A chicken egg is a remarkable biological package. While many eggs purchased in stores are unfertilized and cannot develop into a chick, eggs intended for hatching undergo a specific biological process. Understanding how an egg becomes fertilized provides insight into the beginnings of avian life.
The Hen’s Reproductive System
A hen’s reproductive system consists primarily of the ovary and the oviduct. Most hens have only one functional ovary, usually the left, which contains thousands of tiny ova, or potential yolks. As an ovum matures, it grows into a yolk and is released from the ovary through ovulation.
Upon release, the yolk enters the oviduct, a long, muscular tube. The oviduct is divided into five distinct sections: the infundibulum, magnum, isthmus, shell gland (or uterus), and vagina. Each section plays a specific role in forming the egg around the yolk. Fertilization takes place within the infundibulum, the first part of the oviduct.
The Fertilization Event
For a chicken egg to be fertilized, a rooster must mate with a hen, transferring sperm through cloacal contact into her reproductive tract. Hens possess specialized sperm storage tubules (SSTs) located primarily in the uterovaginal junction of the oviduct. These glands can store viable sperm for an extended period, up to two weeks, enabling the hen to lay fertilized eggs consistently after a single mating.
When a yolk is released from the ovary, it enters the infundibulum. Fertilization must occur within a narrow window, approximately 15 to 20 minutes after ovulation, before the yolk moves further down the oviduct. A single sperm penetrates the blastodisc, a small white spot on the yolk’s surface, transforming it into a blastoderm.
The newly formed single cell, called a zygote, begins to divide. This cell division starts within hours and continues as the egg travels through the oviduct. By the time the egg is laid, the blastoderm may consist of hundreds or thousands of cells.
Egg Formation After Fertilization
After fertilization, the developing egg continues its journey through the oviduct, acquiring its various layers. The fertilized yolk moves into the magnum, the longest section of the oviduct. Here, albumen, or egg white, is added around the yolk over approximately three hours.
The egg then proceeds to the isthmus, where it remains for about 75 minutes while the inner and outer shell membranes form around the albumen. Next, the egg enters the shell gland, or uterus, spending the longest period, typically 20 hours or more. During this time, the hard outer shell, composed mainly of calcium carbonate, is formed, and shell pigments are deposited. The entire process, from ovulation to the laying of a complete egg, takes approximately 24 to 26 hours. Embryonic development initiated by fertilization continues throughout this formation period, even before the egg is laid.
Distinguishing Fertilized Eggs
If you obtain eggs from a flock with a rooster, they may be fertilized. A key way to identify a fertilized egg is by examining the yolk. All eggs, whether fertilized or not, have a small white spot on the yolk called a germinal disc.
In an unfertilized egg, this spot, known as a blastodisc, appears as a small, dense white dot. In a fertilized egg, this spot is called a blastoderm and looks different, appearing larger, less dense, and often resembling a bullseye, with a faint white ring and sometimes a central spot. Fertilized eggs are safe to eat and do not differ in taste or nutritional value from unfertilized eggs, provided they have not begun incubation.