AUSTRALIAN POULTRY FORUM™

One of Natures mysteries is the transformation of the egg into the chick. A chick emerges after only three weeks of incubation. The complexity of the development is not always understood. Hopefully this is an easy explanation.

When 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 differentiation is observed. Some cells may become vital organs such as the heart, liver or brain; others become a wing or leg.

Soon 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’ (some refer to it as the ‘primordial 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 eyes begin.

On day 2 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 starts beating (it appears to flutter at this early stage). Two distinct circulatory systems are established; an embryonic system for the embryo, and a vitelline system extending into the egg proper.

Either during or near the end of day 3 of incubation, the beak begins to develop. And limb buds for the wings and legs are seen.

Through day 4 torsion and flexion continue. The chick's entire body turns 90deg 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 the first parts of the digestive and respiratory systems. The heart continues to enlarge but it has not been enclosed within the body yet. It can be seen beating if the egg is opened carefully. ( NOT recommended if you want the chick to hatch as opening the shell will kill the chick) The other internal organs continue to develop.

By the end of day 4 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 is indistinguishable from that of a mammal embryo at this stage.

The embryo grows and develops rapidly.

At day 7, digits appear on the wings and feet, the heart is completely enclosed in the thoracic cavity, and the embryo looks more like a bird.

From day 10 of incubation, feathers and feather tracts are visible, and the beak hardens.

On day 14, the claws are forming and the embryo is moving into position for hatching.

Around day 20, the chick is in the hatching position, the beak has or is in the process of piercing the air cell, and pulmonary respiration has begun.

After 21 days of incubation, the chick starts its journey of escaping 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 egg tooth, the sharp horny structure on the upper beak, and the muscle on the back of the neck help cut the shell. The chick rests a number of times, changes position, and keeps cutting until its head falls free of the opened shell. Occasionally the chick will rest again at this time. Then it will use it legs to push free of the bottom portion of the shell. The hatching process exhausts the chick and it rests while the navel openings heal and it’s down dries. Gradually, it regains strength and starts to walk.

The incubation and hatching is complete.

The egg tooth will fall off the beak within a few days.

EVENTS IN EMBRYONIC DEVELOPMENT

Before Egg Laying:
Fertilization
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:

1st 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 - start of nervous system
22 hours - beginning of head
24 hours – initial formation of eye

2nd day
25 hours - development of heart
35 hours - beginning of ear
42 hours - heart starts to beat

3rd day
60 hours – first hint of nose
62 hours - formation of legs
64 hours - beginning of wings

4th day - development of tongue

5th day - formation of reproductive organs and differentiation of sex

6th day - development of beak

8th day – first sign of feathers

10th day - beginning of hardening of beak

13th day - appearance of leg scales and claws

14th day - embryo moves into a position suitable for breaking shell


16th day - scales, claws and beak become firm and horny

17th day - beak turns toward air cell

19th day - yolk sac begins to enter body cavity

20th day - yolk sac is completely drawn into body cavity. The embryo occupies about 7/8th of the space within the egg except the air cell

21st day - hatch of chick