Sophie Bennett
The embryology of a chicken is the development of a chicken inside an egg. It is a complex process that usually takes 21 days of favourable incubation conditions for the chicken to develop and hatch. The transformation of an egg into a chick is considered one of the greatest miracles of nature. The embryonic development of a chick can be broken down into several stages, starting with the appearance of embryonic tissue on day one, the appearance of blood vessels and a heartbeat on day three, and the development of the beak and limbs by day five. By day seven, digits appear on the wings and feet, and by day 10, feathers and feather tracts are visible. After 20 days, the chick is in the hatching position, and by day 21, it begins to break free from its shell.
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What You'll Learn
Embryonic development begins before the egg is laid
The embryology of a chicken is the development of a chicken inside an egg. The embryonic development of a chick begins before the egg is laid. When the sperm cell carrying half the required chromosomes fertilises the female egg cell (which carries the other half of the required chromosomes), it forms a zygote, a single cell with the correct number of chromosomes. This fertilisation takes place in the infundibulum about 15 minutes after the holding follicle releases the yolk. Cell division to create the new embryo starts about five hours after fertilisation and continues while the egg passes along the oviduct and after the egg is laid.
Before the egg is laid, the cells divide into groups of special functions (tissues). Once the egg is laid, embryonic development usually stops until proper cell environmental conditions are established for incubation to resume. When the temperature of the egg is below 20°C, the embryo becomes dormant and most development stops. When the temperature rises above 20°C, embryonic activity re-initiates.
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. At the end of the third day of incubation, the beak begins to develop, and limb buds for the wings and legs are seen. Torsion and flexion continue through the fourth day. The chick's entire body turns 90 degrees 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. By the end of the fourth day of incubation, the embryo has all the organs needed to sustain life after hatching, and most of the embryo's parts can be identified.
On 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. After 20 days, the chick is in the hatching position, and after 21 days of incubation, the chick finally begins its escape from the shell.
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The embryo takes on a C-shape on day 5
The development of a chick embryo is a complex process that usually takes about three weeks, or 21 days, of incubation. During this time, the embryo undergoes rapid changes, transforming from a fertilized egg into a fully formed chick.
On the fifth day of incubation, the embryo undergoes significant development, taking on a distinct C-shape. This occurs when the embryo's head moves closer to its tail, resulting in a curved structure. This is a notable change from its previous position, where the embryo lay on its left side.
The C-shape formation is indicative of the embryo's overall growth and the development of its musculoskeletal system. By this stage, the embryo has already developed vital structures such as the brain, eyes, and cardiac structures. The heart, in particular, continues to enlarge, even though it remains outside the body.
Additionally, the fifth day marks the beginning of sex differentiation in the embryo. The respective reproductive organs for each gender start to form, indicating the future chick's biological sex. This process is integral to the embryo's overall development and future reproductive capabilities.
The C-shape formation on day five is a critical milestone in the embryo's journey towards hatching. It signifies the embryo's rapid growth and the development of its musculoskeletal structure, setting the foundation for its eventual emergence as a chick.
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The yolk sac is drawn into the body on day 20
The yolk sac is an important part of the chick embryo's development. It envelops the yolk and produces an enzyme that changes the yolk material into a form that can be used as a food source by the developing embryo.
By the end of the third day of incubation, the embryo's beak begins developing and limb buds for the wings and legs are visible. By the fourth day, the chick's entire body has turned 90 degrees and lies with its left side on the yolk. The head and tail come close together so the embryo forms a "C" shape.
On day 20, the yolk sac is completely drawn into the body of the chick embryo. The embryo becomes a chick, breathing air with its lungs, and internal and external pipping occurs. The embryo is now in the hatching position, the beak has pierced the air cell, and pulmonary respiration has begun.
After 21 days of incubation, the chick begins to escape from its shell. It pushes its beak through the air cell and uses its egg tooth and the muscle on the back of its neck to cut the shell. The chick rests, changes position, and keeps cutting until it is free of the shell.
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The embryo has all organs to sustain life after hatching by day 4
The development of a chick embryo is a complex process that usually takes around three weeks or 21 days of incubation for the chicken to develop and hatch. The embryology of a chicken refers to the development of the chicken inside an egg. On the very first day, embryonic tissue appears, and by the second day, the development of this tissue is very visible, along with the appearance of blood vessels.
By the third day, the heart has begun to beat, and the blood vessels are very prominent. On day four, the embryo's eye is pigmented, and by day five, the elbows and knees are visible. On day six, the embryo's beak begins to appear, and it starts to move voluntarily.
Remarkably, by the end of day four of incubation, the embryo has developed all the organs necessary to sustain life after hatching. At this stage, most of the embryo's parts can be identified, although it cannot be distinguished from the embryo of a mammal. The embryo continues to grow and develop at a rapid pace.
By day seven, digits appear on the wings and feet, and the heart is enclosed within the thoracic cavity, giving the embryo a more bird-like appearance. After ten days of incubation, feathers and feather tracts are visible, and the beak hardens. On day 14, the claws begin to form, and the embryo moves into position for hatching.
After 20 days, the chick is in the hatching position, and pulmonary respiration has commenced. Finally, after 21 days of incubation, the chick breaks free from its shell.
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The chick fully hatches after 21 days of incubation
The embryology of a chicken refers to the development of a chicken inside an egg. This process takes 21 days, with the first day spent in the oviduct and the remaining 20 days spent in an incubator, nest, or under a hen.
On the first day of incubation, embryonic tissue appears. On the second day, blood islands begin linking and form a vascular system, while the heart is being formed elsewhere. By the end of the second day, tissue development is very visible, and blood vessels appear. By the 44th hour, the heart and vascular systems join, and the heart begins to beat.
On the third day, the embryo's entire body turns 90 degrees and lies down with its left side on the yolk. The head and tail come close together, and the embryo forms a "C" shape. The mouth, tongue, and nasal pits develop as parts of the digestive and respiratory systems. The beak also begins to develop, along with limb buds for the wings and legs.
On the fourth day, the embryo has all the organs needed to sustain life after hatching, and most of its parts can be identified. Torsion and flexion continue through the fourth day. By the seventh day, digits appear on the wings and feet, and the heart is completely enclosed in the thoracic cavity. After the tenth day, 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. After 20 days, the chick is in the hatching position, and pulmonary respiration has begun.
Finally, after 21 days of incubation, the chick fully hatches and begins its escape from the shell. This process can take up to 24 hours, and it is not uncommon for chicks to take long 10-hour naps during this time. It is important to never help a chick out of its egg unless you are absolutely sure what you are doing, as there is a risk of drowning if they pip somewhere other than the air sac or very close to it.
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Frequently asked questions
A chick emerges after a brief three weeks (21 days) of incubation. However, this development takes 22 days – one day in the oviduct and 21 days in the incubator or nest.
The yolk sac envelops the yolk and produces an enzyme that changes the yolk material into a form that can be used as food by the developing embryo.
The amnion forms a sac that is filled with fluid in which the embryo floats, providing a shock-absorbing environment. The allantois develops an extensive circulatory system connected to that of the embryo and is driven by the new embryonic heart.
