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When most people purchase eggs from chickens at grocery stores they are unfertilized. These eggs represent the female ovum, and they cannot develop into chickens because the chicken have not been exposed to the sperm of male chickens (roosters), which would create fertilized eggs. Sometimes shoppers may notice that stores sell fertilized eggs and there are a lot of questions about what these are and their purpose.
Fertilized eggs are eggs resulting from chickens that have mated with roosters or that have been exposed to rooster sperm. The fertilization actually occurs before the egg is fully formed and has a shell. Chickens that are free range and are kept with roosters will often lay fertilized eggs, though not every egg a free-range chicken exposed to roosters produces is technically fertilized. Given the right conditions, like placing the eggs in an incubator or allowing then hen to incubate the eggs, the eggs usually become baby chicks. Incubation takes approximately 21 days.
There are a few mistaken impressions about fertilized eggs. When a fertilized egg is collected and refrigerated instead, the egg will not become a chicken in the fridge. Many people believe that chicken eggs that have blood spots on them are fertilized. This actually occurs during ovulation when there is breakage of blood vessels on the egg’s surface. It is not a baby chicken and hens that have no contact with roosters produce these about 1% of the time. These eggs are fine to eat, though some people avoid them. Egg producers tend to use various techniques to spot these, called candling, and remove them.
Another mistaken impression about fertilized eggs is that the white ropey strands in an egg are either a chicken embryo or rooster sperm cells. Actually these strands, called chalazae, occur in all chicken eggs. The freshest egg has more chalazae than do eggs that have been sitting for several weeks.
Instead, a fertilized chicken egg will have a slightly different appearance. The yolk has tiny red lines that start at the top and run to all sides of the egg. This appearance indicates that the egg could turn into a baby chick under the proper circumstances, but since these eggs are usually collected within a few hours of being laid, they don’t have fully formed chicks in them.
There has been much argument about the health benefits of fertilized eggs with some claiming that they are nutritionally superior to unfertilized ones. There may be some health benefits to eating very fresh eggs, fertilized or not, though these may be extremely minimal. Eggs do start to lose protein the longer they sit, though refrigeration helps to arrest protein loss. Most scientific nutritional organizations claim there is little to no difference in nutritional benefits between fertilized and non-fertilized eggs.