home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
The Epic Interactive Encyclopedia 1998
/
Epic Interactive Encyclopedia, The - 1998 Edition (1998)(Epic Marketing).iso
/
H
/
Human_reproduction
/
INFOTEXT
< prev
next >
Wrap
Text File
|
1992-09-04
|
3KB
|
77 lines
Human reproduction is very inefficient by the
standards of most other animals. The most
fertile race of humans is the Hutterites of
North America, whose women usually produce
between 9 and 13 children in one lifetime.
Some species of mice can produce this number
in one pregnancy! In Britain, most women have
1, 2 or 3 children. Conception occurs when
sperm cell from the male meets the egg cell
from the female. In humans this happens in
the Fallopian tube, which connects the ovary
(where the egg cells are produced, usually
one per month) to the uterus (womb). The
sperm cell and egg cell fuse together to form
a zygote. Each sperm cell and egg cell only
carry half the amount of genetic material
(DNA) needed to make a complete cell, so that
when they fuse with each other, the zygote
has exactly the right amount of DNA. The
zygote is the first single cell of a new
baby, and after a period of only a few hours
it begins to divide into 2 cells, then 4
cells, and so on, as it grows into a baby
which will have several hundred million
cells. Once the zygote has divided a few
times, it becomes a little ball of cells
called an embryo. The embryo is carried down
the Fallopian tube into the uterus, whose
walls are soft and velvety. The embryo buries
itself in the soft wall, a process called
implantation. As the embryo grows, it forms
membranes of tissue around itself. At the
same time, the walls of the womb change to
form membranes around the embryo. These
membranes differentiate and change to form
the placenta, which is the supply of
nutrients for the growing baby. As the embryo
grows and develops, it begins to resemble a
tiny human being. At this stage, it is called
a foetus. The foetus is attached to the
placenta by the umbilical cord, through which
it receives oxygen and nutrients. It floats
free in a watery fluid which it produces
called amniotic fluid, which surrounds and
protects it. The foetus makes its own blood,
which it pumps round the placenta with its
own heart. The blood of the foetus and the
mother is therefore kept completely separate,
which may defend the baby from certain types
of infection which the mother may have. In
humans, pregnancy lasts 266 days, or about 40
weeks. During this time, the mother undergoes
many changes. Her uterus grows and becomes
much more muscular, ready to deliver the
baby. Her breasts swell and start to produce
milk to nourish the new baby. Her bone marrow
produces lots more blood. She also finds
herself very hungry, as she has to feed not
only herself but her new baby as well. At the
time of birth, the uterus begins to
powerfully contract, and eventually pushes
the baby out through the cervix (neck of the
womb) and down through the vagina into the
outside world. This process may take many
hours and is very painful and exhausting for
the mother, which is why the term "labour" is
appropriate. The new baby rapidly adjusts to
breathing air for the first time, and when
the umbilical cord is cut the baby becomes an
independent person for the first time.
Sometimes the mother may give birth to twins,
which happens in about 1 in every 60
pregnancies worldwide. Triplets are born to
one pregnancy in every 3600.
Subject by: Dr Aidan M. O'Donnell