As promised in a previous post, this Fowl Friday we’re going to take a little diversion, and look more closely at how chicken eggs are formed.


Eggs are the product of a carefully coordinated, complex series of events

If you read our Pullet Eggs post, I mentioned that young chickens first coming into lay can often have little egg-laying mishaps.  Sometimes a young pullet will just lay an egg yolk wrapped in a membrane, and no shell.  Some eggs are strangely shaped, or may contain no yolk, or a double yolk.

By the time we’re done with this post, we hope you have a slightly better understanding, from the hen’s perspective, as to what is involved in producing wonderful farm-fresh eggs.

To better understand where eggs come from, and how they are formed, it’s necessary to start with some basic anatomy.  The egg itself is composed of key structures, each of which is formed at a different point during it’s journey from the hen’s ovary, through the oviduct.


Anatomy of a hen's egg (click image to enlarge) Image Source: Wikimedia Commons

Birds and mammals have some significant differences in oviductal anatomy.  The sketch below represents the left oviduct of a typical hen, and the path each egg takes, from the ovary, until it is ready to be laid.



Sketch of the process of egg formation within a hen's oviduct (click image to enlarge) Image © Curbstone Valley Farm


Where mammals typically have paired ovaries and oviducts, most birds, including chickens, only have a functioning left oviduct.


An egg’s journey begins at the ovary, which contains the developing follicles, collectively resembling a bunch of grapes.  Each hen has a finite number of follicles, as she has all the follicles she will ever have at the time she hatches.  Hens are most productive during their first few years of life, typically laying an egg every 1-3 days, depending on breed and age.   Hens will continue to lay eggs as they get older, however, total egg production, and egg quality, decreases with age.


Illustration of a hen's ovary (Image Source: Poultry Success (Vol 16-17) - 1905)


Once the ovum is released from the ovary, it is drawn into the infundibulum, at the proximal end of the oviduct.


The ovum is released from the ovary into the infundibulum

From here, the ovum is propelled distally through the oviduct.  In the presence of a rooster, fertilization of the ovum takes place in the infundibulum, before most other components of the hen’s egg are formed.   Chalaza, the white protein ‘ropes’ that anchor the yolk in the center of the egg white, first appear as fine filaments while the ovum is in the infundibulum.

Chalaza first begin to form as the egg passes through the infundibulum


It takes less than 30 minutes for the egg to be moved from the infundibulum to the magnum region of the oviduct.


Within in the magnum, the egg receives a coating of albumen (egg white).


Albumen is deposited in the magnum region

This process takes approximately three hours, at which point the egg makes its way to the Isthmus.


In the isthmus, shell membranes are deposited around the egg, and its albumen coating.


The shell membranes, which contribute to the air cell formation, are deposited around the egg in the Isthmus region


This is a relatively quick process, taking approximately one hour, before the egg makes its way to the uterus, or shell gland region.


In the uterus the outer shell, and shell color, is deposited.


The hard outer shell, and shell pigments, are deposited around the shell membranes in the Uterus region

The egg spends the greatest period of time in the uterus region, as it takes approximately 20 hours to complete egg shell, and pigment, deposition.


Shell pigment, that determines the color of the egg, is deposited in the shell gland

The shell is composed primarily of calcium carbonate derived from the hen’s own body stores of calcium (47%), and dietary calcium (53%).  As more than half of the required calcium comes from food, it is important that a hen’s layer-diet is rich in calcium to help ensure that strong eggshells are formed.


The vagina isn’t involved in egg formation per se, however, a protective shell coating, called the cuticle, or ‘bloom’, is deposited around the egg just prior to laying.  This layer is somewhat oily in texture, and facilitates egg laying, and helps protect the egg from bacterial invasion as it is passed through the cloaca.


The air cell forms as the inner and outer membrane separate while the egg cools, soon after laying

The total time to form an egg is approximately 25 hours.  For hens that lay an egg each day, after an egg is laid in the morning, between 30-75 minutes later, the next ovum is released into the infundibulum, and the process begins again.

When things go Awry

Now that we have a better understanding of egg formation, we can also better interpret where in the process things are amiss when an anomalous egg is laid.

Double Yolks

Occasionally, a hen will produce double-yolked eggs. Double-yolking sometimes occurs in older hens, but may occur in young hens that release two ova in rapid succession.  In our first flock of buff orpingtons, one pullet routinely would lay over-sized eggs, containing two yolks.  Double yolked eggs are perfectly edible, but are not suitable for breeding. Although the eggs tend to be larger, they may lack sufficient nutrients and space to support the development of two chicks, and twin chicks rarely successfully hatch without intervention.

Yolk-less Eggs

A young pullet may produce a yolk-less egg.  This usually occurs when a fragment of ovarian tissue, or oviductal lining sloughs, and stimulates the secreting glands in the magnum to produce albumen.  The rest of the egg-formation process may then continue as normal, except that an egg without a yolk is laid.

Blood Spots and Meat Spots

These are rarely seen in commercially produced eggs, as when the eggs are candled, those containing blood spots and meat spots are rejected.  The eggs are still perfectly edible, but are rejected for aesthetic reasons.  Blood spots are normally associated with the egg yolk. Rupture of tiny vessels during ovulation is typically the cause of blood spots. If you raise your own hens for eggs you may notice these from time to time.


Blood spots may appear on the yolk of some eggs, a by-product of ovulation

Meat spots are usually brown in color and are typically associated with the egg white.  Because the albumen is deposited around the egg in the magnum of the oviduct, these spots aren’t due to vessel rupture at ovulation.  They are formed when small pieces of the of the oviductal lining slough as the egg passes through.

Thin-Shelled or Shell-less Eggs

Occasionally eggs are laid with thin shells, or may be lacking a shell entirely. A shell-less egg doesn’t typically look like an egg would if you cracked an egg open.  Usually the yolk and albumen are at least encased in the shell membrane, but the calcium carbonate hard shell is not deposited.  This usually results in an egg that has the appearance of a small water balloon.  These are occasionally found in young pullets first coming into lay, but providing the pullet is otherwise healthy, this problem should quickly resolve.  If the problem persists however, a veterinarian should be consulted, as there are some poultry diseases, and some poultry medications, that can cause this phenomenon.  An occasionally shell-less egg isn’t necessarily cause for alarm though, and shell-less or thin-shelled eggs are more likely to be related to diet and/or aging.  As these eggs lack a complete protective outer coating, they should not be consumed.

This egg was recently laid by one of our older orchard hens.

The dark circular area on this side of the egg is an area of thin shell

The thin shell was easily removed, revealing that the rest of the egg was formed normally.

The thin shell was easily peeled away, revealing a normal yolk and white. This egg was discarded.

On the other side of the same shell it is clear that pigment was not evenly deposited.

Note the lighter region on the shell where the pigment was not evenly deposited, and the granular aggregate on the shell surface

This is the first time we’ve seen an egg like this from the orchard hens, one of whom is coming back online after a molt.  As a one-off, this isn’t particularly concerning, but if this hen continues to lay eggs with these defects, it warrants further investigation.  As we know that both shell and pigment are deposited within the uterus region, if any more eggs are laid with these defects, it may suggest a problem within that hen’s uterus.



Chicken or the Egg?

Hopefully you now know a little more about how chicken eggs are formed.  One question still remains though, who came first…?