That reminds of a long lost memory. Once in awhile my mother was mistaken when she culled the hens for non-layers and while we were butchering we'd find several neatly formed and encased eggs in the egg tract. As Rap said, the shells are soft and leathery- kind of like turtle eggs - until they've been exposed to the air. Oddly the eggs are formed from the outside in- the shell gets larger and larger until it is ready for the trip.

Mom was always rueful- laying hens were a prized commodity.