Structure and function of the shell and the chorioallantoic membrane of the avian egg: Embryonic respiration

The evolution of a cleidoic (self-supporting) egg in the amniotes was pivotal to the transition of animal life from water to land: embryonic development could occur without the direct presence of water. For birds, remarkable adaptive radiation occurred after the achievement of volancy. It has culminated in ~10,000 species. Among the air-breathing vertebrates, the taxon Aves is the most speciose. In the developing avian egg, exchange of respiratory gases (O2 and CO2) and water vapour occurs entirely by passive diffusion across the shell and the chorioallantoic membrane (CAM) along existing concentration gradients. A multifunctional structure, the shell displays compromise design. On one hand it has to be thin enough to allow optimal flux of O2 and CO2 and loss of water which is generated by metabolism of the developing embryo. On the other hand, the shell must not be too thin to admit pathogens and injurious substances and susceptible to failure (breaking). Also, the shell must not be too strong for the chick to be unable to break out of at hatching. The thickness of the shell and the numbers, shapes and sizes of the pores determine the hatchability of the egg and probably the incubation period of the egg. The CAM of the developing chick embryo is analogous to the placenta of the viviparous animals. While the number of pores may be fixed at the formation of the eggshell, during incubation, the surface area and vascularization of the CAM increase considerably. Here, the structure and function of the avian eggshell and that of the CAM are succinctly outlined. Particular consideration is given to the recent observations made using X-ray microcomputer tomography, a highly instructive technique for studying biological structures. The impact of the evolution of the amniotic egg on the diversification of the animal life especially that of birds, is highlighted.