What are the use of symbols in this story?

In "The Lottery," Shirley Jackson uses several symbols to provide the readers clues to the tragic events that would occur at the end of her story. Each of these symbols is subtle but, in retrospect, deeply frightening.
The first symbol in the book is the rocks the boys gather:

Bobby Martin had already stuffed his pockets full of stones, and the other boys soon followed his example, selecting the smoothest and roundest stones; Bobby and Harry Jones and Dickie Delacroix—the villagers pronounced this name "Dellacroy"—eventually made a great pile of stones in one corner of the square and guarded it against the raids of the other boys.

Boys playing with rocks could symbolize the innocence of childhood. What are rocks to boys other than toys? Later, Jackson flips this symbol of childhood on its head. Instead of symbolizing the innocence of childhood, these rocks are used as murder weapons which the boys fling at Tessie Hutchinson. 
The box from which names are drawn is the second major symbol in "The Lottery." The box symbolizes both the origins and the future of this lottery. The original box, like the actual reason for the lottery, "had been lost long ago" and, every year, Mr. Summers thinks about a new box (new tradition?). However, talk always faded "off without anything's being done."
While there are other symbols in the short story, these are two of the most important ones.