The female reproductive system in the domestic fowl consists of the ovary and the accompanying oviduct. While the female embryo chicken has two sets of reproductive organs, only one of these, the left survives and reaches maturity to produce eggs. The single surviving ovary is located in the laying hen just in front of the kidneys in the abdominal cavity and is firmly attached to the wall of the cavity. The ovary is well endowed with blood vessels to ensure there is no hindrance to the transport of nutrients to the developing yolk.
Ovary
The ovary consists of a mass of yellowish, rounded objects called follicles, each containing an ovum or yolk. There are many such follicles but only a small number in comparison will ever reach maturity to produce an egg. When the hen is in lay the ovary will be active. The size of the follicles will vary from very small to those approaching the normal yolk size in the egg - up to 40 millimetres in diameter and containing a fully matured yolk ready for release into the oviduct.
It is possible to find five stages of development in the active ovary:
- Primary follicles - follicles that have not yet commenced to grow
- Growing follicles
- Mature follicles - follicles ready or nearly so for release
- Discharged follicles - where the yolk has just been released
- Atretic follicles - those from which the yolk has been released some time ago
Yolk
It takes approximately 10 days for a yolk to develop from the very small to the normal size found in eggs and during this time it is contained in the follicle. The follicle acts as a sack during this period of development supplying it with the nutrients required for its growth. When a mature follicle is examined an elongated area virtually free of blood vessels will be found on the distal surface of it. This area, called the stigma, is where the follicle normally splits to release the yolk into the oviduct. If, for some reason, the follicle splits at other than the stigma, the numerous blood vessels that rupture will result in free blood being found in the egg i.e. a blood spot will form.
Oviduct
The function of the oviduct is to produce the albumen, shell membranes and the shell around the yolk to complete the egg. It is a long tube well supplied with blood via numerous blood vessels. There are many glands found in its walls that produce the albumen, the shell membranes and the shell. In the non-layer the oviduct is quite short and small in diameter. However, once the reproductive system becomes active, it grows to a length of 70-80 centimetres with a variable diameter depending on the function of the section being examined.
The oviduct consists of five distinct parts or sections, each having different functions:
- Infundibulum (or funnel) - located adjacent to the ovary and with long segments enclosing the ovary, the infundibulum collects the yolk after it’s release from the follicle as a funnel and directs it into the oviduct. This section has very thin walls and is 6-9 centimetres long. Fertilisation of the ovum by the male sperm occurs here.
- Ampulla or magnum - at approximately 40 centimetres long it secretes more than 40% of the albumen.
- Isthmus - at about 12 centimetres in length, it secretes some albumen and the shell membranes.
- Uterus or shell gland - at approximately 12 centimetres in length it secretes about 40% of the albumen and the egg’s shell.
- Vagina - at approximately 12 centimetres in length, it secretes the egg’s outer cuticle and possibly the shell pigment.
