§ 170. Commissioner; deputy. The commissioner of public welfare may appoint one or more deputies, and such other subordinates as may be prescribed by the board of estimate and apportionment. In case of the absence or disability of the commissioner or of a vacancy in the office, a deputy shall discharge the duties of the office until the commissioner returns, his disability ceases or the vacancy is filled. The commissioner, and the deputies before entering upon the discharge of the duties of their respective offices, shall each execute and file with the city clerk an official undertaking in such penal sum as may be prescribed by the common council.