Sexual Reproduction in
Animals
1
One of the most important
characteristics of living things is their
ability to REPRODUCE
All living things will eventually die
because of a limited lifespan. What
would happen if animals died without
reproducing?
They would become extinct.
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Reproduction in animals ensures that
– there will always be animals of their own
kind around.
– there will be continuity of their kind.
There are basically two modes by which new
individuals are produced, namely
1. ASEXUAL REPRODUCTION AND
2. SEXUAL REPRODUCTION
What is asexual reproduction ?
• In asexual reproduction, a new individual
develops or grows from a single parent.
• All the new individuals formed are identical to
the parent which produced them.
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AMOEBA
PARAMECIUM
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What is sexual reproduction?
•In sexual reproduction, new individuals
are formed by the fusion of a male
reproductive cell (male gamete) and a
female reproductive cell (female gamete).
•This fusion is known as fertilization.
– In plants, we now know that the male gamete
is found in the
• pollen grains.
– and the female gamete is found in the
• Ovules
•What about animals?
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Sexual Reproduction in Animals
•In animals, the male gametes are
known as
– Sperms
•The female gametes are known as
– ova or eggs
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Mrs Theresa Teo/2003
Asexual Reproduction
Adult parent
Detached portion of
Parent
Sexual Reproduction
Adult parent
Male gamete
Adult parent
Female gamete
fertilization
new adult
zygote
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new adult
FERTILIZATION
•For the new individual to start developing
through sexual reproduction, fertilization
must take place.
•Fertilization happens when the sperm
meet and fuse with the ovum (egg) to
create a new cell called a zygote.
•The zygote will have some characteristics
from the male parent and some from the
female parent.
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Sexual Reproduction leads to
Variations in Offspring
Children
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FERTILIZATION
• Fertilization can take place internally or
externally (inside or outside the female’s body).
• In internal fertilization, the sperm from the male
fuses with the egg inside the female’s body.
• In external fertilization, the sperm fuses with the
egg outside the body of the female.
– The female deposits the eggs outside the
body and the male releases million of sperm
into the water nearby. The sperm swim to
the eggs and fuse with them.
– Therefore, this method is used by most
aquatic animals.
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After fertilisation, what happens to
the fertilised egg?
Animals which reproduce
through sexual reproduction
External
fertilization
Internal
fertilization
Eggs develop in
the mother’s body
and the young is
born alive
mammals
There are
exceptions in
each category.
Eggs are laid and the
young develop outside
the mother’s body
Reptiles, birds,
insects11
Eggs are laid and the
young develop outside
the mother’s body
Most aquatic animals,
from sea-snails to
amphibians and fish
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Human Reproduction
•As with flowers, for human reproduction,
we need to know where the male gametes
(sperms) and the female gametes (eggs)
are stored.
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MALE REPRODUCTIVE SYSTEM
1.
Testis (Testes – plural)
–
–
–
–
–
2.
Epididymis
–
–
–
–
3.
It lies against the testes.
It consists of about 6m of coiled and looped tubes.
They store the sperms temporarily.
Its walls produce a secretion
Sperm duct
–
4.
Sperms are produced here.
Sex hormones are also produced here at puberty
They contain over 1 km of tiny tubes.
The walls of these tubes divide to produce cells.
These cells eventually mature to become sperms.
It conducts the sperm with the secretion from the
epididymis to the based of the penis where the sperms
receive other secretions.
Penis
–
Introduces the sperms into the female during mating
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MALE REPRODUCTIVE SYSTEM
5.
Scrotum
–
6.
7.
Urethra
–
A canal through the penis to lead the sperm to the outside.
–
Includes the prostate gland, Cowper’s glands, seminal
vesicle
These secrete slippery fluid to nourish the sperms,
lubricate the urethra and stimulates the activity of the
sperms passing through it.
This slippery fluid which contains the secretions is known
as semen.
Glands
–
–
8.
A bag which protects the testes.
Urinary Bladder
–
–
–
Not part of the reproductive system but the excretory
system.
Stores urine temporarily
Also uses the urethra to discharge the urine
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Reproductive system - Male
/Cowper’s gland
epididymis
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Reproductive system - Male
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FEMALE REPRODUCTIVE SYSTEM
1.
Ovary (Every female has two ovaries.)
– It contains the ova or eggs.
– It also produces sex hormones at puberty.
2. Oviduct (or Fallopian tube)
–
–
3.
It leads the mature egg from the ovary to the uterus.
It is also the place where fertilisation of the egg by
sperm takes place.
Uterus (womb)
–
–
–
It is a pear-shaped muscular organ.
It has a soft mucus membrane which lines the inner
wall of muscles.
This is where the embryo gets embedded and
continues to develop
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FEMALE REPRODUCTIVE SYSTEM
4. Cervix
–
–
The uterus opens to a ring of muscles called the
cervix.
Connects uterus to vagina.
5. Vagina
–
–
–
–
It is a muscular tube to receive sperms when mating.
It is also the birth canal for the baby.
Note that the vagina is not used to discharge urine.
The urethra for females is a separate tube unlike
males.
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Reproductive system –
Female
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Reproductive
System
- Female
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We know that the male gamete is found in the
testes and the female gamete is found in the
ovaries.
The question is how do we get the sperm and the
egg to fuse together so that the egg can be
fertilised and what happens after the egg is
fertilised in human beings (in mammals).
This cannot happen at any age. It can only start
happening after the human being matures or
reaches puberty.
Physical maturity is one thing. Mental maturity is
another. What do you think?
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Puberty (Physical maturity)
The age at which secondary sexual characteristics appear
and gametes (sex cells) begin to be produced or released
Some general changes in both girls and boys.
• Height increases
• Face changes
• Armpit hair and pubic hair start to grow
• Sex organ develops and is capable of releasing the sex
cells.
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Sex Cells
• Male Sex Cells
– Called sperms
– Produced by testes
– Produced until old
age
– Capable of moving
on their own
– Millions of sperms
released during
each ejaculation
• Female Sex Cells
– Called eggs/ova
– Produced by
ovaries
– Produced until
menopause
– Not capable of
moving themselves
– Only one is
released during
ovulation
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Human
Sperm
Contains mitochondria
which provides energy for
the sperm’s activity
Enables the sperm to swim
towards egg
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Human Egg
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Puberty
•Specific changes in the male
 Boys usually reach puberty at an average age of
13 years.
 Sex organ starts to produce sperms
 Shoulder and chest become broader
 Voice breaks and deepens
 Moustache and beard start to grow
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Puberty
 Specific changes in the female
 When a baby girl is born, the ovaries already contain
around 400 000 ova or eggs.
 Girls tend to mature two years earlier than boys
(average age of 11)
 When she matures, a small fraction of these eggs
mature (approx. 450) and is released from alternate
ovaries each month.
 Breasts develop
 Hips widen
 Undergoes the menstrual cycle
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MENSTRUAL CYCLE
1.
It is a cycle that mature girls go through.
2.
It is the sequence of changes which occurs in the wall of the
uterus in the course of 28 days (approx. once every month)
3.
The changes are controlled by hormones secreted by the ovary.
4.
3 main events occur in the cycle.
a.
The lining of the uterus wall thickens to prepare for the
coming of the embryo.
b.
Ovulation – the release of ovum from the ovary once
every month
c.
Menstruation – lining of uterus wall breaks down and
flows out of the vagina as blood.
–
Menstruation happens only if the ovum (i.e the egg)
does not get fertilised by a sperm.
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Menstrual Cycle
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What if fertilisation does occur?
HOW DOES FERTILISATION OCCUR?
1.
There is mating .
2.
Sperms are introduced into the female through the
vagina.
3.
The sperms swim through the uterus to the oviduct
towards the egg.
4.
A sperm bumps into the egg.
5.
The sperm penetrates the egg membrane.
6.
The tail drops.
7.
The egg membrane thickens to prevent other sperms
from entering.
8.
The sperm nucleus fuses with the egg nucleus.
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Fertilisation
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Mrs Theresa Teo/2003
Fertilisation
+
sperm
egg
fertilised egg
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What happens after fertilisation?
1. The egg is now a zygote.
2. The zygote divides into hundreds of cells to become an
embryo as it passes further down the Fallopian tube or
oviduct on the way to the uterus.
3. The embryo then becomes embedded in the uterine wall
which is now thick and rich in blood vessels. This is
called implantation.
4. This embryo continues to divide and the blood, blood
vessels and heart are among the first features to develop.
5. A fluid-filled sac called the amniotic sac protects the
embryo.
6. The embryo gets nutrients, oxygen and antibodies from
the blood vessels in the lining of the uterus called the
placenta.
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What happens after fertilisation?
7. The embryo passes the urea and carbon dioxide back to
the mother.
8. The embryo is attached to the placenta by the umbilical
cord.
9. Eventually, the cells develop into different types of tissues
and organs.
10. When all the main parts of the body and the internal
organs are formed, it is called a foetus.
11. Growth and development continues till around the 40th
week when the muscles of the uterus start to contract and
the process of giving birth begins.
12. The amniotic sac breaks and the contractions increase in
intensity and frequency, and the baby is pushed through
the vagina and out of the female’s
body.
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Foetus Development
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5 months
9 months
Fetus Development
and Structure
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Mrs Theresa Teo/2003
Birth of baby
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