Animals do the darndest things. While rare, it is well-known that a hen will occasionally lay an egg inside of an egg. Called a counter-peristalsis contraction, a double egg occurs while the hen is in the process of forming an egg in her oviduct.
Source: Backyard Poultry, January 7, 2020. Link. 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.
INSIGHTS: The counter-peristalsis contraction explains the egg in an egg development but, we’re still unclear whether the chicken or the egg came first.