Whether all eggs are fertilized depends heavily on if the term refers to a biological ovum or a commercial poultry product. In biology, an egg is the female reproductive cell (ovum) released to potentially create offspring. However, the eggs sold in grocery stores are almost universally the unfertilized output of a hen’s reproductive cycle, never intended to develop into a chick.
The Biological Definition of Fertilization
Fertilization is the biological event where male and female gametes fuse, creating a new, genetically unique organism. Specifically, it is the union of a sperm cell and an ovum, which results in a single-celled zygote. This process is distinct from ovulation, which is the release of a mature ovum from the ovary, typically occurring once per menstrual cycle in humans. The ovum is released into the fallopian tube, where it waits for sperm to arrive.
For fertilization to occur, a sperm must successfully navigate the female reproductive tract, penetrate the outer layers of the ovum, and fuse its nucleus with the egg’s nucleus. This process is highly time-sensitive, as the ovum is only viable for fertilization for a short window, generally 12 to 24 hours after its release. The vast majority of ova released by mammals throughout their reproductive lifespan never encounter a sperm and therefore remain unfertilized.
Understanding Commercial Eggs
Commercial eggs purchased for consumption, such as chicken eggs, are almost entirely unfertilized. Commercial egg-laying operations do not house hens with roosters, meaning the female birds produce eggs without male gametes being present. Since fertilization requires the union of sperm and ovum, the absence of a rooster makes it biologically impossible for these eggs to be fertile. Hens naturally produce eggs as part of their reproductive cycle, regardless of whether a male is present.
If a rooster were present and mating occurred, the resulting fertile egg would look visually similar to an infertile one from the outside. When cracked open, a fertile egg displays a blastoderm on the yolk, often described as a “bullseye.” An infertile egg, the standard grocery store product, only shows a small, dense white spot on the yolk called the germinal disc, which contains only the hen’s cells. The practice of refrigerating commercial eggs also halts any potential cellular development that may have begun.
What Happens to Eggs That Are Not Fertilized
When a mammalian ovum is not fertilized within its short period of viability, its biological fate is to degrade and be eliminated by the body. Once the 12 to 24-hour window passes, the egg begins to disintegrate in the fallopian tube. In many mammals, including humans, the unfertilized ovum is simply broken down and reabsorbed by the body’s cells.
The body’s preparation for a potential pregnancy is reversed when no fertilization occurs. The uterine lining, which thickened to support an implanted embryo, is no longer needed. This tissue is shed, along with the disintegrated ovum, during the process known as menstruation. The shedding of the lining signals the completion of one cycle and the beginning of the next, where a new ovum will be prepared and released.