Fertilisation of the ova (yolk) by the sperm takes place in the infundibulum about 15 minutes after its holding sac ruptures and releases it. The combination of the ova and the sperm form a single cell called the zygote. The zygote reaches the isthmus about 5 hours after fertilisation and it is here that the embryo starts to develop, through cell division. By the time it leaves the isthmus it has grown to an eight cell embryo and by the time it leaves the uterus it has grown to 256 cells. The developing embryo will stay at this stage until incubation conditions are right for further development.
Cell division continues as the egg passes along the oviduct and is laid. An egg takes about 25 hours from the release of the ova to the point of lay. This means that in actual fact a chicken egg takes 22 days to hatch, not the 21 days which is usually quoted: 1 day in the oviduct and 21 days in the nest or incubator.
Embryo Development During Incubation
Formation of the ectoderm, endoderm and mesoderm
Once in the incubator or under a nice warm hen, development through cell division starts again. Two layers are formed over the yolk these are the ectoderm (uppermost) and the endoderm (underneath) layers. At about this stage the central cells of the blastoderm ( the layer of cells that develops on the surface of the yolk and gives rise to the germinal disk from which the embryo develops) separate from their contact with the yolk and form a cavity. It is in this cavity that subsequent embryo development occurs. Soon after the formation of the ectoderm and endoderm, a third layer of cells called the mesoderm or middle layer is formed.
The important organs and body tissues develop from these three layers. With the ectoderm producing the nervous system, parts of the eyes, the feathers, beak, claws and skin. The endoderm produces the respiratory system, the digestive system and secretory organs. While the mesoderm produces the skeleton, muscles, circulatory system, reproductive organs and excretory system.
Extra Embryonic Membranes
Not only do the early embryonic germ layers (ectoderm, mesoderm, and endoderm) develop into specialised tissues of the body, but they also form membranes outside the body which help protect and nourish the developing chick embryo. Three extra-embryonic membranes are formed from the these first germ layers and are as follows:
The Yolk Sac
The yolk sac envelops the yolk and draws nourishment from it, producing an enzyme that changes the yolk material to a form that can be used as a food source by the developing embryo. Any remaining, unused yolk material in the yolk sac when the chicken hatches from the egg is drawn into the abdomen for use by the chicken for the first two to three days after hatching, while the chicken learns what to eat and drink and where to find it.
Ectoderm and mesoderm surrounding the embryo, form a protective covering over the embryo called the amnion. The inner layer of cells secretes amniotic fluid in which the embryo floats. This fluid keeps the embryo from drying out and helps absorb shocks when the egg is knocked.
The Chorio-Allantoic Membrane
The chorion which lines the eggshell and performs gas exchange and waste elimination is formed from the ectoderm and mesoderm layer and the allantois is the embryo’s connection to the chorion. The allantois appears as a balloon-like structure coming from the hindgut at about 4 or 5 days. The allantois develops an extensive circulatory system connected to that of the embryo and driven by the new embryonic heart. When the allantois is fully developed it completely surrounds the embryo.
The allantois membrane has a number of functions, including a respiratory role. The growing embryo uses oxygen and produces carbon dioxide, but is unable to carry out respiration itself yet and therefore it is the role of the allantois to oxygenate the blood and eliminate the carbon dioxide.
The allantois serves an excretory role, removing waste that the embryo metabolises. It also has a digestive role, providing a means for the embryo to access the albumen and the calcium of the shell.
Day by Day Summary of Embryo Development
Day 1: Appearance of some tissue development.
Day 2: Tissue development becomes very visible and blood vessels appear.
Day 3: Heart is beating and blood vessels become very visible.
Day 4: Eye becomes pigmented.
Day 5: Knees and elbows appear..
Day 6: Beak can be seen and voluntary movements start.
Day 7: The comb begins to develop as does the egg tooth.
Day 8: Feather start to be visible and the top and bottom beaks line up.
Day 9: Embryo now looks like a bird and the mouth opening appears.
Day 10: The embryo has toe nails and the egg tooth is clearly visible.
Day 11: Comb is fully developed and the tail feathers are now visible.
Day 12: Toes fully formed. .
Day 13: Appearance of scales as the body is now covered with light feathers.
Day 14: Embryo turns its head towards the large end of the egg.
Day 15: Gut is drawn into abdominal cavity.
Day 16: Feathers now cover the complete body and the albumen is nearly gone.
Day 17: Amniotic fluid decreases and the head is now between the legs.
Day 18: Growth of the embryo is nearly complete. Yolk sac is still on the outside of embryo and the head is under the right-wing.
Day 19: Yolk sac now draws into the body cavity and the amniotic fluid is all gone. The embryo now occupies most of space within the egg, excluding the air cell.
Day 20: The yolk sac is now drawn completely into the body. Embryo becomes a chick and breathes in air. Internal and external pip or cracks appear.
Finally after an incubation period of 21 days, the chick is finally ready to make its escape into the world. 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 starts to use its own lungs. The chick continues to push through the shell, using its egg tooth (this will drop off a few days after hatching), which is situated on the upper beak and remarkably strong neck muscles. This can take some time and the chick will stop and rest many times before its head falls free from the shell. It will then kick off the bottom part of the shell and then rest exhausted while its navel opening heals and its downy feathers dry off.