called an embryo. After about eight
weeks the baby is called a fetus.
In the uterus the baby grows inside a
pouch called the amniotic sac. The
amniotic sac is filled with clear liquid.
The liquid protects the baby and lets it
move around. A bundle of blood
vessels, called the umbilical cord,
connects the baby’s belly to the
placenta. The placenta is a structure
A human pregnancy lasts about nine months. A developing baby goes through many
changes during the first three months.
BRITANNICA STUDENT ENCYCLOPEDIA Reproductive System 49
that lines part of the uterus. The
placenta brings nourishment from the
mother’s body to the baby. It also takes
away wastes from the baby.
After about nine months of development
the baby is ready to leave the
woman’s body. The bottom end of the
uterus, called the cervix, expands to create
a wide opening into the vagina. The
muscles of the uterus contract, or
tighten, to push the baby downward.
The baby moves slowly through the
vagina and out of the mother’s body.
When the baby is born the umbilical
cord and the placenta also leave the
mother’s body. The newborn baby is cut
free from the cord and the placenta.
After several days the stump of the cord
dries up and falls off the baby’s belly. It
leaves behind an indentation called a
navel, or belly button.
#More to explore
Egg • Gestation
Reptile
A reptile is an air-breathing animal that
has scales instead of hair or feathers.
Reptiles have lived on Earth for more
than 280 million years. Scientists consider
them to be the ancestors of birds
and mammals.
Over the years many kinds of reptiles
have become extinct, or died out. The
dinosaurs are well-known reptiles that
became extinct about 65 million years
ago. Today there are about 6,000 species,
or types, of living reptile. They
From the fourth month to the ninth month of pregnancy, the developing baby grows much
larger. The mother’s uterus and belly greatly expand.
50 Reptile BRITANNICA STUDENT ENCYCLOPEDIA
include snakes, lizards, turtles, alligators,
and crocodiles.
Where Reptiles Live
Reptiles are found all over the world,
except in very cold areas. The greatest
number and variety are in the hot,
humid tropics. Snakes and lizards live in
many different kinds of places. They can
live on land, in trees, in underground
tunnels, or in water. Most turtles live in
water but spend some time on land.
Tortoises live only on land. Alligators
and crocodiles spend most of their time
in water, but they can make short trips
on land.
Physical Features
Reptiles are vertebrates, or animals with
a backbone. Except for snakes, most
reptiles have four limbs. Reptiles vary
greatly in size. Some snakes and lizards
are less than 2 inches (5 centimeters)
long. The largest crocodiles, pythons,
and anacondas can grow to more than
30 feet (9 meters) long. The largest lizard
is the Komodo dragon, which
reaches a length of more than 10 feet (3
meters). Leatherback turtles grow to
more than 7 feet (2 meters) long.
Reptiles have dry skin that is covered
with scales. Scales are hardened folds of
the upper layer of skin. The scales of
many lizards overlap. They are arranged
side by side in snakes and alligators and
crocodiles. Scales also cover the bony
shells of turtles.
Reptiles shed and replace their scales
throughout their lifetime. Some species
shed often. Others shed several times a
year. Snakes shed all their scales at once.
Lizards, turtles, alligators, and crocodiles
shed their scales one by one or in
patches.
Behavior
Reptiles are cold-blooded animals. This
means that their body temperature
changes when the temperature around
them changes. Reptiles warm or cool
themselves by moving to warmer or
cooler places. Most reptiles are inactive
during winter. This is called hibernation.
Crocodiles lie on grass in Madagascar, a
tropical island in the Indian Ocean.
The flashy pattern of an adder’s scales may
warn predators to stay away. Adders are
venomous, or poisonous, snakes.
BRITANNICA STUDENT ENCYCLOPEDIA Reptile 51
Some are inactive during parts of the
summer. This is known as estivation.
Most reptiles are meat eaters. They eat
many kinds of animals, including
insects, birds, frogs, mice, and fish.
Some reptiles follow their prey. Others
hide and wait for the prey to approach.
Certain snakes shoot venom, or poison,
into their prey. A few lizards and turtles
eat only plants.
Reptiles have different ways of protecting
themselves from their enemies.
Rattlesnakes shake their tail rattle when
they feel threatened. A cobra spreads its
neck to look bigger. Some turtles hide in
their shells. Crocodiles, turtles, and
some lizards and snakes hiss loudly.
Most reptiles bite when threatened.
Senses
Alligators, crocodiles, and most lizards
hear fairly well. Snakes and turtles have
trouble hearing. Instead they can feel
vibrations in the ground. Some reptiles
have poor eyesight. This is true of snakes
and lizards that tunnel underground.
Reptiles taste and smell using special
organs in the mouth or nose. Some
snakes also have organs that are sensitive
to heat. These organs can sense prey
even in total darkness. This is because
