Membranes, Water, and Shell

Once the albumen proteins have been applied to the yolk, it spends an hour in the next section of the oviduct being loosely enclosed in two tough, antimicrobial protein membranes that are attached to each other everywhere except for one end, where the air pocket will later develop to supply the hatching chick with its first gulps of air. Then comes a long stretch—19 or 20 hours—in the 2-inch-/5-cm-long uterus, or shell gland. For five hours, cells in the uterus wall pump water and salts through the membranes and into the albumen and “plump” the egg to its full volume. When the membranes are taut, the uterine lining secretes calcium carbonate and protein to form the shell, a process that takes about 14 hours. Since the embryo needs air, the shell is riddled (especially at the blunt end) with some 10,000 pores that add up to a hole about 2 mm in diameter.