That the directors, once selected, shall serve without pay, one director from each county is to serve for a period of one year from the date of his selection, one director from each county is to serve for two years from the date of his selection and one director from each county is to serve for three years from the date of his selection, and the respective successors of the first directors to serve for a period of three years; that any vacancy in said board of directors shall be filled by the board of county commissioners of the county wherein the director whose office is to be filled resided.