Earlier this week a Reddit user boiled an egg, only to slice it open and find something truly "eggceptional": A COMPLETE TINY EGG embedded in the white.
This egg-inside-an-egg situation is not unheard of: A British man cracked open an egg in January and an entire second egg fell out. Some eggsperts (sorry) have discovered scholarly mentions of "eggs within eggs" dating as far back as 1250 AD. But it is highly unusual, as Gregoy Bedecarrats, an editor for the journal Poultry Science and associate professor in animal and poultry science at Canada's University of Guelph, told BuzzFeed. "To see that tiny egg inside a bigger is really rare. I've never seen it personally, but it is possible."
In normal egg production, he says, "Once the chicken ovulates, that develops the yolk, and as the yolk passes through the oviduct, it gets covered by membranes and albumen. Normally after that it enter uterus, where actual shell gets deposited, and then it gets laid. As the egg goes through the entire reproductive sytem, there are contractions that push it through towards the opening."
For one egg to end up inside another, the first egg's odyssey down the pathway has to be reversed after its white and shell have formed. "Sometimes, if you have another ovulation before the first egg is laid, you have a 'counter-peristalsis contraction,' which means the oviduct will contract upward, and the first egg will be moved back up. When you see a tiny egg incorporated [in a larger one], it means the egg was still present in the shell gland, and got sucked back up the oviduct to where the other egg was having its white deposited."
To summarize: Egg No. 1 is going on its merry way down the reproductive tract, starting in the ovary (far left, above), then developing its white, then hanging out in the uterus forming its shell, when all of a sudden it get pushed backward and ends up embedded inside egg No. 2, which was still developing its white.
What remains a mystery is what kind of event might cause a backward contraction; you'd have to ask the chicken about that.
Next question: Why is the inner egg so small in this case?
Hens ovulate yolks daily in order of size, starting with the largest, says Michael Hulet, Associate Professor of Poultry Science at Penn State University. But there are smaller ova forming at the same time, and, "Sometimes you will have these smaller ova travel down the tract and a "tiny" egg will be laid," he says. Backward contractions could also happen to a normal-size egg, as it did in this previous instance of eggception. This just happened to be a very tiny yolk, Bedecarrats says, that traveled down and back up the oviduct.
And just out of curiosity, what if the object going backward up the oviduct was... not an egg?
"That used to be a joke with veterinarians a long time ago: If you drop anything in the oviduct, the hen will make an egg around it," Bedecarrats says. "So, if they want to propose to their girlfriend, drop a little ring in."
Note: This is a weird chicken joke, NOT an actual fact. Please do not attempt to propose with an egg-ring. There are better ways: