The ceramic glower is not enclosed in a glass envelope as it does not oxidize like metal and does not need to be run in a special atmosphere.
The glower in the lamp could be replaced and often outlasted incandescent lamps of the era, making it popular in places where maintenance was an issue.
The lamp has a simple resistive
Electricity is applied to the heater tubes, sat adjacent to the glower, which raise in temperature to the point of glowing slightly red.
This heated the glower until it reached a temperature where the resistivity of the glower material dropped to the point where current would begin to flow through it. A contactor or switch would cut the power to the heater tubes, leaving power to run through only the glower.