If, within thirty days after receipt of a certified copy of an ordinance from a municipal corporation proposing annexation designating its three commissioners, the legislative authority of the municipal corporation with which annexation is proposed fails to pass an ordinance designating three commissioners to represent it in such negotiations, then, on receipt of a petition signed by resident electors of a number not less than twenty-five per cent of the number of electors voting at the last previous regular municipal election of the municipal corporation with which annexation is proposed, petitioning the legislative authority thereof to take such action as is necessary to initiate such proceedings and to appoint three commissioners to represent it therein, the legislative authority shall pass an ordinance appointing such commissioners.