Sentence:  Although the standard deviation values on which spread of the lines are based <WSD>are</WSD> relatively larger for those centers which begin to ossify early ( Table 1 ) , there are considerable differences in this value between centers having the closely timed Onsets . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)