The release by the mortgagee or that mortgagee's assigns, executed at the instance of the mortgagor, that mortgagor's heirs or assigns, of any part of the mortgaged premises shall not be deemed or taken to operate as a release or discharge of any other part of the lands included in such mortgage, but such other lands shall be and remain subject to the lien of the mortgage, and execution may be had thereof in the same manner as if the mortgage had originally included only such lands. Every such release shall be under hand and seal, and shall be acknowledged in the same manner as provided by law for the acknowledgment of deeds, and shall become effective upon the date of filing in the office of the recorder of deeds in and for the county in which such lands so released are situated.