The creation of a chicken egg is a complex biological process. This intricate journey, orchestrated by the hen’s reproductive system, typically unfolds over approximately 24 to 26 hours. It begins with the development of the yolk, followed by the careful layering of white and membranes, culminating in the formation of the protective shell before the egg is laid.
The Beginning: Yolk Development
The egg’s journey commences in the hen’s left ovary, which contains thousands of potential yolks. A mature yolk develops within a follicle over a period of about 7 to 9 days. During this growth phase, the yolk accumulates proteins, fats, vitamins, and minerals through a process called vitellogenesis, with nutrient precursors supplied by the hen’s liver.
Once fully mature, the yolk is released from the ovary through a process called ovulation, entering the infundibulum, the first segment of the oviduct. The yolk spends about 15 to 17 minutes in this section. If fertilization is to occur, it happens here, as sperm from a rooster would be present in the infundibulum. As the egg begins its descent, specialized protein structures called chalazae start to form that will later anchor the yolk within the egg white.
Adding the White and Membranes
After leaving the infundibulum, the developing egg enters the magnum, the longest section of the oviduct. Here, layers of albumen, commonly known as egg white, are secreted around the yolk. This process takes approximately three hours, during which the magnum’s glandular tissue produces the various thick and thin albumen layers that constitute the bulk of the egg’s interior. The albumen is primarily composed of water and proteins like ovalbumin, providing both protection and nutrition.
Following its passage through the magnum, the egg moves into the isthmus. In this segment, which takes about 75 minutes, the inner and outer shell membranes are formed. These two thin, protective layers provide an initial barrier against bacterial invasion and serve as the foundation upon which the hard eggshell will later be deposited.
Creating the Shell and Laying
The final and longest stage of egg formation occurs in the uterus, also known as the shell gland. Over approximately 20 hours, the hard outer shell, primarily composed of calcium carbonate, is deposited around the membranes. The hen mobilizes calcium from both her diet and her bones, contributing around 8-10% of her total body calcium for shell formation.
During this period, a process called “plumping” occurs, where water and minerals pass through the shell membranes, causing the egg to swell and achieve its characteristic oval shape. If the egg is to have color, pigments such as protoporphyrin and biliverdin are deposited onto the shell during the last few hours in the uterus. Finally, the fully formed egg passes through the vagina, where a protective, fast-drying coating called the bloom or cuticle is applied, sealing the shell’s pores. The egg then exits the hen’s body via the cloaca, typically rotating to present the blunt end first for easier passage.