Hairline cracks, i.e. very fine cracks, usually run lengthwise along the shell. As they are diffi cult to detect, candling efficiency needs to be maximised (see page 24). Their presence in fresh eggs can be revealed by careful squeezing or tapping. The crack becomes more obvious as the egg ages.
The egg in the photograph has been placed over a bright light. This is the way it would look to a candling operator.
The incidence of this problem varies with flock age, but is usually 1 to 3% of total production.