Childrens Illustrated Encyclopedia

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434

FOR LIFE TO CONTINUE on Earth, humans

and other animals must produce young.

The process of creating new life is called

reproduction. Human beings reproduce

in much the same way as other mammals.

From birth, a woman has many tiny

pinhead-sized ova (egg cells) in two organs

inside the abdomen called ovaries. From

puberty onward, one of these egg cells is

released each month as part of the menstrual

cycle. Throughout life, a man produces small

tadpole-shaped cells called sperm in sex organs

called the testes. During sexual intercourse, sperm

cells leave the man’s body and enter the woman’s

body, swimming toward her ovaries. If a sperm meets a

ripe egg cell, the two join together. This is called

fertilization. The egg cell can only be fertilized for

about 24 hours after ovulation. Once fertilized, the

egg travels to the uterus to continue its development.

During the following nine months the tiny egg

develops into a fully formed baby, ready to be born.

SExUaL INTERCOURSE

During sexual intercourse, the man’s

penis becomes stiff enough to insert

into the woman’s vagina, which also

enlarges. after a while, muscular

contractions squeeze sperm cells from

the man’s testes out of the penis and into

the vagina, in a fluid called semen. This

process is called ejaculation. The sperm

cells swim through the uterus, propelled by

their tails, and travel along the Fallopian tube.

Sometimes, one of these sperm cells reaches the

egg cell and fertilizes it, resulting in pregnancy.

Fallopian tube

(oviduct)

Penis

Vagina

FERTILIzaTION

an egg cell begins to divide and develop into a baby only when it is

joined by a sperm cell. after intercourse, hundreds of sperm cells may

reach the egg, but only one breaks through the outer layer. Once this

occurs, genetic material in the sperm—the instructions needed to make

a new human—joins the genetic material inside the egg. The coming

together of sperm and egg and their genes is called fertilization.

Seminal

vesicle

Penis

Scrotum

Testis

Epididymis

Prostate

gland

Bladder

Sperm cells cluster

around egg cell in

Fallopian tube.

Vagina

Uterus (womb)

FEmaLE SEx ORgaNS

about 300,000 eggs are

stored in each ovary.

During one menstrual

cycle, an egg ripens and

leaves the ovary, moving

into the Fallopian tube, or

oviduct. If it is not fertilized

by a sperm, it reaches the

uterus, dies and breaks

down, then leaves the body

during the process called

menstruation.

maLE SEx ORgaNS

Each testis makes more than 250

million sperm cells every day. The

cells are stored in the testis itself

and in a long, winding tube called

the epididymis. If they are not

released, they break down and are

reabsorbed into the bloodstream.

Urethra

Reproduction 434-

SEx ORgaNS

The main female sex organs, the ovaries, are

inside the abdomen. The main male organs, the

testes and penis, hang outside the abdomen.

Other differences between males and females,

such as the woman’s breasts, are called

secondary sexual characteristics.

Only one sperm

penetrates egg to

fertilize it.

Embryo

enters

uterus about

four days after

fertilization as a

solid ball of 32 cells.

Fertilized egg divides into

two cells within 36

hours, four cells

within 48

hours, then

eight cells ,

and so on.

Uterus

Testis

Ovary

Male

bladder

FETUS

a developing baby, or

fetus, lives inside the

uterus, cushioned from

bumps, bright lights, and

noise by a surrounding

fluid called the amniotic

fluid. However, the baby

can hear the regular thump

of the mother’s heartbeat

and the gurgling of food

in her intestines.

Ovary

Male urethra

Fallopian tube

Female

bladder

Ductus

deferens

Barrier around dividing cells

keeps out other sperm cells.

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