Chicken egg fertilization requires the presence of a rooster. Not all eggs laid by hens are fertilized; only those from a hen that has recently mated with a rooster can develop into a chick.
The Rooster’s Contribution to Mating
The rooster plays a specific role in the reproductive process. Sperm transfer occurs through brief physical contact between the cloacas of the rooster and the hen, commonly referred to as the “cloacal kiss.” The cloaca is a multi-purpose opening in birds, serving as the exit point for digestive, urinary, and reproductive tracts.
During mating, the rooster mounts the hen. Both birds evert their cloacas, pressing them together to transfer semen. This ensures that sperm is deposited into the hen’s reproductive tract. Even a brief mating can lead to successful fertilization of multiple eggs.
The Journey to Fertilization Within the Hen
Once sperm enters the hen’s cloaca, it begins a journey through her reproductive system. The hen’s reproductive anatomy includes an ovary, where the yolk (ovum) develops, and an oviduct, which is responsible for egg formation. Sperm travels from the cloaca up the oviduct, eventually reaching specialized structures called sperm host glands, primarily located near the uterovaginal junction, but also found in the infundibulum.
These sperm host glands are remarkable, allowing the hen to store viable sperm for several days, and sometimes even weeks, after a single mating. This storage mechanism ensures that a hen can lay a series of fertilized eggs without needing daily mating. When a yolk is released from the ovary, it enters the infundibulum, the uppermost part of the oviduct. It is in the infundibulum where fertilization typically occurs, as sperm migrate from the storage glands to meet the newly released ovum. The fusion of a single sperm with the ovum’s germinal disc marks the moment of fertilization, forming a zygote.
From Fertilized Cell to Laid Egg
After fertilization in the infundibulum, the now-fertilized ovum, or zygote, continues its passage down the hen’s oviduct. This journey is where the various components of the egg are sequentially added around the yolk. The first major addition occurs in the magnum, where layers of albumen, or egg white, are secreted around the yolk. This process takes approximately three hours.
Following the magnum, the egg moves into the isthmus, where the inner and outer shell membranes are formed. This stage usually takes about one and one quarter hours. Finally, the egg reaches the uterus, also known as the shell gland, where the hard, calcified outer shell is formed over approximately 19 to 20 hours.
The entire process, from ovulation and fertilization to the egg being ready for laying, typically takes about 24 to 26 hours. If the egg is fertilized, the embryo begins early development within the egg even before it is laid. An unfertilized egg, while still edible, will not develop into a chick.