rh systems have been described . 
Meaning of the word marked with <WSD> tags:  a graded change in the magnitude of some physical quantity or dimension