We change from an embryo to a fetus with many physical changes occurring during the nine month gestation period. Sex of the organism is determined by the chromosome combination that is supplied by the father. After birth, the person undergoes the developmental stages of infancy, childhood, adolescence, adulthood, and older adulthood. Environment, as well as heredity, influences the expression of traits in an individual. Most people stop growing between the ages of 18 and 30.
Animals begin as one fertilized egg.
It then divides to form a tiny ball of cells which after entering the uterus, becomes attached to its lining. An organism is known as an embryo while in its first stages of development. The stomach, brain, heart, liver, eyes, and ears have formed during the first month. At this stage of development the embryo has a mass of 0.02 grams. The embryo doubles in size and has a mass of 1.0 gram.