The document summarizes the formation of the egg in chickens. It describes how the yolk forms in the ovary over 10 days as materials are deposited in layers. When mature, the yolk is released through ovulation into the oviduct. As it passes through the oviduct over 1-3 days, different sections add the albumen, shell membranes, calcium carbonate for the shell, and cuticle. Factors like lighting, diet, and genetics can influence yolk size, ovulation timing, and egg quality. The oviduct has specific sections for picking up the yolk, secreting albumen layers, adding other layers, and passing the completed egg.

1. 5
Formation of the Egg
by Donald D. Bell
The avian egg consists of a minute reproductive cell quite comparable
to that found in mammals. But in the case of the chicken, this cell is located
on the surface of the yolk and surrounded by albumen, shell membranes,
shell, and cuticle. The ovary is responsible for the formation of the yolk;
the remaining portions of the egg originate in the oviduct.
5-A. OVARY
At the time of early embryonic development, two ovaries and two ovi-
ducts exist, but the right set atrophies, leaving only the left ovary and
oviduct at hatching. Prior to egg production, the ovary is a quiet mass of
small follicles containing ova. Some ova are large enough to be visually
seen; others require magnification. Several thousand are present in each
female chicken, many times the number that will eventually mature into
full-size yolks necessary for egg production during the life of the bird.
Formation of the Yolk
The yolk is not the true reproductive cell, but a source of food material
from which the minute cell (blastoderm) and its resultant embryo partially
sustain their growth.
When the pullet reaches sexual maturity, the ovary and the oviduct un-
dergo many changes. About 11 days before she is to lay her first egg, a
sequence of hormonal changes occur. The follicle-stimulating hormone
(FSH) produced by the anterior pituitary gland causes the ovarian follicles
59
D. D. Bell et al. (eds.), Commercial Chicken Meat and Egg Production
© Springer Science+Business Media New York 2002

2. 60 FORMAnON OF THE EGG
to increase in size. In tum, the active ovary begins to generate hormones:
estrogen, progesterone, and testosterone (sex steroids). Higher blood
plasma levels of estrogen initiate development of the medullary bone,
stimulate yolk protein and lipid formation by the liver, and increase the
size of the oviduct, enabling it to produce albumen proteins, shell mem-
branes, calcium carbonate for shell formation, and cuticle.
The first yolk (ovum) to begin maturing does so as major amounts of
the yolk material produced in the liver are transported by the circulatory
system directly to the developing ovary. A day or two later, the second
yolk begins to develop, and so on, until at the time the first egg is laid,
five to ten yolks are in the growth process. About 10 days are required
for an individual yolk to mature. Deposits of yolk material are very slow
at first and light in color. Eventually the ovum reaches a diameter of 6
mm at which time it grows at a greatly increased rate, with the diameter
increasing about 4 mm per day. A greater number of yolks are under de-
velopment at one time in the broiler breeder hen than in the egg-type hen,
but the broiler breeder hen does not have the ability to produce as many
complete eggs.
The color present in the yolk is xanthophyll, a carotenoid pigment de-
rived from the diet. The pigment is transferred first to the bloodstream,
then quickly to the yolk, as well as other parts of the body. Consequently,
more is deposited in the yolk during the hours when the hen is eating
than during dark hours when she is not. This gives rise to deposits of
dark and light layers of yolk material, depending on the dietary pigment
available. From seven to eleven concentric rings are found in each yolk.
Yolk formation is rather uniform and the total thickness of both dark and
light deposits during 24 hours is about 1.5 to 2.0 mm.
Egg yolk is composed mainly of fats (lipids) and proteins, which com-
bine to form lipoproteins, of which 60% of the dry yolk weight is of low
density lipoproteins (LDL), and are known to be synthesized by the liver
through the action of estrogen. In the laying hen, LDL is removed from
the blood plasma as intact particles for direct deposition in the developing
ova.
What influences growth rate of the yolk? Yolks vary greatly in size be-
tween individual chickens in the flock at the same age, and are usually
associated with body weight differences. Yolk size is not associated
with rate of lay, but probably more with the length of time required
for the ova to reach maturity. The yolks from an individual hen in-
crease in size over the production cycle. Furthermore, the first egg
laid in a clutch will usually contain a larger yolk than the remaining
ones. Eggs laid later in the day are 0.5 grams lighter for each addi-
tional hour in the day; this is also associated with smaller yolks. The
inclusion of added fat and protein in the diet has also been shown to
increase the size of the developing yolk.

3. 5-A. OVARY 67
Location of the germinal disc. The yolk material is laid down adjacent to
the germinal disc that continues to remain on the surface of the globu-
lar yolk mass. Once the egg is laid, the yolk rotates so the germinal
disc remains in the large end of the egg.
Ovulation
At maturity the ova are released from the ovary to enter the oviduct by
a process known as ovulation. Each ovum hangs on the ovary by a narrow
stalk containing the arteries that supply the blood to the developing yolk.
The arteries undergo much branching in the surface membranes of the
yolk and the follicle appears highly vascular except for the stigma, a nar-
row band surrounding the yolk that is almost void of blood vessels.
When an ovum is mature, the hormone progesterone, produced by the
ovary, stimulates the hypothalamus to cause the release of the luteinizing
hormone (LH) from the anterior pituitary, which, in turn, causes the ma-
ture follicle to rupture at the location of the stigma releasing the ovum
from the ovary. The yolk is then surrounded only by the vitelline mem-
brane (yolk membrane).
Delayingfirst ovulation. Sexual maturity, as indicated by the first ovula-
tion, may be accelerated or retarded. Restricting feed or decreasing
day lengths during the pullet's growing period are the two main pro-
cedures used (see Cage Management for Raising Replacement Pullets,
Chapter 51, and Managing the Breeding Flock, Chapter 34).
What initiates ovulation? It is not known what sets the hour for the
bird's first ovulation, but both the nervous system and hormonal se-
cretions are of primary importance. The second ovulation is regulated
by oviposition (laying) of the first egg and occurs about 15 to 40 min-
utes after the first egg passes through the vent. Future ovulations oc-
cur at about the same frequency after subsequent eggs are laid.
Eggs laid in clutches. Chickens lay eggs on successive days known as
clutches, after which none are laid for one or more days. The length
of the clutch may vary from 2 days to more than 200 before a day is
missed, but most commercial egg-type chickens can produce more
than 50 eggs in succession without a pause during the early stages of
production in the first lay cycle. The length of clutches is quite consis-
tent with individuals; poor producers have shorter clutches, good pro-
ducers have longer clutches. Once the clutch length is established, the
hen will not ovulate for one or more days and then will produce an-
other clutch. Poor egg producers have a longer rest period between
clutches than do good producers.
Time necessary to produce an egg. The time necessary for an egg to trans-
verse the oviduct varies with individuals. Most hens lay successive
eggs with time intervals of 23 to 26 hours. If the time is greater than

4. 62 FORMAnON OF THE EGG
24 hours, each successive egg will be laid later in the day, and the
ovulation of the yolk for the next egg will also occur later in the day.
Eggs laid in the afternoon have spent several more hours in the ovi-
duct than those laid in the morning. Eventually eggs are laid so late
that the rhythm is broken and an ovulation is skipped.
Time of ovulation. Hens that produce long clutches lay their first egg
of a clutch early in the day, an hour or two after the sun rises or the
artificial lights are turned on. Ovulation of the next yolk comes
quickly after an egg is laid, with only a slight time lag. Those hens
with shorter clutch lengths lay their first egg of the clutch later in the
day, ovulation of the next yolk is slower, and the time lag for laying
is greater. Most ovulations occur during the morning hours, as it is
not natural for ovulations to occur in the mid to late afternoon.
Egg production at start of lay. During the first week of lay, ovulation is
quite irregular; as the hen's hormonal mechanism is not in balance.
Often, only two to four eggs are produced in the first clutch. But by
the second or third week, ovulation is progressing at its peak rate,
only to drop slowly each week throughout the remainder of the laying
cycle.
Light and ovulation. Light, either natural or artificial, has an effect on
the pituitary gland, stimulating it to secrete an increased quantity of
the follicle stimulating hormone (FSH), which in turn, activates the
ovary. Both duration and intensity of light are important. The proce-
dure for correctly lighting a flock of laying hens is complicated and
is discussed in Cage Management for Layers, Chapter 52, and Fundamen-
tals of Managing Light for Poultry, Chapter 10.
Nesting as an indication of ovulation. On most occasions the hen seeks
a nest about 24 hours after ovulation, leading scientists to theorize
that nesting can be used as an indicator of ovulation. Evidently, the
presence of a fully formed egg in the cloaca has nothing to do with
the hen's desire to seek a nest. For example, some hens will ovulate,
but because of a malfunction, or for some other reason, the ovum does
not reach the oviduct, these hens will still seek a nest a day later.
Double ovulation. Normally, only one yolk is ovulated per day, but oc-
casionally two may be released and on rare occasions there may be
three. If two are ovulated at the same time normally only one enters
the oviduct, but if both are picked up simultaneously by the oviduct,
a double yolk egg will result. About two-thirds of the double-yolk
eggs are the result of ovulations within 3 hours of each other. If there
is a great difference in ovulation time, two eggs may be produced on
the same day, but usually the second is soft-shelled.
Double-yolk eggs are more common during the first part of the egg
production period because of an overactive ovary, and are more often
associated with meat-type strains than with egg-type ones. The inci-
dence is an inherited trait since some birds produce higher percent-

5. 5-A. OVARY 63
ages of double-yolk eggs than others. Spring- and summer-housed
pullets also produce a greater number of double-yolk eggs than fall-
and winter-housed pullets.
Defective Eggshells
When the normal interval of about 23 to 26 hours between ovulations
is broken, more eggs are produced with defective shells, including those
with sandpaper texture, white bands, calcium splashing, and chalky white
deposits. The occurrence is greater in meat-type than in egg-type breeds.
From 5 to 7% of the eggs produced have some form of defective shells.
These defects are mostly associated with the age of the flock, with some
strains more prone to the problem than others. Various egg shell defects
are described in Egg Handling and Egg Breakage, Chapter 56.
Yolk Size Affects Egg Size
The size of the completed egg is more closely associated with yolk size
than with any other factor, although variations in albumen secretions in
the oviduct have some influence. The yolk-albumen relationship changes
throughout the laying cycle. Eggs produced at the beginning of the laying
period have yolks that comprise about 25% of the total weight of the egg,
while yolks make up about 30% of egg weight when hens are near the
end of their laying period. In other words, as egg size increases, yolk
weight increases more rapidly than the weight of albumen. In younger
flocks when egg size is small, increasing the level of protein in the diet
may increase the total weight up to 1.5 oz/doz (3.5 g/ea).
Blood Spots and Meat Spots
Often, when the yolk sac ruptures along the stigma, small blood vessels
near the area of the rupture are broken, leaving a clot of blood attached
to the yolk. The frequency of hemorrhages can be related to a number of
factors: genetics, feed, age of the hen, and others. Blood spots are two to
three times more common in brown-shelled than in white-shelled laying
hens.
Any tissue sloughed from the follicular sac or the oviduct can be in-
cluded in the developing egg as it passes through the oviduct. These bits
of tissue darken with age and are known as meat spots. Many blood spots
darken too, and are often incorrectly classified as meat spots. This problem
is especially prevalent in brown-shelled eggs where 15% or more of the

6. 64 FORMATlON OF THE EGG
eggs can be affected, compared to less than 1% in white shell eggs (Carey,
1988).
5-B. PARTS OF THE OVIDUCT
The oviduct is a long tube through which the yolk passes and where
the remaining portions of the egg are secreted. Normally, the oviduct is
relatively small in diameter, but with the approach of the first ovulation
its size and wall thickness expand greatly. The segments of the oviduct
and their purpose are summarized below and are illustrated in Figure 5-1.
A
B OVIDUCT
1 Infundibulum
2 Magnum
3 Isthmus
4 Uterus
5 Vagina
6 Cloaca
7 Vent
A OVARY
1 Mature yolk within
yolk sac or follicle
2 Immature yolk
3 Empty fOllicle
4 Stigma or suture line
Figure 5-1. Ovary and Oviduct

7. 5-8. PARTS OF THE OVIDUCT 65
1. Infundibulum
The funnel-shaped upper portion of the oviduct is the infundibulum.
When functional, its length is approximately 3.5 inches (9 cm). Normally
inactive except immediately after ovulation, its purpose is to search out
and engulf the yolk causing it to enter the oviduct. After ovulation, the
yolk drops into the ovarian pocket or the body cavity, from which it is
picked up by the infundibulum. The yolk remains in this section for only
a short period of about 15 minutes, then is forced along the oviduct by
multiple contractions.
Malfunction ofthe infundibulum. To be completely functional, the infun-
dibulum should pick up all the yolks dropped into the body cavity.
However, it has been found that an average of 4% are not drawn into
the infundibulum, but remain in the body cavity where they are reab-
sorbed within a day. The percentage varies with strains of chickens,
some of which retain up to 10% of their yolks in the body cavity. Meat-
type birds are more often affected than egg-type strains.
Internal layers. Sometimes the infundibulum loses its ability to pick up
a high proportion of the yolks, and they accumulate in the body cavity
faster than they can be reabsorbed. Such hens are known as "internal
layers," although the term does not define the condition well. The
abdomen in such layers becomes distended, and the hen stands in an
upright position.
2. Magnum
The magnum is the albumen-secreting portion of the oviduct, and is
about 13 inches (33 cm) long in the average laying hen. It takes approxi-
mately 2 to 3 hours for the developing egg to pass through the magnum.
Albumen. The albumen in an egg is composed of four layers (see Shell
Eggs and Their Nutritional Value, Chapter 57). The names and percent-
ages are:
Chalazae 2.7%
Liquid inner white 16.8%
Dense white 57.3%
Outer thin white 23.2%
While all four are produced in the magnum, the outer thin white is
not completed until water is added in the uterus.
Chalazae. Upon breaking an egg, one notices two twisted cords, known
as chalazae, extending from opposite poles of the yolk through the
albumen. The chalaziferous albumen is produced when the yolk first
enters the magnum, but the twisting to form the two chalazae seems

8. 66 FORMAnON OF THE EGG
to occur much later as the egg rotates in the lower end of the oviduct.
Twisted in opposite directions, the chalazae tend to keep the yolk cen-
tered in the egg after it is laid.
Liquid inner white. As the developing egg passes through the magnum,
only one type of albumen is produced, but the addition of water plus
the rotation of the developing egg gives rise to the various layers, one
of which is the liquid inner white.
Dense white. The dense white makes up the largest portion of the egg
albumen. It contains mucin that tends to hold it together. The amount
of thick white generated in the magnum is large, but the breakdown
of mucin and the addition of water as the egg moves through the
oviduct tend to reduce the amount of thick white while increasing the
amount of thin white. At the time the egg is laid, it has about one-
third of its original content of thick white, but what remains still com-
prises over half the albumen in the egg.
Egg quality deterioration. After laying, there is a constant change in the
internal contents of the egg. The thick white gradually loses its viscous
composition and its volume decreases, while the thin white becomes
more watery, and the amount increases. These conditions are affected
by holding temperature, relative humidity, time and certain diseases.
The increasing amount of thin white is one of the best indicators of
the age (freshness) of the egg.
3. Isthmus
Next, the developing egg is forced into the isthmus, a relatively short
section approximately 4 inches (10 cm) in length, where it remains for
about 75 minutes. Here the inner and outer shell membranes are formed
in such a manner as to represent the final shape of the egg. The contents
at this time do not completely fill the shell membranes, and the egg resem-
bles a sack only partially filled.
The shell membranes are a papery material composed of protein fibers.
The inner membrane is laid down first, followed by the outer membrane,
which is about three times as thick as the inner membrane. The two mem-
branes are held closely together until the egg is laid; then at the large end
of the egg, the two membranes separate to form the air cell. In a small
percentage of the eggs, the air cell will form in the small end or on the
side.
Air cell is important. When the egg is first laid there is no air cell. How-
ever, it soon appears and increases in diameter to about 0.7 inches
(1.8 cm). As the egg ages, moisture within the egg evaporates through
the shell pores and the air cell increases in diameter and depth. The
size of the air cell can be affected by various storage conditions. High
surrounding temperature and / or low humidity increase the size of

9. 5-8. PARTS OF THE OVIDUCT 67
the air cell. The size of the air cell, as determined by candling, is used
in grading programs to judge the age of the egg. Larger air cells are
indicators of poorer interior quality.
Shell membranes act as a barrier. The shell membranes act as a barrier
to the penetration of organisms such as bacteria. Eggs laid by young
hens have thicker shell membranes than eggs laid by older hens.
4. Uterus (Shell Gland)
The uterus is from 4.0 to 4.7 inches (10 to 12 cm) long in the laying hen.
The developing egg normally remains in the uterus from 18 to 20 hours,
much longer than in any other section of the oviduct.
Outer thin white deposited after shell membranes. When the egg first enters
the uterus, water and salts are added through the shell membranes
by the process of osmosis to plump out the loosely adhering shell
membranes and to liquefy some of the thin albumen to form the fourth
layer, the outer thin white.
The shell. Eggshell calcification begins just before the egg enters the
uterus. Small clusters of calcium appear on the outer shell membrane
just before the egg leaves the isthmus. These are the initiation sites
for calcium deposition in the uterus. Their number is probably inher-
ited and plays a part in the amount of calcium deposition later. They
disappear a short time after the egg enters the shell gland.
The first shell is deposited over the initiation sites to form the inner
shell, a layer composed of calcite crystals, a sponge-like material. This
layer is followed by the addition of the outer shell which is made up
of a layer of hard calcite crystals that are chalky and about twice as
thick as the inner shell surface. The longer the calcite columns, the
stronger the shell. The completed eggshell is composed almost en-
tirely of calcium carbonate (CaC03), with small amounts of sodium,
potassium, and magnesium.
Source ofcalcium for eggshell. There are only two sources of calcium for
eggshell production: the feed and certain bones which act as storage
sites in the body. Normally, most of the calcium for egg formation
comes directly from the feed, with some being derived from the med-
ullary bone which serves as the calcium reservoir. The reservoir is
particularly important at night when the bird is not eating and egg-
shell is being deposited.
Formation of calcium carbonate. Calcium carbonate is formed when cal-
cium ions from the blood and carbonate ions from both the blood and
the shell gland combine in the shell gland. Anything that reduces the
supply of either of these ions interferes with CaC03 formation and
eggshell development, many times resulting in poor shell quality. It

10. 68 FORMA nON OF THE EGG
is thought that high environmental temperatures may also contribute
to this problem inasmuch as eggshells are thinner during hot weather.
Poor shell quality. Many factors may cause a deterioration in eggshell
quality, and their influence mayor may not be due to an inadequate
supply of calcium or carbonate ions. Shell quality is generally defined
as the shell's ability to withstand shock, its overall appearance, and
smoothness. Shell strength can be measured by several different tech-
niques, including resistance to breaking, specific gravity, shell defor-
mation, and shell thickness (see Shell Egg Quality and Preservation,
Chapter 60). Several factors lower eggshell quality; for example:
1. Quality is reduced as the bird ages and continues to lay,
as the hen cannot produce as efficiently an adequate
quantity of calcium carbonate to cover the larger eggs
produced during the latter part of the laying cycle.
2. Increased environmental temperatures.
3. Eggs laid in the morning have poorer shell quality than
those laid in the afternoon.
4. Stress experienced by birds in the flock.
5. Practically all misshapen eggs and eggs with body
checks are laid between 6:00 and 8:00 a.m.
6. Certain poultry diseases (Infectious Bronchitis, Newcas-
tle disease).
7. Certain drugs.
Calcium requirements are high during production. The demand of the lay-
ing hen for calcium is extremely high. A 4-lb (1.8-kg) hen producing
250 2-oz (56.7 g) eggs per year requires about 1.25 lb (0.56 kg) of cal-
cium. Since this is about 25 times the amount of calcium in the bird's
skeleton, it is evident that the dietary need for calcium is great. Most
laying rations contain from 3 to 4% calcium to meet the requirements
and to allow for the inefficiencies of absorption.
Pores in the eggshell. Both the inner and outer shell layers contain small
openings called pores. There may be as many as 8,000 per egg.
Through these pores, air passes into the egg to supply oxygen to the
developing embryo. Also, carbon dioxide and moisture is removed
from the egg by passing through these same pores. In the freshly laid
egg, the pores are almost completely closed, but as the egg ages or is
washed, the number of open pores is greatly increased.
Color of eggshell. Eggshells are predominantly white or various shades
of brown. However, a South American breed, the Araucana, produces
eggs with green or blue shells. Pigments produced in the uterus at
the time the shell is produced are responsible for the color. The shade
of coloring is quite consistent for each bird, with the color intensity

11. 5-8. PARTS OF THE OVIDUCT 69
being a derivative of the genetic makeup of the individual. Some
strains of birds lay eggs with very dark brown shells, while others
may vary all the way to pure white. The brown pigment in eggshells
is porphyrin, uniformly distributed throughout the entire shell.
The cuticle. The cuticle is laid down on the outside of the shell in the
uterus and represents the last of the concentric layers of egg forma-
tion. The cuticle is composed primarily of organic material. Con-
taining a high percentage of water, it acts as a lubricant during the
laying process. But once the egg is laid the cuticle material soon dries,
sealing many of the pores of the eggshell to help prevent too rapid
an exchange of air and moisture and to aid in preventing bacteria
from entering the egg. Various shell cleaning processes (washing and
sanding) will reduce the effectiveness of the cuticle. To counteract this,
egg processors commonly apply a coating of mineral oil to the shell's
surface during processing. The mineral oil helps to slow down the
loss of moisture and maintain interior egg quality (see Processing and
Packaging Shell Eggs, Chapter 58).
5. Vagina
The final section of the oviduct is the vagina, which is about 4.7 inches
(12 cm) in length in a bird during egg production. Normally, the egg is
held in the vagina for only a few minutes, but in some instances may be
held there for several hours. The vagina has no role in egg formation and
only serves to expel the egg once it leaves the shell gland.
Eggs are laid large end first. Although the egg transverses the oviduct
small end first, if the hen is not molested or frightened, the egg will
rotate horizontally just prior to oviposition and will be expelled large
end first. The rotation requires less than 2 minutes, and makes it possi-
ble for the uterine muscles to exert greater pressure on more surface
area during oviposition. However, if something disturbs the bird prior
to rotation, the egg will be laid quickly and forced through the vent
small end first.
See Shell Eggs and Their Nutritional Value, Chapter 57, for more information
on the composition and characteristics of the chicken egg.