How does the bird embryo breathe inside the egg if it does not have a placenta?

I share the link to my channel: / @mauriciogiuliodori In this respiratory physiology video, it is described how bird embryos breathe while they are inside the egg since they do not have a placenta. That is, they cannot exchange gases with maternal blood as we and other mammals do inside the womb. The embryonic membranes are described and what the gas exchange is like between those membranes of the embryo (which replace the placenta) and the air in the atmosphere. That is, chorioallantoic respiration. The air chamber inside the egg and its importance at the end of the incubation period are described. Finally, the characteristics of the eggshell are shown, such as its pores and the role they play in the diffusion of respiratory gases and in containing the entry of germs.