When a rooster and hen mate, a fertilized egg is produced. The chicken lays the egg and, if exposed to the right conditions, it continues to develop for 21 days after which a chick hatches.
Hens are born with as many as 4,000 small ova in their ovaries, each of which can develop into a yolk. A rooster mounts a hen to bring their sex organs, both called cloaca, together, and the rooster’s sperm is transferred to the hen where it combines with the ova, resulting in fertilization. When the egg is laid it needs warmth and humidity for a chick to grow; this can be accomplished by the hen sitting on the egg or by artificial incubation. One mating delivers enough sperm to produce as many as five eggs.