embryology of the chick

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					                                        Embryology of the chick

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.

The zygote

When the sperm cell fertilises the female egg cell ,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
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.

Extra-embryonic membranes

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
   4. Chorion: The chorion fuses the inner shell membrane to the allantois and helps that membrane to
      carry out its functions.

Daily embryonic development

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
      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
      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
      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.

24 Hour Chick Embryo

The notochord is evident ventral to the neural folds. Ventral to the notochord, the foregut appears as a smile-
shaped cavity delimited by thin endodermally derived walls. Note that the mid portion of the floor of the
foregut is slightly thickened. Below this region is a region of slightly thickened ectoderm. This region is the
oral plate which will become perforated at a later date to form the mouth. Below the head fold is the
subcephalic space and extraembryonic germ layers. Note the proamnion (ectoderm and endoderm) below the
head fold. Lateral to the proamnion, layers of mesoderm are visible between the ectoderm and endoderm.
The cavity that is evident between the layers of mesoderm is the coelom.


1. Two subdivisions of Area Opeca i.e. Area Vitellina and Area Vasculosa can be distinguished
 2. Hensen's Node is distinct.
3. There is no somite is present.
4. The clear head fold is anteriorly present.
5. Primitive streak is widened from posterior part.
6. Area Vasculosa is with traces of yolk.
7. At the cephalic region neural fold is present.
                     24 Hour Chick embryo


1. Three major divisions of brain i.e. Procencephalon, Mesencephalon and Rhombencephalon are
2. Notochord is distinct.
3. There are 10 pairs of somites and elongated foregut is present.


      First somite becomes dispersed (and not included in count).
      Cranial flexure present
      3 primary brain vesicles visible
      optic vesicles are not constricted at base heart
 33 Hour Chick embryo

72 Hour Chick embryo