An egg within an egg

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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.

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