Upon the appointment of any officer or member of the police force, it shall be the duty of the city secretary to issue to him a certificate of such appointment and upon receiving same such person shall, before entering upon the duties of his office, take and subscribe to the oath of office required by the constitution of the state, before any officer legally authorized to administer oaths, which oath shall be endorsed on the certificate of appointment and filed with the city secretary. (Code 1941, Art. 109-3)