Marsupials are mammals whose babies
usually develop in a pouch after birth.
Some examples are kangaroos, koalas,
and opossums. Marsupials do not have
true placentas. For this reason, marsupial
gestation is relatively short. Even the
largest type of kangaroo is born after
about 40 days.
Marsupial babies are born tiny and
undeveloped. Right after they are born
they crawl inside a pouchlike flap of skin
at the front of their mother’s body.
There they continue to develop until
they are ready to leave the pouch.
Gestation in Placentals
Placentals are mammals that have placentas
during gestation. Humans, cats,
dogs, deer, whales, and most other
mammals are placentals. The gestation
of placentals is usually longer than the
gestation of marsupials. This means that
the babies of placentals are born more
developed than marsupial babies.
However, the babies of different placentals
are born at different levels of development.
Placentals also have different
lengths of gestation. Horses have a gestation
of about 11 months. Their babies
can walk hours after birth. Chimpanzees
have a gestation of about eight months.
Their babies are helpless for a few
months after birth.
Human Gestation
Human gestation, or pregnancy, is
divided into three periods called trimesters.
Each lasts about three months.
In the first trimester, the fetus develops a
heartbeat and the beginnings of all the
other body parts. The fetus also grows
an umbilical cord, which connects it to
the placenta. At three months, the fetus
weighs less than 1 ounce (28 grams). In
the second trimester, the fetus starts to
move around in the uterus. At six
months, it weighs up to 2 pounds (900
grams). In the third trimester, the baby
continues to grow bigger and stronger.
By the time of birth, the baby weighs
about 7.5 pounds (3.5 kilograms).
#More to explore
Mammal • Reproductive System
The Virginia
opossum has
the shortest
known
gestation—
about 12
days.
All animals go through a period of development
before they are born. For mammals
this takes place inside the mother’s body
during gestation. For other animals this may
take place inside an egg. The developing
animal is called an embryo. The embryos of
many animals look alike in the earliest
stages. But they soon start to develop into
their different forms.
BRITANNICA STUDENT ENCYCLOPEDIA Gestation 109
Ghana
Ghana is a country in western Africa. In
1957 it became the first African colony
south of the Sahara Desert to win independence.
Ghana then led the way in
establishing industries and a modern
educational system. It also produced
important leaders, including Kofi
Annan, a secretary-general of the United
Nations. Ghana’s capital and largest city
is Accra.
Geography
Ghana is bordered by Cote d’Ivoire on
the west, Burkina Faso on the northwest
and north, and Togo on the east. On the
south Ghana has a coastline on the Gulf
of Guinea, which is part of the Atlantic
Ocean. Ghana’s climate is warm yearround,
with dry and rainy seasons.
Ghana’s land is mainly flat. The southwest,
northwest, and north contain
higher plateaus and some hills and valleys.
Low mountain peaks rise in the
east. The Black Volta and White Volta
rivers flow southward from Burkina
Faso. The two rivers feed Lake Volta,
one of the largest artificial lakes in the
world. It was formed behind the Akosombo
Dam, which produces electricity
for Ghana.
Plants and Animals
Tall grasses and scattered trees cover the
northern two thirds of the country. Forests
cover most of the southern third.
Ghana’s trees include the silk cotton
tree, the wawa tree, and the African
mahogany.
Ghana’s large mammals include lions,
leopards, hyenas, antelope, elephants,
buffalo, wild hogs, chimpanzees, and
monkeys. Crocodiles and hippopotamuses
live in the rivers. Ghana also has
numerous birds, including parrots,
hornbills, and kingfishers. Among its
snakes are cobras, pythons, puff adders,
and horned adders.
Women paint the walls of a village in Ghana.
110 Ghana BRITANNICA STUDENT ENCYCLOPEDIA
People
The people of Ghana belong to one
broad group divided into about 75 different
tribes. The largest tribe is the
Akan. Other large tribes include the
Mole-Dagbani, the Ewe, the
Ga-Adangme, and the Gurma. English
is the official language, but each tribe
has its own language. More than half of
the people are Christian. Many others
practice traditional religions or Islam.
Economy
Ghana’s economy is based on farming
and mining. More than half of its workers
are involved in agriculture. The
country’s main exports are gold and
cocoa. Other crops include cassava,
yams, bananas, corn, sorghum, rice, and
millet. Ghana also mines diamonds,
manganese, and bauxite, which is used
to make aluminum. Logging, fishing,
and manufacturing are also important.
History
Most of Ghana’s present ethnic groups
lived in the region when Portuguese
explorers arrived in 1471. Because of its
gold mines, the area became known as
the Gold Coast. In 1482 the Portuguese
began building forts on the coast as
headquarters for the gold trade. In the
1600s traders from other European
countries set up their own forts.
Soon the Europeans began trading slaves
