Fertilization is an important event in sexual reproduction. Heterogamy is a type of sexual reproduction that involves the fusion of gametes that are different in size, structure, and physiology. This process is also known as anisogamy.

Zygote is the product of fertilization. The unicellular zygote undergoes repeated cell division to develop into a multicellular structure. The process through which zygote divides mitotically and forms a hollow, spherical, multicellular structure is called cleavage. This process is also known as blastulation.

In boys and girls, some changes start to appear when they become teenagers. These changes are due to the secretion of sex hormones from testes and ovaries. It indicates that sex hormones are produced and sex organs are matured and this age is called puberty. It is the period between childhood and adulthood is called adolescence.