At breakfast one day, my 4-year-old son was peeling his boiled egg when he noticed something unusual. On the side of the egg was an almost perfect white square with sides about 1 centimetre long. This was enclosed in an oval patch of yellow matter (see photo). Although the yellow patch looked like egg yolk, the egg appeared normal when I cut it in half, and I could not see any connection between the egg yolk in the centre and the yellow patch on the outside. Does anyone know how this formation was created?
The answer from Paul Hudgins was favoured by several other people. But if this is the explanation, why isn鈥檛 this pattern seen more often, and what caused the mysterious yellow colouring? The case remains open 鈥� Ed
鈥� Some eggs have a very soft shell when laid. The imprint was likely caused by the pressure of the hen sitting on the egg and pressing it against the wire mesh forming the bottom of its cage. This would have damaged the membrane that surrounds the egg, producing the square pattern.
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Paul Hudgins, Jacksonville, Florida, US
鈥� One can tell from the picture that at one time the egg had been standing on a surface where a square support had been in contact with the shell, but where a surrounding pool or pad had applied a fluid that had diffused through. Whether the invading substance was itself yellow or whether it just changed the colour of the is hard to say. We can, however, be sure that no internal influence could have created such a precise and shallow pattern.
The architecture of an eggshell is amazing. It protects the contents from outside threats, whether microbial, chemical or mechanical. It permits carbon dioxide to escape and lets oxygen in. It supplies calcium for the chick鈥檚 skeleton. Eventually it lets the hatchling out.
The protection is not perfect, however. By writing on the shell with suitable inks, or using vinegar or alum, one can cause marks that become visible on the hard-boiled egg white as the dyes seep through. The picture clearly shows an accidental example of some such effect. It鈥檚 possible, for instance, that some fluid had spilled onto the scale that grades eggs by weight.
Jon Richfield, Somerset West, South Africa