Tue. Sep 27th, 2022
Reproductive system

Eggs contain all the nutrients essential for growth of a zygote to a fully developed embryo (chick) inside egg. It is packed with all the nutrients essential in human diet except Vitamin C.

The yolk is synthesised in the liver of the female parent in soluble form. Through circulation it is transported to the follicle cells that surround the maturing ovum

The yolk of the egg is formed in the ovary of hen and rest of the component is formed in the oviduct – the egg factory. The ovary is a cluster of many follicles, each of which is independently attached by a slender stalk. Each sphere is a more or less developed ovum or yolk, enclosed in a thin membrane of follicle. The mature yolk is about 40 mm in diameter.

In a laying hen the oviduct appears as a large, much coiled tube occupying a large part of the left side of the abdominal cavity. The oviduct is in continuous movement during the time when an egg is formed.

An oviduct is a tube like organ around 60 to 86 cm in length divided into five areas, which perform the function of formation of different parts of egg.

The oviduct is divided in five clearly defined regions

  1. The funnel or infundibulum nearest to ovary.
  2. The magnum, where egg white is secreted.
  3. The isthmus, which secretes shell membranes.
  4. The uterus or shell gland.
  5. The vagina, which leads to cloaca.

The yolk is formed in the ovary and rest of the process occurs in the oviduct. The ovum or the yolk when reaches full diameter of 40 mm is shed from the ovary and is picked up by infundibulum. The albumen secreted from the magnum is homogenous and gel like in structure due to its mucin content. The chalaziferous layers thin white, middle thick white and the outer thin white layers emerge out of this homogenous gel of albumen. Fertilization takes place immediately after ovulation. The spermatozoa make their way through the entire length of oviduct after mating has occurred by a cock. When the egg is laid, the small end of egg comes first as it moves down the oviduct.

Oviduct of Chicken with parts labelled
Parts of OviductLength of PartTime Spent ThereFunction of Part
Infundibulum2 inches15 minPicks up yolk, egg fertilized
Magnum13 inches3 hours40-50% of white laid down-thick albumen
Isthmus4 inches1.25 hours10% albumen shell membrane laid down, shape of egg determined
(shell gland)
4.2 inches20.75 hours40% of albumen, shell formed, pigment of cuticle laid down
Vagina 4 inchesEgg passes through as it is laid


The ovary is a cluster of developing ova, and is located midway between the neck and the tail of the bird and attached at the back. The ovary is fully formed when a pullet chick hatches but is very small until the chick reaches sexual maturity. At hatch, a pullet chick has tens of thousands of ova, or potential eggs that theoretically could be laid, although most never develop to the point of ovulation. The maximum number of eggs a hen can lay is determined when it hatches because no new ova form after the chick hatches. Mature follicles break in the thin region called stigma and yolk is released.

Stigma on mature follicles

The ovary is attached to the abdominal cavity wall by the meso-ovarian ligament. It carries anything from 2,000 to 12,000 small ova in miniature follicles on its surface, plus hormone producing cells in its body. Not all of the ova found on the immature ovary develop and only approximately 200 to 350 reach maturity under normal modern commercial practice. Each yolk or ova takes about 10 days to grow and reach maturity when it is approximately 31% of the weight of the egg

It is possible to find five stages of development in the active ovary:

  1. Primary follicles – follicles that have not yet commenced to grow
  2. Growing follicles
  3. Mature follicles – follicles ready or nearly so for release
  4. Discharged follicles – where the yolk has just been released
  5. Atretic follicles – those from which the yolk has been released some time ago

The yolk is laid down in concentric rings of darker and lighter coloured material, the colour being produced by xanthophylls that are yellow/orange/red pigments occurring in many plants, plant products and other naturally occurring materials. The bulk of the yolk material provides a source of food for the developing embryo that originates by the fertilising of the germ disc or blastoderm usually located on the upper surface of the yolk of the broken out egg. It lies in the surface segment of the latebra which is a vase-shaped segment of different yolk with its base in the centre of the yolk, the lips on the surface and the stem joining the base to the lips.

Yolk development in the maturing pullet is initiated by follicle stimulating hormone (FSH) produced by the anterior lobe of the pituitary gland. The compounds in the yolk material are formed in the liver and, on the appropriate signal, are transported by the blood stream to the target follicle and into the yolk. The appropriate signal for this development comes from the hormones oestrogen, progesterone and testosterone which are produced by the ovary after receiving the signal of the FSH. These ovarian hormones also provide the stimulus for the formation of the development of the oviduct.


The release of the yolk (the process of ovulation), is the major controlling factor influencing the subsequent steps in the formation and laying of the egg. As a consequence, factors that influence ovulation are of critical importance to the various aspects associated with egg production. The presence of a mature yolk in a follicle causes hormones from the ovary to stimulate the release of luteinising hormone (LH) by the pituitary gland. The presence of LH in the blood stream causes the follicle that contains the mature yolk to split along the stigma thus releasing it into the oviduct abdominal cavity adjacent to the oviduct.


When ovulation occurs, the ovum (yolk) enters the oviduct. The oviduct is a twisted tube that is 25 to 27 inches long when fully developed and is divided into five major sections. These sections are the infundibulum, magnum, isthmus, shell gland, and vagina.


This segment is funnel-shaped and lies adjacent to the ovary. It is up to 9 centimetres long in the laying hen and has the function of searching for and engulfing the yolk that has just been released from the follicle into the adjacent ovarian pocket or body cavity. The yolk remains in the infundibulum for about 15 minutes and it is here that fertilisation takes place.

If the infundibulum malfunction and does not engulf the yolk, the yolk will remain in the ovarian pocket from where normally they will be absorbed within three days. If the number of such occurrences reaches a high level, the yolks will accumulate in the ovarian pocket faster than they can be absorbed. Such birds’ are called internal layers as the abdomen becomes distended and the hens adopt a very upright stance.

  • The first part of the oviduct
  • It is 3 to 4 inches long
  • Engulfs the ovum released from the ovary
  • The muscular infundibulum moves to surround yolk
  • The yolk remains in the infundibulum for 15 to 17 minutes
  • Fertilization takes place in the infundibulum.

Magnum Or Ampulla

The magnum is the longest segment at up to 40 centimetres long. Its function is to add approximately 40% of the albumen to the developing egg that takes about three hours to move through. These percentages vary considerably depending on quite a few factors including the genetics of the hen, age of the bird, the egg’s age and/or storage conditions. However, in a good quality, freshly laid egg the above relationship mostly applies.

The chalazae are two twisted chords of albumen extending from the opposite sides of the yolk into the remaining albumen in the broken out egg. These two cords extend into the ends of the egg along the longitudinal axis and are parts of a very thin envelope of special albumen that surrounds the yolk and holds it in its position. The yolk has to remain centrally located for the survival of the embryo. The yolk turning or rotating as it passes along the oviduct causes the twisted effect of the chalazae.

While the bird produces only dense albumen, as the egg moves along the oviduct, water is added thus making liquid albumen. The rotation of the developing egg causes the albumen to separate into the inner liquid and the dense layers. The outer liquid layer is caused by the addition of more water when in the uterus. The dense layer contains significant amounts of mucin that binds it together in a jelly like form. As an egg stales, the amount of dense albumen decreases as it changes to the liquid form. The liquid form increases in volume and becomes even more fluid.

  • Second section of the oviduct
  • 13 inches long
  • It is the largest section of the oviduct
  • Magnum is a latin word means “large”)
  • The yolk remains here for 3 hours
  • The thick albumen (egg white) forms here


The isthmus is approximately 12 centimetres long and has the functions of adding approximately 20% of the albumen and the shell membranes to the egg. There are two shell membranes:

  1. The inner shell membrane – laid down first
  2. The outer shell membrane – laid down last and about three times the thickness of the inner membrane

The isthmus takes approximately 75 minutes to carry out its tasks. While the egg is still in the oviduct the shell membranes appear as one over the total surface of the egg, so close, they are associated with each other. However, as the egg cools after it has been laid, the membranes separate, usually at the larger end to form the air cell. The air cell in the new laid egg is approximately 1.5 centimetres in diameter and approximately 0.5 centimetres deep.

As the egg ages, the interior contents lose water and the air cell increases in size. This change in size is an indicator of egg quality as related to the age of the egg and the holding conditions. The shell membranes consist of a fibrous protein material and act as a barrier to bacteria and fungi penetration into the egg. They also help reduce the rate of evaporation of water from the egg thus slowing the rate of deterioration of the egg. The isthmus also lays down the foundation for the shell by forming the first crystals of calcium carbonate on the outer shell membrane

  • Third section of the oviduct
  • 4 inches long
  • The isthmus is slightly constricted (the term isthmus referring to a narrow strip of land joining two larger tracts of land)
  • The inner and outer shell membranes forms here
  • The developing egg remains here for 75 minutes.

Uterus (Shell Gland)

The uterus is a relatively short, bulbous gland up to 12 centimetres in length. The developing egg remains in the uterus for 18-20 hours while approximately 40% of the albumen and all of the shell is added. It is for this reason that the organ is often called the shell gland. Shell formation really begins by the deposition of small clusters of calcium carbonate crystals onto the outer shell membrane while in the isthmus. These are the initiation grains for the subsequent calcium carbonate deposition in the uterus. The number of these grains is genetically controlled and is related to the subsequent shell thickness as the more grains deposited in the isthmus, the thicker will be the final shell.

  • The fourth section of the oviduct
  • 4 to 5 inches long
  • The shell formation takes place.
  • The hen’s body mobilizes 8 to 10 percent of body calcium from its bones to make the egg’s shell. Bone calcium provides 47 percent of the calcium required to make a shell, and the hen’s diet provides the remainder
  • Pigment deposition occurs in the shell gland
  • The egg remains here for 20 or more hours.


The last part of the oviduct is the vagina, which is about 4 to 5 inches long. The vagina does not really play a part in egg formation but is important in the laying of the egg. The vagina is made of muscle that helps push the egg out of the hen’s body. The bloom, or cuticle, forms on the egg in the vagina prior to oviposition (the laying of the fully formed egg). The egg travels through the oviduct small end first but turns in the vagina and comes out large end first.

Near the junction of the shell gland and the vagina deep glands known as sperm host glands present. These glands store sperm for long periods of time, typically 10 days to 2 weeks. (One of the unique things about birds is that the sperm remain viable at body temperature.) When a hen lays an egg, sperm can be squeezed out of these glands into the oviduct and then can migrate to the infundibulum to fertilize an ovum.

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