There are two main parts of the reproductive system of the hen-the ovary and the oviduct. Only the left ovary and oviduct mature in a bird. The ovary is attached underneath the backbone, about midway between the neck and tail. When a female chick is hatched, the ovary contains all of the ova (in miniature) it will ever have.
The ovary begins to convert ova to egg yolks at the appropriate time. In order for this to happen, the pullet (young female chicken) must have reached the correct stage of physical development. Then, if the appropriate light stimulation is present, hormones cause ova to develop in sequence to yolks.
Yolks are released from the ovary into the body cavity when they reach the correct size. The ovulated yolk is retrieved by the infundibulum, the first part of the oviduct. Completion of the egg will require approximately 24 more hours. Passage through the magnum, isthmus, and uterus of the oviduct results in the addition of egg white, shell membranes, and shell.
Soon after an egg is laid (oviposition), the process starts again. Another yolk is released, and the next egg is formed. Some chickens and ducks can lay an egg every day for more than 300 consecutive days. Other birds may lay only a few eggs, and then only every second day.
Sperm cells to fertilize the ovum are produced in the testes of the rooster. The testes are inside the body, attached approximately in the middle of the back. They are shaped like beans, and are cream colored in immature birds and white in mature birds.
When roosters reach about two-thirds of their mature body size, they are capable of producing sperm. Important factors in stimulating them to produce sperm are the number of hours of light each day and whether the time of exposure to light is increasing or decreasing. Natural lighting conditions in spring stimulate sperm production, and conditions in autumn depress production.
Sperm cells are carried to the outside of the rooster in a thick white fluid called semen. Semen is carried from the testis to the cloaca by a small tube called the vas deferens. Each vas deferens exits through a small sexual organ called a papilla.
When a bird is laying eggs, and a few days prior, she is receptive to mating. The male indicates his interest by courting behavior. If the female is receptive, she will crouch. The male steps on her back, squats down so that the vents can touch, and releases the semen. Sperm cells must then migrate the whole length of the female reproductive tract to join with the egg cell that is on the yolk. Fertilization must occur in the infundibulum. As a result, the blastodisc on the yolk becomes a blastoderm. During the 24 or more hours that the egg is being formed by the hen, the embryo completes several series of cell division. Instead of the one cell present at fertilization, the developing embryo has 256 cells when the egg is laid. Figure 1 shows the difference between the blastodisc of an infertile egg and the blastoderm of a fertile egg. After the egg cools to room temperature, development of the embryo stops.