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Development of an embryo
In the right conditions, the individual embryo develops into a chick in 21 days. There are three development stages. The first week is the differentiation phase, which already starts in the hen’s body after fertilisation. The specific tissues are formed during this phase. At this moment, the embryos are very sensitive to temperature fluctuations, jolts/vibrations and disinfectants. This phase requires a uniform embryo temperature to ensure all the embryos hatch at the same time later on. At the end of this first phase (day 10/day 11) all the organs and external characteristics have been formed. In the second phase, a period of growth follows. During the last 3-5 days of the incubation process, the body functions mature. These include the thermoregulation ability, the immune system, the digestive system and the metabolism. How the body functions when mature depends on the conditions of incubation. Under optimal incubation conditions, all these functions will be better developed.
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Day 1 Day 2 Day 3
The germinal disc grows in size and the light-coloured ring around the embryo (arrow) means that the extraembryonic membranes are developing. These membranes transport `water’ from the albumen to the yolk under the embryo (subembryonic space). The first blood islands are formed; the head and heart are recognisable. The embryo ‘floats’ on the sub-embryonic liquid. The extraembryonic membranes continue to develop. The blood ring (area vasculosa) is complete, head turned to the right, heart beats.
Day 4 Day 5
The embryo is partly lying on its left side.
Head starts to move.
The eye pigment becomes visible; formation of wings and legs is clearly recognisable.
Embryo takes on a C-shape, lying on its left side. The heart takes on final form with chambers (ventricles and atria). The volume of the sub-embryonic liquid is almost maximum.copyright protected
Day 6
Beak becomes visible. The embryo is protected in the closed amnion.
The tip of the egg contains a jelly-like mass: the residues of protein (albumen).
Almost all the water is now stored in the sub-embryonic cavity.
The yolk has disappeared.
Day 7
Egg tooth and comb visible; legs move, fingers and digits (toes) are recognisable but not yet separated. Calcium starts to be deposited in the bones. The amnion absorbs water, so the yolk sac becomes firmer. Three rows of tail feather buds are clearly recognisable. The amnion absorbs more fluid and increases in size. A chorioallantoic membrane covers 80% of the embryo and the yolk sac. The beak opening is visible. The maternal antibodies are mainly found in the yolk sac.
Day 8 Day 9
Day 10
Active, but aimless, movement of the embryo in the amnion that now has a maximum volume.
The toes are completely separated. Leg scales appear, first feathers visible. Chorioallantoic membrane surrounds the inside of eggshell. Volume of the amniotic fluid decreases. The legs are bent and albumen protein is absorbed in the yolk sac.
Day 11 Day 12
Day 13 Day 14 Day 15
Head in the yolk sac. Heat production and oxygen consumption increase exponentially. The embryo moves for longer periods. Movement stimulates bone development. Embryo starts to extend the head towards the air cell. Residues of the protein containing a small number of maternal antibodies are mixed with the contents of the amnion. Movement decreases.
Day 16 Day 17
The protein is completely absorbed by the embryo.
The down is fully developed. Embryo’s head faces the air cell, the volume of the amnion decreases.copyright protected Head moves under right wing, beak towards air cell, legs over the head. Intestines move into the abdominal cavity.
Day 18
Day 19
The yolk sac is absorbed into the body cavity. The yolk sac is fully retracted via the navel, the chick pierces the inner membrane and starts peeping. Chick breaks through the eggshell, respiratory gas exchange through the shell has stopped and the navel can close. Chick pushes itself free from the eggshell.
