fungal structure fungal primordium 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.