Whenever a position in the competitive classified civil service is to be filled, the appointing authority shall notify the commission of that fact, and the commission shall certify the names and addresses of the 3 candidates standing highest on the eligible list for the class or grade to which the position belongs, and the appointing authority shall forthwith appoint to the position 1 of the 3 persons so certified. The appointment shall be for a probationary period to be fixed by the rules, but not to exceed 6 months. At or before the expiration of the probationary period, the appointing officer, by presenting specific reasons for such action in writing, may discharge a probational appointee, or, with the approval of the civil service commission, transfer him to another department. If not discharged prior to the expiration of the period of probation and if no complaint has been made as to the service rendered, the appointment shall be deemed complete. To prevent the stoppage of business or to meet extraordinary conditions or emergencies, the head of any department or office, with the approval of the commission, may make a temporary appointment to remain for not exceeding 60 days, and only until regular appointment under the provisions of this act can be made.