Chickens produce eggs through a complex biological system. The egg’s journey, from its formation within the hen to its expulsion, involves specialized internal structures working in coordination.
Internal Structures for Egg Formation
Egg formation begins within the hen’s reproductive system, specifically with the ovary and the oviduct. A hen possesses only one functional ovary, typically the left one, which contains thousands of tiny ova, or potential egg yolks, from birth. As a hen matures, these ova develop into yolks, with one yolk maturing and being released approximately every 24-26 hours. This mature yolk then enters the oviduct, a long, coiled tube that can stretch up to 80 centimeters in length.
The oviduct is divided into five distinct sections, each playing a specific role in adding components to the developing egg. The infundibulum, the first section, captures the released yolk and is where fertilization can occur if sperm are present. Following this, the yolk moves into the magnum, the longest part of the oviduct, where the thick albumen, or egg white, is secreted around it over approximately three hours. Next, in the isthmus, inner and outer shell membranes are added, and the egg begins to take on its characteristic oval shape.
The egg then proceeds to the shell gland, also known as the uterus, where the hard, calcified shell is formed. This is the longest stage of egg formation, lasting about 19 to 20 hours, during which calcium carbonate is deposited to create the shell. Pigments are also added here, determining the egg’s color. Finally, the egg passes into the vagina, the last section of the oviduct, where a protective waxy layer called the bloom or cuticle is applied just before laying.
The Journey of an Egg
After formation, muscular contractions within the oviduct propel the egg towards expulsion. The entire process, from ovulation of the yolk to the laying of the completed egg, typically takes about 25 to 26 hours.
As the egg moves through the oviduct, its orientation can shift. While it generally travels small end first, the egg often rotates in the vagina just before being laid. This rotation usually results in the blunt end exiting first, which may help in reducing the likelihood of cracking upon laying.
The Common Exit Point
Chickens possess a single, multi-purpose external opening called the cloaca, which is also referred to as the vent. This anatomical feature serves as the common exit point for the chicken’s reproductive, digestive, and urinary systems. Unlike mammals, chickens do not have separate openings for waste elimination and egg-laying.
When a hen lays an egg, the vagina, the final segment of the oviduct, temporarily protrudes through the cloaca. This action effectively seals off the intestinal opening, preventing the egg from coming into contact with fecal matter as it exits. While waste and eggs share this common exit, the hen’s anatomy is designed to keep the processes separate during the moment of laying.