The chicken egg, a remarkable biological package, represents a complex and highly specialized reproductive process unique to birds. Fertilization is an internal event that must occur early in the egg’s formation, long before the familiar shell and white are added. This biological cascade starts with the hen’s ovary releasing a mature ovum, which is the yolk, into the reproductive tract. For a chick embryo to begin development, this ovum must encounter viable sperm, setting off a rapid sequence of cellular and physiological changes within the hen’s oviduct.
The Journey to Fertilization
Fertilization requires the hen’s specialized mechanism for sperm storage, which begins before ovulation. Following mating, spermatozoa ascend the reproductive tract and are sequestered in microscopic invaginations called Sperm Storage Tubules (SSTs). These tubules are primarily located in the utero-vaginal junction, the area where the uterus meets the vagina.
This unique storage system allows the hen to maintain fertility for an extended period, typically two to three weeks, from a single mating. The SSTs protect the sperm and provide conditions necessary for their long-term survival. When the mature ovum (yolk) is released from the ovary, a hormonal signal triggers the release of small numbers of sperm from the SSTs, which then travel upstream toward the site of fertilization in the upper oviduct.
The Moment of Union
Fertilization occurs immediately after the yolk is released from the ovary, taking place in the infundibulum, the funnel-shaped entrance of the oviduct. The ovum’s genetic material is contained within a small, whitish spot on the yolk’s surface known as the germinal disc. This entire event must be completed rapidly, as the ovum spends only about 15 minutes in this section before moving on.
Avian fertilization is distinct because it involves a process called physiological polyspermy, where multiple sperm successfully penetrate the ovum’s outer layer and enter the cytoplasm. Typically between 20 and 60 sperm enter the yolk. Despite this multi-sperm entry, only one sperm nucleus achieves functional monospermy by fusing with the nucleus of the ovum to create the diploid zygote. The remaining supernumerary sperm nuclei degenerate as the early embryo develops.
Encasing the Embryo in the Oviduct
Once fertilized, the ovum, now a zygote, embarks on a roughly 24-hour journey down the oviduct, during which all external egg components are sequentially added. The first stop is the magnum, the longest section, where the bulk of the albumen (egg white) is secreted around the yolk. This albumen acts as a water source, a nutrient supply, and a physical shock absorber for the developing embryo.
The egg then moves into the isthmus, where it spends about 75 minutes while two layers of protein fibers form the inner and outer shell membranes. These membranes provide a protective lining and help define the egg’s overall shape. The final stage of formation occurs in the uterus, or shell gland, where the egg remains for 18 to 20 hours. Here, the hard, calcareous shell, composed mainly of calcium carbonate, is deposited, providing robust external protection necessary for development outside the mother’s body.
Early Development and Laying
Embryogenesis begins immediately after fertilization; the fertilized cell starts dividing rapidly after the fusion of the nuclei. This rapid cell division, known as cleavage, occurs while the egg is still traveling through the oviduct. By the time the shell is fully formed, the zygote has developed into the blastoderm, a multi-cellular structure containing hundreds of cells.
This blastoderm represents the earliest stage of the chick embryo when the egg is laid. Upon exiting the hen’s body, the egg cools, causing the embryo to enter a state of suspended animation known as embryonic diapause. Development ceases until the egg is subjected to the sustained high temperature of incubation. The visible difference between a fertile egg and an infertile one is the appearance of the blastoderm as a multi-ringed structure, contrasting with the simple, solid white spot of an unfertilized egg.