The members of the present Planning Commission shall continue to serve until December 31, 1959. Their successors, except the member selected by Council as hereinabove provided, shall serve for terms of six (6) years except that two (2) of the electors first appointed by the Mayor shall be designated by the Mayor to serve for three (3) years and the other two such electors for a term of six (6) years commencing January 1, 1960.