The Journey Begins: Fertilization and Egg Formation
The creation of a chick begins with the fertilization of an ovum within the hen’s reproductive system. Unlike mammals, the fertilization process in chickens occurs internally before the egg is fully formed. A rooster’s sperm fertilizes the yolk shortly after it is released from the hen’s ovary. This union forms the single cell that will develop into an embryo.
Following fertilization, the yolk travels down the hen’s oviduct, a tube where various layers are added. First, layers of albumen are secreted around the yolk, providing protein and water for the developing embryo. Next, the shell membranes are formed, creating a protective barrier just inside the hard shell. Finally, the shell gland deposits the hard, porous calcium carbonate shell around the entire structure, completing the egg’s formation before it is laid.
Inside the Shell: Embryonic Development
Once a fertilized egg is laid and incubation begins, under a hen or in an incubator, embryonic development proceeds rapidly over approximately 21 days. The initial single cell divides repeatedly, forming an organized structure called the blastoderm, differentiating into various tissues and organs. During the first few days, the circulatory system starts to form, and the heart begins to beat, establishing a network for nutrient and waste exchange.
By around day four, the eyes and brain begin to develop, visible within the embryo. Limb buds that will become wings and legs also start to appear around this time. Over the subsequent days, feather follicles emerge across the embryo’s skin, and the beak hardens, preparing for emergence. The embryo utilizes the yolk as its primary food source, absorbing its rich nutrients, while the albumen provides additional protein and moisture.
As development continues, the embryo grows larger, filling the egg. Important membranes like the amnion, which surrounds the embryo in a fluid-filled sac, and the allantois, involved in respiration and waste storage, develop and expand. By the final week of incubation, the chick is nearly fully formed, with distinct feathers, fully developed limbs, and a functional respiratory system, ready for hatching.
The Big Day: Hatching
The process of hatching is a significant effort for the chick, marking the end of its 21-day development inside the egg. Around day 19, the chick typically begins to internally pip, breaking into the air cell at the blunt end of the egg. This allows it to take its first breaths of air from within the egg, transitioning from embryonic to pulmonary respiration. The yolk sac, providing nourishment, is fully absorbed into the chick’s body just before hatching, offering a final energy reserve for its first few days outside the shell.
Equipped with a temporary growth on its beak called an “egg tooth,” the chick begins external pipping, tapping and breaking through the shell. It rotates slowly within the egg, using its egg tooth to create a circular line of cracks around the shell’s circumference, known as zipping. This strenuous activity can take several hours, sometimes even a full day, as the chick pushes against the shell until it breaks free.
First Steps: Life After Hatching
Immediately after emerging from the shell, a newly hatched chick appears wet and exhausted. Its downy feathers are matted, and it often rests to recover its strength. Within a few hours, the down begins to dry and fluff out, allowing the chick to regulate its body temperature.
Once dry, the chick starts to gain coordination, taking its first steps. At this stage, the chick relies on external sources for warmth, like a brooder lamp or a mother hen, as it cannot yet regulate its own body temperature. Access to fresh water and chick starter feed is necessary, providing hydration and nutrients for rapid growth and adjustment to life outside the shell.