Page 111 - Science Class 08

egg and also for menstrua on in the
absence of pregnancy.
REPRODUCTIVE PHASE OF LIFE IN HUMANS
You have studied that a boy and a girl become
capable of reproduc on at the age of puberty.
Do you know how long a male or a female
remain capable of reproduc on?
The testes of a man starts producing sperms and
the ovaries of a woman starts producing ovum
at this stage. But, the capacity for menstrua on
and produc on of gametes lasts for a much
longer me in males than in females.
In females, the reproduc ve phase of life
begins at puberty (10 to 12 years of age) and
generally lasts ll the age of approximately 45
to 50 years.
You have studied that the ovaries of a girl
release one mature ovum once in about 28 to
30 days. During this period, the wall of the
uterus becomes thick so as to receive the egg,
in case it is fer lised and begins to develop.
This leads to pregnancy. But, if fer lisa on
does not occur, the released egg and the
thickened lining of the uterus along with its
blood vessels are shed off. This results in
discharge of blood through the vagina. This is
called menstrua on. It occurs once in about 21
to 30 days and con nue for two or three days.
The first menstrual flow begins at puberty and
is termed menarche. It stops at the age of 45 to
50 years of age. Stoppage of menstrua on is
termed menopause. In some girls, menst-
rua on may be irregular at ini al stage, but it
becomes regular over a period of me.
Menstrual cycle is also controlled by hormones.
Science-8
1. Why do our body undergoes changes
during the puberty?
2. What is a gland and what is its func on?
3. What do you mean by exocrine glands?
4. Why the pituitary gland is also called the
master gland?
5. What is the func on of oxytocin?
6. What is the func on of insulin?
7. What is the func on of adrenal gland?
HOW IS THE SEX OF THE BABY DETERMINED?
We generally hear that women are blamed for a
girl baby. You will be surprised to know that this
concept is baseless and absolutely incorrect.
Let us study about it :
The sex of a baby is determined by thread like
structures called chromosomes. All human
beings have 23 pairs of chromosomes in the
nuclei of their cells. Two chromosomes out of
there are the sex chromosomes named X and Y.
A female has two X chromosomes, while a male
has one X and one Y chromosomes.
The gametes (egg and sperm) have only one set
of chromosomes. The unfer lised egg always
has one X chromosome. But sperms are of two
kinds. One kind has a X chromosome and the
other kind has a Y chromosome.
Mother Father
X X X Y
EGGS SPERMS
XX XY
Fer lised Egg
Sex of the Baby Determined
111