The chicken egg is a self-contained biological system, built layer by layer within the hen’s reproductive tract over approximately 25 to 27 hours. This continuous process moves through specialized sections of the oviduct, where the initial reproductive cell is encased in proteins, membranes, and a hard shell. The transformation begins in the ovary, creating the nutrient core, before moving through the oviduct for the addition of protective outer components.
The Beginning: Yolk Development in the Ovary
The process starts within the hen’s single functional ovary, which contains thousands of tiny ova, each a potential yolk. The yolk is the ovum, which develops within a follicle. This development involves a rapid growth phase called vitellogenesis, where the yolk accumulates nutrients for a developing embryo.
The liver synthesizes vitellogenin and lipids, which are transported via the bloodstream to the ovary. These compounds are absorbed by the growing oocyte, forming the lipoprotein-rich yellow yolk. This nutrient-loading process is completed about 24 hours before release.
Once the follicle reaches its mature size, ovulation occurs, releasing the yolk into the oviduct. The first segment, the infundibulum, engulfs the yolk, and fertilization can take place here if sperm are present. The yolk then begins its journey down the oviduct, where the remaining structures will be deposited.
Building the Egg White and Inner Membranes
After the yolk passes through the infundibulum, it enters the magnum, the longest section of the oviduct, where the albumen, or egg white, is added. This stage takes about three hours. Glandular cells in the magnum secrete large quantities of protein-rich fluid, providing a protective cushion and a source of water and protein for the potential embryo.
As the egg rotates through the magnum, structural fibers called chalazae are formed, anchoring the yolk in the center of the dense albumen. The egg mass then moves into the isthmus. The isthmus is responsible for forming the inner and outer shell membranes, a fibrous, protective layer that surrounds the albumen.
The formation of these two shell membranes takes approximately 1.25 hours and occurs before the hard shell is deposited. The membranes attach to one another except at the blunt end, where they separate to form the air cell after the egg is laid and cools. The egg then proceeds to the uterus for the final, most time-consuming stage.
The Final Stage: Shell Hardening and Laying
The final stage of assembly takes place in the uterus, or shell gland, where the egg resides for about 18 to 20 hours. Here, the hard, calcified shell is deposited around the shell membranes. The shell is composed primarily of calcium carbonate, which the hen secretes from her blood and bone reserves into the uterine fluid.
Calcium is rapidly transported to the shell gland, as the hen deposits between 100 to 150 milligrams of calcium every hour during this phase. The process of calcification is regulated by hormones, with the hen drawing from both her diet and her medullary bone reserves to supply the necessary mineral.
The final layer, the cuticle or bloom, is a thin, protective organic coating added just before the egg is laid. This layer seals the shell’s pores, limiting water loss and bacterial entry. Finally, the fully formed egg passes through the vagina and the cloaca, a process known as oviposition, completing the roughly 25-hour cycle.