fungal structure fungal primordium BTO:0005533 FAO:0001052 The nodule from which a mushroom develops. The primordium, less than two millimeters in diameter, resembles a pinhead, and 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. fungal_anatomy_ontology primordium