When a man cracked open an egg bigger than its normal size from one of his hens, it had garnered various skepticism. It is quite uncertain whether the video is real or fake, but the possibility of it happening is definitely plausible.
Though quite rare, it is well-known that occasionally a hen will lay a rather large egg with another fully-formed egg inside of it. The cause of this phenomenon is called a counter-peristalsis contraction and occurs while the hen is in the process of forming an egg in her oviduct.
A hen typically releases an oocyte (the ovum that becomes the yolk of an egg) from her left ovary into the oviduct every 18-26 hours. The oocyte travels slowly through the oviduct organ adding layers of the egg along the path to the chicken’s vent from which it will lay the egg.
Theoretically speaking. a counter-peristalsis contraction is when a second oocyte is released by the ovary before the first egg has completely traveled through the oviduct and been laid.The release of a second oocyte into the oviduct system while a first oocyte is in the eggshell-gland portion of the oviduct (the eggshell gland is also called the uterus in a hen and is where the shell is deposited over the egg) causes a contraction. This counter-peristalsis contraction, resulting from the premature release of a second oocyte into the oviduct, causes the first egg in the eggshell gland to reverse its course and be pushed back up to the top of the oviduct.
Consequently, the first egg (i.e. the previously released egg which was in lower portion of the oviduct before reversing course) is typically added to the oocyte that was just release into the oviduct. The second oocyte then travels down the oviduct and has albumen and a shell deposited over it and first egg together. This creates a very large egg for your poor hen to lay. Ouch!
So, when you crack open a giant egg, there is the normal yolk and whites as well as another fully formed, normal-sized egg inside.
To see that tiny egg inside a bigger is really rare. I’ve never seen it personally, but it is possible,” said Gregoy Bedecarrats, an associate professor in animal and poultry science at the University of Guelph.
via Backyard Poultry Mag