Life In 21 Days

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Post  FinchG on Sun Feb 24, 2013 5:56 pm

Life in Twenty-One Days

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.

Cell division begins soon after fertilization, even while the rest of the egg is being formed. Cell division will continue if the egg is kept warmer than 67 degrees F. The first cell division is completed about the time the egg enters the isthumus. Additional cell divisins take place about every 20 minutes; so, by the time of lay, several thousand cells form two layers of cells called a "gastrula".

At this time the egg is laid, it cools, and embryonic development usually stops until proper environmental conditions are established for incubation. After incubation begins, the cellular growth resumes. 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 a leg.

Soon after incubation is begun, 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. Before the first day if incubation is through, many new organs are forming. The head of the embryo becomes distinguishable; a precursor of the digestive tract, the foregut, is formed; blood islands apear and will develop into the neural groove; and the eye begins.

On the 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, and embryonic system for the embryo and a vitelline system extending into the egg.

In later stages of embryonic development, there are two disinct extra-embryonic blood systems. One system, the vitelline system, transports nutrients from the yolk to the growing embryo. Before the fourth day, it oxygenates blood. The other blood system, made of allantoic vessels, is concerned with respiration and the storage of waste products in the allantois. When the chick hatches, both circulatory systems cease to function.

On the second day, the neural groove forms and the head portion develops into the parts of the brain. The embryo is developed enough that flexion and arching of the embryo begins, the ears begin development, and the lens in the eyes are forming.
At the end of the third day of incubation, the beak begins developing and limb buds for the wings and legs are seen. Three visceral clefts (gills) have formed on each side of the head and neck. These formations are important in the development of the arterial system, eustachian tube (in the ear), face, jaw, and some ductless glands. The fluid-filled amnion has surrounded the embryo to protect it: it helps maintain proper embryonic development. The tail appears, and the allantois is seen. The allantoic vesicle is a respiratory and excretory organ. Nourishment from the albumen and calcium from the shell are transported to the embryo throught the allantois.

Torsion and flexion continue through the fourth day. The chick's entire body turns 90 degrees 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 care fully. The other internal organs continue to develop. By the end of the fourth day fo 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.

Many complex physiological processes take place during transformation from the egg to the chick. They include: the use of highly nutritious food materials in the egg; the respiration of gases, or the taking in of oxygen and the removal of carbon dioxide; and the building of living energy within the chick.
The 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. The supply of albumen is exhausted by the sixteenth day, so the yolk is the sole source of nutrients. After twenty days, the chick is in the hatching position, the beak has pierced the air cell, and pulmonary respiration has begun. The yolk sac is contained completely withing the body cavity in preparation for hatching.

The normal position of the chick for hatching is with the head in the large end of the egg, under the right wing, with the legs drawn up toward the head. If the head is positioned in the small end of the egg, the chick's chances of survival are reduced by at least one-half. This is a serious malposition, or wrong position, for hatching. Just as a wrong position makes birth more difficult in mammals, a wrong position of the chick makes hatching more difficult, or impossible.

After 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 its 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.

The yolk is largely unused by the embryo and is deposited within the chick's body on the 9th day, just before it hatches. The yolk is highly nourishing and provides proteins, fats, vitamins, minerals, and water for several hours after hatching. This yolk is consumed gradually during the first ten days of the chick's life.
Tom W. Smith, Ph. D.
Professor of Poutry Sciences

Befor Egg Laying:

Division and growth of living cells
Segregation of cells into groups of special function (tissues)

Between Laying and Incubation
No growth; stage of inactive embryonic life

During Incubation:

First Day
16 hours - first sign of resemblance to a chick embryo
18 hours - appearance of alimentary tract
20 hours - appearance of vertebral column
21 hours - beginning of nervous system
22 hours - beginning of head
24 hours - beginning of eye

Second Day
25 hours - beginning of heart
35 hours - beginning of ear
42 hours - heart beats

Third Day
60 hours - beginning of nose
62 hours - beginning of legs
64 hours - beginning of wings

Fourth Day - beginning of tongue

Fifth Day - formation of reproductive organs and differentiation of sex

Sixth Day - beginning of beak

Eighth Day - beginning of feathers

Tenth Day - beginning of hardening of beak

Thirteenth Day - appearance of scales and claws

Fourteenth Dau - embryo gets into position suitable for breaking shell

Sixteenth Day - scales, claws and beak becoming firm and horny

Seventeenth Day - beak turns toward air cell

Nineteenth Day - yolk sac begins to enter body cavity

Twentieth Day - yolk sac completely drawn into body cavity; embryo occupies practically all the space within the egg except the air cell

Twenty-First Day - hatching of chick

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