The last word–

TWIN CHICKS Upon cracking open my breakfast boiled egg, I found a whole new egg inside. It was not a double-yolked egg, it was a double-egged egg – a completely new egg with a shell and yolk inside another. Can anybody explain it?

An egg within an egg is a very unusual occurrence.

Normally, the production of a bird’s egg starts with the release of the ovum from the ovary. It then travels down the oviduct, being wrapped in yolk, then albumen, then membranes, before it is finally encased in the shell and laid.

Occasionally an egg travels back up the oviduct, meets another egg travelling down it, and then becomes encased inside the second egg during the shell-adding process, thus creating an egg within an egg. Nobody knows for sure what causes the first egg to turn back, although one theory is that a sudden shock could cause this. Eggs within eggs have been reported in hens, guinea fowl, ducks and even Coturnix quail.

Incidentally, it is especially unusual to encounter this phenomenon in a shop-bought egg, because these are routinely candled (a bright light is held up to them to examine the contents), and any irregularities are normally rejected.Alex Williams

Haverfordwest, Pembrokeshire, UK

As the curator of the British Natural History Museum egg collection, I’ve come across quite a few examples of

egg oddities. Double eggs (as opposed to multiple-yolked eggs) are less common than some other oological anomalies and consequently the ovum in ovo, as the phenomenon described here is known, has attracted specific scholarly attention for hundreds of years.

The Dominican friar and polymath Albertus Magnus mentioned an “egg with two shells” as far back as 1250 in his book De animalibus, and by the late 17th century pioneering anatomists like William Harvey, Claude Perrault and Johann Sigismund Elsholtii had also given the phenomenon their attention.

Four general types occur – variations of yolkless and complete eggs – but this form in which a complete egg is found within a complete egg is relatively rare. Several theories have been proposed for the origin of double eggs, but the most likely suggests that the normal rhythmic muscular action, or peristalsis, that moves a developing egg down the oviduct malfunctions in some way.

A series of abnormal contractions could force a complete or semi-

complete egg back up the oviduct, and should this egg meet another developing egg travelling normally down the oviduct, the latter can engulf the former; more simply, another layer of albumen and shell can form around the original egg.

Often when no yolk is found

within the “dwarf” or interior egg a foreign object is found in its centre. This object has served as a nucleus around which the albumen and shell were laid down, in a process not dissimilar to the creation of a pearl.

The Delaware Museum of Natural History in the US has a fantastic example in its collections (see Photo).

Anybody interested in learning more about this subject should try to find a copy of The Avian Egg by Alexis Romanoff and Anastasia Romanoff (John Wiley & Sons, 1949) and turn to pages 286 to 295. Douglas Russell

Curator, bird group

Department of Zoology

The Natural History Museum

Tring, Hertfordshire, UK

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“Ovum in ovo, as it is known, has attracted specific scholarly attention for hundreds of years”

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