Within sixty (60) days after the adoption of this Act by the electors of one or more cities, villages and incorporated towns within the metropolitan area having a population in the aggregate of at least 100,000 according to the Federal census of 1940, the Governor, by and with the advice and consent of the Senate, shall appoint three members of the Board for initial terms expiring September first of the years 1947, 1948 and 1949, respectively, at least one of which members shall be a resident of that portion of the metropolitan area which is outside the corporate limits of the City of Chicago, and the Mayor, with advice and consent of the City Council of the City of Chicago, shall appoint four members of the Board for initial terms expiring September first of the years 1946, 1950, 1951 and 1952, respectively. At the expiration of the term of any member appointed by the Governor his successor shall be appointed by the Governor, and at the expiration of the term of any member appointed by the Mayor his successor shall be appointed by the Mayor in like manner, and with like regard as to the place of residence of the appointee, as appointments for the initial terms. All successors shall hold office for the term of seven years from the first day of September of the year in which they are appointed, except in case of an appointment to fill a vacancy. In case of vacancy in the office of any member appointed by the Governor during the recess of the Senate, the Governor shall make a temporary appointment until the next meeting of the Senate when he shall nominate some person to fill such office; and any person so nominated, who is confirmed by the Senate, shall hold his office during the remainder of the term and until his successor shall be appointed and qualified. If the Senate is not in session at the time this Act takes effect, the Governor shall make temporary appointments as in case of vacancies. Each appointment by the Governor shall be subject to approval by the Mayor, and each appointment by the Mayor shall be subject to approval by the Governor and, when so approved, the Governor and the Mayor shall certify their respective appointments and approvals to the Secretary of State. If the Governor or the Mayor does not approve or disapprove the appointment by the Mayor or the Governor, respectively, within 15 days after receipt thereof, the person is appointed. Within thirty days after certification and approval of his appointment, and before entering upon the duties of his office, each member of the Board shall take and subscribe the constitutional oath of office and file it in the office of the Secretary of State.