Now we know how aquatic eggs (amphibians and fish) breathe, let’s explore how eggs that are normally laid on land exchange their respiratory gases. The amniotes (reptiles, birds and mammals) mostly lay their eggs on the land, in burrows or in nests high up in the trees. Like any living thing, the embryo within the egg needs to exchange respiratory gases (oxygen and carbon dioxide) with the environment. To do this they have tiny little pores (holes) in their eggshells. Domestic hen eggs have ~7500 pores. Just beneath the surface of the eggshell lie membranes (amnion, chorion and allantois) that have many little blood vessels where the gases can be exchanged and carried to the embryo. Two of these membranes, the chorion and the allantois become joined in older bird embryos to form the chorioallantoic membrane or CAM which is the major site of gas exchange in bird embryos.
Basically, oxygen can diffuse (move down its concentration gradient) through the pores in the eggshell. It enters the bloodstream of the embryo by diffusing though the membrane just beneath the shell into a blood vessel (e.g. capillary) and binds (joins) to haemoglobin which helps increase how much oxygen the blood can carry. The oxygen is then pushed through the blood vessels by the pumping heart of the embryo and can reach every cell in the embryos body. At the cells the carbon dioxide waste is created (futures blogs will explore exactly why the cells need oxygen and create carbon dioxide waste). The carbon dioxide travels back through the blood vessels (although without the help of haemoglobin which usually only joins to oxygen) diffuses through the membrane and out of the pores into the environment.
to breathe, or not to breathe 