Commercial mushroom growing starts with the production of a starter culture of spawn in a laboratory. A tiny piece of a selected mushroom is placed on a sterilised growing medium of agar, where it develops fine filaments known as mycelia. The threads of mycelia are transferred to jars of sterile cereal grain. They soon spread their fuzzy white growths through the grain, forming a material known as spawn. This is mixed into steam-pasteurised compost and put in trays or bags. These are placed in rooms with controlled temperature and humidity, and mushrooms appear some two to three weeks later.