A primordium, often referred to as a knot, is the first point at which the development of a fruit body is visible to the naked eye. Primordia often appear as a balling up of mycelium strands which will eventually develop a cap and a distinct stipe.

Primordium may also be referred to as Hyphal Knots. Below are a few pictures for your reference.

Distinct Cubensis primordium can be seen in this photo as white knots/balls of mycelium.

Primordium on rhizo myc

Primordial growth as seen on rhizomorphic mycelium.

Primordial growth

Primordium/Hyphal Knots

Roping with knots

"Roping" mycelium with primordial growth/Hyphal knots on the right side of photo.