Respiratory gas exchange by the avian embryo

The mechanism of gas exchange by an avian embryo is discussed in terms of the embryo's changing metabolic rate, a constant egg shell gas permeability and the changing gas tensions inside the shell. Since gas transport across the shell is by diffusion, equations are developed which predict the gas composition in the air cell of the egg as well as the rate of exchange of O₂ CO₂ and water vapor. P_O2 and P_CO2 values were obtained from the air cells of chick embryos of various ages, and when these results are considered together with metabolic rate and measured egg shell permeability data they are found to be consistent with the hypothesis that gas exchange by the avian embryo is limited by diffusion through its porous shell. The implications of this are discussed in terms of water vapor loss from the egg during incubation, development of the embryo's blood buffer system and the importance of the permeability of the egg shell to embryo survival.