“The big egg has another egg in it!”
That surprised reaction by a man when he cracked open an oversized egg from one of his hens has garnered plenty of skepticism. Whether the video is real or fake is uncertain, but the possibility of it happening is definitely plausible.
While extremely rare, a hen can lay an egg within an egg: It is caused by a counter-peristalsis contraction. This happens when an egg is forming in the hen’s oviduct and another oocyte is released too soon, causing a contraction that pushes the first egg back up.
Image Credit: Screenshot from YouTube video
Typically, a hen releases an oocyte (the egg cell that becomes the yolk) from her left ovary every 16 to 26 hours, according to Backyard Poultry Magazine. The oocyte then travels through the oviduct, where layers of membranes and albumen are added along the way.
However, if a second oocyte is released too early, the first oocyte (now in the hen’s uterus) gets sucked back inside by a contraction induced by the second egg. The counter-peristalsis contraction pushes it backwards to where it gets embedded inside egg number two.
“To see that tiny egg inside a bigger is really rare,” said Gregoy Bedecarrats, an associate professor in animal and poultry science at the University of Guelph, to BuzzFeed. “I’ve never seen it personally, but it is possible.”