Rule 13 3

Whenever the claim or defense asserted in the amended pleading arose out of the conduct, transaction or occurrence set forth in the original pleading, the amendments shall relate back to the date of the original pleading. An amendment changing the party against whom a claim is asserted shall relate back to the date of the original pleading if, after complying with the foregoing requirement and within the term provided for, the party brought through the amendment:(1) Knew of the pending cause of action so that he is not prejudiced in maintaining his defense on the merits, and(2) if it had not been for a mistake as to the identity of the person actually liable, the action would have been brought originally against him.