http://www.ck12.org Chapter 1. Human Biology
3 rdweek after fertilization: Cells of different types start to develop. Cells that will form muscles and skin, for
example, start to develop at this time.
4 thweek after fertilization: Body organs begin to form.
8 thweek: All the major organs have started to develop.
Pictured below are some of the changes that take place during the 4thand 8thweeks (Figure1.135). 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 lacks a tail. The face has also started to form, and it is much bigger.
FIGURE 1.135
Embryonic Development (Weeks 4–8).
Most organs develop in the embryo dur-
ing weeks four through eight. (Note:
the drawings of the embryos are not to
scale.)Growth and Development of the Fetus
There are also many changes that take place after the embryo becomes a fetus. Some of the differences between
them are obvious. For example, the fetus has ears and eyelids. Its fingers and toes are also fully formed. The fetus
even has fingernails and toenails. In addition, the reproductive organs have developed to make the baby a male or
female. The brain and lungs are also developing quickly. The fetus has started to move around inside the uterus.
This is usually when the mother first feels the fetus moving.
By the 28thweek, the fetus is starting to look much more like a baby. Eyelashes and eyebrows are present. Hair has
started to grow on the head. The body of the fetus is also starting to fill out as muscles and bones develop. Babies
born after the 28thweek are usually able to survive. However, they need help breathing because their lungs are not
yet fully mature.