Aeration of the shell membranes of avian eggs

The inner and outer shell membranes (ISM, OSM) of fertile hen's eggs become aerated during the first ten days of incubation. The volume of gas in the compound membrane increases from 0 to 65% by day 11 when practically all of the space between the fibers is gas filled. The amount of gas depends on a balance between the capillary tension produced by fluid menisci in the interstices between fibers and the colloid osmotic tension of the albumen. Capillary tension, measured directly with a pressure plate apparatus, and colloid osmotic pressure, measured with a new technique, are in virtual equilibrium during aeration. These tensions increase from 0.15 bar at laying to 2.25 bar on day 11 because of evaporation from the membranes and active removal of water from the albumen by the embryo. Water leaving the membranes by evaporation is only partly replaced from the albumen. Thus the water contents of the membranes and albumen decrease (ISM: 79 to 56%, OSM: 71 to 52%, albumen: 86 to 58%). As the membranes dry, gas enters the larger pores first. Physiological estimates of effective radius show smaller (x = 1.3 micrograms), bimodally distributed pores in the ISM and larger (x = 3.4 micrograms), unimodally distributed pores in the OSM.