P.O. Box 381
Ooltewah, TN 37363
Each female chick starts life carrying the beginnings of thousands of undeveloped yolks inside her body, not all of which develope into eggs. A pullet (female chicken at birth) also starts life with two ovaries. As the pullet grows, the right ovary becomes undeveloped and only the left one becomes fully functionable. The functioning ovary contains all the undeveloped yolks the pullet was born with. In a pullet or nonlayer, all the yolks are small because none have developed in preparation for lay. However, in a hen (female chicken that has been in laying for at least 30 weeks) the yolks will range in size from the head of a pin to nearly full size. This cluster of yolks can be found along the backbone of the bird about halfway between her neck and tail.
When a pullet reaches laying age, one by one the yolks mature. The laying age of a pullet will range from 4 to 6 months depending on the breed. At any given time, the pullets body will contain yolks at various stages of developement. Approximately every 25 hours, a yolk is mature enough to be released into a the funnel of the oviduct. This process is called ovulation. Ovulation usually occurs about an hour after the previous egg was laid. Throughout its passage through the oviduct, the egg leads with its pointed end. While the egg progresses through the oviduct, it is constantly rotating within the spiral tube. This movement twists the structural fibers called the chalazae which form the rope-like strands that anchor the yolk in the thick egg white. Just before it is laid, the egg rotates so that the blunt end comes out first. The whole process takes about 25 hours. This causes a hen to lay about an hour later each day. A hens reproductive system slows down at night, thus she will eventually skip a day of lay altogether. It is then that a whole new multiple day laying cycle will begin the following morning. Below are two photos. One shows the anatomy as it is inside the hen and the other shows the anatomy laid out in real life form.
The egg is fertilized while in journey through the oviduct and just before reaching the shell gland or uterus. The sperm of the rooster (male chicken) is encased in various layers of egg white and wrapped in protective membranes. Once it reaches the shell gland, it is sealed within the shell. It is then enveloped in a fast drying fluid coating called the bloom or cuticle. In the photo below, this is referred to as the germinal disc. When a rooster mates with a hen, the sperm travels quickly up the oviduct to fertilize a developing yolk. If the hen laid an egg shortly before, the mating will likely fertilize her next egg. The number of additional eggs that will be fertilized by one mating varies with the hens productivity and breed. Highly productive hens generally remain fertile longer than hens that lay at a slower rate. Single combed breeds remain fertile longer than rose combed breeds. The average duration of fertility is about 10 days. If a certain rooster is desired for mating, it is advisable to wait about 3 weeks after placing the desired cock with the hen to collect eggs for incubation. An egg must be fertilized by a rooster to hatch a chick.