Grade 8 Science CBSE

2. Introduction
All living things ultimately die . However ,life continues from generation to generation
because living things can produce young ones of their own kind.
Reproduction is the process by which living things produce young ones of their own
kind.
In a living organism , if one of the organ systems such as digestive , circulatory or
excretory stops working , the organism will die.
But the reproductive system is not needed for the organism to stay alive.
However without it , the organism cannot produce offspring and therefore, can not
continue its species .

3. Introduction through video

4. Modes of Reproduction
Have you seen the young ones of different animals? Try to
name some of the young ones by completing Table 9.1 shown
in examples at S. No. 1 and 5.
You must have seen the young ones of various animals being
born. Can you tell how chicks and caterpillars are born?
How are kittens and puppies born? Do you think that these
young ones looked the same before they were born as they do
now?

6. Just as in plants, there are two modes by which animals
reproduce.
These are: (i) Sexual reproduction, and (ii) Asexual
reproduction.

7. Sexual Reproduction
The method of reproduction in which two parents --- one male
and other female are involved is called sexual reproduction.
Animals such as frog , fishes , reptiles , birds , humans and
all others mammals reproduce sexually.
Though the reproductive organs may differ in different
animals, the basic steps in sexual reproduction are the same.

8. The male and females produce reproductive cells called gametes.
The male gamete is called sperms . The female gamete is called ovum.
In some organisms such as earthworm and leeches , both male and female
gametes are present in the same individual . Such organisms are called
hermaphrodites.
The male gamete and the female gamete fuse to form a zygote. The
zygote is the first cell of the new organism , which eventually develops into a
new individual.

9. Reproduction in Human Beings
Male Reproductive System :
The male reproductive organs include a pair of testes (singular,
testis), two sperm ducts and a penis . The testes produce the male
gametes called sperms. Millions of sperms are produced by the
testes.
Though sperms are very small in size, each has a head, a middle
piece and a tail. Does it appear to be a single cell? Indeed, each
sperm is a single cell with all the usual cell components.
Penis: The penis is the organ that transfers sperms into vagina of
the females body.

12. Sperm
A Sperm is a very small in size . Its body contains a
head , middle piece and a tail.
Each sperm is a single cell
Sperm ducts are narrow tubes which transfer sperms
from the testes to the penis.
Semen: The mixture of fluids and sperms is called
semen.

13. Video on Male Reproductive organ

15. Female Reproductive system
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.

16. The ovaries produce the ovum . Usually one ovum is released from one
of the ovaries every month.
The release of the ovum from the ovary is called ovulation.
The ovum is a single cell. The ovaries also secretes the female hormones
Oestrogen and Progesterone.
The ovum travels through the oviduct to the uterus (or womb) .
The uterus is a muscular bag within which the baby develops after
fertilization.

17. The uterus is a muscular bag within which the baby
develops after fertilization .
The uterus opens to the outside through the vagina

18. Video on Female Reproductive System

19. Types of Fertilization
There are two types of fertilization they are
1. Internal Fertilization
2. External Fertilization.

20. Internal Fertilization
In birds reptiles and mammals , the male deposits the sperms
inside the reproductive system of the female.
Fertilization occurs and the zygote is formed inside the female’s
body .
This type of fertilization that occurs inside the body of the female is
known as internal fertilization.

21. In earthworms , fertilization is internal . Ann earth worm ,
though a hermaphrodite , does not fertilize itself . Two
earthworms come in contact with each other and exchange
sperms .
The eggs are fertilized internally with the sperm received from
the partners.

22. External Fertilization
Sea animals , such as starfish and jellyfish , gather in large groups in water
They lay their eggs and sperms together on the surface of the water.
The sperm fuse with the eggs on the water to form zygotes .
The zygotes develops into new individuals in the water. This type of
fertilization as ot occurs outside the animals body .
Animals living in or near ponds and lakes also have external fertilization.

23. During spring and the rainy season , frogs and toads move to ponds and
slow moving streams .
The male and female clasp each other in water . The female lays
hundreds of eggs . The eggs are delicate and do not have a shell.
A layer of jelly –like substance holds them together and provide
protection to them.
The male ejects sperms directly on the cluster of floating eggs. The
sperms swim in water with the help of their long tail , and fertilize the eggs
In fishes too, the fertilization is external.

24. Fertilization in Humans
The first step in the process of reproduction is the fusion of a sperm and
an ovum.
When sperms come in contact with an egg, one of the sperms may fuse
with the egg. Such fusion of the egg and the sperm is called fertilisation .
 During fertilization, the nuclei of the sperm and the egg fuse to form a
single nucleus. This results in the formation of a fertilised egg or zygote
. Did you know that the zygote is the beginning of a new individual? The
process of fertilization is the meeting of an egg cell from the mother and a
sperm cell from the father.

25. Fertilization

26. So, the new individual inherits some characteristics from the
mother and some from the father.
 Look at your brother or sister. See if you can recognize some
characters in them similar to those of your mother or your
father.

27. Fertilization in Human(Video)

29. Development of Embryo
Fertilization results in the formation of zygote which begins to develop into
an embryo .
The zygote divides repeatedly to give rise to a ball of cells
The cells then begin to form groups that develop into different tissues and
organs of the body.
This developing structure is termed an embryo. The embryo gets
embedded in the wall of the uterus for further development .

30. The embryo continues to develop in the uterus. It gradually
develops body parts such as hands, legs, head, eyes, ears etc.
The stage of the embryo in which all the body parts can be
identified is called a foetus .
When the development of the foetus is complete, the mother gives
birth to the baby.
Soon after fertilization, the zygote divides repeatedly and travels
down the oviduct.

31. As it travels down, many protective layers are formed around
it.
In human beings ,the zygote develops into a complete
individual in about 9 months.
The mother then gives birth to the baby.

32. Development of Embryo(Video)

33. IVF
Have you heard of test tube babies?in the class that in some women
oviducts are blocked.
These women are unable to bear babies because sperms cannot reach
the egg for fertilisation.
In such cases, doctors collect freshly released egg and sperms and keep
them together for a few hours for IVF or in vitro fertilisation (fertilisation
outside the body).
In case fertilisation occurs, the zygote is allowed to develop for about a
week and then it is placed in the mother’s uterus.

34. Complete development takes place in the uterus and the
baby is born like any other baby.
Babies born through this technique are called test-tube
babies. This term is actually misleading because babies cannot
grow in test tubes.

35. Viviparous and Oviparous
In animals where the fertilization is internal , there are different ways
in which the fertilized egg develops into a new organism.
Humans and most other mammals such as cows , goats and tigers
give birth to fully formed young ones.
The animals that give birth to young ones are called Viviparous
animals.
A chick comes out of egg in due course of time . Snakes and
crocodiles also lay fertilized eggs which develops into young ones.
Animals that lays eggs are called oviparous animals.

36. Examples all birds .
In oviparous animals , for example , a hen , the process of development is
different .
After fertilization , the zygote divides repetedly and travels down the
oviduct as in humans.
However ,as it travels down , many protective layers are formed around it.
The hard shell of hens egg is one such layer . After the hard shell is
formed around the embryo , the hen lays the egg.
The hen then sits on the egg to keep it warm .

37. Development of the chick takes place inside the shell.
It takes about 3 weeks for the embryo to develop into a chick.
After its development is complete , the chick comes out by
bursting open the egg shell.

38. Video on Oviparous and Viviparous
ANIMALS

39. Young Ones to Adults
The new individuals which are born or hatched from
the eggs continue to grow till they become adults.
In some animals, the young ones may look very
different from the adults. Recall the life cycle of the
silkworm (egg → larva or caterpillar → pupa → adult)
you studied in Class VII.
Frog is another such example .

40. Metamorphosis
New born mammals and newly hatched birds have similar shape and
structure as their parents.
However , this is not so through out the animals kingdom. A frog , mosquito
or butterfly have different stages of development between hatching of the
eggs and formation of the individual .
The complete change of form of an animal as it develops from a young
onto into an adult is called metamorphosis.
A frog passes through three stages during its development from an egg to
an adult frog:

41. Egg → tadpole(larva) → adult frog
The tadpole that hatches out of the egg looks very different
from an adult frog .
It undergoes changes to finally become a frog

44. In mosquito , when the eggs hatch , a worm- like larva
comes out of each egg.
In about two weeks , it changes into an encased form
called pupa which has the shape of a comma (,) .
The pupa changes into a mosquito inside the case ,
and in about a week the mosquito wriggles out of the
case.

46. Butterflies and moth also pass through the larva and
pupa stages before the winged adult is born:
Egg →caterpillar (larva) →pupa →adult

47. Asexual Reproduction
Asexual reproduction involves the production of new
organisms by just one parent .
It does not require two sexes .There are many forms of
asexual reproduction . Some of the asexual reproduction are
discussed below.
Binary Fission
Budding

48. You have already learnt about the structure of amoeba. You will recall that
amoeba is a single-celled organism .
 It begins the process of reproduction by the division of its nucleus into
two nuclei .
This is followed by division of its body into two, each part receiving a
nucleus .
 Finally, two amoebae are produced from one parent amoeba.
 This type of asexual reproduction in which an animal reproduces by
dividing into two individuals is called binary fission. Apart from budding and
binary fission,

50. Multiple Fission
The process of reproduction in which a parent spilts into
many individuals identical cell is called multiple fission.
A protective cover called cyst is formed around the cell within
which the cell divides many times to produce daughter cells.
The cyst is then broken down to release the cells.
Plasmodium reproduce through multiple fission.

52. Fragmentation
The method of reproduction in multicellular organisms
in which the body of the parent breaks into several
parts and each part then develops into a complete
organism is called fragmentation.
Animals such as flatworms , starfish , and sponges
reproduce by fragmentation.

54. Budding
In budding , a new organism grows in the form of a bulb – like projection
called a bud.from the parent organism.
It grows and may eventually break away from the parent.
Yeast and hydra reproduce by budding .In hydra , a bud appears on the
body wall which grows into a complete hydra in a day or two and separate
from the parent.
In corals and sponges , buds do not separate out but remain attached to
the parent organism.
They grow to full size and reproduce again and again producing a colony.

55. Budding in Hydra