The formation of an egg is an intricate biological process, transforming a tiny cell into a complete, nutrient-rich product. This remarkable journey takes approximately 24 to 26 hours for a hen to complete, from the initial development of the yolk to the final laying of the shelled egg. This process highlights the efficiency of a hen’s reproductive system, designed to produce eggs almost daily.
The Hen’s Reproductive System
A hen’s reproductive system consists of two main parts: the ovary and the oviduct. While female chicken embryos are born with two ovaries, only the left ovary typically develops and becomes functional in adult hens. This single ovary, located near the backbone in the abdominal cavity, contains thousands of tiny ova, which are potential future egg yolks.
The oviduct is a long, coiled tube, roughly 24 to 26 inches in length, that connects to the ovary and leads to the cloaca. It is divided into five distinct sections, each playing a specific role in adding components to the developing egg. These sections include the infundibulum, magnum, isthmus, uterus (also known as the shell gland), and vagina.
Developing the Yolk
Egg formation begins in the ovary with the development of the yolk. The ovary contains numerous ova, or oocytes, in various stages of maturity, resembling a cluster of grapes. The largest and most mature yolk undergoes a growth phase called vitellogenesis, accumulating layers of fat and protein.
During this period, which can last approximately 7 to 9 days, the hen’s liver produces vitellogenin, a protein-rich substance. This substance travels through the bloodstream to the ovary, providing the proteins, fats, vitamins, and minerals that form the nutrient-dense yolk. Once the yolk reaches its mature size, typically around 35-40 millimeters in diameter, it is released from the ovary through ovulation.
Assembling the Egg Layers
After ovulation, the mature yolk is captured by the infundibulum, the funnel-shaped entrance of the oviduct, where the vitelline membrane forms around it. The yolk then moves into the magnum, the longest section of the oviduct, where the egg white, or albumen, is added. This process takes approximately 3 hours, with the magnum secreting proteins that constitute the albumen.
Next, the developing egg enters the isthmus, where the inner and outer shell membranes are formed around the albumen. These fibrous membranes, primarily composed of proteins like collagen, provide structural support and take about 1 to 1.5 hours to develop. The egg then proceeds to the uterus, also known as the shell gland, where the hard outer shell is formed.
The Final Stages and Laying
The egg spends the longest period in the uterus, approximately 18 to 21 hours, during which calcium carbonate is deposited to create the shell. The hen requires a significant amount of calcium, about 2.0-2.5 grams per egg, which is supplied through her diet and from bone reserves. During the last few hours in the uterus, shell pigment, determining the egg’s color, is added to the outer layers of the shell.
As the egg nears completion, a protective outer coating, the cuticle or bloom, is applied to the shell in the uterus. This thin, water-soluble layer, composed mainly of glycoproteins, lipids, and fat, serves several purposes. The bloom seals thousands of tiny pores on the eggshell, preventing moisture loss and acting as a barrier against bacteria and fungi, preserving the egg’s freshness.
The bloom also acts as a lubricant, facilitating the egg’s passage through the oviduct during laying. Once fully formed, the egg moves into the vagina, a muscular tube that helps push the egg out of the hen’s body through the cloaca. Hens typically lay eggs large end first, even though the egg travels through the oviduct small end first. A hen can lay an egg every 24 to 36 hours.