Egg formation in the ovary: Eggs are formed in the reproduction tracts of the mature female bird. The tract consist of the ovary and oviduct. The yolk is formed in the ovary, while the albumen, shell  membranes and shell are formed in the oviduct.

The ovary is located under the back done. The ovary consists of follicles (yolk sacs) which envelope the ova. A hen can produce up to 400 ova before molting but only very few of these (about 200 – 600) grow to maturity and are ovulated.

The production of ova is usually stimulated by an hormone called follicle stimulating hormone (FSH). Light from the sun or artificial light striking the hen’s eyes stimulates the anterior pituitary gland to release FSH into the blood stream. The FSH moves via the blood stream follicles. A follicle containing an ovum is stimulated to grow at a time. The following day another ovum is stimulated to commence growth. It takes 10 – 11 days for an ovum to grow into maturity before ovulation. Therefore during ovulation, there are usually about ten ova at different stages of development.

The growth of an ovum is usually assisted by some hormones. When FSH reaches the ovary, the ovary is stimulated to produce hormones like estrogen, progesterone and testosterone. The estrogen stimulates the formulation of yolk protein and fat by the liver. These nutrients are eventually transported by the blood to the developing ovum. The estrogen additionally assists in the development of the medullary bone as well as increasing the size of the oviduct. The colouring pigment, mainly xanthophylls comes directly from the digested feed and is carried similarly via the blood to the developing ovum to colour the yolk.


Ovulation is usually induced by an hormone after the ovum has matured. The hormone, progesterone produced in the ovary excites the hypothalamus to release luitenizing hormone (LH) from the anterior pituitary gland. LH stimulates the mature follicle to rupture. The repture occurs along a whitish avascularised suture line on the follicle called stigma. Sequel to the rupture, the yolk is released by the ovary into an ovarian pocket. Form here the yolk is picked up by the infundibulum. This process of releasing mature ovum by the ovary is called ovulation. In the process of ovulation, if any blood vessel accidentally rupure in the follicle, the egg will have blood spot, which shows up in the albumen. If part of the follicle chips off and accompanies the ovulated yolk, there will be meat clot in the albumen. After ovulation, the ruptured follicle is absorbed by the body.

The second ovulation, which occurs the following day usually takes place 15 – 40 minutes after an egg is laid. Mature yolk ready for ovulation is orange red in colour. Other immature follicle vary in size and colour from pale white to orange.

Sometimes there can be double ovulation where two ova are released simultaneously. In this case, only one ovum is often picked up by the infundibulum. Where both are picked, it results in a double yolk egg. Double ovulation is usually due to an overactive ovary. Incidence of double yolk egg is largely genetic because not all birds lay double yolk eggs. The size of yolk often determines the size of eggs. Large sized eggs usually have big yolk. Ovulation most commonly takes place early in the morning and rarely in the afternoon. This is why most eggs are laid in the morning hours before noon.

Eggs Formulation in the Oviduct

The oviduct consists of the following parts:

  1. Infundibulum or funnel
  2. Magnum
  3. Isthmus
  4. Uterus or shell gland
  5. Vagina and
  6. Cloaca.

The summary of the egg formation process in the oviduct is shown in table form below;

Part of OviductAverage LengthTime SpentActivity
Infundibulum9cm1/4HrEgg fertilization takes place.



Magnum33ck3HrsAlbumen (thick white) is secreted and added to the ovum


Isthmus10cm11/4 Hrs(a)    Water and some salts are added.

(b)   Inner and outer shell membranes are formed

Uterus10 – 12 cm18 – 22 Hrs(a)    Water is added osmotically to outer thin white

(b)   Shell formation and calcification takes place

(c)    Cuticle is deposited on ready-to-be-laid shell

Vagina10cmEgg passes through
Cloaca1 – 2 minutesEgg rotates horizontally turning the large end out towards the Cloaca for oviposition.



This is the funnel shaped upper protion of the oviduct, which engulfs ovulated ova. The infundibulum usually remains inactive without ovulation. It has glands called sperm nests for storing sperm cells awaiting the ova. Such spermatozoa can remain viable in the nest for two to three weeks. The ovum remains in the infundibulum for 15 minutes during which fertilization takes place if sperm cell is available. Malfunctioning of the infundibulum may make it to pick the ova irregularly e.g. skipping days in picking the ova. This anomaly makes the affected bird to lay irregularly. Such unpicked ova are usually re-absorbed into the body cavity within 24 hours.


This is the largest protion of the oviduct, hence the nomenclature, magnum, meaning ‘great’. It is about 33cm long. The yolk moves from the infundibulum to the magnum by peristaltic action caused by coordinated expansion and contraction of the magnum muscles. Egg stays in the magnum for about three hours. The thick white of the albumen is primarily secreted in the magnum by albuminous glands. This thick white then separates into the four layers of albumen;

  1. The inner thick white plus the chalazae;
  2. The inner thin white which becomes thin as a result of osmotically absorbed water;
  3. The outer thick white or firm and
  4. The outer thin white

The formation of the outer thin white is incomplete in the magnum. It gets completed in the uterus later when water is osmotically added to it through the shell membrane.


This is a narrow protion of the oviduct; hence it is called isthmus. The dictionary meaning of isthmus is a narrow piece of land with water on both sides joing two larger bodies of land. The isthmus is about 10 cm long. Egg remains for one hour, fiftenen minuties. Very little quantity of water as well as some salts are secreted and added to the albumen here. The inner and outer shell membranes are also formed to enclose the albumen.

Uterus or Shell Gland:

The primary function of the uterus in egg formation is deposition of shell. A few other activities however occur in the uterus. Immediately the egg reaches the uterus, water and salts secreted in the uterus are added. These enter the shell membrane by osmosis. The water helps to complete the formation of the last albumen layer (the outer thin white) and also makes the egg in the shell membrane plump. Due to insufficient fluid in the isthmus, the shell membranes formed are loosely held. Deposition of calcium on the membranes then follows.

There seems to be few crystals of calcium deposited on the shell membrane in the isthmus. These deposits serve as initiation sites for calcium deposition in the uterus. The shell is composed mainly of calcium. Some of the calcium is derived from feed while some come from the medullary bone, a type of calcium reservoir. The calcium used in shell formation is usually first deposited in the long bones such that less than 50% of the shell calcium comes directly from the feed.

Egg remains in the uterus of chickens for 18 – 22 hours. During the last 5 hours in the uterus, the brown pigment of the shell, porphyrin, from the uterus is deposited on the shell. The intensity of shell pigmentation is usually influenced by genetics, nutrition as well as environmental conditions of the hen e.g. heat and other environmental stresses. Genetically, some chickens lay dark brow-shelled eggs always while others lay light ones. Good nutrition usually provides more pigments for dark brown shells. Stresses generally make the colour light depending on the intensity of the stress.

Shortly before the egg leaves the uterus, cuticle is deposited on it. This fluid, among other functions, assists as lubricant during the oviposition process.


Vagina plays no part in egg formation. It only serves as a passage for egg. It is about 10cm long. There is a sphincter (valve) between uterus and vagina where utero-vaginal glands are located. These glands help to store sperm cells temporarily during mating. Eggs do not stay in the vagina.


The cloaca is the small space with elastic muscles through which an egg is laid. It is a kind of junction outlet for the urinogenital system of the fowl. The ureters and left oviduct open into the cloaca. (The chicken does not have the right ovary and oviduct. Both left and right ones are usually present during the early embryonic development but only the left ones develop. The right ones normally atrophy except in a few cases that have been reported).

Egg does not normally remain any long in the cloaca before oviposition. The egg, which has been moving in the oviduct with the small end in front rotates horizontally within 1 – 2 minutes in the cloaca turning the large end forward for oviposition. If the hen is disturbed in any way before the rotation, she may hastily lay the egg with the small end first. The egg rotation is necessary because the muscular pressure needed to expel the egg is safer applied to the stonger end which is the small end. The egg is normally laid at the hen’s body temperature. At oviposition, the body temperature of the hen usually reaches its peak of 41.9°C.

Egg Defects and their Causes

(1) Blood Spot in Egg: This is caused by accidental rupure of one or more blood vessels in the follicle during ovulation

(2) Meat Spot or Meat Clot in Egg: this can be due to any of the following condition:

  • Presence of blood spot that has changed colour
  • Chipping off of tissue of the follicle during ovulation
  • Chipping off of the wall of the oviduct in the process of egg formation.

(3) Double Yolk Egg: This may be due to either of the following conditions:

  • Double evulation and the infundibulum picks both ova together,
  • Double ovulation, but infundibulum picks one ovum while the second ovum joins the next day’s ovum and both are picked together,
  • Normal ovulation but ovum is retained in ovarian pocket only to join the following day’s ovum and both are picked together by the infundibulum.

(4) Yolkless Egg: this condition is caused by the presence of foreign body in the oviduct e.g. chippedtissue from the ovary or oviduct. The tissue simulates yolk such that the oviduct is fooled to take it for a small yolk and normal egg formation processes continue on it. Since such foreign body are usually small in size, the volume of egg white secreted is correspondingly small thereby resulting in unusually small sized yolkless eggs.
(5) Thin or Soft-Shelled Egg: This situation occurs if egg spends insufficient time in the uterus for normal calcification to complete.
(6) Shelless Egg: Any of the following can cause this happen:

  • Egg spending no time at all in the uterus
  • Deficiency of calcium dietarily for shell formation
  • Poor calcium metabolism by the hen due to any of the following conditions:
    • Disease, like Newcastle or Chronic respiratory disease
    • Intense heat in the poultry house
    • Heredity

(7) Glassy, Chalky or Rough-Shelled Egg: This is usually caused by malfunctioning of the uterus.
(8) Mal-shaped Egg:  This often occurs if the hen is subjected to some stresses e.g. handling stress while transporting birds, and other disturbances.
(9) Egg Within Egg: Reversal in the movement of egg within the oviduct causes this condition. After shell formation in the uterus, instead of moving to the cloaca for ovipositon, the egg abnormally reverses back to the magnum. Taking it for a yolk at the magnum, albumen is deposited on ot and it moves to the isthmus to receive shell membranes and thereafter to the uterus for re-calcification before it is finally laid as an “egg within egg”. Such egg normally has only one yolk in the inner egg.
(10) Off-coloureed Yolk: This is usually caused by some undesirable substances in the feed or by a disease.
(11) Off-flavoured Egg: The condition for off-coloured yolk can cause off –flavour in egg. Too muchfish in the diet for example can produce fishy flavor in the egg.
(12) Extremely Small Egg: Such egg have yolk and still be very small, through in must cases, extremely small eggs are yolkless, the cause of which is explained above. The cause of extremely small egg can be genetic or due to environmental or disease stress
(13) Misplaced or Loose Air-cell: This may be due to vigorous shaking of the egg, accidental misplacement during egg formation, genetics or disease.

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