When the ovum is fertilised by a male's sperm, it becomes a zygote, which develops into a new organism. The ovum is fertilized inside the female body, and the embryo then develops inside the uterus, being fed by the mothers placenta.
The ovum is the largest cell in the human body. You can see it without a microscope. The human ovum is between 100 and 200 µm long. They are nevertheless much smaller than the cleidoic eggs laid externally by reptiles and birds, which is why they need a long period of internal development in the womb.
The sexual cycle is quite different in other mammals, whose females are only receptive during the 'heat' part of their estrous cycle when eggs are released from the ovary. Sexual activity then continues for a few days, then ceases entirely until the next heat.
In many plants, ova are made inside archegonia through meiosis. The archegonium has a long 'neck' with the egg cell inside. When the egg is mature, the neck opens and sperm swims in to fertilize the egg.
In flowering plants, the female gametes are made of only eight cells, called the embryo sac, inside the ovule. The cell closest to the opening of the embryo sac becomes the egg cell. When pollinated, sperm swims into the embryo sac and fertilizes the egg. The zygote then develops into an embryo inside the ovule.
- The Ovarian Kaleidoscope Database Describing the genes involved in making eggs