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Human beings have a limited lifespan but are able, through

reproduction, to produce a new generation to succeed them.

Male and female reproductive systems produce specialized sex

cells – called sperm and eggs – that unite during fertilization and

combine their genetic instructions to make a new, unique human. After

fertilization the embryo – later called a fetus – grows and develops inside its

mother’s uterus (womb) for the nine months of pregnancy.

Reproduction

FERTILIZATION ▲

Inside a woman’s reproductive system,

sperm swim towards and surround an egg

and attempt to penetrate its outer layer.

Eventually one succeeds, loses its tail, and its

head fuses with the egg’s nucleus, thereby

combining two sets of genetic instructions (DNA).

Dark retina of a

developing eye on the

side of the embryo’s head

Male sperm

with beating tail

penetrates the outer

layer of the egg

80

◀ CELLS MULTIPLY

As the fertilized egg, or pre-embryo, is swept towards

the uterus it divides repeatedly, producing first two

cells, then four, then eight, and so on. Six days after

fertilization it embeds itself in the uterus lining.

▲ FOUR WEEKS

This pea-sized, four-week-old

embryo – its head is on the left

• is growing in its mother’s
  uterus. Its tiny heart is beating
  and the nervous system and
  vital organs are forming.

▲ FIVE TO SIX WEEKS

The embryo’s brain continues to develop, as do other

organs, such as the intestines. Its skull and face are taking

shape, and limbs, in the form of buds, are starting to grow.

The embryo floats within a sac filled with protective fluid.

▲ SEVEN WEEKS

The bones of the grape-sized

embryo start to harden and muscles

develop and get stronger. Wrists and

ankles are now visible and fingers

and toes are just starting to appear.

Internally, the kidneys start to remove

waste to make urine.

The liver makes the

embryo’s blood cells

until bone marrow

can take over

72 hours after fertilization

the pre-embryo is a

ball of 16 cells

The head becomes

more erect and the ears,

including the organs of

balance, develop

Female egg is

large and

spherical. It

cannot move of

its own accord

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