How Chicken Eggs Are Formed
Lots of people wonder, just where do eggs come from. Sure they come from chickens (hens to be specific), but how do they 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, and then gets its “bloom” a thin coating which helps it pass through the cloaca smoothly, just before the egg is laid.
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.
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.
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 breed's 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 hen's 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.