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| Description: Table 42.2 Stages of Human Development Prenatal period Zygote Single cell resulting from fusion of sperm nucleus and egg nucleus at fertilization. Blastocyst Ball of cells with surface layer, fluid-filled cavity, and inner (blastula) cell mass. Embryo A|| developmental stages from two weeks after fertilization until end of eighth week. Fetus A|| developmental stages from ninth week to birth (about 41 weeks after fertilization). Postnatal period Newborn Individual during the first two weeks after birth. Infant Individual from two weeks to fifteen months. Child Individual from infancy to about ten or twelve years. Pubescent Individual at puberty, when secondary sexual traits develop. For girls, occurs between 10 and 15 years; for boys, between 11 and 16 years. Adolescent Individual from puberty until about 3 or 4 years later; physical, mental, emotional maturation. Adult Early adulthood (between 18 and 25 years); bone formation and growth finished. Changes proceed slowly after this. Old age Aging processes result in expected tissue deterioration.
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