In mammals, the embryo is supplied with oxygen via the placenta and the umbilical cord.
But how do the embryos of birds or reptiles get oxygen?
In: Biology
The embryo only begins to breathe a few days before hatching. There’s a tiny air bubble inside the egg that is pierced by a tiny calcium horn in the embryo to begin breathing. Within days, the embryo has grown up enough to break the outer shell and hatch.
If it prematurely breaks the inner bubble, it can literally drown and fail to hatch.
Egg shells are actually permeable to oxygen. It’s pretty much that simple. The shell itself is made of calcium carbonate with thousands and thousands of microscopic pores that allow gasses to go in and out. The outside of the shell is coated in a membrane that allows gasses through, but not pathogens. In the US, eggs are washed to remove any fecal matter or dirt that may have gotten on the eggs, which also removes that membrane. That leaves nothing to stop bacteria from getting through the pores in the shell, which is why eggs need to be refrigerated in the US. In the UK, they do not wash their eggs and as such the membrane protects the eggs and they do not need to be refrigerated.
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