hard structure surrounding the ovum, embryo, yolk, albumen, and membranes, comprised primarily of calcium carbonate (calcite crystals), which are embedded in a small component of organic matter; Bald Eagle eggshells are ~ 0.56-0.6 mm thick (cf. ~ 0.28-0.4 mm for a chicken); shell thins during incubation as the embryo absorbs some calcium for its bones; shell is produced in the uterus of the oviduct in layers: base (mammillary) layer embedded in the outer shell membrane, has cones or knobs (Latin mammilla = nipple) with organic material that produce calcite crystals, which grow upward to form columns comprising the palisade and vertical layers, with spaces between the columns reflecting cellular structure of the outer shell membrane, finished with the thin (~ 0.005-0.01 mm) cuticle layer of proteins and organic material covering the egg surface; thousands of pores allow oxygen, carbon dioxide, and evaporated water to pass through the shell