PROCESS OF EGG FORMATION IN BIRDS
The process of egg formation is controlled by hormones. The egg is formed partly in the ovary and partly in the oviduct.
The egg is formed in the reproductive tract of a female chicken, called a hen. The reproductive tract is divided into two major parts: the ovary and the oviduct. The ovary is where the yolk is added. When the yolk reaches the right size, it is released from the ovary by a process called ovulation. The released yolk is then picked up by the infundibulum. It is here that fertilization must take place.The yolk then passes to the magnum, where the albumen is added. It then goes on to the isthmus for the addition of the shell membranes
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 burst to release the yolk.
The yolk released by the ovary is taken up by the infundibulum, the internal terminam 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 fertilized or not. The egg spends 15minutes in the infudibulum before it moves to the 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 75minutes 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 outer opening.
hen outer opening:
The egg stays here for a very short time before it is laid through the cloaca or vent. It takes almost 26hours for a complete egg to be formed and laid.
Immediately after the outer shell are outer and inner membranes. Both membranes give protection to the egg.
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.
This is also called the egg white. It accounts for over 50% of the total body weight of the egg. It is rich on protein
Yolk: This is located at then 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.
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.
How a Hen Makes an Egg
The egg is formed in the reproductive tract of a female chicken, called a hen. The reproductive tract is divided into two major parts: the ovary and the oviduct. The ovary is where the yolk is added. When the yolk reaches the right size, it is released from the ovary by a process called ovulation. The released yolk is then picked up by the infundibulum. It is here that fertilization must take place.The yolk then passes to the magnum, where the albumen is added. It then goes on to the isthmus for the addition of the shell membranes. The developing egg spends most of its time in the shell gland, where the shell and any shell pigments are added. As the egg is being assembled it travels down the oviduct small end first. In the outer opening, it is pushed out, the large end first. This prevents the egg from being contaminated by fecal material when it is laid.
Formation of the egg
Reproductive organs of the hen
Reproductive organs of the hen
The egg is formed gradually over a period of about 25 hours. Many organs and systems help to convert raw materials from the food eaten by the hen into the various substances that become part of the egg.
The hen, unlike most animals, has only one functional ovary – the left one – situated in the body cavity near the backbone. At the time of hatching, the female chick has up to 4000 tiny ova (reproductive cells), from some of which full-sized yolks may develop when the hen matures. Each yolk (ovum) is enclosed in a thin-walled sac, or follicle, attached to the ovary. This sac is richly supplied with blood.
The mature yolk is released when the sac ruptures, and is received by the funnel of the left oviduct (the right oviduct is not functional). The left oviduct is a coiled or folded tube about 80 cm in length. It is divided into five distinct sections, each with a specific function, as summarized in table 1.
Functions of various different sections of the hen’s oviduct
Section of oviduct Approximate time egg spends in this section Functions of section of oviduct
1 Funnel (infundibulum) 15 minutes Receives yolk from ovary. If live reproductive cell is present, fertilization occurs here (commercially produced table eggs are not fertilized)
2 Magnum 3 hours Albumen (white) is secreted and layered around
3 Isthmus 1 hour Inner and outer shell membranes are added, as are some water and mineral salts
4 Shell gland (uterus) 21 hours Initially some water is added, making the outer
white thinner. Then the shell material (mainly
calcium carbonate) is added. Pigments may also
be added to make the shell brown
5 cloaca, less than 1 minute The egg passes through this section before
laying. It has no other known function in the
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