The chicken egg undergoes a complex biological transformation within the hen before it is laid. This process involves a series of timed stages, where each component is developed and assembled. Understanding this journey reveals the sophisticated physiology of a laying hen.
Yolk Development
The initial phase of egg formation begins in the hen’s ovary with the development of the yolk. Each hen is born with thousands of immature yolks, known as ova. Once a yolk is selected, it spends approximately 10 days growing, accumulating layers of fat and protein that contribute to its size and yellow color.
As the yolk matures, it is enclosed within a follicle. A thin vitelline membrane forms around the yolk, separating it from the future egg white. When the yolk reaches its mature size, it is released from the ovary in a process called ovulation. This released yolk is then captured by the infundibulum, the first section of the oviduct, where fertilization would occur if a rooster were present.
Albumen and Membrane Formation
After the yolk enters the infundibulum, it moves into the magnum, the longest section of the oviduct. Here, the thick albumen, or egg white, is rapidly added around the yolk in successive layers. This process takes approximately three hours. The albumen provides protein and water for a developing embryo.
As the yolk and albumen travel through the magnum, chalazae begin to form, anchoring the yolk in the center of the egg. These chalazae stabilize the yolk, preventing it from touching the shell. Following the magnum, the egg enters the isthmus, where the inner and outer shell membranes are formed around the albumen, providing another layer of protection. This stage takes about 1 to 1.5 hours.
Shell Creation
The developing egg then proceeds to the shell gland, also known as the uterus, where the outer shell is formed. This is the longest stage of egg formation, lasting around 20 hours. During this time, calcium carbonate is deposited onto the shell membranes in a process called calcification, creating the rigid structure. A chicken eggshell contains about 2.2 grams of calcium carbonate, sourced from the hen’s diet and mobilized from her bones.
Just before the egg is laid, a non-calcified layer called the cuticle, or bloom, is applied to the shell. This thin coating seals the shell’s pores, serving as a barrier against bacterial entry and regulating gas exchange. Shell color, determined by the hen’s genetics, is also added during this stage. Pigments like protoporphyrin create brown hues, deposited on the shell’s surface, while oocyanin results in blue or green colors, permeating the entire shell.
Egg Laying
Once the shell is formed and the cuticle applied, the egg is ready to be laid. The hen’s reproductive system, specifically the vagina, pushes the egg through the cloaca, a single opening that serves both reproductive and excretory functions. The egg rotates just before laying, exiting blunt end first, which helps minimize breakage.
The entire process, from yolk release to egg laying, takes 24 to 26 hours. Shortly after laying an egg, within 30 to 75 minutes, the hen’s body is prepared to release the next yolk, initiating the cycle anew.