The embryology of the chicken is the development of the chicken inside of the egg. Hatchery managers need to be able to differentiate between normal and abnormal embryos and identify possible causes of embryo mortality during incubation.
Three stages of the early development of the chicken embryo
Fertilisation and early development
Fertilisation of the germinal disc by the sperm takes place in the infundibulum about 15 minutes after its holding follicle has released 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 is eventually laid. It is generally said that the hen’s egg 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.
When the sperm cell (with half the required chromosomes) fertilises the female egg cell (with half the required chromosomes) it forms the zygote - a one-cell individual with the proper total chromosome number. About five hours after fertilisation the zygote enters the isthmus and it is here that the first cell divisions occur. The new embryo starts to develop by simple cell division. By the time the egg leaves the isthmus the zygote, now called the blastoderm or embryo, is eight cells and after four hours in the uterus it has grown to 256 cells.
Early development of the chick embryo (2 days)
Formation of ectoderm, endoderm and mesoderm
Initially the dividing cells form one layer over the yolk, but as cell division continues two layers are formed. These are called the ectoderm (uppermost) and the endoderm (underneath) layers. At about this stage the central cells of the blastoderm separate from their contact with the yolk to 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.
From this stage on, the organs and tissues will develop from these three layers of cells.
i. The ectoderm produces the nervous system, parts of the eyes, the feathers, beak, claws and skin
ii. The endoderm produces the respiratory system, the digestive system and secretory organs
iii. The mesoderm produces the skeleton, muscles, circulatory system, reproductive organs and excretory system
Another important development at this stage is the way the cells change to allow the production of the different types of cells that make up the tissues. By the time the egg is laid the embryo consists of many cells differentiating into the various systems, organs and tissues.
The fowl retains some vestiges of the habits of its reptilian ancestors; one such habit is the influence of ambient temperature during the post laying period on embryonic development. When the temperature of the egg is below 20°C the embryo becomes dormant and most development stops. When the temperature rises above about 20°C embryonic activity starts again. This temperature of about 20°C when embryonic activity starts or stops is often referred to a physiological zero.
Fluctuating temperatures above/below 20°C will create a start/stop response in embryonic development, each succeeding response progressively weakening the embryo. The temperature must be increased to the required 37-38°C for optimum development to occur. Failure to satisfy this need leads to significantly weaker embryos. To retain maximum viability of the embryo, hatching eggs should be processed and placed in cool storage below 20°C as soon as possible after collection and held at that temperature until the pre-warming process just prior to setting the eggs in the incubator. Once in the incubator the temperature must be controlled within very close parameters.
Because the avian embryo has no anatomical connection to the hen, all of its nutritive requirements except oxygen must be contained in the egg. The embryo very early in its development develops special membranes external to its body to access the nutrients in the egg and to carry out essential body functions.
There are four of these special membranes and their names and functions are as follows:
1. Yolk sac: This sac envelops the yolk and produces 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/drink and where to find it.
2. Amnion: The amnion forms a sac that is filled with fluid in which the embryo floats. In this way it provides a shock-absorbing environment in which the fragile embryo can develop without harm from normal day to day knocks.
3. Allantois: 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. This membrane has a number of functions:
o Respiratory - the developing embryo uses oxygen and produces carbon dioxide i.e. it has respiration. It is unable to carry out this function itself and hence the allantois oxygenates the blood and eliminates the carbon dioxide.
o Excretory - it removes the wastes that result from the embryo’s metabolism and deposits it in the allantoic cavity.
o Digestive - it provides the means for the embryo to access the albumen and the calcium of the shell.
4. Chorion: The chorion fuses the inner shell membrane to the allantois and helps that membrane to carry out its functions.
Extra embryonic membranes of the chicken
To better carry out an investigation into poor hatchability it is necessary to have knowledge of the way the embryo develops from day to day. This will allow the investigator to compare notes and to identify at what age/stage they may have died. This is important information when attempting to identify the cause of any poor results.
· DAY 1: There are several important body functions and organs that become evident during the first day of incubation:
o Hour 4: heart and blood vessels start to develop.
o Hour 12: heart starts to beat; blood circulation begins; blood vessels of the embryo and the yolk sac join.
o Hour 16: embryo takes on the chicken shape.
o Hour 18: the alimentary tract appears.
o Hour 20: the vertebral column appears.
o Hour 21: the nervous system starts to develop.
o Hour 22: the head begins to take shape.
o Hour 24: the eyes start to develop.
· DAY 2: The ears start to develop.
· DAY 3: Early on day 3 the beak, legs and wings begin to develop. At 3.5 days the embryo begins to rotate so that it lies on it’s left side. The circulatory system development increases at a rapid rate.
· DAY 4: The tongue starts to develop and all body organs are evident. The vascular system is clearly visible.
· DAY 5: The reproductive organs separate and the sex can be determined. The heart begins to take on a definite shape. The embryo now is obviously a chicken.
· DAY 6: The embryo starts to move in the egg.
· DAY 7: The body starts to develop at a rate much greater than the head.
· DAY 8: The first sign of feathers appear.
· DAY 10: The beak hardens and the toes and scales appear.
· DAY 11: The abdomen appears and the intestines are seen developing.
· DAY 13: Most organs have developed and growth is the major activity. Down is present; skeleton starts to calcify.
· DAY 14: The embryo moves to lie lengthwise in the egg with the head towards the larger end.
· DAY 17: The head turns to locate under the right wing and with the beak directed towards the air cell.
· DAY 19: The yolk sac containing the remaining yolk material starts to enter the chicken’s abdominal cavity. The chicken adopts a position to facilitate pipping of the shell by the beak.
· DAY 20: The yolk sac is fully drawn into the abdominal cavity and the navel or umbilicus starts to close. The chicken’s beak penetrates the air cell and it takes it’s first breath i.e. pulmonary respiration commences.
· DAY 21: After first pipping the shell the chicken rests for several hours. It then starts to break out of the shell by cutting around the shell from the first pip in a clockwise direction. It may take up to 20 hours from the first pip till the chicken escapes. This is a major undertaking for the cicken and any that are weak are unlikely to succeed or if they do they will be weak.
The normal hatching position is:
i. Forepart of the body towards the large end of the egg
ii. Head under the right wing
iii. Legs up under the head
With natural incubation, while the eggs are laid in the nest over a period of several days, and the hen is setting on the different eggs for different lengths of time depending on when the eggs are laid, the chickens hatch over a relatively short period of time. This indicates that there is some system that synchronises the hatching process. It is now known that the different embryos communicate with each other by a series of clicking sounds, the rate of clicking being the important feature. Ensuring the eggs on the hatching trays are in contact with each other facilitates the synchronisation of hatching where the eggs are incubated in a modern machine. This will assist in reducing the time between when the first and last chicken hatches.
