An egg within an egg
QWe recently had an egg within and egg! This egg was laid by a four-year-old Cuckoo Marans. It’s apparently unusual, although not unknown, for this to happen, but what particularly interested me was the colour of the small egg. While this older hen sometimes lays a lighter coloured egg, the small egg inside is showing the intense, distinctly Marans colouring. Does this disprove the theory that the shell colour is applied just before the egg is laid?
AAnne Perdeaux said: Thank you for a really interesting question! Under normal circumstances an egg is created when a yolk is released from the hen’s ovary and travels down the oviduct, collecting the white, shell and yes, last of all, the colour. In this case, a second yolk has been released while the first egg is in the latter part of the oviduct where the shell is made. A contraction, probably due to stress or shock, has sent the first egg back up the oviduct, where it’s been added to the second yolk. The whole lot then has then travelled back down the oviduct in the usual way, gathering white and shell as it goes, thus resulting in an egg within an egg. This strange phenomenon is known as a ‘counter-peristalsis contraction’, and was even mentioned in 1250AD by the Dominican Friar, Albertus Magnus in his book De Animalibus. Incidentally, in most cases, the inner egg is full-sized, making the outer egg very large – I believe that finding a perfect tiny egg inside a normal one is quite rare.