|Photo from Wikimedia Commons|
Once incubation begins, the embryo starts to develop. It needs food, water and oxygen to grow. The yolk provides most of the food, the albumen provides protein and water and the oxygen passes through the shell and the membranes.
During the last two days the chick inside the egg begins to breathe with its lungs for the first time. To do this it needs an air supply inside the egg shell. At the round end of the egg there is an air space. if you examine an egg carefully with a magnifying glass, you'll see that there are tiny little holes, called pores, that let the bird breathe. The chick also calls through the shell to communicate with its momma before it hatches.
Birds Embryonic development video: https://youtu.be/-Ah-gT0hTto
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