The following shall be the respective starting points or dividing lines for numbers on buildings fronting on streets heretofore laid out or hereafter to be laid out or extended, viz.: from east to west, First Street; from north to south, Main Street from the south boundary of the City to a point on the west bank of the Los Angeles River, and the Los Angeles river; thence to the north boundary of the City, numbers shall begin at the above named starting point or dividing lines and continue to the terminal of the respective streets; provided that on all streets lying east of the Los Angeles River and extending north or northerly from First Street, the numbers shall continue to the northerly terminus of the street, or to Macy Street or Mission Road; and provided that on all streets extending south or southerly from Downey Avenue, the numbers shall begin at Downey Avenue as the dividing line and continue south to the terminus of the street or to Macy Street or Mission Road; and provided, further, that on all streets extending north or northerly from Downey Avenue, the numbers shall begin at Downey Avenue and continue to the northerly terminus of the street.