"I contain multitudes," says a line in Walt Whitman's poem "Song of Myself." Whitman was not thinking in biological terms, but the line has biological resonance. Recent studies suggest that each of us possesses in addition to the trillions of cells descended from the fertilized eggs we once were - a cadre of cells we have acquired from other, genetically distinct individuals. In utero we receive an infusion of them from mom. And women who become pregnant also collect a sampling shed by the developing embryo.