Section 3


• Fertilization occurs in a fallopian tube. Pregnancy begins when a blastocyst implants itself in the lining of the uterus.

• The three primary germ layers—the ectoderm, mesoderm, and endoderm—form early in embryonic development. Four membranes—the amnion, yolk sac, allantois, and chorion—also form early in embryonic development.

• Nutrients, gases, and other substances pass through the placenta by diffusion from the mother to the fetus.

Vocabulary gestation (p. 1056) blastocyst (p. 1056) implantation (p. 1057) pregnancy (p. 1057)

trimester (p. 1057) embryo (p. 1057) amniotic sac (p. 1057) chorionic villus (p. 1058)

• For the first eight weeks of pregnancy, the developing human is a called an embryo. From the eighth week until birth, a developing human is known as a fetus.

• Unnecessary drug use can negatively affect an embryo or fetus.

• During childbirth, contractions of the uterus initiated by prostaglandins and oxytocin push the baby from the mother's body through the vagina.

placenta (p. 1058) umbilical cord (p. 1059) human chorionic gonadotropin (p. 1059)

fetus (p. 1059) labor (p. 1060) afterbirth (p. 1060)

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