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Ask Dr. Knowledge

What are the white things that hang off an egg yolk?

June 28, 2010

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What are the white things that hang off an egg yolk? A friend claims it is chicken sperm and another says they are tiny chicken embryos. Are either of them right?

Neither of your friends has it quite right, and it’s amazing how few people know what these things are even after having cracked thousands of eggs.

An egg yolk is basically a bag of concentrated food for the development of a chicken embryo if the egg is fertilized. It doesn’t float around freely inside the clear egg white, but is anchored to the shell by two little twisted ropes called chalazae (pronounced cuh-LAY-zee), and these are the white things you are talking about. One chalaza connects the yolk at the more pointed end of the egg and the other at the rounder end. This tethering ensures that the yolk is protected against hitting the inner walls of the egg if the egg is moved around.

A lot of people pick these things out when they crack an egg, but they are quite safe to eat. They can also tell you something very useful. As eggs age, these structures start to disappear, so clearly visible chalazae are a good sign your eggs are fresh.

You can also see the spot that could develop into a chicken as a little white region on the yolk which is not one of the chalazae. This is called the germinal disk. If the egg were fertilized, it would have genetic information from the mother hen and from the rooster’s sperm, but it wouldn’t actually be rooster sperm. Anyway, most chicken eggs are unfertilized.

Some people wonder why chickens keep laying eggs that aren’t fertilized. The way to understand this is to realize that humans ovulate once a month, and that happens regardless of whether or not the egg will be fertilized. Chickens are similar, but they ovulate about once a day and then push the eggs out of their bodies.

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.