(1) The development of the embryo during egg formation

The egg begins to develop shortly after fertilization in the fimbria of the fallopian tube. It takes about 24 hours until the egg is produced outside the body. Divides to form a multicellular blastoderm. The blastoderm of a fertilized egg is white and disc-shaped. The thin transparent part in the center of the blastoderm is the bright area, and the surrounding thicker opaque part is the dark area. Infertile eggs also have white dots, but they are smaller than the blastoderm of fertilized eggs, and there is no distinction between light and dark areas. The embryo begins to develop in the bright zone of the blastoderm and forms two different cell layers, the outer layer is called ectoderm and the inner layer is called endoderm. This stage of embryonic development is the gastrula stage. The egg is laid after the chicken embryo forms two germ layers, and its development temporarily stops when it is cold.

(2) Development of the embryo during incubation

After the fertilized egg is incubated, due to the rise in temperature, the blastoderm begins to divide, and mesoderm soon forms between the endoderm and the endoderm. . The three germ layers ultimately form all the tissues and organs of the chicken embryo. The ectoderm forms the skin, feathers, beak, claws, nervous system, retina of the eye, ears, and oral and cloacal epithelium; the mesoderm forms the muscles, bones, circulatory system, and reproduction. And excretory organs, connective tissue; endoderm forms the epithelium and endocrine glands of the digestive tract, respiratory organs. It grows into a complete chick in 21 days (Figure 2-2).

Figure 2-2 The anatomy of chicken embryo development day by day

①External characteristics of embryonic development: From a morphological point of view, chicken embryo development is roughly divided into the following stages.

Internal organ development stage: The first 4 days of incubation are the internal organ development stage.

On the first day of incubation, the embryo begins to develop again. The mesoderm enters the dark zone, and many red spots appear on the edge of the blastoderm, called "blood islands". When looking at an egg, a dark dot with bright surroundings will appear on the surface of the yolk, which is called a "fish eye bead" or "white light bead."

On the second day of incubation, the yolk sac, amniotic membrane and serous membrane begin to form, the heart begins to form, the blood islands merge into blood vessels, and the rudimentary heart begins to beat. The embryonic blood vessels connect with the yolk sac blood vessels to initiate blood circulation. When you look at the eggs, you can see the yolk sac vascular area, which looks like a cherry and is commonly known as "cherry beads".

On the 3rd day of incubation, the allantois begins to grow. The head and eyes of the embryo are extremely large, and the neck is very short. At the end of the 3rd day, the limbs become hilly. The back grows extremely rapidly, causing the embryo to be curved. . When the eggs are photographed, the embryo and stretched yolk sac blood vessels are visible, resembling a stationary mosquito, commonly known as "mosquito beads."

On the 4th day of incubation, the amniotic cavity is formed and the embryo and yolk sac are completely separated. Due to the rapid growth of the midbrain, the head is significantly enlarged and the embryonic body is significantly curved. When shining an egg, the yolk does not easily rotate with the egg, commonly known as "nail shell". And because the blood vessels of the embryo and yolk sac are shaped like spiders, they are also called "little spiders."

External organ formation stage: The 5th to 14th day of incubation is the external organ formation stage.

On the 5th day of incubation, the peripheral nervous system, gonads, liver, spleen, etc. develop significantly, forming the mouth and limbs. The protein gradually decreases, the yolk gradually expands, the embryo is extremely curved, and the eyes are heavily pigmented. When looking at the eggs, you can clearly see black eyespots, called "beads", "single beads" or "black beads".

On the 6th day of hatching, the beak and beak tip begin to form, and the legs and wings are roughly differentiated. The allantois reaches the surface of the eggshell membrane, and the yolk sac is distributed over more than half of the yolk surface. Due to the contraction of the smooth muscle of the amniotic membrane wall, the embryo moves regularly, the trunk of the embryo increases, and the embryo body begins to straighten. When looking at the eggs, you can see two small round balls on the head and enlarged trunk, commonly known as "double beads".

On the 7th day of incubation, the embryo has shown the characteristics of birds, the gizzard is formed, the villi begin to form, the neck is straightened, and the wings and beak are obvious. When the egg is photographed, the embryo sinks in the amniotic fluid and is difficult to see clearly, which is called "sinking". At this time, the surface of half the egg is covered with blood vessels.

On the 8th day of incubation, the upper and lower beaks are clearly separated, the limbs are fully formed, the abdominal cavity is healed, the ribs, liver, lungs, and stomach are clearly visible, and the right ovary of the mother chick begins to degenerate. When looking at the egg, you can see from the front that the embryo is floating in the amniotic fluid, commonly known as "floating"; from the back you can see that the yolks on both sides are not easy to shake when the egg rotates, commonly known as "hard edges".

On the 9th day of incubation, the beak begins to become horny, the nostrils are obvious, and the eyelids have reached the iris. The cartilage begins to ossify and the toes are completely separated. The whole body is covered with feathers and nipples. When the embryo is dissected, the heart, liver, stomach, esophagus, intestines and kidneys are well developed. The allantois surrounds almost the entire embryo. When looking at the egg, it can be seen that the sides of the yolk tend to shake, and the allantoic blood vessels on the back stretch across the yolk sac, commonly known as "hairy edges" or "trending tendons."

On the 10th day of incubation, the keel process forms and the allantoic blood vessels reach the sharp end of the egg. When looking at the egg, it can be seen that the allantoic blood vessels are closed at the sharp end of the egg. Outside the degassing chamber, the entire egg is covered with blood vessels, which is commonly known as "closing".

On the 11th day of incubation, villi appear on the back of the embryo, coronal teeth appear on the crown, and the allantoic fluid reaches its maximum amount. When you look at the egg, you can see that the blood vessels are thickened and the color is darker.

On the 12th day of incubation, the protein is partially absorbed, the body is covered with villi, and the toes are fully formed. When you look at the egg, you can see that the blood vessels are thickened and the color is darker.

On the 13th day of incubation, most of the head and body are covered with down, and scales appear on the soles and toes. When illuminating the egg, the bright part at the sharp end of the egg gradually decreases as the embryo age increases.

On the 14th day of incubation, the whole body is covered with villi, the head is facing the air chamber, and the long axis of the embryo is parallel to the egg.

Embryonic growth stage: The 15th to 20th day of incubation is the growth stage of the embryo.

On the 15th day of hatching, the wings take shape and the beak is close to the air chamber and is in a closed state.

On the 16th day of incubation, the crown and beard are obvious, and most of the protein has been absorbed.

On the 17th day of incubation, pulmonary blood vessels are formed, but there is no blood circulation. The legs hug the head tightly and the beak turns to the air chamber. Amniotic fluid and allantoic fluid begin to decrease. When looking at the egg, the sharp end of the egg can no longer see the shiny part, which is commonly known as "sealed door".

On the 18th day of incubation, the amniotic fluid and allantoic fluid decreased significantly, and the embryonic growth was almost completed. The head is bent right down, the beak faces the air chamber, and the eyes begin to open. When looking at the egg, you can see that the air chamber is tilted to one side. This is because the embryo is turning around, which is called the "slant mouth".

On the 19th day of incubation, the yolk sac contracts and is absorbed into the abdominal cavity along with the yolk. The beak enters the air chamber and begins breathing. The allantoic artery and vein begin to degenerate. When looking at the eggs, you can see the black shadows on the wings, beak and neck flashing in the air chamber, commonly known as "flash hair".

On the 20th day of incubation, the allantois wilts completely, and all the remaining yolk and yolk sac enter the abdominal cavity. The chick pecks through the air chamber and begins breathing with its lungs. The cries of the chicks inside the shell can be heard. The chick has already pecked the shell. When pecking the shell, first use the "shell breaker" to peck a round hole near the air chamber. Then, while turning the head, peck open the eggshell in a counterclockwise direction near the air chamber, and then extend the head. The feet break out of the shell. Some chicks hatch.

② Development of fetal membranes and material metabolism: Chicken embryo development includes intraembryonic and extraembryonic parts. The development of the embryo itself is the intraembryonic development part, and the formation of fetal membranes is the extraembryonic development part. The nutrition and respiration of the chicken embryo mainly rely on the fetal membrane. Therefore, the development of the fetal membrane is particularly important for the development of the chicken embryo. Four fetal membranes are formed in the early stages of chicken embryo development, namely vitelline membrane, amniotic membrane, serosa and allantois.

Yolk sac: The yolk sac is a membrane sac wrapped around the yolk. It begins to form on the 2nd day of incubation and gradually grows to cover the surface of the yolk, covering 1/3 on the 4th day, 1/2 on the 6th day, and almost covering the entire yolk surface on the 9th day. The yolk sac is connected to the fetus by the stipe. The yolk sac is densely distributed with blood vessels and has many villi, which helps the fetus absorb nutrients from the yolk. The yolk sac is not only the nutritional organ of the embryo, but also the early respiratory organ and hematopoietic organ. On the 19th day of incubation, the yolk sac and remaining yolk begin to enter the abdominal cavity, and completely enter the abdominal cavity on the 20th day.

Amnion: The amnion is a membrane sac surrounding the fetus. It begins to appear about 33 hours after hatching. On the 2nd day, it covers the head of the embryo and gradually surrounds the body of the embryo. On the 4th day, the amniotic membrane closes to surround the embryo, and then enlarges and is filled with transparent liquid, that is, amniotic fluid. During incubation, the protein flows into the amniotic membrane, making the amniotic fluid thicker. At the end of incubation, the amount of amniotic fluid becomes less, so the amniotic membrane sticks to the fetal body and remains on the shell membrane after hatching.

Smooth muscle cells on the amniotic membrane continue to contract rhythmically. Because the amniotic cavity is filled with amniotic fluid, which acts as a buffer, the chicken embryo is not damaged, prevents adhesions, and promotes fetal movement.

Serosa: The serosa and the amnion are formed at the same time. On the 6th day of incubation, it is close to the outside of the amnion and yolk sac. Later, it is separated from the amnion due to the development of the allantois, attached to the inner shell membrane, and separated from the allantois. The outer layers unite to form the allantoic serosa. The serosa is clear and avascular, so the individual serosa cannot be seen when opening an incubating embryo.

Allantois: The allantois is located between the amniotic membrane and the yolk sac. It begins to appear on the second day of incubation, then grows rapidly, and is close to the inner surface of the shell membrane on the sixth day. On the 10th to 11th day of incubation, it surrounds the entire contents of the embryo and closes at the sharp end of the egg. The allantois is connected to the intestine, and the fluid excreted by the fetus is accumulated in it, and then evaporates out of the egg through the stomata. The surface of the allantois is covered with blood vessels. The embryo absorbs nutrients in the protein and minerals in the eggshell through the blood circulation of the allantois, inhales oxygen from the outside through the air chambers and stomata, and excretes carbon dioxide. The allantois gradually dries up at the end of incubation, and there is yellow-white nitrogen-containing excrement in it, which remains in the eggshell after hatching.

The substance metabolism of the embryo during incubation mainly depends on the development of fetal membranes. In the first two days of incubation, fetal membranes have not yet formed, there is no blood circulation, and material metabolism is extremely simple. The embryo absorbs nutrients from the yolk through osmosis, and the required gas comes from the decomposition of carbohydrates. Two days later, blood circulation in the yolk sac is formed, and the embryo begins to absorb nutrients and oxygen from the yolk. After 5 to 6 days of incubation, the allantoic blood circulation is also formed. At this time, the fetus not only relies on the yolk sac to absorb nutrients in the yolk, but also relies on the allantoic blood vessels to absorb nutrients in the protein and eggshell, and also passes through the stomata through the allantoic circulation. Absorb oxygen from the outside. When the allantois is closed, the embryo's material metabolism and gas metabolism are greatly enhanced, and the temperature inside the egg rises. After 18 to 19 days of incubation, the protein is exhausted, the allantois withers, the air chamber is penetrated, and the embryo begins to breathe with the lungs. The embryo only relies on the yolk sac to absorb nutrients in the yolk, fat metabolism is strengthened, and the respiratory volume increases.