The display panel comprises a gas-filled envelope which includes a base plate on which a plurality of groups of cathodes and their conductors are formed, each group of cathodes being operable to display a character. The envelope also includes a face plate, spaced from the base plate, and carrying transparent conductive anodes, each associated with one group of cathodes. The base plate of the panel is provided with a hole and a short, smalldiameter tube axially aligned with the hole and extending away from the outer surface of the base plate. The usual tubulation is secured to the outer surface of the base plate enclosing the hole and the short tube, and the desired gas and mercury vapor are introduced into the panel envelope through the tubulation and through the short tube and hole in the base plate. The short tube prevents large mercury globules, which might be present in the sealed-off tubulation, from accidentally getting into the panel envelope.