Bartelio home page about the yolk

On the first day, a pointed thickened layer of cells becomes visible in the caudal or tail end of the embryo. This pointed area is the primitive streak and is the longitudinal axis of the embryo. On the second day, the blood islands begin linking and form a vascular system, while the began to form.

By the 44th hour, the heart and vascular systems join, and the heart begins beating. Two circulatory systems, an embryonic system and a vitelline system are formed. They are used for the embryo and extending into the egg respectively. Later, the neural groove forms and the head portion develops into the parts of the brain. The embryo is developed enough that flexion and arching of the embryo begins, the ears begin development, and the lenses in the eyes are forming.



In later stages of embryonic development, two distinct extra-embryonic blood systems will be established. The vitelline system transports nutrients from the yolk to the growing embryo and also oxygenates blood before the fourth day. The other blood system which is concerned with respiration and the storage of waste products in the allantois, is made of allantoic vessels. Both circulatory systems stops functioning only when the chick hatches. At the end of the third day, the beak begins developing and limb buds for the wings and legs are seen.

Three visceral clefts, gills, have formed on both sides of the head and neck. These formations are important in the development of the arterial system, eustachian tube (in the ear), face, jaw, and some ductless glands. the embryo has been surrounded by the fluid-filled amnion to protect it. This is done by helping to maintain proper embryonic development. The tail appears, and the allantois becomes visible. The allantoic vesicle is a respiratory and excretory organ. Nourishment from the albumen and calcium from the shell are transported to the embryo through the allantois. Torsion and flexion continue through the fourth day.

The chick's entire body turns 90° and lies down with its left side on the yolk. The head and tail come close together so the embryo forms a "C" shape. The mouth, tongue, and nasal pits develop as parts of the digestive and respiratory systems. The heart continues to enlarge, even though it has not been enclosed within the body. It can be seen beating if the egg is opened carefully. The other internal organs continue to develop. By the end of the fourth day, the embryo has all organs needed to sustain life after hatching, and most of the embryo's parts can be identified. Many complex physiological processes take place during the change from the egg to the chick. They include the use of highly nutritious food materials in the egg; the exchange of gases, and the building of living energy within the chick. By the seventh day digits appear on the wings and feet, the heart is completely enclosed in the thoracic cavity, and the embryo looks more like a bird.

After the tenth day of incubation, feathers and feather tracts are visible and the beak hardens. On the fourteenth day, the claws started forming and the embryo started to move into the hatching position. By the sixteenth day, the supply of albumen will be used up, thus the chick gets its nutrients solely from its yolk. On the 20 days the chick is in the hatching position, the beak has pierced the air cell, and pulmonary respiration has begun. The yolk sac is contained completely within the body cavity in preparation for hatching. The correct hatching position is with the head in the large end of the egg, under the right wing, with the legs drawn up toward the head. If the head on the opposite side, the chick's chances of survival are reduced by at least one-half. This is a serious malposition for hatching. Just as a wrong position makes birth more difficult in mammals, a wrong position of the chick makes hatching more difficult or impossible. After 21 days of incubation, the chick finally begins its escape from the shell. The chick begins by pushing its beak through the air cell. The allantois, which has served as its lungs, begins to dry up as the chick uses its own lungs. The chick continues to push its head outward.

Formation and Parts of the Egg in the womb. The embryo develops in the blastoderm. Albumen surrounds the yolk and protects this potential life. Both the yolk and albumen are prepared to sustain life for three weeks in the chicken egg. This entire mass is surrounded by two membranes and a shell. The shell allows the exchange of gases and waste products to take place. When an egg is fertilised, you can see a light round spot on top of the yolk. This is the germinal disc or female egg. At the time of lay, it is hard to tell whether it is fertile. Each yolk is attached to the ovary by a slender stem or peduncle. It is encased by a thin membrane sac or follicle having a fine network of blood vessels. In the follicular sac, the yolk is formed by depositing continuous layers of yolk materials. Within 7-9 days before the laying of the egg, 99% of the yolk material is formed. The germinal disc remains on the surface of the yolk throughout yolk formation. When the yolk matures, the follicular sac ruptures or splits along a line,stigma with few blood vessels. The blood spots on yolks are formed when any blood vessels cross the stigma, depositing a small drop of blood as it is released from the follicle. The yolk is kept intact by the vitelline membrane surrounding it when it is released from the follicle.

The yolk falls into the hen's abdominal cavity after it is released from the folicle. After that, it is engulfed by the infundibulum of the oviduct with its thin, funnel-like lips. However, if the infundibulum is unable to pick up the yolk from the body cavity, the body will reabsorb the yolk. A hen that consistently fails to pick up the yolks from the body cavity is called an internal layer. After the yolk is surrounded by the infundibulum, fertilization should occur right away. Next, the yolk enters the magnum section of the oviduct, where the dense portion of the albumen is added. The albumen serves as a shock-absorbing substance and feeds the developing embryo, while the shape of the egg is also determined here. The magnum is now divided from the isthmus by a narrow, translucent ring without glands. The two shell membranes form here. The shell membranes loosely contain the yolk and dense white until the rest of the albumen is added in the uterus. The shell is composed mainly of calcium carbonate, and it takes about 20 hours for the egg shell to form. If the hen lays brown eggs, the brown pigments are added to the shell in the last hours of shell formation. A thin coating called "bloom" is applied to the shell in the vagina, keeping harmful bacteria or dust from entering the egg shell pores. The egg is laid large end first. Just before laying, the egg is turned horizontally . If the hen is disturbed, the egg may be prematurely laid small end first. Oviposition is the process of pushing the egg from the oviduct. When the egg is warm, it fills the whole shell, but when cooled, the inner portion of the egg contracts and forms an air cell between the two shell membranes.The air cell is formed after the egg has been laid. A fresh egg can identified by looking at the size of the air cell. If the air cell is small, it shows that the egg was collected soon after being laid and was stored properly. The large end of the egg is more porous and air can enter easily. Therefore, the air cell is usually located there. The chick punctures and breathes through this air cell just before hatching.

The Egg-laying Process

Chickens lays only one egg daily. Chickens lay eggs in the morning, but each day a little later. Sometimes the last egg of a series is produced in the early afternoon. When it gets too late, they take the next day off.

Perspective on Laying Eggs

1. The hen goes towards the nest in a very hesitant way. When it finally enters, she sits quietly for about half an hour. She closes an eye or calmly puts a straw on her back, and she gets more excited. The hen raises her tail now and then and spreads the feathers of her bottom. These movements increase gradually.

2. Beneath her tail, between the feathers, there is an inch wide opening in the form of a horizontal slit surrounded by a ribbed rim, with skin and feathers further outside . It is called the vent.

3. The hen stands up suddenly with her feet wide apart, tail raised, bottom feathers spread out, and back feathers upright. As her vent opens a little, you begin to see a red membrane.

4. As the hen lowers her bottom, her vent widens rapidly and the rim is stretched further. The membrane forms a pinkish dome around the egg which is not yet visible at this stage. 5. The vent is now wide open and the ribbed rim has become narrow and far stretched. Through the opening bulges a pink hemisphere of tissue revealing distinct blood vessels. Its top is pointed downward where a new opening arises. The egg appears as a much lighter-colored disk. Even brown eggs are much lighter than the surrounding membrane.

6. The hen strains at intervals. Each time, the egg comes a little further out. As it does, the membrane opens to form a red collar around the wider, middle portion of the egg. The membrane will protrude a little ways from the ribbed rim.

7. The moist egg pops out. Sometimes it will come out blunt end first, and sometimes, the vice-versa. After few seconds, a small red cone still remains outside, but it is retracted almost immediately and the vent is closed again. The bird stands high above the egg and rests, beak open and panting after the heavy work.

The entire process (from rising to dropping the egg) is quite fast and is finished within half a minute. Therefore, it is hard to observe. After a while, the hen looks back, inspects the egg with her beak and leaves the nest under loud cackles. The hen then eats and drinks and goes her usual ways. She seems to have forgotten her egg completely.

The Hen's Perspective on Laying Eggs

1. Chickens, as well as other birds, have a vent for reproduction, and the evacuation of stools and urine. The bladder was absent because their urine is not a fluid. It is a white paste, urates, that you can see surrounding the droppings. The intestine, ureters and oviduct come together into a common chamber called the cloaca. This is a rather dirty place, whereas the egg is always clean and almost sterile when laid.

2. The hen turns part of the cloaca and the last segment of the oviduct inside out, "like a glove." The described red membrane is then everted inside of these organs. The egg emerges far outside, at the end of the bulge. So it cannot contact the walls of the cloaca and get contaminated by stools or urine. Moreover, the intestine and inner part of the cloaca are kept shut by the emerging egg, and their contents cannot leave when the hen strains to deliver the egg. Therefore, eggs are always clean as they are laid. However, sometimes a hen, stomping around the nest with dirty feet, will get the egg dirty.

Why Chickens Leave the Nest

All chickens lay one egg per day. If the eggs are not collected, and enough eggs remain in the nest, the hen may stop laying eggs and start brooding. When the hen leaves the nest after laying an egg, it cools which stops the development of the embryo inside. If the ambient temperature remains between 45F and 65F, the embryos will remain viable for as long as two weeks. When the hen becomes broody and sits on her eggs for three weeks, all of the eggs will hatch at about the same time. This is why it is important for the hen to leave the nest after laying.

Breaking Broodies

When we remove the eggs, the hen thinks that there are not enough eggs, thus she continues to lay. The hens of some breeds will get broody anyway after some time. This condition can be cured by placing the hen in a "broody pen". This is a small cage, hung in the hen house. The cooling of the bottom, the swinging movements, and the lack of eggs to sit on are said to cure this condition. The broodiness trait has been removed from most commercial laying breeds through selection. The modern laying hen has delegated her responsibility of hatching, raising, and educating chicks to humans. In nature, the poor creatures would soon become extinct.