Eggs are formed by secretions of the oviduct as the ovum or yolk is passed through the oviduct. Hens are also known to occasionally lays eggs with two yolks or

Eggs are formed by secretions of the oviduct as the ovum or yolk is passed through the oviduct. Hens are also known to occasionally lays eggs with two yolks or no yolk at all, with the latter often containing a blood clot or random tissue where the yolk is expected to be. So it was clear that a foreign object planted at the right place and right time could induce the animal to form an egg around it. I'm just not sure if this is reliable enough to create a business around the idea.