If a sexually active rooster is placed into a flock of hens, fertile eggs can be produced by the second day after introducing the rooster. If mating occurs within a short period, the next egg yolk released by the hen's body can be fertilized.
The reproductive system or genital system is a system of sex organs within an organism which work together for the purpose of reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system.[dead link] Unlike most organ systems, the sexes of differentiated species often have significant differences. These differences allow for a combination of genetic material between two individuals, which allows for the possibility of greater genetic fitness of the offspring.