Facts and Myths about Fertilized Eggs

 There are a few common misconceptions about fertile eggs that will be cleared up in this article, but first, it is important to understand the differences between fertile and infertile eggs as well as incubated and un-incubated fertile eggs.

There are a few common misconceptions about fertilized eggs that I hope to clear up in this article, but first, it is important to understand the differences between fertilized and unfertilized eggs as well as incubated and un-incubated fertilized eggs.

UNFERTILIZED EGGS 

A hen must mate with a rooster in order for her egg to contain both the male and female genetic material necessary to create an embryo inside the egg. An unfertilized egg contains only the hen’s genetic material, which means a chick can never hatch from that egg. The hen’s genetic material, termed the blastodisc, can be identified on an egg yolk as a light-colored dot with irregular borders. Every egg contains a blastodisc.

FERTILIZED EGGS

When an egg is fertilized by a rooster, the blastodisc becomes known as the blastoderm, which is the first stage of embryonic development. The blastoderm is identified by its bullseye appearance, having regular, concentric circles. The blastoderm will remain in a state of suspended animation, so to speak, forever unless warmed at particular temperatures for several hours. When a fertile egg is incubated under precise, steady temperatures and humidity levels for 21 days, the blastoderm may develop into a chick.

INCUBATED FERTILIZED EGG

A fertilized egg must be kept at a temperature of at least 85°F for several hours in order for the blastoderm to begin developing into an embryo.

This fertile egg has been kept at the proper incubation temperature for 24 hours.
This fertilized egg has been kept at the proper incubation temperature for 24 hours.
This fertile egg has been kept at the proper incubation temperature for 2 days.
This fertilized egg has been kept at the proper incubation temperature for 2 days.

UN-INCUBATED FERTILIZED EGG

A fertilized egg that is never incubated will never contain an embryo and will never look like anything other than common breakfast food.

A fertile egg that is never incubated will never contain an embryo and will never look like anything other than common breakfast food.
Only fertile eggs that have been incubated under proper conditions can become an embryo. Freshly collected eggs can never contain a chick.

MYTHS & FACTS ABOUT FERTILIZED EGGS

MYTH: A fertilized egg has a baby chick in it.
FACT: Freshly laid eggs can never contain a chick. Only fertilized eggs that have been incubated under proper conditions can become an embryo and develop into a chick. To see exactly how an embryo develops, from the inside and out, each of the 21 days until it hatches, click here.

There is no scientific evidence that fertile eggs are nutritionally superior to infertile ones. Fertile eggs have remnants of the male's sperm and a small layer of cells that could form the embryo. The proportion of these to the total egg is so small that it is impossible to detect chemical differences between fertile and infertile eggs.
There is absolutely no flavor difference between fertile and infertile eggs.

MYTH: Fertilized eggs are more nutritious than unfertilized eggs.
FACT: There is no scientific evidence that fertilized eggs are nutritionally superior to unfertilized ones. Fertilized eggs have remnants of the male’s sperm and a small layer of cells that could form the embryo. The proportion of these to the total egg is so small that it is impossible to detect chemical differences between fertile and infertile eggs.1

There is absolutely no flavor difference between fertile and infertile eggs.

MYTH: Fertilized eggs taste different from infertile eggs.
FACT: There is absolutely no flavor difference between fertilized and unfertilized eggs.

A blood spot inside an egg can occur at various points in a hen's reproductive system as a result of a blood vessel rupturing. It can be the result of a genetic predisposition, a vitamin A deficiency, or a random event. There is no correlation between blood spots and fertile eggs. The misconception may have come about due to the appearance of incubated, fertile eggs developing veins at or around day four into incubation. Veining looks nothing like a blood spot, however.

MYTH: A blood spot inside the egg means the egg is fertilized.
FACT: A blood spot inside an egg can occur at various points in a hen’s reproductive system as a result of a blood vessel rupturing. It can be the result of a genetic predisposition, a vitamin A deficiency, or a random event. There is no correlation between blood spots and fertilized eggs. The misconception may have come about due to the appearance of incubated, fertilized eggs developing veins at or around day four into incubation. Veining looks nothing like a blood spot, however.
The blood in the following photo of an unincubated egg is NOT a developing embryo. The blood has nothing to do with the egg being fertilized or not fertilized, it was caused by a glitch that occurred while the yolk was being released from the hen’s ovary and would have occurred whether or not a rooster mated with the hen that laid this egg.

The blood in the following photo of an unincubated egg is NOT a developing embryo. The blood has nothing to do with the egg being fertilized or not fertilized, it was caused by a glitch that occurred while the yolk was being released from the hen's ovary and would have occurred whether or not a rooster mated with the hen that laid this egg.
Only eggs that are incubated and begin developing can be identified as fertile after a minimum of 3 days. The blastoderm and blastodisc cannot be seen through the shell. It is possible for an incubated egg to be fertile and appear infertile when candled if the egg failed to develop. The only way to determine whether an unincubated egg is fertile is to crack it open and identify the blastodisc or blastoderm.

MYTH: Candling an egg will reveal whether the egg is fertilized or not. (Candling is the term used for shining a light through an eggshell to see what’s inside.)
FACT: Only eggs that are incubated and begin developing can be identified as fertilized after a minimum of 3 days. Neither the blastoderm nor a blastodisc can be seen through the shell. It is possible for an incubated egg to be fertilized and appear unfertilized when candled if the egg failed to develop. The only way to determine whether an unincubated egg is fertilized is to crack it open and identify the blastodisc or blastoderm.

This egg was candled after 4 days of incubation.
This egg was candled after 4 days of incubation.

Sources & further reading

1 http://urbanext.illinois.edu/eggs/res04-consumer.html

Rachel Divider

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Beth Swift
Beth Swift
3 years ago

I bought fresh eggs from a farm yesterday, mind you I have not had farm fresh eggs before. I noticed when making eggs this morning the shell was harder to crack then store eggs. Why is this? Also why do some eggs has a white stringy thing attached to the yoke? Store eggs have a white dot on the yoke, why is this. I’m sorry for all the question but I was told your the one to go to. Thank you

Barclay Wallace
Barclay Wallace
2 years ago

Thank you for this article! I’ve raised chickens ever since I was a teenager and this article just answered the questions I wanted to know. I want some chicks from a new rooster so I read waiting 3 weeks for the old roosters sperm to go away. Very informative! Thanks

Hollie
Hollie
2 years ago

Hi, Kathy. I would like to clarify something as there may be a typo in this article. See the word I underlined in red? I believe you meant to say blastoDERM here since you defined a blastoderm as the spot in an egg which contains both the male and female genetic material.

Hollie
Hollie
2 years ago

You’re welcome! I appreciate this site very much. Love the videos on FB too. 🙂

Ahmad
Ahmad
1 year ago

Amazing

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