A hen requires about 24 to 26 hours to produce and egg. Thirty minutes later, she starts all over again.
The reproductive system of the hen consists of the ovary, the organ where the yolk develops, and the oviduct where the egg is completed. The ovary is attached to the back about halfway between the neck and the tail. The oviduct, a tubelike organ approximately 26 inches long, is loosely attached to the backbone between the ovary and the tail. Most female animals have two functioning ovaries, but the hen uses only one, the left. The right ovary and oviduct remain dormant.
Ovary
A female chick is born with a fully formed ovary containing several thousand tiny ova, or future yolks.
These begin to develop, one at a time, when the pullet reaches sexual maturity. Each yolk is enclosed in its own sac or follicle.
The follicle contains a highly developed system of blood vessels which carry nourishment to the developing yolk. At ovulation, the follicle ruptures to release the yolk into the oviduct. A double-yolked egg is the result of two yolks being released at the same time. Rupture occurs at the stigma line, an area of the follicle which has no blood vessels. ~see Blood Spots
Oviduct
The infundibulum, also know as the funnel, captures the ovulated yolk. This is where fertilization, if it occurred, would take place. After about 15 minutes, the yolk passes along to the magnum. Here, in approximately 3 hours, the albumen is deposited around the yolk. As the albumen is formed, the yolk rotates, twisting the albumenous fibers to form the chalazae.
The next site of activity is the isthmus where the two shell membranes are formed in about 1¼ hours.
The egg has now reached it full size and shape. It passes along to the uterus to acquire, after 19 hours, it shell, shell color and bloom. After a few minutes' pause in the vagina, the uterus inverts through the vagina, the cloaca (the junction of the digestive, urinary and reproductive systems) and the vent to release the egg outside the hen's body. Laying of the egg is known as oviposition.
During formation, the egg moves through the oviduct small end first. Just before laying, it is rotated and laid large end first. A young hen lays small eggs. The size increases as she gets older.