The formation of a chicken egg is a continuous biological process in the hen, and fertilization must occur early in this cycle before the egg is fully encased. The hen’s reproductive system, consisting of the ovary and the oviduct, transforms a single yolk cell into a complete egg structure in approximately 25 hours. This internal process happens long before the egg is laid, ensuring the developing embryo is protected and nourished by the subsequent layers added as the yolk travels down the reproductive tract.
The Rooster’s Contribution and Sperm Storage
The journey to fertilization begins when the rooster transfers semen to the hen via a brief physical contact known as the cloacal kiss. Chickens do not rely on internal penetration for mating, instead aligning their cloacas to transfer the sperm. The sperm must then ascend the hen’s oviduct, traveling in the opposite direction of the forming egg, to reach the fertilization site.
The hen possesses a unique adaptation that makes ongoing mating unnecessary for every single egg. Specialized structures called sperm storage tubules (SSTs) are located primarily at the uterovaginal junction (UVJ), the boundary between the uterus and the vagina. These tiny, cylindrical glands act as biological reservoirs, allowing the hen to store viable sperm for an extended period, typically two to three weeks following a single mating.
These storage tubules protect the sperm from the harsh environment of the oviduct before they are released in small, steady amounts. The long-term viability of the sperm in the SSTs is supported by surrounding female cells, which may transfer lipids to sustain them. This storage mechanism ensures a consistent supply of male gametes for future eggs.
Fertilization: The Critical Timing and Location
The process of egg formation starts when the ovary releases a mature yolk (ovum) into the oviduct, a process called ovulation. This yolk contains the female genetic material in a tiny white spot called the blastodisc. The yolk is immediately captured by the funnel-shaped entrance of the oviduct, known as the infundibulum.
Fertilization must happen here, at the very beginning of the oviduct, and the timing is extremely narrow. Sperm released from the SSTs must be present in the infundibulum to meet the yolk within a window of about 15 to 20 minutes after ovulation. If the yolk passes this initial section without encountering sperm, the opportunity for fertilization is lost for that particular egg.
Once a single sperm successfully penetrates the blastodisc, the fertilized yolk is now a zygote. After this union, the ovum’s surface rapidly forms a secondary membrane that prevents any other sperm from entering, known as the block to polyspermy. The resulting blastoderm immediately begins dividing into multiple cells as it continues its journey down the oviduct.
Layering the Egg: Post-Fertilization Development
Immediately after fertilization in the infundibulum, the yolk moves into the next section of the oviduct, the magnum. This is the largest part of the tract, and the fertilized ovum spends approximately three hours here while the majority of the egg white (albumen) is secreted and layered around it. The albumen provides both cushioning and a rich source of protein and water for the developing embryo.
The egg then proceeds to the isthmus, where it remains for about an hour as the inner and outer shell membranes are added. These two thin, fibrous layers provide a barrier against bacterial contamination and will eventually separate at the blunt end of the egg to form the air cell. The final and longest stage occurs in the uterus, often called the shell gland, where the egg spends about 20 to 21 hours.
It is in the uterus that the hard, calcified shell is deposited around the egg. During this entire 24 to 26-hour journey through the oviduct, the fertilized blastoderm continues its rapid cell division. By the time the hen lays the egg, the embryo is ready to continue growing once incubation temperature is applied.