The mammillary layer Robinson and King (1970) hypothesised that eggshells classified as weak could be explained in part by the presence of abnormal mammillary cores. These specific nucleation sites on the outer surface of the outer shell membrane attract calcium salts and so initiate the formation of the mammillary layer in that region of the oviduct termed the tubular shell gland. Their results referred only to observations of changed morphology at light microscope level and so were somewhat speculative. They also used EDTA to decalcify the shell and separate the membranes prior to examination. Further analyses would show that this treatment also alters the topography of nucleation sites. These criteria notwithstanding the hypothesis concerning an
abnormality in structure had validity. It has since been shown that the nucleation sites vary not only in their distribution but also in their protein composition and so exert modifying effects on mineralisation which in turn can lead to malformed mammillary bodies and by implication poorly constructed subsequent shell layers. In 1977, Bunk and Balloun provided a detailed description of the mammillary cores and their firm association with the outer shell membrane. Prior to 1983, the separation of the shell membrane fibres from the ‘‘true shell’’ had involved the use of sodium sulphide and other acid treatments, for example dilute HCl. The strength of the membrane/shell bond varies between individual eggs and using chemical treatment to dissociate the two layers was considered undesirable insofar as inadvertently passing the end-point of membrane removal could influence the morphology of the calcified layer. Reid (1983) was the first to employ plasma ashing to separate the shell from the membrane attachment and showed that this low temperature process had no deleterious effect on the morphology of the mammillary layer, thus preparing the way for the subsequent descriptions on the diversity of structure in the foundation layer of the shell. The nature and incidence of the 12 structural modifications consistently observed in the basal layers of the shell (Solomon, 1999) would in time be shown to vary with factors as diverse as diet, bird age and housing and to influence not only the appearance and texture of the shell but explain the observed changes in both the specialised and non-specialised measures of shell quality (Bain, 1992).