The chicken egg undergoes a complex formation process within the hen’s body. This journey, from initial cell development to a fully formed egg ready for laying, typically spans about 24 to 26 hours.
The Hen’s Internal Anatomy
Egg formation begins within the hen’s reproductive system, involving the ovary and a specialized tube called the oviduct. Most birds, including chickens, possess only one functional ovary, usually the left one. The oviduct, a long and coiled structure, is divided into five distinct sections, each with a specific role in egg assembly.
These sections include the infundibulum, magnum, isthmus, shell gland (also known as the uterus), and the vagina, which connects to the cloaca. The infundibulum acts as a funnel, capturing the yolk after its release. The magnum adds the majority of the egg white, while the isthmus forms the protective shell membranes. The shell gland deposits the hard outer shell, and the vagina facilitates the egg’s passage to the cloaca, the common exit point for reproductive and excretory systems.
Yolk Formation and Release
Egg development starts in the hen’s ovary, where thousands of tiny ova are present from birth. Each ovum is enclosed within a follicle, richly supplied with blood vessels. As a hen matures, individual follicles grow, accumulating nutrient-rich material that forms the yellow yolk. This growth process for a single yolk can take approximately 10 days.
Once a yolk reaches maturity, it is released from its follicle in a process called ovulation. This mature yolk then enters the infundibulum, the first segment of the oviduct. This capture occurs quickly, typically within 15 to 17 minutes after ovulation, marking the beginning of the egg’s journey. If fertilization is to occur, it typically happens in this initial section.
Albumen and Membrane Development
After the yolk enters the infundibulum, it moves into the magnum, the longest section of the oviduct. Here, the yolk spends about three hours as albumen, commonly known as egg white, is secreted around it. Albumen is primarily composed of water and proteins, providing protection and nutrition. The twisting motion of the egg as it travels through the magnum also forms the chalazae, rope-like structures that anchor the yolk in the center.
Following the magnum, the egg moves into the isthmus, where it remains for approximately 75 minutes. In this section, two thin, protective shell membranes are formed around the albumen. These membranes act as a barrier against bacterial contamination and provide structural integrity. The egg also begins to take on its characteristic oval shape during its time in the isthmus.
Shell Formation and Egg Laying
The final and most time-consuming stage of egg formation occurs in the shell gland, or uterus, where the egg spends about 18 to 20 hours. Here, the hard outer shell, primarily composed of calcium carbonate, is deposited around the shell membranes. The hen’s body mobilizes calcium from its diet and bone reserves to create the casing. Pigmentation, if present, is also applied to the shell in this region during the last few hours of shell formation.
Once the shell is complete and a protective outer coating called the bloom or cuticle is applied, the egg is ready to be laid. The egg then moves into the vagina and exits through the cloaca. Before laying, the egg typically rotates so that the larger, blunt end emerges first, which can help prevent cracking. The entire process, from ovulation to laying, typically takes around 25 hours.