Figure22.6: Thisspermisreadytopenetratethemembraneofthisegg. Noticethedifference
in size of the sperm and egg. What will happen if the sperm manages to break through the
egg’s membrane? ( 12 )
Pregnancy and Childbirth
Once the ball of cells implants in the uterus, it is called anembryo. The embryo stage
lasts until the end of the 8th week after fertilization. After that point until birth, the
developing baby is called afetus. To see how an embryo and fetus grow and develop, go to
http://www.pennhealth.com/health_info/animationplayer/fetal_dvlp_tool.html
Growth and Development of the Embryo
During the embryo stage, the baby grows in size. It also develops different types of cells
and organs. Cells of different types start to develop by the 3rdweek after fertilization. They
form structures that suit them for different roles in the body. Cells that will form muscles
and skin, for example, start to develop at this time.
In 2004, French scientist Nicole Le Douarin won the first Pearl Meister Greengard Prize in
Science. This award has been called the “American Nobel Prize for women scientists.” Dr.
Le Douarin received the prize for developing a method to follow the path of individual cells
during the development process. This, in turn, has helped scientists discover how different
organs develop.
During the 4thweek after fertilization, body organs begin to form. By the end of the 8th
week, all the major organs have started to develop. Figure 2 shows some of the changes that
take place during the 4thand 8thweeks. Look closely at the two embryos in the figure. Do
you think that the older embryo looks more human? Notice that it has arms and legs and