The hen’s reproductive system, divided into the ovary and the oviduct, produces an egg designed to nourish and protect a developing embryo. The entire transformation from a single cell into a fully formed egg takes an average of 24 to 26 hours. Each section of the reproductive tract adds a specific component to the growing structure.
The Beginning: Yolk Formation
The egg’s journey begins in the ovary, positioned high in the hen’s body cavity. A hen is born with thousands of potential ova, contained within protective sacs called follicles. The yolk is a single reproductive cell that rapidly accumulates nutrients, preparing to be the primary food source for a potential chick.
When the hen reaches maturity, one follicle is selected to develop into a mature yolk, a process taking about 7 to 10 days before ovulation. During this period, the yolk increases in size through the deposition of fats and proteins, synthesized in the hen’s liver and transferred via the bloodstream. This accumulation creates the distinct yellow color, influenced by dietary pigments like xanthophylls.
Once fully mature, the yolk is released from the ovary through ovulation, where the follicle ruptures along a specific, bloodless line. The released yolk is quickly captured by the funnel-shaped infundibulum, the first section of the oviduct. Ovulation usually occurs about 30 minutes after the hen has laid the previous egg.
Building the Egg: Albumen and Membrane Deposition
After the yolk is captured, it moves through the oviduct, where the remaining layers are added. The yolk first enters the magnum, the longest section of the oviduct, where the egg white, or albumen, is deposited. This three-hour process involves secreting proteins, such as ovalbumin, which provide a nutritious layer and an antimicrobial defense for the embryo.
The developing egg then moves into the isthmus, a shorter section of the oviduct where the two shell membranes are formed. These inner and outer membranes act as the scaffolding for the final hard shell layer. The egg spends about 75 minutes in the isthmus, where it also begins to take on its characteristic oval shape.
The Final Stage: Shell Creation and Laying
The structure now moves into the uterus, also known as the shell gland, which is the most time-consuming stage of egg formation. The egg spends approximately 20 hours here while the hard outer shell, primarily composed of calcium carbonate, is constructed through calcification.
The hen must mobilize calcium, often around 2 grams per egg, to complete the shell. This calcium is sourced from the hen’s diet and, during nighttime hours when feeding stops, from specialized bone reserves. Crystalline calcium is layered onto the outer shell membrane during formation.
In the final hours, any shell pigmentation is applied to the outermost layer. The shell gland also secretes the cuticle, or bloom, which seals the shell pores to reduce bacterial contamination and moisture loss. Finally, the egg passes into the vagina, which serves as a passage for the laying process, known as oviposition. Muscular contractions help to expel the completed egg through the cloaca, concluding the 24-hour cycle.