albumen as possible between the embryo and
the shell, and prevents premature contact
between shell and embryo, which could distort
the embryo’s development.
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