Introduction

The eggshell, underlying shell membranes and chorio-allantois serve the avian embryo as a gas exchange organ, and the air spaces contained within the shell membranes are functionally analogous to the alveolar spaces of mammalian lungs (Rahn & Paganelli, 1985). However, gas exchange through the pores in the shell occurs primarily by diffusion, while alveolar ventilation depends in large part on convective gas flow. This chapter examines the applicable laws of diffusion which govern gas exchange in the avian tgg and the consequences of these laws for the developing embryo. Recent additions to our knowledge of the gas phase pathway for oxygen (O2), carbon dioxide (CO2) and water vapour exchange are reviewed together with the fluxes of these gases in this pathway and the partial pressures in the air space which drive the fluxes. The regional differences in gas tensions in the air space and convective gas flow induced by non-equimolar diffusion are also considered. Cardiogenic pressure pulses which are transmitted through the pores are also described.

The laws of diffusion in the gas phase

The seemingly impervious shells of bird eggs have been known to contain pores for more than 140 years, according to reports cited by Romanoff (1943). Through the pores and the underlying shell membranes must pass O2, CO2 and water vapour as these gases enter or exit the embryonic circulation and tissues during development. The pores in the shell, and the gas-filled spaces between the fibres of the outer and inner shell membranes, provide the pathway for gas exchange (and hence the resistance) between the chorio-allantoic capillary blood and the ambient atmosphere.