Formation

A hen requires about 24 to 26 hours to produce an egg. After the egg is laid, the hen starts all over again about 30 minutes later.

The hen’s reproductive system consists of the ovary, the organ where the yolk develops, and the oviduct where the egg is completed. The ovary is attached to the hen’s back, about halfway between the neck and the tail. The oviduct, a tubelike organ about 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. The ova begin to develop, one at a time, when the pullet (a hen less than 1 year old) 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. Typically, about 15 minutes after the last egg was laid, ovulation occurs. At ovulation, the follicle ruptures to release the yolk into the oviduct. A double-yolked egg results when two yolks are 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 known as the funnel, captures the ovulated yolk. The infundibulum is where fertilization, if it occurred, would take place. After about 15 minutes, the yolk passes along to the magnum where, in about 3 hours, the hen deposits the albumen around the yolk. As the albumen is formed, the yolk rotates, twisting the albumenous fibers to form the chalazae. Next, in about 1 1/4 hours, the two shell membranes are formed and some water and minerals are added in the isthmus.

The egg has now reached its full size and shape and passes along to the uterus (shell gland) where, after 19 to 21 hours, it acquires its 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, the egg rotates to be laid large end first. A young hen lays small eggs. The size increases as she gets older.

– See Bloom; Chalazae; Color, shell; Composition; Shell