Mia Patel
The development of a chicken embryo is a complex process that usually takes about three weeks, or 21 days, for the chicken to hatch. The fertilisation of the germinal disc by the sperm occurs about 15 minutes after its holding follicle releases the yolk, and cell division begins about five hours later. Over the next few days, the embryo's circulatory system and vascular system develop, and by the fourth day, the embryo has all the organs it needs to survive after hatching. By the fifth day, the embryo takes on a C-shape, and by the sixth day, it can move voluntarily. By the 20th day, the embryo is ready to hatch, and on the 21st day, it does.
What You'll Learn
Development of the embryo's circulatory system
The development of a chicken embryo typically takes 21 days of favourable incubation conditions for the chicken to develop and hatch. However, this development takes 22 days—one day in the oviduct and 21 days in the incubator or nest.
The circulatory system becomes functional early in the embryo's development, often before other organs and systems are obvious. This is because the embryo requires an adequate supply of oxygen and metabolites for its development. In its early stages, oxygen and metabolites may be provided by diffusion. However, beyond a certain size, the circulatory system becomes necessary for providing these essential elements.
The development of the circulatory system in higher vertebrate embryos, such as those of birds and mammals, generally follows a sequence of seven main events. Initially, a tubular heart bends into an "S" shape. Blood then flows from back to front through the sinus venosus, atrium, ventricle, and bulbus cordis. The atrium and ventricle then subdivide, as does the opening between them. The sinus venosus is incorporated into the right atrium, and the pulmonary veins are segregated to open into the left atrium. The bulbus cordis is subdivided into a pulmonary trunk from the right ventricle and a systemic trunk from the left ventricle. Finally, an embryonic set of six arterial arches is reduced to three in adult birds, and their relationships are further complicated by asymmetrical loss and development of certain parts.
The allantois, a membrane that develops an extensive circulatory system connected to that of the embryo, plays a crucial role in the embryo's development. It is driven by the new embryonic heart and, when fully developed, completely surrounds the embryo. The allantois serves several functions, including respiration, as the developing embryo uses oxygen and produces carbon dioxide.
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Embryo's growth and sex differentiation
The development of a chicken embryo takes approximately 21 days, with the first 24 hours considered as Day 1. On this day, the fertilized egg's germinal disc is at the blastodermal stage, with a central area that is lighter in colour, housing the embryo. The segmentation cavity takes on the shape of a dark ring, and the primitive streak—a pointed thickened layer of cells—becomes visible, marking the embryo's longitudinal axis. The head and backbone of the embryo begin to develop from this structure.
On Day 2, the first groove appears at the centre of the blastoderm, and the vitelline membrane, crucial for embryo nutrition, makes its first appearance. Blood islands, precursors to the vascular system, also begin to form and link together.
By Day 3, the embryo is lying on its left side, and the heart has begun to beat. Blood circulation commences, and the vitelline membrane spreads over the yolk surface. The head and trunk can be discerned, and the brain and cardiac structures are visible.
On Day 4, the amniotic cavity, filled with amniotic fluid, develops around the embryo, providing protection and facilitating movement. The embryo's entire body turns 90 degrees, and the mouth, tongue, and nasal pits develop as parts of the digestive and respiratory systems. The heart continues to enlarge, and by the end of Day 4, the embryo has all the organs necessary for survival after hatching.
On Day 5, the embryo undergoes a sensible increase in size and takes on a C shape as the head moves closer to the tail. This is also when sex differentiation occurs, with the respective reproductive organs for each gender beginning to form. The fingers of the inferior limbs differentiate, and fissures appear between the digits of the upper and lower limbs.
By Day 6, the embryo has the aspect of a chick. The vitelline membrane continues to expand, covering more than half of the yolk. The beak has begun to form, and the embryo can now move voluntarily. The brain progressively enters the cephalic region, shrinking proportionally to the embryo's size.
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Development of the embryo's nervous system
The development of a chicken embryo typically takes 21 days of favourable incubation conditions for the chicken to develop and hatch. However, this development takes 22 days in total—one day in the oviduct and 21 days in the incubator or nest.
An electron microscope study of developing nervous tissue has been conducted on chick embryos, with the results correlated to those obtained by other techniques. The focus of this study was the relationship between ectoderm and mesoderm cells at the time of neural induction and the formation of the Nissl substance.
The embryonic growth and development of the nervous system are influenced by both endogenous and exogenous factors. The neural induction process involves the differentiation of ectoderm cells into neural ectoderm cells, which then migrate and form the neural tube. This neural tube eventually differentiates into the brain and spinal cord.
The development of the nervous system is a critical stage in chick embryonic development, along with the functional maturation of numerous organ systems and the acquisition of thermoregulation. The ambient temperature during the post-laying period also influences embryonic development. When the temperature of the egg is below 20°C, most development stops, and the embryo becomes dormant. Embryonic activity resumes when the temperature rises above 20°C.
Furthermore, the hatching process is not just a biological mechanism but also a social one. Embryos communicate with each other through a series of clicking sounds, adjusting their hatching times accordingly. This synchronisation helps to reduce the time between the first and last chicks hatching.
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Formation of the embryo's beak
The development of a chicken embryo takes around 21 days of favourable incubation conditions. However, this can take 22 days in total, with one day in the oviduct and 21 days in an incubator or nest.
The formation of the embryo's beak occurs on the sixth day of incubation. On this day, the embryo's beak appears, and voluntary movements begin. The embryo takes on a C-shape, with its head moving closer to its tail, and the neck noticeably thinning to separate the head from the body. The brain progressively enters the cephalic region, becoming smaller in proportion to the embryo's size.
By the eighth day, the embryo's upper and lower beak are differentiated and equal in length. The embryo starts to look bird-like, and its mouth opening occurs. On the tenth day, the embryo's egg tooth becomes prominent, and by the fourteenth day, the embryo is moving into position for hatching.
On the twentieth day, the chick is in the hatching position, and the beak has pierced the air cell. The allantois, which has served as the chick's lungs, begins to dry up as the chick uses its own lungs. The chick continues to push its head outward, using the sharp horny structure on the upper beak (the egg tooth) and the muscle on the back of its neck to cut the shell.
The horny cap will fall off the beak within days after the chick has hatched.
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The embryo's hatching process
The embryology of a chicken is the development of a chicken inside an egg. The process of hatching starts with fertilisation, which takes place in the infundibulum about 15 minutes after the holding follicle releases the yolk. Cell division to create the embryo starts about five hours after fertilisation and continues while the egg passes along the oviduct and after the egg is laid.
On the first day of incubation, a group of cells becomes what is called the primitive streak, which is the longitudinal axis of the embryo. From this, the head and backbone of the embryo begin to develop. The nervous system also starts to form, along with the alimentary tract and blood islands.
On the second day, the blood islands start linking and forming the vascular system, while the heart is being formed elsewhere. By the 44th hour of incubation, the heart and vascular system join together, and the ear begins to form. The first groove also appears at the centre of the blastoderm, and the vitelline membrane, which will play a major role in embryo nutrition, makes an appearance.
On the third day, the embryo is lying on its left side. The heart is now beating, and blood circulation has begun. The vitelline membrane spreads over the yolk's surface, and the head and trunk can be discerned. The brain and cardiac structures are also visible. The allantois, which helps the embryo exchange gases and deal with liquid waste, begins to form.
On the fourth day, the embryo's 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. The heart continues to enlarge, and the other internal organs continue to develop. By the end of the fourth day, the embryo has all the organs needed to sustain life after hatching, and most of the embryo's parts can be identified.
On the fifth day, there is a sensible increase in the embryo's size. The head moves closer to the tail, and the limbs extend. The fingers of the inferior limbs differentiate, and sex differentiation occurs.
On the sixth day, the embryo can move voluntarily. The vitelline membrane continues to grow and now surrounds more than half the yolk. The beak has begun to form, and the brain progressively enters the cephalic region.
On the seventh day, the neck thins, clearly separating the head from the body. The brain continues to grow smaller proportionally to the embryo's size. The digits appear on the wings and feet, and the heart is completely enclosed in the thoracic cavity.
On the eighth day, the vitelline membrane covers almost the whole yolk. The embryo starts to look bird-like, and the mouth opens.
On the ninth day, the embryo occupies practically all the space within the egg except the air cell.
On the 11th day, the embryo now has the aspect of a chick.
On the 12th day, feather follicles surround the external auditory meatus and cover the upper eyelid.
On the 13th day, claws and leg scales appear.
On the 14th day, down covers almost the whole body and grows rapidly.
On the 15th and 16th days, few morphological changes occur, but the chick and down continue to grow. The head moves toward the pipping position, under the right wing.
On the 17th day, the embryo's renal system produces urates.
On the 19th day, the remaining yolk sac begins entering the body cavity.
On the 20th day, the chick begins pipping through the shell.
Finally, on the 21st day, the chicks hatch and dry.
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Frequently asked questions
A chicken embryo develops and hatches in about 21 days under favourable incubation conditions. This development takes 22 days – one day in the oviduct and 21 days in the incubator or nest.
During the first 24 hours, there is not much to see, but there is activity going on inside the egg. A group of cells becomes what is called the primitive streak, from which the head and backbone of the embryo develop. The nervous system starts to form, and blood islands appear, which will develop into the vascular or blood system.
By the third day, the embryo's heart begins to beat and blood circulation begins. The embryo is lying on its left side, and the head and trunk can be discerned. By the fifth day, there is a sensible increase in the embryo's size, and the head moves closer to the tail, forming a C-shape.
By the 15th day, the embryo grows rapidly and assumes a hatching position with its head under its right wing. The remaining yolk sac begins to enter the body cavity. By the 20th day, the chick begins pipping through the shell, and it hatches and dries out on the 21st day.
