Chicken eggs are a common part of many diets, yet the biological processes behind their formation and potential for life are not widely understood. For an egg to develop into a chick, it must undergo internal fertilization within the hen.
Understanding Chicken Eggs
Most eggs found in grocery stores are unfertilized, meaning they will not develop into a chick. These eggs come from hens kept without roosters, and hens lay eggs regularly regardless of mating. Such eggs are safe and nutritious for human consumption. A chicken egg comprises several components, each serving a specific purpose.
The yolk contains a rich supply of nutrients that would nourish a developing embryo. Surrounding the yolk is the albumen, or egg white, which provides protein and a protective cushion. The hard shell offers physical protection and allows for gas exchange. On the surface of the yolk, a small, whitish spot known as the germinal disc is visible; this is where fertilization takes place if sperm is present. If fertilized, this disc develops into an embryo.
The Fertilization Process
For an egg to become fertilized, a rooster must mate with a hen. Sperm transfer occurs through a brief physical contact between their cloacas. During this contact, the rooster deposits sperm into the hen’s cloaca. The sperm then travels up the hen’s reproductive tract, into the oviduct.
Hens can store sperm within specialized structures called sperm host glands. These glands can keep sperm viable for about 10 days to 2 weeks. When a mature yolk (ovum) is released from the hen’s ovary, it is captured by the infundibulum, the first section of the oviduct. Fertilization occurs here shortly after the yolk enters the infundibulum. Sperm released from the host glands migrate to this site to fertilize the newly ovulated yolk, forming a single-celled zygote.
From Fertilization to Laying
Once fertilization occurs in the infundibulum, the now-fertilized ovum, or zygote, continues its journey through the hen’s oviduct. The next section, the magnum, adds layers of albumen, forming the egg white, a process that takes approximately three hours. Following this, the egg enters the isthmus, where the inner and outer shell membranes are formed around the developing egg.
The egg then proceeds to the shell gland, also known as the uterus, where the hard outer shell is deposited. This shell formation is the longest stage, lasting approximately 19 hours, and gives the egg its final shape and rigidity. Embryonic development begins with cell division shortly after fertilization, continuing as the egg moves through the oviduct, even before the egg is laid. The complete process, from ovulation to the laying of a fully formed egg, takes approximately 25 to 26 hours.