An egg (in the female) or sperm (in the male) cell. See also Somatic cell.

A functional unit of heredity that is a segment of DNA found on chromosomes in the nucleus of a cell. Genes direct the formation of an enzyme or other protein.

Fertilization of an egg stimulates cell division, and the resulting cells are organized into three different layers, called germ layers. The three layers are the ectoderm, the mesoderm, and the endoderm.