The creation of an egg within a chicken is a complex biological journey. This continuous process showcases the hen’s remarkable physiology, allowing her to produce eggs regularly. Each stage, from yolk development to final protective layers, is carefully orchestrated within the hen’s reproductive system.
Yolk Development
The egg’s journey begins in the hen’s ovary, where thousands of tiny ova, or potential yolks, are present. In most hens, only the left ovary fully matures and becomes functional for egg production. As a hen reaches maturity, around 18 to 22 weeks of age, hormonal changes trigger the growth of these ova into yolks.
A single yolk develops within a follicle on the ovary, accumulating a rich supply of fats, proteins, and vitamins from the hen’s bloodstream and liver. This growth phase for an individual yolk takes approximately 10 days. Once a yolk reaches its mature size, it is released from the follicle in a process called ovulation, entering the oviduct, a long, coiled tube where the rest of the egg will form.
Formation of Albumen and Membranes
After ovulation, the yolk enters the infundibulum, the first oviduct section, where fertilization, if it occurs, takes place. The yolk then travels into the magnum, the longest oviduct part. Here, the albumen, or egg white, is rapidly secreted around the yolk over about three hours. This albumen provides both protection and nourishment for a potential embryo.
The egg then progresses to the isthmus, where it spends about an hour. In this section, two shell membranes, an inner and an outer, are formed around the albumen-covered yolk. These membranes provide an additional layer of protection and help the egg takes on its characteristic oval shape. The sequential addition of these layers ensures the egg’s structural integrity.
Shell Production
Following the formation of the membranes, the egg moves into the shell gland, or uterus, the most time-consuming part of egg formation. The egg resides here for about 20 hours as the hard outer shell is deposited. This shell is primarily composed of calcium carbonate, which is secreted in crystalline layers.
The hen’s body requires a significant amount of calcium for this process, which it draws from her diet and, if necessary, from her bones. During the final hours of shell formation, pigments are added, giving the egg its characteristic color, which can range from white to various shades of brown, blue, or green depending on the chicken breed. Before the egg is laid, a protective outer coating called the cuticle or “bloom” is applied. This layer seals the pores in the shell, helping to prevent bacterial entry and reduce moisture loss.
Laying the Egg
Once the shell and cuticle are complete, the fully formed egg is ready to be laid. The entire process from ovulation to laying takes 24 to 26 hours. The egg passes through the vagina and is then expelled from the hen’s body through the cloaca.
Hens possess a natural instinct to lay eggs regularly, laying an egg every 25 to 26 hours. As soon as one egg is laid, the hen’s reproductive system begins forming the next egg within 30 minutes. This continuous cycle allows healthy hens to produce eggs daily.