Chapter 16: Regulation and Reproduction

Figure 10 The development of fraternal and identical twins is different.
Fraternal twins develop
from two different eggs
that have been fertilized
by two different sperm.
Midwives Some women
choose to deliver their
babies at home rather than
at a hospital. An at-home
birth can be attended by a
certified nurse-midwife.
Research to find the educational
and skill requirements
of a nurse-midwife.
Fraternal Twins
480 CHAPTER 16 Regulation and Reproduction
(l)Tim Davis/Photo Researchers, (r)Chris Sorensen/The Stock Market/CORBIS
Fertilization
Fetal
stage
Identical Twins
Identical twins develop
from one egg that has been
fertilized by one sperm. The
zygote divides into two
separate zygotes.
Multiple Births
Sometimes two eggs leave the ovary at the same time. If both
eggs are fertilized and both develop, fraternal twins are born.
Fraternal twins, as shown in Figure 10, can be two girls, two
boys, or a boy and a girl. Because fraternal twins come from two
eggs, they only resemble each other.
Because identical twin zygotes develop from the same egg
and sperm, as explained in Figure 10, they have the same hereditary
information. These identical zygotes develop into identical
twins, which are either two girls or two boys. Multiple births
also can occur when three or more eggs are produced at one
time or when the zygote separates into three or more parts.
Development Before Birth
After fertilization, the zygote moves along the oviduct to the
uterus. During this time, the zygote is dividing and forming into
a ball of cells. After about seven days, the zygote attaches to the
wall of the uterus, which has been thickening in preparation to
receive a zygote, as shown in Figure 11. If attached to the wall of
the uterus, the zygote will develop into a baby in about nine
months. This period of development from fertilized egg to birth
is known as pregnancy.