Buzz
You may not think about it as you're cooking them sunny side up on a Sunday morning, but chicken eggs—and those of other birds and reptiles—actually come in some quite clever and complex packaging.
Hard-shelled bird eggs contain albumen, or the 'egg white,' along with the yolk. Inside the yolk, the fertilized egg cell, or embryo, starts to grow, absorbing nutrients from both the yolk and albumen. The baby bird is sheltered and nourished within the egg, having almost everything it needs—except, it seems, for a little fresh air.
Animals that grow inside their mothers, like mammals, rely on the umbilical cord to get oxygen. A bird egg, on the other hand, doesn’t have a direct means of taking in oxygen or removing carbon dioxide, but the egg still ensures the embryo’s needs are met.
Directly beneath the shell of the egg, there are two membranes. When the mother lays the eggs, they are initially warm, and as they cool down, the contents inside the egg contract slightly. This causes the two membranes to separate a bit, forming a small air pocket. As the baby bird develops, it forms an allantois, a hollow pouch that grows from the lower part of the chick's gut. This pouch eventually fuses with the second membrane, the chorion, surrounding the yolk, creating a chorioallantoic membrane. Similar to lung tissue, this membrane links the baby bird’s circulatory system to the air outside the egg.
As the embryo matures, it uses this membrane to take in oxygen and release carbon dioxide (CO2). The hollow part of the allantois also acts as a storage space for nitrogen-containing waste.
The surface of the bird egg is covered with thousands of microscopic pores that allow CO2 to escape and fresh air to enter. These pores also enable moisture to enter the egg, which prevents the developing bird and other parts of the egg from drying out. This explains why hard-boiled eggs always feel slightly heavier than raw ones.
What happens to the chorioallantoic membrane after the baby bird hatches? It remains attached to the eggshell. If you're hoping to spot it in an unfertilized egg—like the one you're scrambling for breakfast—your chances are quite low, as it only forms when a fertilized embryo begins to develop. Without a baby bird, there’s no chorioallantoic membrane to be found.
A version of this article was originally published in 2013 and has been updated for 2024.
