As We're Told

At the start, something must be arbitrarily excluded.
The saline solution. Call it an apple. Call this a test
or a joke. From now on, apple will mean arbitrary
choice or "at random." Any fence maintains the other
side is "without form." When we're thrown out, it's onto
the lap of our parent. Later, though, Mother puts
the apple into Snow White's hand,
and then it's poison!