DEVELOPMENTAL STAGES Development actually begins prenatally and extends through life. In prenatal development, there are three physiological stages: germinal, embryonic, and fetal. The baby develops differently during each of these stages. In the germinal stage, which is at 1 to 2 weeks gestation, conception occurs with the fertilization of the egg to form a zygote. The zygote is a single-celled structure with a certain genetic makeup. The zygote will divide through mitosis to make four, eight, sixteen, and more cells. Fewer than half of all zygotes will survive past this stage. Cells gradually become more specialized in order to form certain tissues and organs. The germinal stage is prior to implantation. The embryonic stage is from weeks three through eight. It starts with implantation and the formation of the placenta, which connects the embryo to the uterine wall. The basic structures will develop, such as the head, chest, and abdominal cavity. Immature organs form and the heart beats. The brain and spinal cord develop. In the fetal stage, weeks 9 through 40, the embryo becomes a fetus. The tail disappears and the sex organs develop. There is maturation of all of the major organs, including the brain. Senses such as hearing and the sensation of pain develop and the size of the fetus increases dramatically. Figure 12 shows the different stages of fetal development:
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