The unusual occurrence is called a counter-peristalsis contraction.
A hen releases an oocyte from her left ovary into her oviduct every 18 to 26 hours,
Usually, the oocyte travels down the oviduct, adding layers of the egg until it’s laid through the hen’s vent.
But in a counter-peristalsis contraction, a second oocyte is released by the ovary before the first egg has completed its journey.
When the second oocyte’s released, the first is in the eggshell-gland portion of the oviduct, were the shell is made.
It’s thought that a shock, or stress causes a contraction, forcing first egg to reverse its course.
This means that the first egg is added to the second oocyte, which then travels down the oviduct and is covered in a shell, creating a giant egg around the first one.
The phenomenon has been mentioned as far back as 1250 AD by the Dominican friar and polymath Albertus Magnus in his book De Animalibus.
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