A retirement system is hereby created and established in each city of the first class in each county with a population of one hundred twenty-five thousand or more to be known as the "employees' retirement system". This chapter shall become effective as to any such city when by ordinance of the city duly enacted its terms are expressly accepted and made applicable thereto. This section shall not be construed as preventing performance before July 1, 1939, of any preliminary work which any city council, city commission or board of administration shall deem necessary.