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When egg shells are embedded in plastic it would appear that the plastic can form a clean mould of the inner surface of the shell with its knob-like projections, it can also penetrate into the pore channels and even into the body of the shell itself. The shell can then be dissolved away ldaving the plastic moulds behind. It is thus possible to obtain a clear picture of the shape and size of pores and their relationship to the air spaces in the mammillary layer. The egg shells of various species of birds have been examined and differences in pore size and shape clearly demonstrated. The plastic residue from the shell itself also seems to indicate that the shell consists of layers of material, the layers being arranged differently in different species.

Studies on the egg shells of a selection of Sphenisciformes, similar to previous ones on ratites and the Anatidae, have been carried out. The true shells, after removing the membrane and cover, were analysed for insoluble and soluble nitrogen, and studies were also made using histological and plastic embedding techniques. The combined studies indicate that, in addition to the large crystals running through the shell, there is often a thin layer of fine vertically orientated crystals above these. Then resting on this layer is a thin organic cuticle and usually, above this, a cover which is chiefly organic matter but contains some inorganic material. This cover is very marked in shells of Aptenodytes sp. and is thicker at the poles than at the equatorial region. The histological picture of the true shell is very similar to that of the Anatidae. Most of the pore channels seem to be, at least partially, blocked with organic matter. Unlike the thickest Anatidae shells, there are few branched pores, although in the thicker shells the mouths of the channels have a shape like an arrow head. Plotting true shell thickness against parts per 10,000 of nitrogen in the true shell gives some separation of genera and to a lesser extent of species, but the results are not so clear cut as in the Anatidae.

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