Download Human Development Remember that when gametes combine, a

Human Development
Remember that when gametes combine, a zygote is formed. Once this occurs in the fallopian tube,
it takes approximately six days for the fertilized egg to travel to the uterus where it attaches to the
uterine wall. During this time, mitosis (mitotic division), also known as cleavage, is occurring and the
cells form a ball of cells known as an embryo. During cleavage, the cells increase in number only, not in
size. Once implanted in the uterine wall, the embryo will develop into a fetus and then continue to grow
and develop until it is ready to be born.
During the first part of pregnancy, cells continue to divide by mitotic division and begin to
differentiate, forming tissues and organs. The embryo's cells multiply and start to take on specific
functions. This is called differentiation. This is when they become different types of cells such as blood
cells or nerve cells. The placenta and the amniotic sac that cushions and protects the developing embryo
both form at this time, also through differentiation. The placenta is attached to the umbilical cord which
is attached to the embryo by the “belly button.” The placenta allows for the transfer of material, both
nutrients and waste products, between the mother and child.. There is no mixing of blood from mother to
child.
During the first few months, when essential organs are forming in the embryo, things can go
wrong. Problems associated with either the embryo’s inherited genes or the mother’s exposure to various
harmful environmental factors can
affect the embryo. Harmful
environmental factors that a woman
should avoid at any time during
pregnancy include alcohol, drugs, and
tobacco. Use of these can lead to the
birth of a baby with brain damage, drug
addiction, and/or low birth weight and
the problems associated with it. After
about two months, when all the major
organs have begun to form, the embryo
is called a fetus. An embryo or fetus can
also be harmed if the mother has a poor
diet, is exposed to certain toxic
substances, or gets certain infections,
such as German measles or AIDS.
During the second trimester, the fetus grows in size and continues to develop. The brain develops
and begins to control muscle responses. The baby begins to move and the mother can feel the
movements. During the third and final trimester, the brain and nerves develop. The fetus can hear, smell,
and see light. The lungs mature and the fetus gets into position for birth.
After birth, cell differentiation and body growth continue until adulthood. During adulthood, the
structures of the body slowly begin to age. Eventually, the organism weakens and dies. This process of
birth, growth, development, aging, and death is a predictable pattern that applies not just to humans but
to all organisms.
The following questions will be discussed in small groups:
Why do you think a mother’s poor diet would affect an embryo?
If there is no mixing of blood, how do the mother and fetus exchange materials?
Explain why a fetus does not eliminate feces.