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Reproduction in animals class 8

 

Introduction

  • Every living organism remain alive on this earth for a limited period of time and then dies. So, new organisms have to be produced in place of those who die.
  • The production of new organisms from the existing organisms of the same species is known as reproduction.
  • The process of reproduction ensures continuity of life on earth. For example, human beings reproduce by giving birth to babies (sons and daughters). These babies grows and ultimately become adults. So, when the old parents die, their sons and daughters keep living on this earth. These sons and daughters also reproduce by giving birth to more babies, and this process goes on and on.
  • Thus, reproduction by human beings ensures that the human species will continue to exist on this earth for all the time to come. Similarly, cats reproduce by giving birth to kittens so that their species may live for ever.
  • The dogs reproduce by giving birth to puppies so that their species may continue to live on this earth. Most of the plants reproduce by producing seeds to grow more plants so that their species may continue to live on this earth.

Modes of Reproduction

  • There are many different ways in which new organisms are produced from their parents. Some organisms like Amoeba just split into parts to produce new Amoeba, some organisms like Hydra and yeast grow out of the parent’s body in the form of a bud; some organisms like birds and snakes hatch out of the eggs laid by their parents, whereas some organisms like human babies, cubs, kittens and puppies are born from their mother.
  • This means that each species of organisms reproduces in a different way. All the different ways of reproduction can be divided into two main groups :
  • Asexual reproduction
  • Sexual reproduction

 

Sexual Reproduction

 

  • In sexual reproduction, the offspring arises from two parents of different sexes : a male sex and a female sex. The male parent contains male sex cells (or male gametes) and the female parent contains female sex cells (or female gametes). The production of a new organism from two parents by making use of their sex cells (or gametes) is called sexual reproduction.
  • In sexual reproduction, the sex cell of the other parent to form a new cell called ‘zygote’. This zygote then grows and develops to form a new organism. Thus, in sexual reproduction, two parents are needed to produces a new organisms. The humans, fish, frogs, hens, cats, dogs, cows, horses, deer, rabbits, lions and tigers all reproduce by the method of sexual reproduction. Most of the flowering plants also reproduce by sexual reproduction.

Male Reproductive Organs

  • The male reproductive organs include a pair of testes (singular, testis), two sperm duct and a penis. The testes are the oval shaped organs which lie outside the abdominal cavity of a man. Testes are the primary reproductive organs in man (or males). The function of testes is to make the male sex cells (or male gametes) called sperms and also to secrete the male sex hormone called testosterone.
  • The testes of a man make the sex gametes (or sperms) from puberty onwards, throughout his life. The testes of a man lie in small muscular pouch called scrotum, outside the abdominal cavity. The testes are outside the abdominal cavity of the body (and not deep inside the body), because the sperm formation require a lower temperature than the normal body temperature. Being outside the abdominal cavity, the temperature of scrotum is about 2-3ºC lower than the temperature inside the body. In this way, the testes provide an optimal temperature for the formation of sperm.

Male Reproductive Organs

 

The Human Sperm

  • The male gamete in animals is called sperm. Sperms are extremely small cells which can be seen only with the help of a high power microscope. The sperm cell is hundreds or even thousands of times smaller than the ovum. It is consists of three parts as head, middle piece and a long tail.
  • The sperms are motile which move independently with the help of their tails. The nuclei of the sperm contain chromosomes which carry the genes (characters).

Human Sperm

Female Reproductive Organs

  • The female reproductive organs are a pair of ovaries, oviducts (fallopian tubes), and the uterus. The ovary produces female gametes called ova (eggs). In human beings, a single matured egg is released into the oviduct by one of the ovaries every month.
  • Uterus is the part where development of the baby takes place. Like the sperm, an egg is also a single cell.

Female Reproductive Organs in Humans

Fertilization

·                     When sexual reproduction to occur, a male gamete must combine (or fuse) with a female gamete. The fusion of a male gamete with a female gamete to form a zygote during the sexual reproduction, is called fertilization.

·                     The zygote is actually ‘fertilised ovum’ or ‘fertilized eggs.’ The zygote grows and develops to form a new baby.

Fertilization
  • The unborn baby in the uterus in the early stages of development (when its body parts are not much developed) is called an embryo.
  • On the other hand, the unborn baby in the uterus in the later stages of development (when all its parts are well developed and can be identified) is called a foetus.

Internal and External Fertilisation

  • The fertilization which occurs inside the female body is called internal fertilization. In internal fertilization, the female eggs are fertilized by sperms inside her body. In mammals (including human beings), bird and reptiles, the fertilization occurs inside the female body.
  • The fertilization which occurs outside the female body is called external fertilization. In this, the female animals eggs are fertilized by sperms outside its body. In amphibians (like frogs and toads) and fishes, the fertilization of eggs occurs outside the female animal’s body. The male and female animals release their sperms and eggs in water where fertilization takes place by fusion between sperms and eggs.

Development of Embryo

  • When the ovum (or egg) is fertilized in the oviduct then a zygote is formed. The zygote divides rapidly by mitosis as it moves down slowly in the oviduct and forms a hollow ball of hundreds of cells.
  • This hollow ball of cells, now called an embryo, sinks into the soft and thick lining of the uterus and gets embedded in it. The embedding of embryo in the thick lining of the uterus is calledimplantation.
  • After implantation, a disc-like special tissue develops between the uterus wall (called uterine wall) and the embryo (or foetus), which is called placenta.

(a) zygote formation and development of an embryo from the zygote,
(b) Ball of cells &(c) Embedding of the embryo in the uterus.

  • The foetusis connected to placenta in mother’s body through umblical cord. It is through the placenta that all the requirements of the developing foetus like nutrition, respiration and excretion etc, are get from the mother’s body. In other words, the exchange of nutrients, oxygen and waste products between the embryo and the mother take place through the placenta.

Viviparous and Oviparous Animals

  • Some animals give birth to young ones while some animals lay eggs which later develop into young ones. The animals which give birth to young ones are called viviparous animals. Those animals which lay eggs are called oviparous animals.

Young Ones to Adults

  • The new individuals which are born or hatched from the eggs continue to grow till they become adults. The life cycle of the silkworms

Egg ®larva or caterpillar ®pupa ®adult

  • The different stages of frog starting from the egg to the adult stage. There are three distinct stages, that is,

Egg ®tadpole (larva) ®adult

 

  • The features that are present in the adult are not found in these young ones.

Life Cycle of Frog

  • The transformation of the larva into an adult through drastic changes is calledmetamorphosis. In human beings, body parts similar to those present in the adult are present from the time of birth.

Asexual Reproduction

  • The asexual reproduction is the simplest method of reproduction. It takes place mainly in those organisms, whose bodies have a simple structure. So the simple animal, simple plants and micro-organisms (like bacteria) reproduce asexual reproduction methods. The asexual reproduction takes place in unicellular animals and plants, simple multicellular animals (like Hydra and Planaria) and some multicellular plants ( likeBryophyllum and rose plants).
  • In asexual reproduction, the offspring arise from a single parent. The production of a new organisms form a single parent without the involvement of sex cells (or gametes) is calledasexual reproduction. It is called asexual reproduction because it does not use special cells called ‘sex cells’ (or gametes) for producing a new organism.
  • In asexual reproduction, a part of the parent organism separates off and grows into a new organism. Thus, in asexual reproduction only one parent is needed to produce a new organism. But no sex cells are involved in asexual reproduction. Some of the examples of asexual reproduction are : binary fission in Amoeba; budding in Hydra ; spore formation in Rhizopus fungus (or bread mould); segregation in Planaria (flatworm); fragmentation in Spirogyra and Vegetative propagation in flowering plants (like rose plants).

 

Binary Fission

  • Binary fission is an asexual method of reproduction of organisms. In binary fission, the parent organisms splits (or divides) to form two new organisms, when this happens, the parent organisms ceases to exist and two new organisms come into existence.
  • The unicellular organisms like Amoeba¸Paramecium, Leishmania, bacteria etc. reproduce by binary fission.

Binary Fission in Amoeba

  • Amoeba reproduce by binary fission by dividing its body into two parts.
  • When the Amoeba cell has reached its maximum size of growth, then first the nucleus of Amoeba lengthens and divides into two parts. After that the cytoplasm of Amoeba divides into two parts, one part around each nucleus.
  • The two daughterAmoeba produced grow to their full size and then divide again to produce four Amoeba, and so on. In the unicellular organisms such as Amoeba, the splitting of parent cell during fission (or cell division) can take place in any plane.

Budding

  • Budding is an asexual method of reproduction. In budding, a small part of the body of the parent organism grows out as a ‘bud’ which then detaches a become a new organism. The asexual reproduction by budding is observed in Hydra and yeast.

Budding in Hydra

 

  • Hydra is a simple multicellular animal. Hydra reproduces by the process of budding (by using its regenerative cells).
  • In hydra first a small outgrowth called ‘bud’ is formed on the side of its body by the repreated mitotic divisions of the cells.
  • This bud then grows gradually to form a small hydra by developing a mouth and tentacles and finally the tiny new hydra detaches itself from the body of parent hydra and lives as a separate organism. The bud formed in a hydra is not a single cell. It is a group of cells.

Formation of Clone Dolly

  • Cloning is the production of an exact copy of a cell, a part or complete body. Dolly was the first clone born on 5th July, 1996 and was the first mammal to be cloned. It is cloned by Ian wilmut and his colleagues.
  • During the process of cloning dolly, a cell was collected from the mammary gland of female Finn Dorsett sheep. Simultaneously, an egg was obtained from a Scottish black face eve. The nucleus was removed from the egg, then the nucleus of mammary gland cell from the Finn Dorsett sheep was inserted into egg of Scottish blackface ease. Development of this egg followed normally and finally Dolly was born.
  • It was found to be absolutely, identical to the finn Dorsett sheep and produced several offspring of her own through sexual means. Dolly died on 14th February 2003 due to certain lung disease.

Formation of Clone Dolly

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EXERCISE

 

  1. I) Very Short Answer Type Questions :

 

  1. What is male gamete?
  2. Give one difference between asexual and sexual reproduction.
  3. What is female gamete?
  4. Which organ produces the ovum?
  5. What type of reproduction takes place in Amoeba.
  6. Do we also undergo metamorphosis?
  7. What is foetus?
  8. What is the other name of oviducts?
  9. Give some examples of oviparous animals.
  10. What type of fertilization takes place in frog?

 

  1. II) Short Answer Type Questions :
  2. Name the processes which are essential for the survival of individuals.
  3. What do you mean by reproduction?
  4. Define the term sexual reproduction?
  5. What do you mean by fertilization?
  6. What are the two modes of reproduction?
  7. What is embryo?
  8. Define the term asexual reproduction?
  9. What is sperm? Explain its structure.
  10. How do chicks born in hens?
  11. Write life cycle of silk worm in brief.

III) Long Answer Type Questions :

  1. What is asexual reproduction? Write various methods of asexual reproduction.
  2. Explain the asexual reproduction in hydra with diagram.
  3. Explain the asexual reproduction in Amoeba with diagram.
  4. What is the name of asexual reproduction method in :

(i) Spirogyra (ii) Leishmania       

  1. What is it an advantage for the testes to be situated in the scrotal sac outside the main body cavity?
  2. IV) Very Long Answer Type Questions :
  3. (a) What does fertilization take place in human females.

(b)      What is meant by implantation?

(c)      At which part sperm can enter in female.

  1. (a) Explain the fertilization in frog.

          (b)      How it is different from fertilization in birds?

  1. Match the item given in Column A with these in Column B suitably.
  Column-A   Column-B
1. Yeast A Asexual reproduction
2. Ovary B Lays eggs
3. Single parent C Budding
4. Testes D Ovum
5. Oviparous E Sperm

 

  1. (a)Write the name of mode of reproduction by which Dolly sheep is          produced.

          (b)      Which type of fertilization takes place in human beings?

(c)      In which type of fertilization animals produce a larger number of eggs and     sperms?

(d)      How many chromosomes are present in single cell of a human zygote?

(e)      Name the hormone which is secrete from primary organs in male and female.       

  1. Make a chart which represents the reproductive parts in male and female.

 

 

 

 

WORKSHEET-1

  1. What do you mean by reproduction?

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  1. Why reproduction is necessary?

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  1. Define sexual reproduction

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  1. Draw a well labelled diagram of a human male reproductive organs and explain it.

 

 

 

 

 

 

 

 

 

 

  1. Draw a well labelled diagram of sperm and write their parts.

 

 

WORKSHEET-2

  1. Draw a well labelled diagram of female reproductive organs.

 

 

 

 

 

 

 

 

 

 

  1. Explain the functions of the female reproductive organs.

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  1. What do you mean by fertilization?

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  1. What do you mean by external fertilization. Give example.

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  1. By giving examples, explain internal fertilization.

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  1. What do you mean by zygote formation?

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WORKSHEET-3

  1. How the embryo is developed?

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  1. What do you mean by a foetus?

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  1. Explain the term viviparous by giving examples.

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  1. Define the term oviparous animals. Give examples.

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  1. Define metamorphosis. How the young ones gets converted intoadults?

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WORKSHEET-4

  1. Define asexual reproduction. Give examples.

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  1. How the binary fission occurs in Amoeba, explain with proper diagrams.

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  1. Differentiate between asexual and sexual reproduction.

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  1. Define budding. How the budding process happens in Hydra.

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  1. How the clone dolly is formed?

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  1. In which female reproductive organs does the embryo get embedded?

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