Why does the air sac get bigger as a chick is developing inside an egg?

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The yolk and albumen together contain about 65% water and when this evaporates through the porous shell and membranes from the air sac content changes occur which can be identified as the process of ageing. The rate of evaporation is affected by the area of shell surface to volume of the contents. Certain egg shapes lose water quicker than others. External conditions too, affect the water loss. For example, eggs stored in a warm, dry environment will age faster than those kept in cool, humid conditions. It is well known that eggs will keep longer in a refridgerator and if this also contains a dish of water, little evaporation will occur at all. As evaporation proceeds, the air sac increases in size and the egg contents decrease in volume. These changes cause the egg membranes to stretch in some places and relax in others, thereby allowing the contents to 'flop' within the shell when the egg is shaken sharply. This becomes more evident by the white being less firm as certain proteins within the albumen {denature;} a process related to changes in the OH (acidity) brought about by the absorption of carbon dioxide. Interstingly, an old fashioned way of storing eggs was to place them in lime water which absorbed the carbon dioxide so preventing it from entering the egg and changing the nature of the contents. Mar 20 02, 6:04 PM