Adapted from an article by Marsh Farms

KEY WORDS: calcium carbonate, porous, shell, membranes, air cell, albumen, yolk, chalazae

All parts of an egg have a role to play in the development of the duckling. The shell is made of CALCIUM CARBONATE. It produces a hard protective cover for the duckling. The shell is also POROUS. Porous means many tiny holes through which fluids, air or light can pass. The pores at the large end of the egg are larger and more numerous than those at the small end of the egg. These pores allow carbon dioxide and moisture to be released from the egg. At the same time, the pores allow the same gases and oxygen we breathe, to enter the shell.

Just inside the shell are two thin skins or MEMBRANES. These membranes protect the contents of the egg from bacterial invasion and prevent the liquid inside the egg from evaporating too rapidly. The membranes will separate from the shell at the large end of the egg. The freshly laid egg is the temperature of the hen, about 107 degrees. As the egg cools to the temperature of its surroundings, the contents of the egg shrink. The hard outer shell remains the same size. Since there are more pores at the large end of the egg the air enters to fill the space left by the shrinking contents. This air space is called an AIR CELL. The air cell serves as a tiny shock absorber during the early development of the duckling. Toward the end of the life cycle, approximately the twenty-seventh day, the duckling will poke a hole in the membrane and draw its first breath from this space.

The membranes also act as a protective wall for the ALBUMEN or the “white” of the egg. The albumen is the liquid in which the duckling will develop. It also serves as a source of food or protein for the duckling, just as the yolk does. The yolk contains large amounts of carbohydrates, fat and protein. The yolk is also a reservoir of the vitamins and minerals essential for the normal growth of the duckling. A rope-like structure, called the CHALAZAE is connected to each end of the yolk. The chalazae acts as an anchor for the growing duckling. When the egg is turned over the chalazae or rope-like structure allows the yolk to turn so that the developing duckling always remains on the top of the yolk.

Let's review the structure of an avian or duck egg:

1. The hard outer protection of an egg is called the shell. It contains thousands of pores that allow material into the shell and to escape from the shell.
2. Just inside the shell are two protective membranes. The membranes act as a protective device preventing bacteria from getting into the egg and also fluids from getting outside the shell.
3. The membranes separate and form an air cell. The air cell is the pocket where the duckling gets its first breath of life.
4. The albumen or white of the egg is the liquid where the duckling will develop. It is also a source of protein for the developing duckling.
5. The yolk is another source of food for the duckling. This source of food consists of carbohydrates, fats, proteins, vitamins and minerals.
6. The chalazae is a rope-like structure that acts as a stabilizer for the duckling. When the egg is turned, this structure will allow the duckling to always remain on top.

This is the physical structure of an avian or duck egg. This is where the embryo will begin life.

KEY WORDS: hen, ovary, oviduct, cloaca, drake, sexual reproduction, fertilization, blastoderm, sperm, testes

The first stage of the development of an egg begins inside the body of a female duck, properly called DUCK but often referred to as the HEN. The part of the egg we call a yolk is formed in the hen's OVARY. The ovary contains many yolks in different stages of growth. When one yolk becomes large enough it moves out of the ovary and into a tube called the OVIDUCT. As the yolk travels within the oviduct, two other basic parts of an egg are formed. The first is the ALBUMEN or white. The second, the hard shell, is also formed there. About twenty-four hours after the yolk enters the oviduct, the completed egg is ready to be laid. It comes out of the hen's body through an opening called the CLOACA. This is the process of development that produces an egg people use as food.

Eggs purchased at the grocery store are not fertilized. An egg must be fertilized to be able to hatch into a baby duckling or chick. How do eggs become fertilized? Birds propagate, or continue the species, by a process known as SEXUAL REPRODUCTION. Sexual reproduction is the joining of two sex cells, the ovum of a female duck and the sperm of the male drake. The union of the two sex cells is known as FERTILIZATION.

To understand how this union is achieved, let's examine the physical make-up of a drake. The drake has two sex organs called TESTES which are hidden inside the drake's body. These testes produce a type of cell called SPERM. These tiny cells travel from the testes through two tubes to an opening called the CLOACA. Do you remember this term from the anatomy study of the hen? The cloaca is found in both the duck and drake.

When the hen and the drake mate, these openings are brought together. the sperm of the drake are passed into the hen and begin moving into the oviduct of the hen. The sperm travel along the oviduct until a single sperm unites with the germ spot or germinal disc on the yolk of an egg in the oviduct. The single sperm cell uniting with the germinal disc is called fertilization. The germ area is generally lighter than the yolk and is raised slightly on the yolk.

As soon as fertilization takes place the newly formed cell begins to divide. The cell divides in multiple numbers: first there is only one, then it divides to form two, then they divide to form 4, then 8, 16, 32, and so on until a tiny cluster of cells appear on the surface of the yolk. This tiny cluster of cells, which appear as a small whitish spot on the yolk, is called a BLASTODERM.

This spot, in a fertilized egg, is the beginning of a duckling, but it is still so small that it can not be seen without a microscope.

KEY WORDS: incubation, embryo, notochord, yolk sac amnion, allantois, vitelline vessels, allantoic vessels, pipping

We have learned about the first stages of embryonic development. After the ovum is fertilized, the cells of the blastoderm continue to divide until the egg is laid. As the yolk travels down the oviduct to the cloaca, the albumen or “white” of the egg is added, and then the shell. When the egg is laid, it cools, and the development of the embryo stops. This does not result in the death of the duckling. On the contrary, it allows the egg to rest. Remember when we began this project, the fertile eggs were stored in a cool ventilated room. We carefully sprinkled the eggs with water so the albumen fluid would not fry up; and we turned the eggs. The cells on the yolk simply stop growing.

In order to continue development, the embryo requires one important ingredient: warmth. Female ducks, like many other birds, sit on their eggs to keep them warm. This is called INCUBATION. People who raise thousands of chickens or ducks on a farm use large machines called INCUBATORS to provide the warmth that eggs need to continue to grow. This is what we are using, an incubator, which provides a favorable environment of warmth and humidity that a mother duck would otherwise have provided herself. The embryo will resume development after several days of rest if it is heated again by the duck or in an incubator.

It takes 28 days of incubation for a fertile duck egg to hatch. Let's discuss the development of the duckling during the incubation process.

All the cells in the blastoderm are alike. As the division of cells progresses, some differences begin to appear. At this point, the collection of cells is called an EMBRYO. This is a word used to describe all plants and animals in the early stages of development.

Gradually the various parts of the cluster of cells begin to group into individual organs that make up the duckling. The development of these cells is a continuous, orderly process. It involves many changes from apparently simple, to new complex structures. From these structures arise all the organs and tissues of the living duckling.

As the embryo is growing, it is using the highly nutritional food material in the egg. The embryo is taking in oxygen and expelling carbon dioxide. Special temporary embryonic membranes are formed within the egg, both to protect the embryo, and to allow its growth. These embryonic membranes include the YOLK SAC, AMNION, and the ALLANTOIS. The yolk sac supplies food material to the embryo. The amnion, by enclosing the embryo, provides protection. The allantois serves as a respiratory organ and as a reservoir for the waste products. These temporary embryonic membranes function within the egg until the time of hatching. The embryonic membranes from no part of the fully developed duckling. While it is still in the egg though, the duckling is attatched to the embryonic membranes by blood vessels much like our own umbillical cord.

These blood vessels form early in the incubation period. One set, the VITELLINE VESSELS, is concerned with carrying the yolk materials to the growing embryo. The other set, the ALLANTOIC VESSELS is chiefly concerned with respiration and with carrying waste products from the embryo to the allantois. When the duckling is hatched, the embryonic vessels cease to function.

Let's take a look at the embryo as it begins to form into a duckling. The first six days are the most critical to the growth of the embryo. The first sign of growth in an embryo takes place approximately 18 hours into incubation. The main body of the duckling is beginning to take composition. At this point, the embryo's body is represented by a faint line called a NOTOCORD. This is the central area that shapes into the SPINAL CORD. This area is surrounded by many blood cells visible in the yolk of the egg.

By the beginning of the second day of incubation the brain is beginning to be visible. The back bone and the spinal cord are also developing. Before the second day is over, much has happened to the growth of the embryo. The brain is rapidly developing. The mouth and digestive systems begin to materialize. The nervous system shows evidence of formation. The head begins to develop and the circulatory system and the eyes start to take shape.

By the fourth day, the ears have begun to form and it is possible to see the heart beating when the egg is candled. By ten days, all of the organs and body systems have formed, and the duckling will spend the rest of its time maturing and growing. At two weeks incubation time the beak is formed along with the egg tooth and the feathers have begun to form, and at three weeks the scales (on the legs and feet), claws and beak are becoming firm and horny. At about this time, the duckling's beak turns toward the air cell and it is big enough that it can no longer turn around in its shell.

The yolk sac begins to be ingested into the body cavity around day 25, and by the 26th day the duckling fills the egg almost entirely. The air cell enlarges at this time and the yolk sac should be completely drawn into the body by the 26th or 27th day.

If conditions have all been right, the duckling should break into the air cell some time on the 27th day, and break out of its shell on day 28.

Let's go back to the end of the third week (about 21-23 days). After the duckling's bill turns toward the air cell, it is fully matured and getting read for hatching. This is a time when the embryo needs rest. The duckling must ready itself for the long process of birth. It is also around this time (a few days later) that the eggs can stop being turned.

Several changes prior to hatching begin to take place. The fluid of the amnion begins to decrease. The neck of the duckling develops a 'double bend' so that its bill is under its right wing and toward the air chamber. The remaining yolk sac is taken into the body cavity of the duckling for use as food in the first days after hatching.

Some time towards the end of the 26th day, the duckling thrusts its head forward. Its bill rapidly pierces the membranes, and enters the air chamber. The duckling begins to slowly breathe the air contained inside. If you watch the egg shell carefully, you can sometimes see the shell expand and contract slightly as the duckling breathes. Listen for its first peeps.

By the 27th or 28th day there should be a crack in the egg shell. This is the first outside evidence that the hatching process has begun. The first break in the shell is made by the EGG TOOTH, a sharp bony structure located near the tip of the bill. The egg tooth was developed especially for cracking the egg shell. The process of cracking the egg shell is known as PIPPING. The pipping duckling will begin to breathe normally now. The allantois dries up and the circulation in the allantois rapidly ceases to function.

The duckling slowly revolves in the shell and chips at the egg shell in a circular path. Then, a vigorous twist of the duckling's neck makes the shell walls begin to shake, and the shell finally falls open along the chipped path. This duckling is now ready to greet the world.