The Avian Embryo

Credits: Mississippi State University and Dr. Tom W. Smith Jr., the author.


The earliest stages of a bird in its egg are amazing and exciting. In only three weeks a small clump of cells that do not seem to resemble any animal species changes into an active, newly hatched chick. A study of this change is educational and interesting and gives us insight into how humans are formed.

This publication will help you study the formation of the egg and the avian (bird) embryo, or chick within the egg. This publication includes plans for two small incubators so you can build one. You may buy small commercially-built incubators at stores selling farm and educational supplies.

Incubation procedures show you the effects of heat, moisture, and ventilation on the development of the chick embryo. You also learn to hatch other fowl such as turkeys, ducks, quail, and pheasants. This publication describes how to observe and exhibit an avian embryo while it is alive and still functioning or as a preserved specimen.

Formation and Parts of the Egg

The avian egg, in all its complexity, is still a mystery. A highly complex reproductive cell, it is essentially a tiny center of life. Initial development of the embryo takes place in the blastoderm. Albumen surrounds the yolk and protects this potential life. The blastoderm is an elastic, shock-absorbing semi-solid with a high water content. Together the yolk and albumen are prepared to sustain life -- the life of a growing embryo -- for three weeks, in the case of the chicken. This entire mass is surrounded by two membranes and an outer covering called the "shell." The shell allows an exchange of gases and is a way to conserve the food and water supply within.

A hen can produce an egg without mating. Such an egg, while edible, is not fertile and will not hatch. If a rooster mates with and fertilizes the hen, the male reproductive cell (sperm) unites with the female reproductive cell (ovum) to form a single cell that can develop into an embryo. This egg is fertile and can hatch. If you open an egg and place it in a dish, you can see a light round spot on top of the yolk. This is the germinal disc, true ova, or female egg. At the time of lay, it is hard to tell whether it is fertile.

The egg is formed in the mature hen by a reproductive system composed of an ovary and oviduct. Most females have two functional ovaries, but chickens and most other birds have only one ovary and one oviduct. In early states of embryonic development, each female chick has two ovaries; only the left one develops into a functional organ. In some birds, such as hawks, the right ovary and oviduct usually develop. A mature ovary looks like a cluster of grapes. It may contain up to 4,000 small ova that can develop into yolks. 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.

The oviduct is a large, coiled tube located in the left side of the abdominal cavity. In this oviduct, all parts of the egg except the yolk are formed. It is divided into five distinct regions: (1) infundibulum or funnel, (2) magnum, (3) isthmus, (4) uterus or shell gland, and (5) vagina.

Each male chicken has two reproductive organs called testes, located within the body about midway of the back. The testes produce sperm cells that are complementary to the egg cells of the hen. Each sperm cell has a long, whip-like tail that propels it forward. The sperm are sent to the cloaca through the vas deferens, a tube between the testes and cloaca. After mating, the sperm travel through the hen's oviduct and concentrate in storage sites of the infundibulum.

The yolk is formed in the follicular sac by the depositing of continuous layers of yolk material. Ninety-nine percent of the yolk material is formed within the seven to nine days before the laying of the egg. The germinal disc of a developing yolk contains the single ovum cell which, after fertilization, develops into the chick. 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 with few, if any, blood vessels. This line is called the "stigma." If any blood vessels cross the stigma, a small drop of blood may be deposited on the yolk as it is released from the follicle. This causes most blood spots in eggs. After the yolk is released from the follicle, it is kept intact by the vitelline membrane surrounding it. The release of the yolk from the ovary is called "ovulation."

After its release from the follicle, the yolk falls into the hen's abdominal cavity. The infundibulum of the oviduct quickly engulfs the yolk with its thin, funnel-like lips. If, for some reason, 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 of the ovum follows almost immediately. Sperm cells from the male are stored in glands or nests located in the infundibulum and are released when the yolk passes by. A sperm cell must penetrate the thin vitelline membrane and reach the female cell to complete fertilization. The vitelline membrane thickens as the rest of the egg is formed.

The yolk quickly 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. The shape of the egg is largely determined in this section.

The magnum is divided from the isthmus by a narrow, translucent ring without glands. The isthmus is smaller in diameter than the magnum. It is here the two shell membranes form. The shell membranes loosely contain the yolk and dense white until the rest of the albumen is added in the uterus.

The shell is added in the uterus or shell gland portion of the oviduct. The shell is composed mainly of calcium carbonate. 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.

The chalazae, two cord-like structures that keep the yolk centered in the egg, first appear in the uterus. The chalazae also function as an axis around which the yolk can rotate and keep the germinal disc uppermost at all times.

In the last portion of the oviduct, the vagina, a thin coating called "bloom" is applied to the shell to keep harmful bacteria or dust from entering the egg shell pores. The egg passes through the oviduct small end first but is laid large end first. In the vagina the egg is turned horizontally just before laying. If the hen is disturbed on the nest, the egg may be prematurely laid small end first. Oviposition is the act of pushing the egg from the oviduct.

When an egg is laid, it fills the shell. As it cools, the inner portion of the egg contracts and forms an air cell between the two shell membranes. A high quality egg has a tiny air cell, indicating the egg was collected soon after being laid and was stored properly. The air cell is usually located in the large end of the egg, where the shell is most porous and air can enter easily. The chick punctures and breathes through this air cell just before hatching.

Life in Twenty-one Days

One of the greatest miracles of nature is the transformation of the egg into the chick. A chick emerges after a brief three weeks of incubation. The complexity of the development cannot be understood without some explanation of embryology.

Cell division begins soon after fertilization, even while the rest of the egg is being formed. Cell division will continue if the egg is kept warmer than 80 °F. The first cell division is completed about the time the egg enters the isthmus. Additional cell divisions take place about every 20 minutes, so by the time of lay, several thousand cells form two layers of cells called a "gastrula."

At this time the egg is laid, it cools, and embryonic development usually stops until proper environmental conditions are reestablished for incubation. After incubation begins, the cellular growth resumes. At first, all the cells are alike, but as the embryo develops, you can see cell differences. Some cells may become vital organs; others become a wing or leg.

Soon after incubation is begun, 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. Before the first day of incubation is through, many new organs are forming. The head of the embryo becomes distinguishable; a precursor of the digestive tract, the foregut, is formed; blood islands appear and will develop later into the vascular or blood system; the neural fold forms and will develop into the neural groove; and the eye begins.

On the second day of incubation, the blood islands begin linking and form a vascular system, while the heart is being formed elsewhere. By the 44th hour of incubation, the heart and vascular systems join, and the heart begins beating. Two distinct circulatory systems are established, an embryonic system for the embryo and a vitelline system extending into the egg.

In later stages of embryonic development, there are two distinct extra-embryonic blood systems. One system, the vitelline system, transports nutrients from the yolk to the growing embryo. Before the fourth day, it oxygenates blood. The other blood system, made of allantoic vessels, is concerned with respiration and the storage of waste products in the allantois. When the chick hatches, both circulatory systems cease to function.

On the second day 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.

At the end of the third day of incubation, the beak begins developing and limb buds for the wings and legs are seen. Three visceral clefts (gills) have formed on each side 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 fluid-filled amnion has surrounded the embryo to protect it: it helps maintain proper embryonic development. The tail appears, and the allantois is seen. 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 is seen beating if the egg is opened carefully. The other internal organs continue to develop. By the end of the fourth day of incubation, the embryo has all organs needed to sustain life after hatching, and most of the embryo's parts can be identified. The chick embryo cannot, however, be distinguished from that of mammals.

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 respiration of gases, or the taking in of oxygen and the removal of carbon dioxide and the building of living energy within the chick.

The embryo grows and develops rapidly. 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 are forming and the embryo is moving into position for hatching. The supply of albumen is exhausted by the sixteenth day, so the yolk is the sole source of nutrients. After 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 normal position of the chick for hatching 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 is positioned in the small end of the egg, the chick's chances of survival are reduced by at least one-half. This is a serious malposition, or wrong position, 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. The sharp horny structure on the upper beak (egg tooth) and the muscle on the back of the neck help cut the shell. The chick rests, changes position, and keeps cutting until its head falls free of the opened shell. It then kicks free of the bottom portion of the shell. The chick is exhausted and rests while the navel openings heal and its down dries. Gradually it regains strength and walks. The incubation and hatching are complete. The horny cap will fall off the beak within days after the chick hatches.

Newly-hatched chicks can be shipped long distances (up to 72 hours travel time) without food. You must provide chicks with feed and water on the first day of life so they can learn to eat and drink immediately. The yolk is largely unused by the embryo and is deposited within the chick's body on the 19th day, just before it hatches. The yolk is highly nourishing and provides proteins, fats, vitamins, minerals, and water for several hours after hatching. The yolk is consumed gradually during the first 10 days of the chick's life.


Before Egg Laying
Division and growth of living cells
Segregation of cells into groups of special function (tissues)
Between Laying and Incubation
No growth; stage of inactive embryonic life

During Incubation

First day
16 hours -- first sign of resemblance to a chick embryo
18 hours -- appearance of alimentary tract
20 hours -- appearance of vertebral column
21 hours -- beginning of nervous system
22 hours -- beginning of head
24 hours -- beginning of eye
Second day
25 hours -- beginning of heart
35 hours -- beginning of ear
42 hours -- heart beats
Third day
60 hours -- beginning of nose
62 hours -- beginning of legs
64 hours -- beginning of wings

Fourth day -- beginning of tongue

Fifth day -- formation of reproductive organs and differentiation of sex

Sixth day -- beginning of beak

Eighth day -- beginning of feathers

Tenth day -- beginning of hardening of beak

Thirteenth day -- appearance of scales and claws

Fourteenth day -- embryo gets into position suitable for breaking shell

Sixteenth day -- scales, claws, and beak becoming firm and horny

Seventeenth day -- beak turns toward air cell

Nineteenth day -- yolk sac begins to enter body cavity

Twentieth day -- yolk sac completely drawn into body cavity; embryo occupies practically all the space within the egg except the air cell

Twenty-first day -- hatching of chick