The air inside the discharge tube is a non-conductor of electricity. So initially the tube looks intact. As the air pressure inside reduces, the gas starts ionizing. Since a potential difference is maintained inside the tube, when one gas atom is ionized, the electron escaping from it, ionizes other gas atoms. There a stream of positive ions and negative electrons gets created. These start moving towards cathode and anode respectively. And thus, in such way, anode and cathode rays are produced.