Once inside the cervix, the sperm continue on their journey.

Some remain here, perhaps to provide a back-up

for those that continue on.

The rest proceed up through the cavity of the uterus

in search of the egg.

Their number is noticeably reduced.

Here from inside the uterus,

we see the openings to the two fallopian tubes.

The sperm swim toward them.

One leads to the waiting egg.

The other to an empty tube.

Only half of the few remaining sperm

will swim up the tube that holds the egg.

The tubes are lined with millions of tiny cilia,

the same cilia which helped draw the egg

from the ovary toward the uterus at ovulation.

These cilia sway constantly,

creating a gentle downward current

that the sperm must swim against.

Some sperm get stuck in among the cells

lining the walls of the tube.

Others lose their sense of direction.

During this part of the sperm's journey

the layer of enzyme inhibitors, at the tips of their heads

is slowly being worn away.

The sperm are fully capacitated and able to fertilize

the egg, if they ever encounter it.

Of the two hundred million sperm that begin

this long journey

only about 50 ever reach the egg.

The egg is surrounded by 2 layers of the nutritive

cells which have nourished it on its journey

in the fallopian tube.

The sperm immediately release their digestive enzymes

to break through these protective, nutritive layers

in order the reach the egg itself.

At best, only one will eventually enter and

fertilize the egg.

The joint force of their exertions starts the egg

rolling around, like a mysterious, celestial body.

All the while, the sperm are dissolving their way

through the egg's outer layers.

The first sperm to reach the egg's membrane

is immediately drawn inside.

This is the tail of the penetrating sperm

seen from the surface of the egg at enormous

magnification.

And this is a picture from inside the egg itself.

It has been fertilized now for only a fraction of a second.

Here is the penetrating sperm with a circle

around it

from a wider perspective.

Only this single sperm will be allowed inside the egg.

A rapid, biochemical change took place in the

egg's membrane.

It is now impermeable to all other sperm.

Once inside the sperm too undergoes a

dramatic transformation.

It loses its mid-piece and tail.

The tailless head spells up almost like rising bread.

This hole in the sperm head's covering

is the first indication that it's about to rupture

and release its precious genetic material.

Here, the first strand of genes is making its way

out of the sperm into the egg.

The genetic material continues to spill out

from the sperm head.

Its tightly packed molecules contain the father's

hereditary message.

They almost seem to have been dispersed by

an explosive force.

This is an extreme close-up of the genetic material itself.

The magnification on the television screen is over

half a million times.

The genetic material of the mother's egg

and the father's sperm

combine.

Two cells have joined to make a single new cell.

And within 24 hours that new cell begins to divide.

These early cell divisions of the human embryo

have never before been filmed.

The fertilized egg now has 2 nuclei,

the small indentations at the center of the cell.

The first division of the egg is beginning.

Now there are 2 identical cells,

still within the same nourishing material

of the original egg.

No growth has occurred,

but, rather, a distinction between cells.

Once it has begun to divide

the fertilized egg is called a zygote.

Now there are 4 cells.

The divisions happen at such an accelerated pace

that there's little opportunity for the new cells

to grow before they divide again.

Now there are 8 cells.

Each new generation of cells of the zygote is

smaller than the one before it.

Another division,

and another.

As the zygote divides, it moves along the tube

toward the uterus.

Now it has become a dense, compact cluster

of many cells.

And after 5 days it is called a blastocyst.

It is still no bigger than the original egg.

Its center is filled with liquid.

Within 10 days of fertilization,

the blastocyst implants itself firmly

in the lining of the uterus.

Already the mother's hormones are directing

changes in her body

to prepare it to support the growing embryo.

Barely visible to the naked eye,

the embryo will be nourished by the lining of the uterus,

and then by the placenta.

After 2 weeks, the embryo is elongated.

It is barely one tenth of an inch long.

At the top, what will become its head and brain.

Below, at the tail, the embryo is firmly attached

to the placenta, which will nourish it.

At 4 weeks, the embryo has arm buds and

is distinctly curled.

It has the beginning of eyes.

At 5 weeks, the nose begins to take shape.

At 6 weeks, leg buds are apparent.

The embryo is less than half an inch long.

It floats inside the fluid-filled amniotic sac.

Its spine is clearly visible.

At 7 weeks, the embryo is three quarters of an inch long.

It can move its hands on which there are

clearly defined fingers.

Its internal organs are visible.

The eyelids are formed.

The skull bones, rich in blood vessels,

are growing together at an angle at the

crown of the head.

At 8 weeks, the fingers of the hand are

well defined.

And the toe joints of the feet are clearly visible.

At about 10 weeks, the embryo is considered a fetus.

It can move actively.

There is a suggestion of an ear.

It is two inches long.

And it still has the stub of a tail.

At 11 weeks, it is two and a half inches long.

At 12 weeks, it is 3 inches long.

Here-the umbilical cord-connecting the fetus

to its food supply.

By 14 weeks, it can bring its hands together.

And suck its thumb.

By 15 weeks, the sensory organs are nearly

completely formed.

And by 16 weeks, it is actively turning inside the mother.

This fetus is 18 weeks old.

It is five and a half inches long.

And is here shown, 15 times larger than its

actual size.

Its mouth and lips are fully formed.

And it has the strange nasal plugs

whose purpose is not yet understood.

The eyes of the fetus are closed,

but it can see.

(pulsing sound)

This sound is made by the fetus as it

breathes in the amniotic fluid

in what is known as fetal respiration.

It brings the fluid in through its mouth,

and then breathes it out again.

The umbilical cord is the fetus' link to its

source of life, the mother.

Here are the fetus' sex organs.

All its important physiological systems have developed.

But it will be at least another 8 weeks before the

fetus has even a remote chance of surviving

outside its mother's womb.

Whatever signals the beginning of birth

is till a mystery.

But when the fetus is ready to be born,

the uterus begins its powerful contractions.

And the process of birth begins.

"Pull your legs right in.

Keep it coming.

Take another quick breath.

Hold it.

Push.

Don't stop.

One more.

Deep breath.

Push....push.

That a girl.

Can you see this baby's head?"

"Ok, now, stop pushing.'

"Oh, beautiful job."

"Here he is...

suck out his little nose and mouth

so he can take his nice, clean, first breath.

Nothing's little about this baby."

(baby cries)

"Congratulations!"

"Another girl!"

(baby cries)

"Happy birthday."