Chicken laid a slug full of membranes

[MuscovyMad]

I went out to the coop today and found a cracked egg and this lying beside it. It looked like a big cream slug and when I opened it, it was full of membranes and pink stuff. Has anybody had something like this before or knows what causes it?

 

[MuscovyMad]

My phone isn't uploading the photo :-(.