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Sexual Reproduction in Animals. 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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Sexual Reproduction in Animals

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Sexual Reproduction in Animals

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.

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



    What is asexual reproduction ?

  • In asexual reproduction, a new individual develops or grows froma single parent.

  • All the new individuals formed areidenticalto the parent which produced them.



    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 asfertilization.

      • 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?

    Sexual Reproduction in Animals

    • In animals, the male gametes are known as

      • Sperms

    • The female gametes are known as

      • ova or eggs

    Mrs Theresa Teo/2003

    Asexual Reproduction

    Sexual Reproduction

    Adult parent

    Adult parent

    Adult parent

    Detached portion of Parent

    Male gamete

    Female gamete


    new adult


    new adult


    • 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.

    Sexual Reproduction leads to Variations in Offspring



    • Fertilization can take placeinternally 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.

    After fertilisation, what happens to the fertilised egg?

    There are exceptions in each category.

    Animals which reproduce through sexual reproduction

    External fertilization

    Internal fertilization

    Eggs are laid and the young develop outside the mother’s body

    Eggs develop in the mother’s body and the young is born alive

    Eggs are laid and the young develop outside the mother’s body

    Most aquatic animals, from sea-snails to amphibians and fish

    Reptiles, birds, insects


    Human Reproduction

    • As with flowers, for human reproduction, we need to know where the male gametes (sperms) and the female gametes (eggs) are stored.


    • Testis (Testes – plural)

      • 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.

    • Epididymis

      • 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

      • 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


    • Scrotum

      • A bag which protects the testes.

    • Urethra

      • A canal through the penis to lead the sperm to the outside.

    • Glands

      • 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.

    • Urinary Bladder

      • Not part of the reproductive system but the excretory system.

      • Stores urine temporarily

      • Also uses the urethra to discharge the urine

    /Cowper’s gland


    Reproductive system - Male

    Reproductive system - Male


    • Ovary (Every female has two ovaries.)

      • It contains the ova or eggs.

      • It also produces sex hormones at puberty.

    • Oviduct (or Fallopian tube)

      • 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


    • Cervix

      • The uterus opens to a ring of muscles called the cervix.

      • Connects uterus to vagina.

    • 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.

    Reproductive system – Female

    Reproductive System- Female

    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?


    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.


    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



    Contains mitochondria which provides energy for the sperm’s activity

    Enables the sperm to swim towards egg


    Human Egg



    • 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



    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



    It is a cycle that mature girls go through.

    It is the sequence of changes which occurs in the wall of the uterus in the course of 28 days (approx. once every month)

    The changes are controlled by hormones secreted by the ovary.

    3 main events occur in the cycle.

    The lining of the uterus wall thickens to prepare for the coming of the embryo.

    Ovulation– the release of ovum from the ovary once every month

    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.


    Menstrual Cycle


    What if fertilisation does occur?


    There is mating .

    Sperms are introduced into the female through the vagina.

    The sperms swim through the uterus to the oviduct towards the egg.

    A sperm bumps into the egg.

    The sperm penetrates the egg membrane.

    The tail drops.

    The egg membrane thickens to prevent other sperms from entering.

    The sperm nucleus fuses with the egg nucleus.




    Mrs Theresa Teo/2003





    fertilised egg


    What happens after fertilisation?

    • The egg is now a zygote.

    • 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.

    • The embryo then becomes embedded in the uterine wall which is now thick and rich in blood vessels. This is called implantation.

    • This embryo continues to divide and the blood, blood vessels and heart are among the first features to develop.

    • A fluid-filled sac called the amniotic sac protects the embryo.

    • The embryo gets nutrients, oxygen and antibodies from the blood vessels in the lining of the uterus called the placenta.


    What happens after fertilisation?

    • The embryo passes the urea and carbon dioxide back to the mother.

    • The embryo is attached to the placenta by the umbilical cord.

    • Eventually, the cells develop into different types of tissues and organs.

    • When all the main parts of the body and the internal organs are formed, it is called a foetus.

    • 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.

    • 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.


    Foetus Development


    5 months

    9 months

    Fetus Development and Structure


    Mrs Theresa Teo/2003

    Birth of baby


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