15. Stages In Chick Embryo
One of the
greatest miracles of nature is the transformation of the egg
into the chick. A chick emerges after a brief three weeks of
incubation. The complexity of the development cannot be
understood without training in embryology.
the egg is laid, some embryonic development has occurred and
usually stops until proper cell environmental conditions are
established for incubation to resume. At first, all the cells
are alike, but as the embryo develops, cell differences are
observed. Some cells may become vital organs; others become a
wing or leg.
after incubation begins, a pointed thickened layer of cells
becomes visible in the caudal or tail end of the embryo. This
pointed area is the primitive streak, and is the longitudinal
axis of the embryo. From the primitive streak, the head and
backbone of the embryo develop. A precursor of the digestive
tract forms; blood islands appear and will develop later into
the vascular or blood system; and the eye begins.
second day of incubation, the blood islands begin linking and
form a vascular system, while the heart is being formed
elsewhere. By the 44th hour of incubation, the heart
and vascular systems join, and the heart begins beating. Two
distinct circulatory systems are established, an embryonic
system for the embryo and a vitelline system extending into the
end of the third day of incubation, the beak begins developing
and limb buds for the wings and legs are seen. Torsion and
flexion continue through the fourth day. The chick's entire body
turns 90o 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 develop as parts
of the digestive and respiratory systems. The heart continues to
enlarge even though it has not been enclosed within the body. It
is seen beating if the egg is opened carefully. The other
internal organs continue to develop. By the end of the fourth
day of incubation, the embryo has all organs needed to sustain
life after hatching, and most of the embryo's parts can be
identified. The chick embryo cannot, however, be distinguished
from that of mammals.
embryo grows and develops rapidly. By 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 of incubation, feathers and feather tracts
are visible, and the beak hardens. On the fourteenth day, the
claws are forming and the embryo is moving into position for
hatching. After twenty days, the chick is in the hatching
position, the beak has pierced the air cell, and pulmonary
respiration has begun.
21 days of incubation, the chick finally begins its escape from
the shell. 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 uses its own lungs. The chick continues to
push its head outward. The sharp horny structure on the upper
beak (egg tooth) 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 the bottom portion of the shell. The chick is exhausted
and rests while the navel openings heal and itís down dries.
Gradually, it regains strength and walks. The incubation and
hatching is complete. The horny cap will fall off the beak
within days after the chick hatches.