The fertilization of a hen’s egg by a rooster is central to chicken reproduction. While hens can lay eggs without a rooster, these eggs are unfertilized and will not develop into chicks. Fertilization requires contributions from both the hen and the rooster, culminating in precise events within the hen’s reproductive system.
The Hen’s Role in Egg Formation
A hen’s reproductive system is adapted for egg production, consisting primarily of the ovary and the oviduct. Hens typically develop one functional ovary, usually the left, containing thousands of potential yolks from birth. As a hen matures, these yolks develop, with one ripening at a time until ready for release.
Once a yolk matures, it is released from the ovary in ovulation. This released yolk then enters the oviduct, a long, tubular organ divided into five distinct sections: the infundibulum, magnum, isthmus, shell gland (or uterus), and vagina. As the yolk travels through these sections, various layers are added. The magnum secretes the albumen, or egg white, around the yolk, followed by the addition of inner and outer shell membranes in the isthmus. The egg then spends the longest period, about 18 to 20 hours, in the shell gland where the hard outer shell is formed.
The Rooster’s Contribution
The rooster’s role involves producing and delivering sperm to the hen. Unlike many mammals, roosters have internal testes that continuously produce sperm. These testes are viable at the chicken’s body temperature. Sperm travels from the testes through deferent ducts to small papillae within the cloaca.
Mating occurs through a brief contact between the cloacas of the rooster and the hen, often called a “cloacal kiss.” During this rapid event, the rooster transfers sperm from his cloaca to the hen’s. The hen possesses specialized sperm storage tubules, primarily in the utero-vaginal junction of her oviduct, which can store viable sperm for up to two to three weeks.
The Fertilization Process Within the Hen
Fertilization occurs internally in the infundibulum, the uppermost part of the hen’s oviduct. This happens shortly after the yolk is released from the ovary and enters the infundibulum. Sperm, released from the storage tubules, travel to meet the freshly ovulated yolk.
Multiple sperm penetrate the vitelline membrane, the protective layer surrounding the yolk. However, only one sperm’s nucleus will fuse with the ovum’s nucleus to form a zygote, initiating embryonic development. Once fertilized, the developing zygote, now termed a blastoderm, continues its journey down the oviduct, accumulating albumen, shell membranes, and the hard shell. This process ensures the egg is fertilized before the shell forms, which would otherwise prevent sperm penetration.
Development of the Fertilized Egg
After fertilization, cell division begins on the yolk’s surface even before the egg is laid. By the time a fertilized egg is laid, the embryo, though microscopic, has already undergone significant initial development, comprising hundreds or thousands of cells. Early embryo development pauses if the egg cools below a certain temperature, known as physiological zero.
For further development and hatching, the fertilized egg requires proper incubation and controlled humidity. Eggs that are unfertilized, or fertilized but not incubated, will not develop into a chick. These eggs are perfectly safe and nutritionally similar to fertilized eggs for human consumption.