The City Planning Commission shall consist of the Mayor, one member of the Council to be appointed by the Council, and three (3) electors of the City, not holding other Municipal office, to be appointed by the Mayor. Members of the City Planning Commission shall be chosen for their especial suitability derived from their professions, such as architects, engineers and attorneys, or specific knowledge or experience helpful to such a Commission in the pursuance of its duties. The terms of the present members of the Planning Commission, other than the Mayor and the Councilman, shall terminate on December 31, 1951, and their successors shall be appointed by the Mayor for a term of six (6) years, except that of the three (3) appointed for the terms beginning January 1, 1952, one shall be appointed for a term of two (2) years, one for a term of four (4) years and one for a term of six (6) years. A vacancy occurring during the term of any member of the City Planning Commission shall be filled for the unexpired term in the manner authorized for an original appointment.