Wow that's pretty different. Not sure but I've read sometimes tissue can get expelled and the body of the chicken will treat it as a egg and it will go through the process just like a egg would and even get a shell formed over it.if I were to take a guess I would gather that that is exactly what happened here. A piece of dislodged tissue went through the egg production process. That is why you found it in the egg
I would agree that its body tissue that got expelled as part of the egg.
I've had hens expel some pretty funky stuff.....but never inside an egg. Sometimes it makes me not want to eat eggs for awhile. haha
Thanks for your thoughts. It seems like this is a very rare event as I can't find anything similar on-line. I'll keep a closer eye on the hens and stop giving the eggs away for a while.