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I Didn't Know That

October 19, 2000

Q: How does a chicken put the shell on an egg?

A: The production of an egg by a hen works much like an assembly line, according to animal scientist Ralph Ernst of UC Davis. Eggs begin as single cells in follicles in the ovary. Triggered by an as-yet poorly understood signal, they grow over a period of several days into the full yolk. Eight to 10 are at various stages of maturation at any time.

The yolk then passes to the infundibulum, where fertilization occurs if the hen has mated. After about 15 minutes, it passes to the oviduct. In the first part of the oviduct, called the magnum, a thick layer of protein called albumen is deposited around the yolk over a period of three hours. In the next segment of the oviduct, called the isthmus, two membranes are formed loosely around the albumen. (As the egg cools and shrinks after being laid, an air bubble forms between these two membranes.) The egg then proceeds to the shell gland, where body fluids diffuse through the membranes and plump the egg up to its full size. Over about 19 hours, crystals of calcium carbonate then precipitate onto the surface of the membrane, leaving tiny pores that allow gases to pass in and out of the egg. If it is a brown egg, the last part of the shell receives the pigment. Finally, a waxy layer called the cuticle is deposited on the surface and the egg is laid. In high-producing chickens, this entire process takes about 24 hours.

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