CLASS 8 REPRODUCTION IN ANIMALS.

 NOTES

1. Reproduction is an essential feature of living beings.
2. It is necessary for the continuity of the species.
3. reproduction is made of two words:-re means again and production means to make ,that means reproduction can be considered as again production of similar species. 
4. their are two types of organism:-

a) VIVIPAROUS ORGANISMS-those organisms which gives birth to their young ones directly they are called  viviparous organisms., like human beings, cats, dogs etc.

b)OVIPAROUS ORGANISMS-those organisms which lay eggs are called oviparous organisms .like hen, birds, fishes etc.

MODES OF REPRODUCTION-there are two types of reproduction 

a)ASEXUAL REPRODUCTION

B)SEXUAL REPRODUCTION

a)ASEXUAL REPRODUCTION- this type of reproduction is found in lower organisms or unicellular organisms.like bacteria ,yeast and amoeba.

• The offspring(children) are exact same as parents.there are two types methods of asexual reproduction.

 Budding. In ,this case of asexual reproduction, the organism, like hydra, develops a bulge called bud. This bud develops into an adult hydra like structure which when gets separated from the parent hydra leads independent life.

      Another, example of asexual reproduction by budding is seen in yeast.

Fig. 9.6 Budding in Hydra.

      Binary Fission: The process of reproduction in which an animal reproduces by dividing into two individuals known as binary fission. For example, Amoeba asexually reproduces by binary fission.

Fig. 9.7 Binary fission in Amoeba

B)SEXUAL REPRODUCTION-This type of reproduction found in higher organisms.like human beings

the off spring is the mixture of both parents.in this reproduction two parents are required.

the gametes of male parent is called sperm where as the gamete of female parents is called the ovum.

GAMETES-gametes are the sex cells found in both the parents which carry genes from both of the parents to the next generation.

REPRODUCTIVE PARTS OF MALE-

Male Reproductive Organs: Male human reproductive organs contain:

      •   A pair of testes (singular-testis), two sperm duct and a penis.

      •   The testes produce the male gametes� called sperms.

      •   The sperm ducts carry sperms to the penis.

      •   The penis is used for ejecting sperms and also passing urine. The tail in sperm helps in movement of the sperm to reach the egg in the female sex organs.

  •   Human Sperm: A sperm is unicellular (single celled), has a head, a middle piece and a tail.

Fig. 9.2 A human sperm.

Female Reproductive Organs: Female human reproductive organs contain:

      •   A pair of ovaries which produce , female gametes called ova (eggs).

      •   Oviduct (fallopian tubes): A single natured egg is released into the oviduct by one of the ovaries every month.

      •   Uterus: Uterus is the part where development of the baby takes place.

      •   Egg (ovum): Like a sperm, an egg is also a single cell. It contains a nucleus and the cytoplasm.
The egg may be very small as in humans, much larger as in ducks and hens. Ostrich egg is the largest.

PROCESS OF SEXUAL REPRODUCTION

 In this process of sexual reproduction, a male and a female gamete (reproductive cells) fuse to form a single cell called Zygote. This zygote gradually develops into an adult, similar to the parents. The individual that grows from a zygote, receives character of both the parents  mother and father.

•   Fertilisation: The fusion of a sperm with an egg is called fertilisation. During fertilisation, the nuclei of the sperm and the egg fuse to form a single nucleus. This results in the formation of a fertilised egg called Zygote.

Fertilisation may be:

      Internal Fertilisation: Fertilisation which takes place inside the body of the female is called internal fertilisation. For example: as in humans, dogs, cows, birds etc.

      External Fertilisation: Fertilisation which takes place outside the body of the female is called external fertilisation. It is very common in aquatic animals, such as fish, starfish and amphibians (animals who live in water as well as on land such as frog).

      IVF or in vitro Fertilisation: The fertilisation of an egg that takes place outside the body in a test tine or in any other, apparatus is known IVF or in vitro fertilisation.

      Test Tube Babies: In this technique freshly released egg and sperms are put together for a few hours for IMF. In case fertilisation occurs, the zygote is allowed to develop for about a week and then placed in the uterus of the mother. Complete development takes place in the uterus and the baby is born like any other normal baby. The term test tube babies is misleading, because babies cannot grow in test tubes.

•   Embryo: The stage, when cells produced hp the division of the zygote begin to form groups that develop into different tissues and organs of the .body is termed as an embryo.

•   Foetus: The stage of embryo in which al the body parts can be identified is known as foetus.

   Life-cycle of Animals: Sexually reproducing animals start their life from a zygote which develops into an embryo that grows into mature adult. The adult produces sex cells (egg and sperm). The fusion of sperm and egg leads to zygote. This whole cyclic order is known as life-cycle.

Fig. 9.4

•   In some animals, the young ones may look very different from the adult. For example, in case of frog, fertilised egg (zygote) develops into tadpole (larva) which later develops into adult frog.

Fig. 9.5

      The tadpole (larva), which is very different from the adult frog and is unable of jumping, transforms into adult frog that is capable of jumping and swimming.

      The process of transformation of the larva into an adult through drastic changes is called metamorphosis.

      Like frog, metamorphosis is also found in sill moth and butterfly.

  Cloning: Cloning is the produotion of an exact copy of a cell, any other living part or a complete organism. For example, production of Dolly Sheep by Ian Wilmut and his colleagues in Edinburg, Scotland. Cloning is an artificial method invented by man to produce organisms without sexual or asexual reproduction.

NCERT EXCERCISE AND QUESTION ANSWERS.-

1. Explain the importance of reproduction in organisms.

Soln:

Importance of reproduction in organisms is as follows

• Reproduction is termed to be a biological process in which organisms give birth to their off springs.
• It helps in maintaining the continuity of a particular race and helps in increasing the population of the species.
• Reproduction is meant for the survival of all living things.

2. Describe the process of fertilisation in human beings.

Soln:

• Fertilization is defined as the process in which there is a fusion of male gamete and female gamete.
• The male gametes or sperms are released from the male reproductive organ called the penis.
• The sperms release and enter the female body through the vagina.
• From the vagina, sperms travel through the fallopian tubes where they meet the eggs.
• From there on, the process of fertilization takes place in the fallopian tube.
• The male gamete cell (sperm) and female gamete cell (egg) fuse together to form a zygote.
• The zygote divides rapidly to form a group of cells called morula, which becomes the embryo after approximately five days. The foetus is present for about eight weeks after the fertilization.

3. Choose the most appropriate answer.

(a) Internal fertilisation occurs

(i) in female body.

(ii) outside female body.

(iii) in male body.

(iv) outside male body.

(b) A tadpole develops into an adult frog by the process of

(i) fertilisation

(ii) metamorphosis

(iii) embedding

(iv) budding

(c) The number of nuclei present in a zygote is

(i) none

(ii) one

(iii) two

(iv) four

Soln:

a) (i) in female body.

b) (ii) metamorphosis

c) (iii) two

4. Indicate whether the following statements are True (T) or False (F).

(a) Oviparous animals give birth to young ones. ( )

(b) Each sperm is a single cell. ( )

(c) External fertilisation takes place in frog. ( )

(d) A new human individual develops from a cell called gamete. ( )

(e) Egg laid after fertilisation is made up of a single cell. ( )

(f) Amoeba reproduces by budding. ( )

(g) Fertilisation is necessary even in asexual reproduction. ( )

(h) Binary fission is a method of asexual reproduction. ( )

(i) A zygote is formed as a result of fertilisation. ( )

(j) An embryo is made up of a single cell. ( )

Soln:

a) False

b) True

c) True

d) False

e) True

f) False

g) False

h) True

i) True

j) False

5. Give two differences between a zygote and a foetus.

Soln:

Zygote:

• It is the earliest stage of development
• It is formed by the fusion of male and female gametes
• It is a single cell
• The zygote divides several times to form an embryo
• The zygote normally lasts a week and then develop into its next stage.

Foetus

• It is the last developmental stage of an organism
• The stage of the embryo that shows all the main recognizable body parts of a mature organism.
• The foetus stage occurs after the embryo stage.
• Foetus mainly undergoes internal development.

6. Define asexual reproduction. Describe two methods of asexual reproduction in animals.

Soln:

In this type of reproduction, the offspring arises from a single organism without the fusion of male and female gametes. It never changes the number of chromosomes.

Binary fission in amoeba

• It is a kind of asexual reproduction in which one cell divides into two halves.
• It is a unicellular organism that has a cell membrane, cell wall and cytoplasm.
• The division of the cell can take place on any plane.
• In this process, the nucleus of the amoeba first divides to form two daughter nuclei by the process called Karyokinesis.
• Finally, the division of the body into two halves having a nucleus respectively takes place.

Budding in hydra

• Organisms such as hydra, use regenerative cells for reproduction in the process of budding.
• The first step is the formation of buds, and it develops as a small outgrowth on the parent’s body.
• As the bud enlarges, it receives the characteristics of the parent organism.
• Once it is developed, it may be detached from the parent body and develop into a new individual.
• In some rare cases, the buds may not be detached forming interconnected buds.
• There are also methods like fragmentation, multiple fission etc.

7. In which female reproductive organ does the embryo get embedded?

Soln:

• In the female reproductive organ, the embryo gets attached to the uterus.
• Once it gets attached, there occurs the formation of body parts like legs, hands, eyes etc.
• The embryo is then called a foetus.

8. What is metamorphosis? Give examples.

Soln:

• It is defined as the process in which an animal’s body structure abruptly changes through cell growth and differentiation.
• It is a biological process.
• Examples of this kind are frog and insects.

The life cycle of a frog:

The tadpole emerging from the egg will have gills, tail, etc.

They can swim easily in water.

It undergoes abrupt changes and develops into a mature frog.

9. Differentiate between internal fertilisation and external fertilisation.

Soln:

Fertilization, in general, is defined as the fusion of a male and a female gamete.

Internal fertilization

It occurs inside the female body.

There are high chances of survival of the offspring.

Internal fertilization protects the fertilized egg or embryo from harsh environments.

Examples are cow, humans, dogs, monkeys, etc.

External fertilization

It occurs outside the female body.

There are low chances of survival of the offspring.

Most aquatic animals use this type of fertilization and the advantage of external fertilization is that it produces a large number of offspring due to external hazards.

Examples are fish, frog, organisms etc.

10. Complete the crossword puzzle using the hints given below.

Across

1. The process of the fusion of the gametes.

6. The type of fertilisation in hen.

7. The term used for bulges observed on the sides of the body of hydra.

8. Eggs are produced here.

Down

2. Sperms are produced in these male reproductive organs.

3. Another term for in vitro fertilisation.

4. These animals lay eggs.

5. A type of fission in amoeba

Soln:

Across

1) Fertilization

6) Internal

7) Buds

8) Ovary

Down

2) Testis

3) Zygote

4) Oviparous

5) Binary