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Objectives

Section 19.1. Development Before Birth. Objectives. Summarize the events that occur during the first week after fertilization. Describe the structures that protect and nourish the embryo and fetus.

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Objectives

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  1. Section 19.1 Development Before Birth Objectives Summarize the events that occur during the first week after fertilization. Describe the structures that protect and nourish the embryo and fetus.

  2. Quick Quiz Which of the following statements made by a married couple do you think are good reasons for having a baby? Choose one or more. “It’s now or never. We’re almost 40.” “We have lots of love to give a child.” “If we have one more, maybe it will be a girl.” “Our marriage will improve if we have a baby.” “With our new jobs, we’ve finally saved enough money to start a family.” In a paragraph, describe the factors that a married couple should consider before they have children.

  3. The Beginning of the Life Cycle • Once a couple has decided to start a family, they may try to conceive, or get pregnant. • In a fertile woman’s body, about once a month an egg enters one of the fallopian tubes and begins its journey to the uterus. • If the egg is on its way to the uterus, a sperm may fertilize it. This moment of fertilization is also called conception.

  4. Fertilization The Beginning of the Life Cycle • Only a few hundred sperm of the hundreds of millions that enter the vagina usually make it to the egg, and only one can fertilize it. • In the first week after fertilization, the fertilized egg undergoes many cell divisions and travels to the uterus.

  5. The Zygote The Beginning of the Life Cycle • The united egg and sperm is called a zygote(ZY goht). • Within 36 hours, while the zygote is still traveling through the fallopian tube, it begins to divide.

  6. Cell Division The Beginning of the Life Cycle • The original cell divides to make two cells. • The two-celled embryo divides into four cells, and so on, until the embryo is made up of dozens of cells.

  7. The Blastocyst The Beginning of the Life Cycle • From the two-cell stage until about nine weeks after fertilization, the growing structure is called an embryo (EM bree oh). • About five days after fertilization, the structure, called a blastocyst (BLAS tuh sist), is no longer a solid mass of cells, but a sphere of cells surrounding a hollow center.

  8. Implantation The Beginning of the Life Cycle • Once the blastocyst forms, it begins to attach itself to the wall of the uterus. • The process of attachment is called implantation.

  9. Cell division Blastocyst Zygote Fallopian tube Fertilization Ovary Implantation Uterus

  10. PlacentaThis structure lines part of the wall of the uterus during pregnancy and nourishes the embryo with substances from the mother’s blood. Wall of uterus Amniotic fluid Amniotic sacThe amniotic fluid contained in this sac cushions the embryo from shock and helps keep the embryo’s temperature constant. Umbilical cordAn umbilical cord carries nutrients and oxygen from the placenta to the embryo, and carries wastes away. Cervix Vagina

  11. Development in the Uterus • While the embryo grows, several other structures also develop. • These structures—the amniotic sac, placenta, and umbilical cord—protect and nourish the developing embryo, and later the fetus.

  12. Amniotic Sac • Soon after implantation, a fluid-filled bag of thin tissue called the amniotic sac (am nee AHT ik) develops around the embryo. • Inside the sac, the embryo floats in amniotic fluid.

  13. Placenta • The attachment holding the embryo to the wall of the uterus develops into a structure called the placenta. • Within the placenta, oxygen and nutrients move from the mother’s blood into tiny blood vessels that lead to the embryo.

  14. Umbilical Cord • About 25 days after fertilization, a cordlike structure called the umbilical cord (UM BIL ih kul) develops between the embryo and the placenta. • Blood vessels in the umbilical cord carry nutrients and oxygen from the placenta to the embryo and wastes from the embryo to the placenta.

  15. The Growing Embryo • During the first two months of development, the major body systems and organs start to form in the embryo. • By the end of eight weeks, the embryo is about an inch long and has recognizable external features.

  16. The Fetus • From the third month until birth, the developing human is called a fetus. • During the third to sixth month, the fetus begins to move and kick. • By the end of the ninth month, the fetus is ready to be born.

  17. Vocabulary zygote The united egg and sperm. embryo The stage of human development from the two- cell stage until about nine weeks after fertilization. blastocyst A hollow, spherical structure made up of about 50-100 cells, formed when a zygote divides and grows. implantation The process in which the blastocyst attaches itself to the wall of the uterus. amniotic sac A fluid-filled bag of thin tissue that develops around the embryo.

  18. Vocabulary placenta The structure that holds the embryo to the wall of the uterus. umbilical cord The cordlike structure that connects the embryo and the placenta. fetus The stage of human development from the third month after fertilization until birth.

  19. End of Section 19.1 Click on this slide to end this presentation.

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