Determining the fertility of an avian egg is a common need for those seeking to incubate and hatch chicks. A chicken egg is considered fertile only if a rooster was present to mate with the hen, allowing the male and female reproductive cells to combine. Unfertilized eggs are nutritionally identical to fertile eggs that have not been incubated. The main distinction lies in the potential for development: a fertile egg contains the potential for an embryo, while an infertile egg does not.
Checking the Egg Before Incubation
The most definite way to check a fresh egg’s fertility is by cracking it open and examining the small white spot on the yolk’s surface. This spot is called the germinal disc, and its appearance changes dramatically depending on whether fertilization has occurred.
In an infertile egg, this spot is known as the blastodisc and appears as a small, dense, and irregular white dot, usually measuring about 2 to 3 millimeters in diameter. The edges of the blastodisc often look ruffled or jagged, and the spot itself is a solid, opaque white color.
The spot in a fertile egg is called the blastoderm, which has begun the process of cell division while the egg was still in the hen’s oviduct. The blastoderm is noticeably larger, often measuring 5 to 9 millimeters across, and has a distinct “bullseye” or donut-like appearance. It consists of a faint white ring surrounding a clearer central area. This characteristic concentric ring structure is the definitive visual sign of a fertile, unincubated egg.
Using Light to Check During Incubation
The technique of candling allows for the non-destructive inspection of an intact egg after incubation has begun, typically between days 3 and 7. Candling involves shining a strong light source, such as a specialized candler or a bright LED flashlight, directly through the eggshell in a completely dark environment.
The ideal time to candle is around day 7 of incubation, as the embryo’s circulatory system is visible by this point. A fertile, developing egg will show a small, dark, spider-web-like network of fine red lines branching out from a central dark spot, which is the embryo itself.
An egg that is infertile will appear uniformly clear when candled, showing only a shadow of the yolk and no sign of blood vessels or a dark spot. An embryo that started developing but died early may present with a blood ring. This blood ring is formed by the early circulatory system separating from the dead embryo and is a sign that the egg should be removed from the incubator to prevent potential contamination.
Key Visual Differences Between Fertile and Infertile Eggs
Determining fertility depends on the stage at which the egg is examined. Before incubation, the germinal disc on the yolk is the sole indicator. An infertile egg displays the small, solid, and irregularly shaped blastodisc. The fertile egg, conversely, exhibits the blastoderm, which is a larger spot with a characteristic bullseye pattern of a faint ring around a translucent center.
Once incubation has been underway for several days, candling provides the next set of visual cues. The infertile egg remains clear, showing only the yolk shadow within the shell. A fertile egg that is developing properly will reveal a small dark embryo surrounded by a radiating network of tiny red blood vessels.
If a blood ring is present, this indicates the egg was fertile but the embryo has died and is no longer viable. These distinct appearances at different stages allow for an accurate assessment of an egg’s potential.