Chicken eggs are a common part of many diets, but the journey from a hen’s reproductive system to a laid egg involves a precise biological process. Understanding how these eggs, particularly those destined to become chicks, are formed begins with the internal workings of the hen. This process highlights the efficiency of avian biology, transforming a single cell into a complex, self-contained unit ready for development.
The Hen’s Reproductive System
A hen’s reproductive system consists of the ovary and the oviduct. The ovary, which resembles a cluster of grapes, contains thousands of tiny ova, or potential yolks, each enclosed within a follicle. As a yolk matures, it grows by accumulating yolk fluid and is eventually released from the ovary through a process called ovulation.
Upon ovulation, the yolk is captured by the infundibulum, the funnel-shaped beginning of the oviduct. The oviduct is a long, convoluted tube divided into five distinct sections: the infundibulum, magnum, isthmus, shell gland (or uterus), and vagina. Each section plays a specific role in the formation of the egg as the yolk travels through it.
The Fertilization Event
Fertilization in chickens occurs in the infundibulum, the initial segment of the oviduct. When a rooster mates with a hen, sperm are deposited into the hen’s cloaca and travel into the oviduct, where they can be stored in specialized glands for up to several weeks. When a mature yolk is released from the ovary and enters the infundibulum, stored sperm fertilize it.
This fertilization typically happens within minutes of the yolk’s arrival in the infundibulum. While many sperm may reach the yolk, only one successfully fuses with the ovum, delivering its genetic material to the germinal disc, a small white spot on the yolk’s surface. This fusion marks the moment of fertilization, forming a zygote, which then begins cell division to become an embryo, or blastoderm. The yolk remains in the infundibulum for roughly 15 to 18 minutes before continuing its journey down the oviduct.
From Fertilization to Laying
After fertilization in the infundibulum, the developing embryo, still encased within the yolk, moves into the magnum. Here, the egg white, or albumen, is secreted and layered around the yolk. This process takes approximately three hours, with the albumen providing both protection and nourishment for the growing embryo.
Following the magnum, the egg enters the isthmus, where the inner and outer shell membranes are formed around the albumen. This stage lasts about 75 minutes. The egg then proceeds to the shell gland, or uterus, where the hard outer shell is deposited. This takes approximately 19 to 20 hours, during which calcium carbonate is applied to form the shell, and any shell pigments are added. The entire journey from ovulation to a fully formed egg takes approximately 24 to 26 hours.
Identifying a Fertilized Egg
Distinguishing a fertilized egg from an unfertilized one is possible through a method called candling. Candling involves shining a bright light through the egg in a darkened room. In a fresh, unincubated fertilized egg, a small, often subtle, bullseye-like pattern or a more developed germinal disc (blastoderm) may be visible on the yolk, contrasting with the smaller, less distinct spot (blastodisc) found in unfertilized eggs.
After several days of incubation, signs of fertilization become clearer. By around day 7 of incubation, a developing embryo appears as a small dark spot with noticeable red, spider-like blood vessels radiating outwards, indicating active growth. Unfertilized eggs, even after incubation, remain clear inside, showing no signs of an embryo or blood vessel development.