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.
As mentioned in past posts, 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.
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! When you crack open such an egg, there is normal yolk and whites as well as another fully formed, normal-sized egg inside (see photo for example).
Recently, a tiny, fully-formed egg was found inside of a regular-sized egg in Britain. This particularly rare, miniscule egg inside an egg was also caused by a counter-peristalsis contraction.However, in this case the oocyte that was released in the first egg (the one that reversed course in the oviduct) was tiny because the ovary had released an oocyte out of order. Usually hens ovulate daily in order of size – laying the largest, most developed oocyte first. The hen’s ovary is simultaneously preparing smaller oocytes for release at a later time. Occasionally, a small, underdeveloped oocyte jumps the queue. In case of the British man who found the tiny egg inside a normal-sized egg – that is what happened. To see a picture and learn more about this extremely rare, tiny egg inside a regular-sized egg LINK HERE.
You can learn more about the formation of eggs and phenomenon of having a chicken lay a fully-formed egg inside of an egg in episode 030 of the Urban Chicken Podcast LISTEN HERE.
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