Abstract The mammillary region of poor quality avian egg shells displays a number of aberrant structural alterations not commonly observed in high quality egg shells. Three general categories of mammillary alterations were observed in radial sections of extremely low quality shells: 1) Shells with a proliferation of discrete rounded calcified bodies situated upon the shell membrane fibers; 2) shells with an extremely disorganized, multinucleate mammillary region without numerous calcified bodies; and 3) mammillary knobs possessing cavernous tips without mammillary core formations. Adhesion discs (fracture surfaces between adjacent calcified structures) were observed and classified into two types, small-rounded (Type I) adhesion discs between adjacent calcified bodies, or between calcified bodies and larger mammillary formations; and large oval (Type II) adhesion discs between adjacent mammillary knob formations. The nucleation zone or external surface of the outer shell membrane fibers, upon which calcium salts initially precipitate, was also observed to vary in low-quality shells. Waves or bulges in the nucleation zone were observed directly beneath the multinucleate unorganized mammillary knob formations. These findings suggest that the initial process of shell mineralization (mammillary knob formation) is responsible for the base upon which the palisade (main shell) layer is deposited. Disorganization of the basal calcified region may give rise to poorly indigitating calcite (palisade) columns and decreased intrinsic shell strength.