An egg ist ein ei, es un huevo, est un oeuf

1. Many of the mechanical tests devised to measure shell quality are inadeq uate because they fail to recognise the complex interaction between the org anic and inorganic aspects of the eggshell. 2. Twelve structural modifications have been observed at the level of the m ammillary layer and their presence correlated with a variety of environment al stress events. Occurring as they do in the basal layers of the shell, th ese morphological variants influence its mechanical properties. 3. The organic matrix proteins which complex with the calcium carbonate der ive from a variety of sites within the oviduct and vary in their location w ithin the fully formed shell. In vitro mineralisation reveals the significa nce of these proteins in the crystal growth mechanism. 4. The isolation and identification of the protein moiety in well-structure d eggshells is an essential prerequisite to understanding the abnormalities in crystal growth observed in the shells of older birds challenged by dise ase and other undesirable 'on farm' events. 5. The eggshell is the daily indicator of the bird's harmony with its envir onment and as such provides a readily accessible and non invasive measure o f welfare. The integration of these data with those derived from behavioura l and biochemical testing should provide industry with a reliable numerical welfare index.