Claire Thompson
The development of a chicken embryo is a complex process that begins with fertilisation. After cell division, the zygote enters the isthmus and develops into an embryo with two layers: the ectoderm and the endoderm. A third layer, the mesoderm, soon forms, and the organs and tissues of the bird develop from these three layers. By the seventh day, digits appear on the wings and feet, and by day 17, the embryo's renal system produces urates. On day 18, the chick starts to absorb the vitellus and there is a reduction in the amount of amniotic fluid. On day 19, the vitellus is absorbed more rapidly, and the beak is against the air cell, ready to pierce it. On day 20, the chick pierces the internal membrane and breathes air for the first time. Finally, on day 21, the chick breaks free of its shell and hatching is complete.
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What You'll Learn
Embryo development begins 5 hours after fertilisation
Embryo development in chickens begins 5 hours after fertilisation. At this point, the zygote enters the isthmus, and the embryo begins to develop through simple cell division. By the time the egg leaves the isthmus, the zygote, now called the blastoderm or embryo, comprises eight cells, and after four hours in the uterus, it has grown to 256 cells. The dividing cells initially form one layer over the yolk, but as cell division continues, two layers are formed: the ectoderm (uppermost) and the endoderm (underneath) layers. At this stage, a cavity is formed as the central cells of the blastoderm separate from the yolk. This cavity is where subsequent embryo development occurs.
Following this, a third layer of cells, the mesoderm or middle layer, is formed. Organs and tissues will develop from these three layers of cells. By the time the egg is laid, the embryo consists of many cells, differentiating into various tissues, organs, and body systems. The embryonic development of a chicken is influenced by ambient temperature. When the temperature of the egg is below 20°C, the embryo becomes dormant, and most development stops. Embryonic activity resumes when the temperature rises above 20°C. Fluctuating temperatures above and below 20°C will create a start/stop response in embryonic development, weakening the embryo with each successive response.
On day 17, the embryo's renal system produces urates, and the beak, which is under the right wing, points to the air cell. On day 18, there is an onset of vitellus internalisation, and there is a reduction in the amount of amniotic fluid. This is the time for transfer from the incubator to the hatcher. On day 19, the vitellus is accelerated in its resorption, and the beak is against the inner shell membrane, ready to pierce it. On day 20, the vitellus is fully resorbed, and the umbilicus is closed. The chick pierces the inner shell membrane and breathes in the air cell, with gas exchanges occurring through the shell, which is porous. The chick is ready to hatch, and piercing of the shell begins.
On day 21, the chick uses its wing as a guide and its legs to turn around and pierce the shell in a circular way with its egg-tooth. It frees itself from the shell in 12 to 18 hours and lets its down dry off. The incubation and hatching are then complete.
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The embryo occupies almost all of the egg by day 20
The development of a chicken embryo is a complex process that occurs over approximately three weeks. During this time, the embryo grows and develops rapidly, transforming from a fertilised egg into a chick. By day 20, the embryo occupies almost all of the egg, marking a significant milestone in its growth and indicating that it is almost ready to hatch.
The development of a chicken embryo begins with fertilisation. When a rooster mates with a hen, the sperm fertilises the female egg cell, forming a zygote. This single cell then divides and grows, eventually forming an embryo. The embryo develops and grows while the egg passes along the oviduct and after it is laid, with favourable conditions required for the embryo to survive and thrive.
In the initial stages of development, the embryo's cells form layers, including the ectoderm, endoderm, and mesoderm. These layers are crucial for the formation of organs and tissues. As the embryo grows, its size increases noticeably, and it takes on a C-shape, with the head moving closer to the tail. The limbs also begin to extend, and the digits of the wings and feet become visible.
By day 10, the embryo's development becomes even more pronounced. Feathers and feather tracts are visible, and the beak hardens. The embryo is actively preparing for hatching, and by day 14, its claws are forming, and it moves into position for hatching. The embryo's growth is rapid, and by day 20, it has fully absorbed the yolk and occupies almost the entire egg.
On day 20, the embryo is in the hatching position, with its beak piercing the air cell and pulmonary respiration initiated. This is a critical stage, as the embryo breathes air and completes the absorption of the vitellus (yolk). External pipping may also occur at this stage, where the embryo pierces the inner shell membrane, signalling that it is ready to hatch.
By day 21, the chick begins its escape from the shell, using its wing as a guide and its legs to turn and pierce the shell. This process typically takes 12 to 18 hours, and the chick then lets its down dry off before fully emerging from the shell. Thus, by day 20, the embryo has undergone remarkable growth and development, occupying almost all of the egg and preparing for its imminent hatching.
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The egg provides energy and protection for the embryo
The egg provides the ideal environment for the embryo to develop and hatch. The fertilisation of the germinal disc by the sperm takes place in the infundibulum about 15 minutes after its holding follicle releases the yolk. The zygote, or fertilised egg, then enters the isthmus, where the embryo starts to develop through cell division. The egg provides the necessary machinery for the embryo to make new proteins to carry out more complex functions as it develops. The egg's mRNA translates information from the DNA to tell structures called ribosomes which proteins to build so the cell can grow and divide into a larger organism.
The egg also provides the energy required for the embryo to develop. The mitochondria in the egg produce energy to help the cell grow and function. The egg's mitochondria supply the energy to develop the embryo and provide the blueprint for every other mitochondrion in the body. The embryo gets its energy by burning the yolk fat. The yolk, along with the albumen, is also used by the embryo to build up its body. The less residual yolk is left at the time of hatching, the better, as it indicates that the embryo has efficiently used its energy to gain a high yolk-free body weight.
The egg also provides physical protection for the embryo. The zona pellucida surrounds the egg to protect its contents. It also plays an important role in fertilisation by ensuring that only one sperm can fertilise the egg. The egg's cytoplasm makes up the bulk of its interior and enables successful fertilisation and embryo development. The proteins within the egg's cytoplasm are involved in many biochemical reactions that prepare the egg for fertilisation and play a key role in maturing the embryo.
The egg also provides protection for the embryo in the form of nutrients and energy stores that the developing embryo will use during its early stages of development. The egg contains stockpiles of all the materials needed for the embryo's initial development until it can begin feeding. The egg contributes DNA as well as numerous other important components of the cell that will ultimately form the embryo.
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Embryonic development occurs in the cavity between the ectoderm and endoderm
Embryonic development is a complex process that transforms an egg into a chick. This development occurs in the cavity between the ectoderm and endoderm, two of the three primary germ layers that form during the first two weeks of embryonic growth. These layers are groups of cells that organise and develop into specific structures in the embryo.
The process begins with fertilisation of the germinal disc by the sperm, which takes place in the infundibulum about 15 minutes after its holding follicle releases the yolk. Cell division to create the new embryo starts approximately five hours after fertilisation and continues as the egg passes along the oviduct and after it is laid. The zygote, comprising eight cells, enters the isthmus, where the new embryo begins to develop by simple cell division. After four hours in the uterus, it has grown to 256 cells.
Initially, the dividing cells form one layer over the yolk, but as cell division continues, two layers are formed: the ectoderm (uppermost) and the endoderm (underneath). At this stage, the central cells of the blastoderm separate from the yolk to form a cavity, and subsequent embryo development occurs in this cavity. The ectoderm and endoderm layers form tightly connected epithelial sheets, and from them, all organs and tissues develop. The ectoderm gives rise to the central and peripheral nervous systems, sensory organs, epidermis, hair, and nails. The endoderm forms the epithelial lining of the gastrointestinal tract, liver, pancreas, and lungs.
Following the formation of the ectoderm and endoderm, a third layer of cells, the mesoderm or middle layer, is formed. The organs and tissues of the bird develop from these three layers of cells. By the seventh day, digits appear on the wings and feet, the heart is enclosed in the thoracic cavity, and the embryo resembles a bird. After 21 days of incubation, the chick escapes from the shell, using its egg tooth to cut through.
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The embryo's development is influenced by the ambient temperature
The embryology of a chicken refers to the development of a chicken inside an egg. The process begins with the fertilisation of the germinal disc by the sperm, which takes place in the infundibulum about 15 minutes after its holding follicle releases the yolk. Cell division to create the new embryo starts about five hours after fertilisation and continues while the egg passes through the oviduct and after the egg is laid. Generally, it takes 21 days of favourable incubation conditions for the chicken to develop and hatch. However, the development of the embryo is influenced by the ambient temperature.
Chicken embryos are poikilothermic, meaning they rely on an external source (a hen or incubator) to provide the heat necessary for their development and the maintenance of normal metabolic functions. Therefore, temperature influences the metabolic rate of yolk and albumen mobilisation and consumption and, subsequently, embryonic development throughout incubation. The ideal temperature inside the incubator is between 37 and 38°C, typically around 37.5-37.8°C. However, the temperature inside the egg, called the "embryo temperature", is not equal to the incubator air temperature. Embryo temperature depends on the balance between embryonic heat production and heat transfer between the eggshell and its environment.
The ambient temperature during the post-laying period significantly impacts embryonic development. When the temperature of the egg falls below 20°C, the embryo becomes dormant, and most development stops. As the temperature rises above 20°C, embryonic activity resumes. This temperature threshold of 20°C is often referred to as a physiological zero. Fluctuating temperatures above and below this threshold create a start/stop response in embryonic development, and each successive response progressively weakens the embryo.
Studies have shown that cyclic increases in incubation temperature during the critical period of incubation can induce adaptive responses and increase the thermotolerance of chickens without affecting hatchability. Additionally, light during incubation can modify embryo development, physiology, and post-hatch behaviour. Light stimulation during incubation has been found to lower stress responses and improve spatial abilities and cognitive functions in chickens. Combining light and cyclic higher eggshell temperatures during incubation increases pineal aralkylamine N-acetyltransferase, an enzyme that plays a role in melatonin hormone production.
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Frequently asked questions
It takes 21 days for a chicken embryo to develop an outer shell.
During the 21 days, the embryo grows and develops rapidly. By the seventh day, digits appear on the wings and feet, and the heart is enclosed in the thoracic cavity. After the tenth day, feathers and feather tracts are visible, and the beak hardens. On day 17, the embryo's renal system produces urates, and the egg white is fully absorbed.
The process is called incubation.
