Stage wise development of embryo of chick / Embryonic development day by day

 The egg of hen is polylecithal type as it contain large amount of yolk. The living part of the egg is restricted to the upper pole of yolk which is known as Blastodisc or Germinal disc.

FERTILIZATION

The fusion of male sperm and female ovum to form zygote. 

CLEAVAGE

The repeated mitotic division of zygote which occurs in the blastodisc is called cleavage .

First two cleavage

The first two cleavage occurs at right angle to each other in a vertical plane which results in the formation of 4 cell.

Third cleavage

It occurs  in a horizontal plane which results in the formation of eight cell.

Remaining  cleavage

The remaining cleavage are irregular as a result of this many cell are formed which are known as blastoderm. This outcome of cleavage called cytoplasmic  localization.

Morula

In this stage many blastoderms are together look like mulberry. It is a 16 celled stage.

 Blastula

Morula divides to form blastula. In this stage the cavity is formed which is known as Blastocoel or segmentation cavity. The upper layer is known as epiblast and lower layer is known as hypoblast.

 Gastrula

 In this stage, the cell of epiblast move downward and they invaginate and form primitive streak on the upper surface. The anterior end of the primitive streak is called primitive knot or hensen’s node. In the primitive streak cell of the epiblastic migrate downward and then lately these cell form mesodem. Migrating cells are attached with the hypoblast and form endoderm and the remaining cell of the epiblast form ectoderm.

Neurulation

In this stage, formation of notochord and neural tube takes place. 

After incubation

 Soon after the inoculation begins a primitive streak; pointed thickened layer of cells becomes visible in the caudal or tail end of embryo. It is the longitudinal axis of the embryo. From the primitive steak the head and backbone of the embryo develop.  Simply on the first 16 hours there is the first sign of resemblance to a chick embryo

There is the appearance of the alimentary canal in 18 hours, vertebral column in 20 hours and beginning of nervous system and head in 21 hours  . In 24 hours  there is the beginning of eye.

On the second day

The blood islands begin linking and form a vascular system, while the heart  and vascular systems join and the heart begins beating . Twodistinct circulatory systems are established, an embryonic system for the embryo and a vitelline system extending into the egg.

On the third day

 The beak begins to develop, the limb buds for the wings and legs are seen . The chick’s entire body turns 90 degree and lies down with its left side on the yolk. The head and tail come close together so the embryo forms a ‘C ’ shape.

The mouth, tongue and nasal pits develops parts of the digestive and respiratory system. The heart continues to enlarge. The other internal organs continue to develop.

On the fourth day

The embryo has all organs needed to sustain life after hatching and most of embryo’s parts can be identified. The chick embryo cannot be distinguished from that of mammals.

The embryo grows and develops rapidly.

On the seventh day

Digits appear on the wings and feet, the heart is completely enclosed in the thoracic cavity and the embryo looks more like a bird.

After the tenth day

Feathers and tracts are visible and the break hardens.

On the fourteenth day to seventh day : Claws are forming an embryo is moving into position for hatching.

On fifteen  to nineteenth day :It’s hard to observe the embryo by candling.

After twenty  day:The chick is in the hatching position beak has pierced the air cell and pulmonary respiration has begun.

After twenty one days of incubation

 The allantois which has  served as its lungs, begins to dry up as the chick uses its own lungs.

The chick continues to push its head outward. The sharp horny structure on the upper beak and the muscle on the back of the neck help cut the shell. The chick rests, changes position and keeps cutting until its head falls free of the opened shell. It then kicks free of bottom portion of the shell. The chick is exhausted and rests while the navel openings heal and its down dries. Gradually it regains strength and walk. The incubation and hatching is complete. The horny cap will fall off the beak within days after the chick hatches

 

And in this way the stage of development of  embryo of chicks get completed.