The process of egg formation is controlled by hormones. The egg is formed partly in the ovary and partly in the oviduct.
- Ovary: The yolk is secreted by the ovary and enclosed in a follicle. The yolk increases in size by accumulating yolk materials carried from different parts of the body by blood stream. The germinal disc is attached to the top of the yolk. The follicle bursts to release the yolk.
- Infundibulum: The yolk released by the ovary is taken up by the infundibulum, the internal terminal part of the oviduct. Fertilization of the egg occurs in this part of the oviduct, before the other components are added. However, complete formation of the egg is independent of whether the egg is fertilised or not. The egg spends 15 minutes in the infundibulum before it moves to the magnum.
- Magnum: In the magnum, the egg stays for three hours and part of the albumen is secreted on the yolk. Chalaza is also formed in this region. The egg now moves to the isthmus.
- Isthmus: The egg stays here for 75 minutes and the two shell membranes are formed. The shape of the egg is also formed at this region after which it moves to the uterus.
- Uterus: The egg stays here for 19 – 20 hours where the shell is formed from calcium carbonate secreted by glands of the uterus. Mineral solutions are also. added to the egg after which it moves to the vagina.
- Vagina: The egg stays here for a very short time before it is laid through the cloaca or vent. It takes almost 26 hours for a complete egg to be formed and laid.
Structure of a Fertile Egg
- Shape: The egg is oval or oblong in shape. It is either brown or white in colour. The differences in colour are due to differences in breeds.
- Shell: The hard shell covers the egg externally. It is rich in calcium carbonate and it protects the egg.
- Membranes: Immediately after the outer shell are outer and inner membranes. Both membranes give protection to the egg.
- Airspace: The airspace is located on one of the pointed ends of the egg. It is found in-between the outer and inner membranes. The airspace is very important for respiration of the embryo.
- Albumen: This is also called the egg white. It accounts for over 50% of the total body weight of the egg. It is rich in protein.
- Yolk: This is located at the centre of the egg. It is a yellowish jelly-like mass. It is rich in proteins, mineral salts, vitamins and other food substances. It supplies the embryo nutrients.
- Embryo: This is also called germinal disc. This is located at the centre of the yolk as a dark spot. The germinal disc is only found in fertile eggs. It develops to form the chick during incubation period.
- Chalaza: The chalaza extends to both sides of the yolk. It is a piece of thick protoplasm. It holds the yolk and the embryo in place within the albumen.