Shell membranes are thin layers of protein that line the inside of an egg shell. They serve as a protective barrier, helping to keep out bacteria and maintain the egg's internal environment. These membranes are made up of two layers: the inner and outer membranes, which work together to provide structural support. In addition to protection, shell membranes also play a role in gas exchange. They allow oxygen to enter and carbon dioxide to exit, which is essential for the developing embryo inside the egg. This function is crucial for the growth of chickens and other birds that lay eggs.