Full Idea
What appears to me most likely is these forms are like patterns set in nature, and other things resemble them and are likenesses; and this partaking of the forms is simply being modeled on them.
Gist of Idea
Probably partaking in the Forms is like being modeled on a pattern
Source
Plato (Parmenides [c.366 BCE], 132c)
A Reaction
This invites the question of whether the things depend for their existence on the Forms, or whether things could survive the loss of their patterns (if Zeus destroyed all the Forms). Plato generally implies dependence.
Book Reference
Plato: 'Parmenides', ed/tr. Gill,M.L./Ryan,P. [Hackett 1996], p.134