How Chicken Eggs Are Formed
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How Chicken Eggs Are Formed

People wonder just where do eggs come from. Sure they come from chickens (hens to be specific), but how do the develop while inside the hen?

Lots of people wonder just where do eggs come from. Sure they come from chickens (hens to be specific), but how do the develop while inside the hen?

The egg actually starts as a yolk, which is correctly called an oocyte at this point, and is produced by the hen during ovulation, which happens almost every day, depending on the breed of hen, her age, and season.

Hens produce eggs whether or not roosters (males) are present, the rooster only fertilizes the egg, so if no rooster is present the egg grows but will not be fertile. If the hen was bred she would have the sperm inside her oviduct and fertilization would occur. The Germinal Disc is a spot on the side of the yolk and is what would grow into the chick if the egg is fertilized.

As the yolk passes down through the spiraling oviduct it twists and thin strands called chalazae form. The chalaze hold the yolk in place, anchoring it to either end of the egg.

The egg shell itself forms last, then gets its “bloom” a thin coating which helps it pass through the cloaca smoothly, just before the egg is laid.

eggs development

photo source, from a butchered hen

The whole process takes about 24 hours and begins again shortly after an egg is laid.

Proper feeding is ultra important to ensure that the egg develops correctly. Hens should be fed a special layer ration, which should either contain bits of oyster shell or they can have it offered separately. The shell provides the calcium needed for the production of their own egg shells. Some people will feed hens crushed up egg shells for this purpose too.

eggs in chicken

photo source

Oddities and Abnormal Eggs

Some people think the color of the egg shell can show if the egg was from a free range hen or not, this is not true!  The color of the egg shell has nothing to do with nutrition or how a hen was kept, it has more to do with the breed of chicken. Typically brown hens lay brown eggs, white hens lay white eggs, however there are some breeds (such as the Araucana) that lay blue or green eggs.  There are some brown breeds that lay white eggs, and visa versa, so this general rule is not always true.  What a hen eats will impact the color (and flavor) of the yolk. Hens on better diets generally produce deeper colored yolks.

colored eggs

photo source

Sometimes an egg will have two yolks. This is simply a matter of ovulation happening too quickly, the yolks, as they pass through the oviduct become stuck together. This is more common in young hens and those of certain cross breeds common in India. If fertilized the growing chicks will likely fight each other and die, as such the eggs do not hatch.

Yolkless Eggs - Even less common are eggs with no yolks. These are called “wind eggs” or “dwarf eggs”. They are typically a hens first effort to produce an egg and are often smaller and misshapen.

Blood Spots - These are more common in brown eggs. This has nothing to do with whether or not an egg is fertile. It is simply a fluke that occurred when a blood vessel ruptured during the development of the yolk. Eggs are typically screened when sold and those with blood spots are removed as people find them undesirable even though they are perfectly safe.

After Being Laid

Once an egg is laid, if fertile, it should grow into a chicken which will hatch in 21 days. Typically not all fertile eggs will hatch. There are many conditions that must be met in order for fertile eggs to hatch.

Eating eggs are collected immediately after being laid in battery hen situations or are gathered throughout the day in smaller operations, such as free range farms.

Related Links

The Cruelty of Battery Hen Cages and Mass Egg Production

Why Won't My Hens Eggs Hatch?

Bantam Breed of Chickens

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Comments (23)

Really interesting article and great pictures!

This was very interesting. I learned a thing or two.

Brenda, this is a signature piece for you! Coincidentally, believe it or not, we were talking about eggs this morning and I told the breakfast group about your last article and someone asked the question, "so how are eggs formed?" And here you are. Timely (lol) and most informative. A great factoid. Marie :)

Ranked #2 in Birds

Marie.. your breakfast club might not feel like eating eggs for a while if you show them the pictures though!

Ranked #8 in Birds

Great article

Very interesting.

Very well written article. Liked the facts here. :)

Very interesting! I'm surprised at how the yolks look at various stages of development. Today when I cooked eggs, I cooked them without the yolks and just scrambled the whites. I love eggs, but I am having to follow a cardiac diet.

Very informative, Brenda. All these years we thought that blood spots were developing embryos.

Fantastic article Brenda, really interesting and something we just don't really think about. Lots of information I wasn't sure about. My son did an experiment last night on an egg. If you cover them in a glass with vinegar and leave for a few hours it is supposed to make the shell soft so they don't break and can bounce! needless to say when I got up this morning there was smashed egg all over the garden, at least he had the sense to try it outside!

wow that is a new outlook on eggs

Mufumbiro Moses

Good info for me as i cause laying poultry to lay 90% to 100% without a deeper thought of how this happens inside the bird.

Best regards


Help! I have a Bard Rock Hen that is laying eggs with partailly formed shells. This has beening going on for a few weeks now. Any suggestion?

Ranked #2 in Birds

to Dave Be sure your hen is on a proper laying ration and gets calcium in her diet too. Calcium may be given in the form of oyster shell which can be purchased in large bags at livestock feed stores. Or you can even feed her broken up shells from good eggs. Very young and very old hens may have this problem as well, but typically for young hens it does not last for more than a few days if they are on a good diet. Other than that it might be a problem with the particular hen and you may wish to cull her.

Mosese Mufumbiro

I do appreciate the contribution given by Brender Nelson, her opinion could be helpful, however note that nutrient shortages and excesses in the diet of a bird both have negative effects on the bird’s productivity and shell quality.

There is need for quantification and tailoring the solution to Dave’s challenge by getting the nutritional profile for the Bard Rock Breed of poultry, the age of your bird and the feed raw materials available in Dave’s location. If this information is availed to me, I will offer to design a formulation (diet) to solve the problem.

I am also conscious that you may fail to get the nutritional profile for the Bard Rock Breed, if this is true let me so that I can afford time to find it. In case I fail, I shall use my experience to develop a formulation for you.

However below is some relevant information as food for thought to enable Dave to do self troubleshooting in case the challenge is nutritional or beyond nutrition?


The importance of adequate nutrition in providing the hen with what she needs to maintain adequate eggshell quality is obvious. A hen lays approximately 250 eggs per year which corresponds to 20 times the quantity of calcium in her bones at any one time. Therefore, the calcium requirement of the laying hen is great. It can be calculated that during the 20 hours that are required to form an eggshell, 25 milligrams of calcium must be deposited on the egg every 15 minutes. This amount of calcium is the total amount of calcium in a normal hen's circulatory system at any given point in time. The laying hen is also not 100% efficient in extracting calcium from the available sources in the diet. Therefore, many times the diet has to furnish in excess of 4 grams of calcium to the hen daily. Calcium availability values are sometimes not known and it must be remembered that higher daily intakes are needed when the availability values are known to be low. High phosphorus content in the feed and excess chlorine may have a negative effect on eggshell quality. It is possible that these two elements act negatively on eggshell quality through their influence on acid-base balance (pH) in the blood. The importance of adequate vitamin D intake by the hen is obvious and it is essential for proper calcium and phosphorus utilization. However, excess vitamin D and its metabolites have not been shown to benefit eggshell quality when normal hens are already consuming adequate vitamin D. Other vitamins and trace minerals, when fed above the hen's requirements, have failed to improve eggshell quality.


Usually, eggshell quality is not as much of a problem in cooler environments as compared to hot environments. One of the contributing factors causing poorer eggshell quality in hot weather is hens not consuming adequate feed. This can lead to problems in body weight, egg production, egg size, and eggshell quality if measures are not taken to assure adequate daily nutrient and energy consumption. When environmental temperature becomes excessively hot, feed intake decreases and energy becomes the first limiting factor to the hen. Inadequate consumption of amino acids, calcium, phosphorus and other nutrients can usually be corrected by adjusting the nutrient density of the diet. However, it must also not be forgotten that in hot weather, unlike cooler weather, the laying hen has to make critical "life sustaining" physiological adjustments in order to cope with the increased environmental temperature. The laying hen, through panting, resists the rise in body temperature during periods of heat stress. At the same time, the acid-base balance in the bird's blood is changed. We sometimes forget that the laying hen has to cool her body in extremely hot environments and this will shift her physiological priorities from producing eggs and maintaining an adequately calcified eggshell to that of staying alive. In such situations maximum egg mass (egg production times egg weight) along with maximum eggshell quality are difficult to achieve with any age of bird.


Not all diseases which affect chickens cause a decline in eggshell quality. However, egg production will usually decline. An example of a disease that can affect the numbers of eggs and not necessarily the Concepts of Eggshell Quality. Other common viral diseases such as egg drop syndrome (EDS), avian influenza (AI), Newcastle disease (ND) and infectious bronchitis (IB) may produce severe effects on eggshell and internal quality. Many times the total number of eggs is not influenced even though the egg records indicate a drop in total collectable eggs. This is due to the increase in non-collectable eggs (shell-less or ultra-thin shells) that are lost under the cages. This is a common occurrence with EDS. It has been established that the EDS virus affects only the shell gland but with ND and IB every portion of the reproductive tract can be affected. If one disease had to be singled out as being the one responsible for the majority of the economically significant production losses in egg layers it would have to be Infectious bronchitis. Infectious bronchitis virus, a coronavirus, has a preference for the mucus membranes of the respiratory and reproductive tracts. The kidney is also affected by certain IB virus strains. Not only is eggshell quality affected but internal quality also declines. Watery whites are very common and can persist for long periods after egg production returns. Also, an IB outbreak can result in a pale colored shell in brown shell.


your suggesting for it to be called a period the rooster has to be present.

the amount of time between a period maybe different but then so is the reproductive architecture between a human woman and a hen.

Also you mention "ovulation" within 4 lines of the start of your article, if you look up on wiki, it says;

"Ovulation is the process in a female's menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum"

and further down the page;

"Chickens have an ovulation almost every day"

Notice there is no mention of difference between the ovulation of a chicken or female woman.

Ranked #2 in Birds

Sam your comment confuses me, I certainly did not mean to suggest that it is a period if a rooster is present. The rooster is required for fertlization only, but hens lay eggs if there is a rooster there or not, a "period" as defined by the 2-5 bleeding phase in a human female is very different and not at all related to the chicken. The period in a human is when her uterine lining sheds. This does not occur in chicken or most other animals.

Blair Gomez


I hope this comment/email finds you well. I recently helped build an infographic about Amazing Egg Facts and thought you might like to use it on your blog/site. Here's a link to the infographic(

There's code below the IG that helps you post it, but if you need another image size or would like some help, just let me know. : )



So it's ok to eat the by-product of another animals reproductive system...

Women have eggs, do we suppose to scramble that up also and put it in our food...

I forgot, we in America do that to. smh

Who ever came up with the idea to eat such an item in the first place? and then we will turn around and claim in has nutritional benefits...really. Under a microscope the rat can have nutritional benefits. Bottom line is that chicks come from eggs and so some are aborted fetuses that are being ate and some are not, just fluids waiting on sperm to create a animal.

We were having a conversation yesterday afternoon about how eggs (for eating) were actually formed within the chicken and how long this process took.  I said that I would google it, and although it took a while to find, this article explains it all perfectly. Thanks!

The color of the hen has no more to do with the color of the egg shell than a cows color has on the color of the milk. The breed of the hen determines the color of the egg shell.  A white plymouth rock hen will lay an egg with a brown shell.  A brown leghorn hen will lay a white shelled egg.

YOU are saying here that egg is produced as a result of OVULATION,andOvulation is the process in a female's menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum (also known as an oocyte, female gamete, or colloquially, an egg).

How can you say then it's not a menstrual produce you are contradicting your own statement.IS IT EVER POSSIBLE TO HAVE AN EGG WITHOUT A MENSTRUAL

Wow some of the comments are really goofy. Thanks for the article it was very informative and I hope I can use it as a reference on my research paper.