The New Childrens Encyclopedia

288

The start of life

Human life begins after a sperm fertilizes an egg

and the egg develops in the uterus (the womb).

A fertilized human egg takes about nine months to

grow into a baby ready for birth. In the uterus, the

fetus depends on the placenta (the tissue that links

the mother’s and fetus’s blood) for all its needs.

FERTILIZATION

Millions of sperm swim toward the egg,

propelled forward by flexible tails, but usually

only one will fertilize it. On contact, the sperm

and egg merge to create a single cell—the

fertilized egg. The cell then begins to divide.

O

This is a scan of a fetus inside the womb,

taken between four and six months into

pregnancy. It was produced using sound waves

to form a picture, which was then turned into a

three-dimensional (3-D) image. 3-D scans first

appeared in 1987.

1 WITHIN 36 HOURS the

fertilized egg has divided into two

cells. Twelve hours later it has

divided into four cells, and so on.

 REPRODUCTIVE ORGANS

Women have two ovaries (A), where eggs,

or ova, are stored, and a uterus (B), where

a baby is nourished and grows until birth.

Men have two testes (C), where sperm are

made. They also have a penis (D) through

which the sperm travel to get to the eggs.

2 THREE TO FOUR DAYS

after fertilization, there is a cluster

of 16 to 32 cells. The cluster enters

the uterus.

3 ABOUT SIX DAYS after

fertilization, the cell cluster forms

a hollow cavity. It attaches itself

to the lining of the uterus with

rootlike growths.

4 ABOUT EIGHT DAYS after

fertilization, an embryo begins to

form. New cells will form tissues

and organs as a baby develops.

After an egg is

fertilized it begins

to form a barrier

to other sperm.

Each sperm has a

rounded head and

a long tail.

ULTRASOUND SCAN

D

A

B

C

A

C

THE HUMAN BODY