19.1: Development of the Fetus Animation Video
Transcript:
Development of the Fetus
At the beginning of any pregnancy, the female body goes through the process of ovulation at the midpoint of her menstrual cycle. This means that a mature egg is released from the ovary and enters the fallopian tube.
Humans reproduce through sexual intercourse. For a pregnancy to take place, a male’s sperm must enter the female’s vagina. The sperm then swim from the vagina to the fallopian tube, where an egg may be located. There, one sperm and the mature egg combine in a process called fertilization.
When fertilization takes place, a single sperm breaks through the outer layers of the egg. The fertilized egg is called a zygote. Since a zygote is formed from both a sperm and an egg, it includes the genes and chromosomes from both the mother and the father. At that point, a pregnancy begins.
In the first two weeks, the single-celled zygote goes through a process of dividing itself into many cells. First, the single cell divides into two. These two cells then each divide, producing four cells. These cells also divide, and so on. Within 5 days, the zygote forms a ball of 128 cells.
Once this mass of cells implants itself in the lining of the uterus, it is called an embryo. Over the next six weeks, the embryo develops and takes shape. A membrane grows and surrounds the embryo, the placenta forms in the uterus, the umbilical cord forms to connect the baby to the mother, and the embryo’s major organs begin to form.
By the ninth week of pregnancy, the baby is called a fetus.
During the last stage of pregnancy, the fetus grows considerably. The fetus develops most of its organs, bones, and muscles. After about nine months, the baby will be ready to be born.