ANIMAL DEVELOPMENT IS a process in which a single cell (a fertilized egg or zygote) is transformed into an adult organism. The animal development is a lifelong process including stages of fertilization, embryogenesis, organism development after birth, sexual maturation, adulthood development, aging, and death. Because the formation of a multicellu—lar embryo is key to early development, many cell types must be differentiated from the fertilized egg to form organs and tissues in the body. Stem cells play important roles and are involved in every step of development, from early embryogenesis to adulthood replenishment of damaged cells and maintenance of body function. A zygote is considered to be the earliest—stage stem cell, as every cell and organ in the body stem from zygote division and differentiation.