Section 90. Commissioner and deputy commissioner of public works

The
  commissioner of public works shall be the  head  of  the  department  of
  public  works.  He  shall appoint, to hold office during his pleasure, a
  deputy 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,  the  deputy  commissioner
  shall discharge the duties of the office until the commissioner returns,
  his  disability  ceases  or  the vacancy is filled. The commissioner and
  deputy commissioner 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.