A spermatozoon joins an ovum to form a zygote. This phenomenon occurs when a fertilized ovum or partially formed embryo splits incompletely. Each of them contains a single oocyte (immature ovum or egg cell). The first being a female gamete, known as the ovum (egg/oocyte). It results from cleavage of the oocyte (ovum) after fertilization. The process of fertilization involves a sperm fusing with an ovum. The follicle ruptures and the ripe ovum is expelled into the abdominal cavity. Each ovary contains hundreds of egg cells or ova (singular "ovum"). If the ovum divides after fertilization, identical twins are produced. Dolly's embryo was created by taking the cell and inserting it into a sheep ovum. The ovary is an organ found in the female reproductive system that produces an ovum. Only one sperm is required to fertilize the ovum. Afterwards, he placed the "fertilized" ovum back into the rabbit. He was the first to show that fertilisation requires both spermatozoa and an ovum. They almost always occur following dispermic fertilisation of a normal ovum.