Porosity of the Avian Eggshell

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Abstract
The avian eggshell is considered as a mediating boundary that operates along with the nest microenvironment and the behaviour of the brooding parent(s) to isolate the embryo from the external environment. Particular importance is attached to shell modifications that could assure that the pores are not flooded or blocked with debris. It is proposed that the mechanical properties of the shell that ensure the integrity of the diffusion pathways throughout incubation are of equal importance to those that protect the embryo from mechanical dimage. Five broad categories of pore types in avian eggshells have been defined. 1) simple pore systems—a tube open at both ends traverses the true shell, 2) occluded pore systems—the outer surface of the shell is coated with featureless material of unknown origin and chemical composition fissures in this material presumably permit gaseous diffusion, 3) plugged pore systems—the outer orifice of the pore orifice contains a plug of organic or crystalline inorganic material, 4) capped pore systems—the outer surface of the true shell and pore orifices are covered with a stratum of spheres formed from organic material, vaterite or non crystalline materials rich in calcium and phosporus, 5) reticulate pores—the outer portion of the palisade layer is modified so that a plexus of tubules comprises much of the shell. Although five pore systems have been defined so far, all probably share a common origin in that cones formed in the distal part of the isthmus provide not only the opportunity for pore formation but also foundation for the palisade layer.

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