Before the Egg is Laid: • The egg is fertilized. • The zygote divides and begins to grow. • The cells segregate into groups of specialized function. • The embryo nearly stops growing between laying and incubation. During Incubation: • The area pellucida and area opaca of the blastoderm develop. Major events: • 18 hours: The alimentary tract appears. • 19 hours: The brain crease begins to form. • 20 hours: Somites appear. • 21 hours: The brain and nervous system begin to form. • 22 hours: The head fold begins to form. • 23 hours: Blood islands appear. • 24 hours: The eyes begin to form.
Day 1: The Journey Begins The Yolk
The formation of the egg starts in a hen's ovary. (Watch for more on the formation of an egg on Day 5) What appears to be the yolk of an egg is actually a single cell also known as the ovum. The cell membrane of the ovum is surrounded by a non-cellular vitelline membrane produced by the ovum. The ovum stores nutrients in its cytoplasm as yolk granules for use by the embryo if the ovum is fertilized. (Watch for more on egg nutrition on Day 8). There are two kinds of yolk: Yellow yolk is produced during the day and contains more fat. White yolk is produced at night and contains more protein. Most of the white yolk is located directly below the nucleus (the position of the possible future embryo) in the latebra and the nucleus of Pander. But as egg laying time grows closer,yolk is produced continually and begins to accumulate rapidly. For the last seven or eight days before the ovum is ovulated, yellow and white yolk are laid down in rings like those of a tree. When the ovum has accumulated enough yolk to grow a chick, it is released from the ovary.
Text and graphic by Janet Sinn
Day 1: The Journey Begins The Embryo Illustrated This is an illustration of a 24-hour chick embryo lying on top of its yolk. The various structures and layers present at this time are color-coded. The neural tube, derived from ectoderm, is blue. The somites and mesodermal layer from which they are derived are red. The gut, derived from endoderm, is yellow.
The Journey Begins The Egg Yolk
An infertile and a fertile egg. Can you tell the difference? The white chalazae is much more prominent in the infertile egg. However there is no correlation between fertility and the size of the chalazae. The chalazae is composed of mucin fibers; mucin is a special kind of structural protein. The chalazae holds the yolk in place within the egg.
In the infertile egg, on the left, the nucleus is merely a light spot on the yolk. The egg on the right is fertilized. In the fertilized egg the ovum has fused with a sperm to begin forming an embryo. By the time the fertilized egg is laid, many cells are divided on the surface of the yolk and formed a blastoderm. Can you see the difference between the nucleus of the infertile egg and the blastoderm of the fertilized egg? Both are indicated by the blue arrow.
After one day in the incubator, the small white spot seen in the fertilized egg above has grown to the size of a nickel or a quarter.
Day 1: The Journey Begins A Microscope's View The image to the right is of a 24 hour old chick embryo seen under a light microscope. This is a top view of the embryo. This is how it would appear if you were looking down on it laying on top of its yolk. At this age the embryo is about 30mm long from the tip of its head to its tail. The embryo has been stained to make it easier to see under the microscope. The image below is the same embryo, enlarged, turned black and white, and labeled.
Day 1: The Journey Begins An MRI's View
In the first day's MRI scan you may not be able to see very much, but the image might look like these.
Yolk formation takes about 18 days. During the last 7 or 8 days of yolk development, the yolk is deposited in layers like the rings of a tree. These layers are visible in your MRI images. Yellow yolk is formed during the day and is high in lipid (fat). In the MRI images, yellow yolk appears dark. White yolk is low in fat and is formed during the night. The white yolk appears as narrow, white bands on the MRI images.
Day 1: The Journey Begins 3-D Reconstruction of a 24 Hour Chick Embryo
The image above was made by slicing a fixed 24 hour chick embryo in very thin sections from head to toe. The sections were then photographed under the microscope and brought into a computer. Special software was used to restack the slices into a three-dimensional volume. The different layers and organs that are already beginning to develop are colored and labeled.The three basic embryonic germ layers are ectoderm (blue), mesoderm (red) and endoderm (yellow). The purple notochord is the embryonic axis where the spinal column will be. The blue neural tube will become the brain and nervous system. The yellow gut will become the digestive tract, and the red somites will become muscle and skeleton. 3-D reconstruction by Jo Ann Eurell
Day 1: The Journey Begins Candled Eggs When the unfertilized egg is candled, a shadow of the yolk is seen at the top of the egg (side view-dark area). If the egg is turned toward the large end, (angle view) the outline of the air space is visible. If the egg is moved on the candler light, the structures within will float around. The 24 hour embryo shows the yolk as the darkest area at the top of the egg. The albumen, or white of the egg, fills the rest of the shell and only the shadow of the air
space can be seen.
Day 2: The Heart of the Matter
Major Events: • 25 hours: The heart begins to form. • 33 hours: The ears begin to form. • 42 hours: The heart starts to beat.
Day 2: The Heart of the Matter The Egg Yolk On the second day of incubation a blood ring appears around the area opaca. The question mark shape in the middle of the ring is the embryo. By 24 hours, pockets of blood cells and developing blood vessels, called blood islands, begin to appear on the surface of the yolk along the outer edges of the area opaca. These are called the vitelline vessels. At the same time, the heart and vessels within the chick embryo have begun to develop. The omphalomesenteric veins can be seen branching out from the embryo in this photo of a 43 hour chick embryo. The vitelline vessels and the branches of the omphalomesenteric arteries and veins grow toward each other. When they meet, channels open and circulation from the yolk to the embryo begins. Vitelline circulation is responsible for bringing food from the yolk to the embryo.
Day 3: Say Cheese!
Major Events: • The head begins to turn onto its left side. • The tongue begins to form. • The amnion completely surrounds the embryo. • The tail has appeared. • Wing and leg buds are visible. • Soon other organs such as the liver, kidneys, and lungs will appear.
Day 3: Say Cheese! The Egg Yolk The embryo is the question mark shape in the center of the image. It has grown considerably since yesterday and the anterior half of its body is curved even more. The heart, located inside the anterior curve of the body is much larger and more developed than in yesterday's photograph. The omphalmomesenteric veins, extending from either side of the embryo to connect with the vitelline vessels, have grown and branched.
Day 4: Setting Up Camp
Major Events: • The somites extend to the tip of the tail. • The toes begin to form. • The allantois is visible coming off the hindgut.
Day 4: Setting Up Camp The Egg Yolk At four days, nearly the entire body of the embryo has risen off the yolk and turned to lay on its left side. The upper portion of the embryo is even more curved and the tail is also beginning to curve. The heart is the dark red area within the upper curve of the embryo. Just a fraction of an inch from the heart, the eye is visible. The large node just above and to the right of the eye is a portion of the brain.
Day 5: What Came First: the Chicken or the Egg?
Major Events:
•
The reproductive organs begin to form. • The bones of the legs begin to form. • The crop begins to form.
Day 5: What Came First: the Chicken or the Egg? Candled Eggs
Visible Features: • Yolk • Eye of embryo • Albumen
Day 6: Who Do I Look Like?
Major Events: • The beak becomes visible. • The wing bends at the elbow. • The allantois begins to fuse with the chorion. • The ribs begin to appear. • The gizzard begins to form. • The intestines begin to loop.
Day 6: Who Do I Look Like? Candled Eggs
Visible Features: • Yolk • Air space • Eye of embryo • Blood vessels
Visible Features: • Yolk • Air space • Eye of embryo • Blood vessels
Day7: Jumpin' Genes.
Major Events: • A row of feather germs appears on the tail. • The sclera of the eye begins to form. • Feather papillae appear on the thigh. • The leg bends at the knee. What Do I Look Like Today?
Day 8: Egg Nutrition
Major Events: • Three rows of feather germs are visible on the tail. • The nictitating membranes (inner eye lids) begins to form. • The egg tooth begins to form. • The bone marrow cavity of the femur begins to form.
Day 9: Name That Artifact
Major Events: • Upper eye lids begin to cover the eyes. • The knee caps begin to form.
Day 10: Egg Chemistry
Major Events:
• • • • •
Claws begin to form. The comb is visible. Flight feathers appear. Feather tracts appear over the sternum. Lower eyelids develop.
Day 11: Birds of a (Colored???) Feather
Major Events: • The toe claws begin to curve downward. • The bottom of the feet become padded. • Feather germs on the back and tail appear as long, tapering cones. • The beak begins to harden. • The comb appears serrated. • The embryo begins to draw calcium from the eggshell for its growing bones.
What Do I Look Like Today?
Day 12: Image Processing
Major Events: • Scales appear on the lower legs. • The embryo continues to grow and move. • The ribs begin to ossify.
Day 13: Yolk! Yolk! Yolk!
Major Events: • Fingers of the wings are covered with feather papillae. • The left and right collar bones fuse to form the wishbone.
Day 14: The Skeleton Major Events: • The embryo turns its head toward the blunt end of the egg. • The skull has begun to ossify.
Day 15: The Respiratory System
Major Events: • The scales, claws, and beak are becoming firm
Days 16: What Shape is an Egg?
Major Events: • The embryo turns its beak toward the air cell. • The chorioallantoic cavity lines most of the inside surface of the egg shell.
Days 17-21: Happy Birthday!
Major Events: • Day 17: The beak turns towards the air cell. • Day 18-19: The beak breaks through the inner shell membrane. • Day 18-19: The lungs begin to function. • Day 19: The yolk sac begins to enter the body cavity. • Day 20: The lungs are using the air cell for breathing completely. • Day 20: The embryo occupies all of the egg except air cell. • Day 21: The neck begins to spasm. • Day 21: The egg tooth pips through the shell. • Day 21: Exhausted and wet - but alive - the chick has hatched!
Days 17-21: Happy Birthday! How the Chick Hatches
HOW THE CHICK EMERGES FROM THE SHELL The head of the chick develops at the large end of the egg. Between the 15th and 16th days, the chick orients itself so that its head is near the air cell at the large end of the egg. Not long before the chick is ready to attempt to make its way out of the shell its neck acquires a double bend so that its beak is under its right wing and pointed toward the air cell. About the 19th day the chick thrusts its head forward. Its beak quickly breaks through the inner shell membrane, and the chick's lungs begin to function. Complete breathing by the lungs usually does not occur until the 20th day of incubation. Using its egg tooth (a tiny, sharp, horny projection on the end of its beak), the chick pecks at the shell thousands of times. Finally, the young bird pips its way through the shell and begins to breathe air directly from the outside. After the chick has made a hole in the shell, it stops pipping for three to eight hours and rests. During this time, it is acclimating its lungs to the outside atmosphere. After the resting stage is completed, the second stage of pipping begins. The chick begins to turn slowly inside the egg. As it turns, usually counter-clockwise, the cutting edge of the chick tooth continues to chip away. In two to five hours, the chick has made about three quarters of a turn inside the egg. As the chick progresses in its movement around the shell, it begins pushing on the egg cap (large end). Squirming and struggling, the chick works feverishly for about 40 minutes pushing at the cap. Finally with a vigorous shove, the chick breaks free from the shell, still wet and panting. When the chick is freed completely from the shell, it lies still. Its energy has been virtually exhausted, and it is extremely tired. After a rest of some few minutes, the chick begins to rise to its feet and gain coordination of its muscles. Within a few days the egg tooth, its usefulness over, will disappear.
STUDYING EMBRYONIC DEVELOPMENT You can study the external form of chicken embryos at various stages of development. Beginning with the 3rd or 4th day of incubation you can, by carefully breaking open an egg each day, observe many events in the development of embryos. And it is easy to preserve the embryos for display or additional study. To study the early embryonic development from the beginning to the 4th day of incubation, you will need to prepare a set of whole mounts and observe them under a microscope. This is a rather exacting procedure which requires considerable time. It should not be attempted by the younger student or 4-H member.
Seven-day old embryo with its embryonic membranes a embryonic blood vessels. (Slightly modified from Duval, A. L. Romanoff, Cornell Rural School Leaflet, September, 1939.) (Fig. 8)
Bone formation and growth can be studied by clearing and staining the embryo. This process requires advanced and exacting techniques, considerable time and equipment, and some financial outlay. Therefore, clearing and staining of embryos should be at tempted only by the advanced student under the supervision of an experienced person. If you plan to study the different stages of embryonic development by one or more of the above methods, be sure to incubate enough eggs for this purpose. The stages of embryonic development are listed in the table below (see also Figs. 9 and 10). You will not be able to see all of these with the naked eye. But just by breaking and observing an egg each day you can identify many of the stages.
Successive changes in the position of the chick embryo and its embryonic membranes. (From A. L. Romanoff, Cornell Rural School Leaflet, September, 1939.) (Fig. 9)
Daily changes in the weight and form of the developing chick embryo - in this case White Leghorn. (Based on A. L. Romanoff, Cornell Rural School Leaflet, September, 1939.) Illustration Copyright Š 1998, Jill Hixon and the University of Illinois. Not the same illustration as appears in the original text, but close and more colorful. (Fig. 10)
Advanced students who want to prepare whole mounts of embryos can find the procedures in such works as Animal Micrology, Introduction to Vertebrate Embryology, and Handbook of Microscopal Technique (see the reference list on page 16).