What types of substances/liquids would be able to penetrate an eggshell?

The shell is a special membrane higher vertebrates have called an amnion. It is waterproof, allowing things like reptiles and birds to breed on dry land. The shell of a brid's egg is composed of Calcium carbonate, which is what limestone, cement and sea shells are made from. It is a basic material and acids will dissolve it. The amnion is a semi-permiable membrane. This means it will allow some substances to diffuse through it and block others. The amnion is permiable to gasses like Oxygen and Carbon dioxide. There is a large space at the end of the egg which stores extra air. A bird's egg is designed to be very sturdy in terms of outside forces applied to it. However, it is very weak in terms of forces from the inside. This allows the egg to hatch easily despite the fact it resists crushing from the weight of the parent incubating it. Mammals have amnions as well, but only three species lay eggs. In the rest, the amnion is the birth sack which ruptures just prior to birth. This is the equivalent of an egg hatching.