The biological function of the egg shell is as a chamber for embryonic development and from which the chick is able to emerge at the appropriate time. The characteristics which serve this purpose in an optimum way are quite different from the requirements of the table egg industry. Among the challenges imposed by commercial egg production and marketing is the ability to resist fracture damage. The industry sustains economic loss from cracked eggs and at least some of the cracking can be attributed to deficiencies in shell structure. While early research concentrated on the thickness of the shell, more recently there has been an increased emphasis on its fine structure and shape. Egg shell strength has been the subject of much research, but relatively little attention has been paid to the relationship of strength and other shell parameters to breakage under commercial conditions. Where this has been studied, the power of shell measurements to predict breakage has been found to be low. The ultrastructure of the shell has been studied during the past 25 years and this has led to extensive gains in understanding. In the case of the mammillary layer, knowledge has been advanced following the use of plasma etching and scanning electron microscopy. Several structural anomalies have been described which significantly affect shell strength and resistance to breakage. The use of finite element analysis has made possible a definitive description of shell shape and led in turn to useful work on crack generation and shell failure. Studies of the protein content of the shell, which may be an important factor in its crystal growth and structure, are also contributing to our understanding of egg shell architecture.

The procedures normally used for the removal of hens from cages at the end of their laying cycle and their subsequent transport to the slaughterhouse are described. Because 30% of live birds arriving at the slaughterhouse have been found to have one or more broken bones, discussion is focused on the principal factors involved in the aetiology of such lesions. These include the reduction in bone strength due to mineral depletion during lay, restricted physical movement and exercise within cages and the methods used to remove birds from their cages, in addition to the other sources of potential trauma associated with transportation from the farm to the processing plant.