part of fungus fungal primordium 2013-11-07T08:49:53Z A mushroom develops from a nodule, or pinhead, less than two millimeters in diameter, called a primordium, which is typically found on or near the surface of the substrate. It is formed within the mycelium, the mass of threadlike hyphae that make up the fungus. The primordium enlarges into a roundish structure of interwoven hyphae roughly resembling an egg, called a button. BTO:0005533 BrendaTissueOBO Marion primordium