The Hen

The reproductive system of the female chicken is in two parts: the ovary and oviduct. Unlike most female animals, which have two functioning ovaries, the chicken usually has only one. The right ovary stops developing when the female chick hatches, but the left one continues to mature.

The ovary is a cluster of sacs attached to the hens back about midway between the neck and the tail. It is fully formed when the chicken hatches and contains several thousand tiny ova, each ovum within its own follicle. As the female reaches maturity, these ova develop a few at a time into yolks.

Left: As the ovum develop and get larger, you can see the stigma line (clear line on each ovum). This is where the yolk is released.
Right: Ovaries and Oviduct / yolk to complete egg

On the surface of every egg yolk there can be seen a tiny, whitish spot called the blastodisc. This contains a single female cell. If sperm is present when a yolk enters the infundibulum, a single sperm penetrates the blastodisc, fertilizing it and the blastodisc becomes a blastoderm. Technically, the blastoderm is the true egg. Shortly after fertilization, the blastoderm begins to divide into 2, 4, 8 and more cells. The first stages of embryonic development have begun and continue until the egg is laid. Development then subsides until the egg is incubated. When sperm and ova unite, this process is called fertilization. After fertilization, the egg can develop and become a chick. Only fertilized eggs grow into chicks. The chicks grow and become adult birds.

The oviduct is a tube like organ lying along the backbone between the ovary and the tail. In a mature hen it is approximately 25 to 27 inches long. The yolk is completely formed in the ovary. When a yolk is fully developed, its follicle ruptures, releasing it from the ovary. It then enters the infundibulum, the entrance of the oviduct (see below).

All of the other parts of the egg are added to the yolk as it passes through the oviduct. The chalazae, albumen, shell membranes, and shell are formed around the yolk to make the complete egg, which is then laid. This complete cycle usually requires a little more than 24 hours. About 30 minutes after the egg is laid, another yolk is released and the process repeats itself. Development takes place as follows:

The Hens reproductive tract. Note the Ovary and the Infundibulum are parts of separate organs.