Within twenty days after a copy of any petition has been served upon any adverse party, he may file with the secretary of the board if the petition has been directed to be heard by the board, or with the referee if the petition has been assigned to a referee, an answer in the form prescribed by the board.
Every fact alleged in a claim petition not specifically denied by an answer so filed by an adverse party shall be deemed to be admitted by him. But the failure of any adverse party, or of all of them, to deny a fact so alleged shall not preclude the board or referee before whom the petition is heard from requiring, of its or his own motion, proof of such fact.
77 P.S. § 1516