Light micrograph (LM) of a human ovum. The egg cell, or ovum, is the female reproductive cell (gamete) in oogamous organisms. When egg and sperm fuse, a zygote is formed, which rapidly grows into a new organism. In all mammals the ovum is fertilized inside the female body. The human ova grow from primitive germ cells that are embedded in the substance of the ovaries. Each of them divides repeatedly to give secretions of the uterine glands, ultimately forming a blastocyst. The ovum is one of the largest cells in the human body, typically visible to the naked eye without the aid of a microscope or other magnification device. Magnification: unknown.