Breathing Inside an Egg

Tucked away inside its shell, a bird embryo has to breathe. But, rather than using lungs to draw air in and push carbon dioxide and water vapor out, a bird embryo relies on“diffusion”- -the natural movement of gases- -much as do insects (which also lack lungs). In fact, both insects and eggs use tiny pores, or holes, and pore canals that connect the outside with their interior. For birds there are hundreds or thousands of tiny pores distributed all over the shell surface. The pores connect, via a narrow tube, the embryo’s blood supply to the outside world. The number of pores per egg varies markedly between species, partly but not entirely related to the size of the egg. Since the pores are fairly straight and run vertically from the inner to the outer surface, their length is usually similar to the thickness of the shell. Generally, the number and size of pores determine how much and how fast oxygen diffuses into the egg. As well as taking away unwanted carbon dioxide, the pores allow water vapor to escape from the developing embryo. As the embryo grows it generates water, referred to as metabolic water, produced as a result of the metabolism of food. Different foods generate different amounts of metabolic water. Fat, for example, yields comparatively high amounts of metabolic water.

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Inside an egg the developing chick generates plenty of metabolic water from the fat-rich yolk (yellow part of the egg) as it grows. This water has to be removed; otherwise the embryo would drown in its own juices, so to speak, and it does this by allowing it to diffuse as water vapor through the pores in the shell. As a result, eggs lose weight during the course of incubation. What is remarkable is that, despite the huge variation across bird species in the size of eggs, the duration of incubation, and the relative size of the yolk, the loss of water between laying and hatching is always about 15 percent of the egg’s initial weight. The water vapor lost during incubation ensures that the relative amount of water in the egg is the same in the chick at hatching as it was when the egg was laid. In other words, the composition of the newly laid egg has evolved through natural selection to ensure that the newly hatched chick has the right composition- -in terms of the amount of water in its tissues, too. This is achieved by adjusting-via natural selection- -the effective pore area such that all the metabolic water produced during development is eliminated before hatching. One consequence of this loss of water vapor is a space in the egg, roughly 15 percent of its volume, that becomes the air cell at the blunt, or flat, end of the egg and provides the amount of air needed by the chick just before it hatches.

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The air cell is formed between the inner and outer shell membranes when the egg is laid. As the egg cools and its contents contract after leaving the female’s body, air is drawn in through the pores and accumulates in a lens-shaped pocket at the blunt end of the egg. If you hold a hen’s egg against a bright light, you can see the air cell. Furthermore, when you peel a hard-boiled egg to eat, the air cell’s presence is revealed by the flattened area of white at the blunt end where the air cell has pressed down on the albumen (egg white). William Harvey in the 1600s was the first to think about the role of the air cell, dismissing the then-widespread belief that its position in the egg signaled the sex of the chick. As development proceeds, the air space increases in size, and it is for this reason that you can assess the age of an egg, or its stage of development, from how it floats in water: a very fresh egg with virtually no air cell sinks; an older egg floats.

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Because gases behave differently under pressure, we might expect the size and number of pores to differ among birds breeding at different altitudes. Specifically, the loss of gases will be less at high altitudes. And this is confirmed by a comparison of birds breeding at different elevations: species breeding at high altitudes have fewer, smaller eggshell pores.

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?Tucked away inside its shell, a bird embryo has to breathe. But, rather than using lungs to draw air in and push carbon dioxide and water vapor out, a bird embryo relies on“diffusion”- -the natural movement of gases- -much as do insects (which also lack lungs). In fact, both insects and eggs use tiny pores, or holes, and pore canals that connect the outside with their interior. For birds there are hundreds or thousands of tiny pores distributed all over the shell surface. The pores connect, via a narrow tube, the embryo’s blood supply to the outside world. The number of pores per egg varies markedly between species, partly but not entirely related to the size of the egg. Since the pores are fairly straight and run vertically from the inner to the outer surface, their length is usually similar to the thickness of the shell. Generally, the number and size of pores determine how much and how fast oxygen diffuses into the egg. As well as taking away unwanted carbon dioxide, the pores allow water vapor to escape from the developing embryo. As the embryo grows it generates water, referred to as metabolic water, produced as a result of the metabolism of food. Different foods generate different amounts of metabolic water. Fat, for example, yields comparatively high amounts of metabolic water.