Porosity of the Avian Eggshell

The avian eggshell is considered as a mediating boundary thatoperates along with the nest microenvironment and the behaviourof the brooding parent(s) to isolate the embryo from the externalenvironment. Particular importance is attached to shell modificationsthat could assure that the pores are not flooded or blockedwith debris. It is proposed that the mechanical properties ofthe shell that ensure the integrity of the diffusion pathwaysthroughout incubation are of equal importance to those thatprotect the embryo from mechanical dimage. Five broad categoriesof pore types in avian eggshells have been defined. 1) simplepore systems—a tube open at both ends traverses the trueshell, 2) occluded pore systems—the outer surface of theshell is coated with featureless material of unknown originand chemical composition fissures in this material presumablypermit gaseous diffusion, 3) plugged pore systems—theouter orifice of the pore orifice contains a plug of organicor crystalline inorganic material, 4) capped pore systems—theouter surface of the true shell and pore orifices are coveredwith a stratum of spheres formed from organic material, vateriteor non crystalline materials rich in calcium and phosporus,5) reticulate pores—the outer portion of the palisadelayer is modified so that a plexus of tubules comprises muchof the shell. Although five pore systems have been defined sofar, all probably share a common origin in that cones formedin the distal part of the isthmus provide not only the opportunityfor pore formation but also foundation for the palisade layer.