22.3 Lesson 22.3: Reproduction and Life Stages
Lesson Objectives
- Explain how fertilization occurs.
- Identify major events of pregnancy and childbirth.
- List important developments of infancy and childhood.
- Outline changes that occur during adolescence.
- Describe the stages of adulthood.
Check Your Understanding
- What are sperm and eggs?
- How many chromosomes do sperm and eggs have?
- What is the role of sex hormones during the teen years?
Introduction
The sperm and egg pictured in Figure 1 below don’t look anything like a human baby. After
these two gametes unite, however, they will develop into a human being. How does a single
cell become a complex organism made up of billions of cells? Keep reading to find out.
Fertilization and Implantation
Sexual reproduction occurs when an egg unites with a sperm. This is calledfertilization.
Sperm are deposited in the vagina during sexual intercourse. They “swim” through the
uterus and enter a Fallopian tube. This is where fertilization normally takes place. A sperm
that is about to enter an egg is shown inFigure22.6. If the sperm breaks through the egg’s
membrane, it will cause changes in the egg that keep other sperm out. It will also trigger
the egg to complete meiosis. Recall that meiosis begins long before an egg is released from
an ovary.
The sperm and egg each have only half the number of chromosomes as other cells in the
body. Therefore, when they unite, they form a cell with the full number of chromosomes.
The cell they form is called azygote. The zygote slowly travels down the Fallopian tube to
the uterus. As it travels, it divides by mitosis many times. It forms a hollow ball of cells.
After the ball of cells reaches the uterus, it embeds in the lining of the uterus. This is called
implantation. It usually occurs about a week after fertilization.