An egg contains a phenomenal amount of nutrition for its size, but you already knew that right?
An average egg contains a mere 75 calories but gives you 7g protein, 5g fat and 1.5g of saturated fat. In addition it gives you vitamins A, D, B12, riboflavin, pantothenic acid, phosphorus, choline, carotenoids and cholesterol!
And as reported by Pennsylvania State University if you eat only pasture raised chicken eggs instead of generic supermarket eggs, you get even more health benefits- making it truly a superfood.
We’ve previously talked about why your hen has stopped laying and also, the best chicken breeds for lots of eggs, but we haven’t discussed where does this incredible food come from? To just say it comes from a chicken doesn’t do justice as to how complex the process is.
After reading this article today you will realize what a small miracle that egg really is.
The Reproductive System of the Hen
A hen is born with two ovaries.
Although after a chick has hatched their right ovary shrinks, becoming non-functioning, so all the egg laying activity is located in their left ovary.
Interestingly, all the eggs a hen will ever lay are stored in their ovary at birth.
The egg machinery (i.e. oviduct) of the hen is about 25-27 inches long. It really looks like a stretched out trumpet. Let’s take a look at each part of the oviduct and how it works together to produce an egg.
Step 1: Follicle
It all starts in the Follicle; a mature yolk is released from the follicle into the infundibulum. The follicle contains all of the yolks a hen will ever lay in her life; she likely will not use all of them. The release of the yolk stimulates hormonal responses throughout the system.
Step 2: Infundibulum
The yolk spends about fifteen minutes in the infundibulum, where it is gently ‘massaged’ and stretched out. If fertilization is going to occur, it happens here.
Step 3: Magnum
Once the yolk leaves the Infundibulum, it passes through to the Magnum. This is the largest part of the reproductive system where approximately 50% of the thick, white albumin is attached. The yolk remains here for about three hours before moving on.
Step 4: Isthmus
After the yolk has passed through the Magnum, it reaches the Isthmus. The shape of the egg is determined here and the inner and outer shell membranes are attached.
Step 5: Uterus
Once the shape has formed the egg passes through to the Uterus (shell gland).
Here the egg shell is made. The shell is largely made from calcium carbonate. The hen pulls roughly half the calcium from her body and the other half is obtained from her diet. If there is any pigment to be added to the egg, this is where it happens. The egg remains in the uterus for around twenty hours.
This is commonly an area where things go awry in the hens’ ability to maintain a good production egg.
Step 6: Vagina
Not really important to formation, the vagina is all important in getting the egg laid properly. The ‘bloom’ of the egg is laid down here and the egg is turned in a process called ‘oviposition’. The egg enters the vagina narrow end first, but is laid ‘blunt’ end first.
Occasionally, an egg will get ‘stuck’ while turning around- this is called egg bound.
The hen can sometimes expel them herself, but on occasion may need assistance from her keeper.
‘Sperm pockets’ are located in the walls of the vagina. During fertilization, some sperm is released and travels up to the infundibulum for the process to begin. Bird sperm remains viable at body temperature for 10-14 days.
Step 7: Cloaca
Finally the egg reaches the Cloaca which is the Thruway exit!
The entire process takes around 25-27 hours, so you average hen will lay slightly less than an egg a day- this is an average of around 270 eggs per year for a hen in their first year of lay!
Once an egg has been laid, within thirty or so minutes another will start on its own journey.
It is rare for a hen to start ovulation after 3pm, so once every 6-7 days she will miss a day of production.
Overview of Egg Laying Process
The Anatomy of the Egg
The egg consists of many separate pieces, all nicely packaged up to give you breakfast!
- Eggshell: This holds everything together. The shell consists almost entirely of calcium carbonate and is perforated by up to 17,000 pores to allow passage of air and moisture between the egg and the air.
- Outer shell membrane
- Inner shell membrane: These membranes sit side by side just under the shell and act as defense mechanisms against bacteria.
- Chalaza: The stringy bits you hate! They are attached to the yolk, twisted in opposite directions and serve to keep the yolk centered. This is vitally important for the correct development of a chick.
- Exterior albumin: Thin outer layer of albumin next to the shell membrane.
- Middle albumin: Thicker, white albumin. It is a good source of riboflavin and protein.
- Vitteline layer: The clear casing that the yolk is wrapped in. The integrity of the vitteline layer is important to prevent ‘splotching’ on the yolk.
- Germinal disk: This is a 2-3mm white spot on the yolk surface. This is where the sperm enters the egg. The embryo will develop from this disk.
- Yellow yolk: This is the essence of life, if you will. It contains all the nutrition an embryo will need to become a chick.
- Air cell: When the egg cools, an air space forms. During aging, moisture and carbon dioxide leave through the pores of the shell and the air cell enlarges. This is why an old egg will float.
- Cuticle/bloom: This is an outer coating, placed whilst the egg is in the vagina. It seals the shell, preventing bacteria from going in and moisture from coming out.
Three Most Common Egg Laying Problems
Soft Shell or No Shell Eggs
This is caused by a malfunction in the uterus. It may simply be that the pullets’ reproductive equipment is too immature yet and requires a slightly longer development time. Or it could be that the hen is aging or it’s simply a genetic ‘oops’.
I have one thirty month old hen who consistently lays ‘shell troubled’ eggs. The pictures below show a parchment shell removed from the vent and an irregular coating of shell. She fairly frequently lays no shells and sometimes a perfect egg!
Although she is currently well, she is in danger of egg yolk peritonitis from one of these episodes.
Egg Yolk Peritonitis
This occurs when fluid from the oviduct leaks into the abdominal cavity and starts an infection. The yolk is a superb place for E. coli bacteria (from the gut) to grow in and the hen will get sick quickly and often dies almost before you notice she is sick.
Unfortunately, many diagnoses of egg yolk peritonitis are made at autopsy.
Normally, when the egg travels into the vagina it turns itself around so that it is laid blunt end first. Sometimes if it is too large, it will get stuck, causing the hen to be ‘egg bound’.
If this happens the egg needs removing or you hen can become very ill.
A warm water bath and some gentle massage of the abdomen can help. You can also lubricate the vent with some Vaseline to assist in the passage of the egg.
If the process goes well you should end up with some beautiful eggs, like these!
So there you have- the miracle of the egg. A little power house of nutrition carefully packaged by the hen. Did you know that eggs were so complex!?