n 1  :  isomerism esp. of chemical compounds of the same type  2  :  the condition of having or the stage of evolutionary development characterized by a body made up of metamers that is usu. held to be an essential prelude to the differentiation of the more highly organized animals (as arthropods and vertebrates) through the disproportionate development and elaboration of some segments together with the coalescence, reduction, or loss of others .
 
 
 
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