Fertilized vs Unfertilized Eggs

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In our modern society, many of us have become so removed from our food that there are lots of myths swirling around about eggs.  One of the most common (and unlikely!) fears surrounding eggs is that you could pull an egg from the fridge, crack it open and out pops a fully or partial formed chick.  So let’s take a look at that incredible, edible egg to sort out fact from fiction concerning fertilized eggs.

Quick chicken biology 101

Hens begin to lay eggs somewhere around the 16th-20th week of life.  Like human females, a hen is born with all the eggs she will ever have.  Thousands of tiny ova (the things destined to be egg yolks) lie within her ovaries.  When she reaches maturity, the ova is released into the oviduct where it will begin it’s journey, along the way adding the egg white, shell membranes and shell.  Immediately after the yolk is released, it goes into a “holding tank” called the infundibulum for about 15 minutes.  If the yolk is to be fertilized, it needs to happen in this short 15 minute window.  Hen’s bodies can store a rooster’s sperm for up to a week after mating, to be used during this window.  The entire process of forming an egg will take about 24 hours to complete.

Roosters are not needed

The process of producing eggs will happen with or without the presence of a male chicken.  The releasing of an ova is regulated not by mating, but by the hen’s pituitary gland. The pituitary gland is located by her eyes, just beneath the thin wall of her skull.  The skull is so thin that light can penetrate it, stimulating the gland below.  The pituitary gland needs 14-16 hours of daylight stored before it will signal to the ovaries to release an ova.  This is why chickens lay less in the short winter days.  So to lay eggs, hens need light NOT a rooster.

Chick Development

If the hen has mated within the past week, as long as the rooster is healthy, it is a good bet the eggs she lays will be fertilized.  But what does that mean for you as the egg eater?  Honestly it means nothing.  Even if you have a rooster, as long as you are collecting eggs every day you will never crack open an egg to find a developing chick.  For a fertilized egg to begin developing, it needs to be kept in the right conditions (generally around 100 degrees with 60% humidity) for several hours.  Even after several hours, the development is so slight, you would likely not notice it.  It take about 3 days of incubating at proper conditions for visible veining to show.  This means if you have a rooster and you forget to collect eggs for three days but they are just sitting out in a nest box with no hen sitting on them, they are not going to be developing (unless it’s been really hot!).  It also means if you have a rooster and a hen that is sitting on eggs, but you are collecting them daily, they are not going to be developing.  Putting the eggs in the refrigerator will completely stop all development, so if you had a fertilized egg sitting under a hen for a week and you collect it and put it in the fridge, it will not continue to develop (although it will likely have some development).  It takes 21 days for a chick to fully develop and hatch.

How can I tell if an egg is fertilized?

Other than having the potential for developing into a chick, there is no difference.  There is no difference in taste or nutritional quality.  When you crack open an egg, in the yolk you will notice a small white dot called a blastodisc.  In a fertilized egg, the small white dot will have a bulls eye type ring around it and is now called a blastoderm.

Normal egg signs that don’t indicate fertility

Red spots or blood spots – this does not indicate fertility, it is just a broken blood vessel.  This is safe to eat.  The very earliest chick development at day four looks like veining, not spots.
White stringy “things” – some people think this is an umbilical cord, it is not.  It is called a chalaza and is a rope like structure that helps keep the yolk suspended in the egg so it isn’t banging against the shell.  The chalaza is more prominent in farm fresh eggs.  As eggs age, the chalaza begin to dissolve so you are less likely to notice these in store bought eggs.  It is totally safe to eat.

Bottom Line

If you are buying commercial eggs from the grocery store, you are not going to ever find a developed or developing chick in your egg.  Large egg farms only keep female hens so there would be no chance of them getting fertilized.  Even if some rogue rooster were to sneak in, the eggs at egg farms are collected everyday and refrigerated so development would not have a chance to start.  If you are eating eggs from a local small farmer or from your own backyard hens, even if a rooster is present, as long as eggs are collected every day, you do not need to worry about cracking open an egg and having a chick pop out!



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One comment

  1. Robin says:

    Great information! I’m always a little taken back by the thought that roosters are needed for egg production. I read it in a forum just the other day.

    Nice to find you through the Homestead Blog Hop!

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