2 thin layers of fibrous proteins lining the eggshell, essential components of the shell both in its development in the oviduct and in protecting and nourishing the growing embryo; surround the yolk, albumen, ovum, and extra-embryonic membranes; serve as a barrier against microbes from outside the shell, facilitate gas exchange and water evaporation from the shell, and may help maintain the shell’s structural integrity; the inner shell membrane wraps around and holds together the albumen; the outer membrane lies tightly over the inner membrane and serves as an anchor for the shell during its development, when organic materials that produce calcium carbonate crystals intertwine with membrane fibers; the inner membrane separates from the outer membrane at the blunt end of the egg, forming an air cell, which increases in volume until it is pierced by the chick’s egg tooth as it nears hatch; this so-called internal pip triggers the chick’s lungs and air sacs to begin their respiratory function; the shell membranes develop in the isthmus of the oviduct before the egg is laid; low air humidity during hatching sometimes can dry the membranes and make them difficult for the chick to tear through