NCERT Summary: Reproduction in Animals | Science Class 8

Male Reproductive System

• It consists of testis, sperm duct and penis.
• Testes are involved in the production of male gametes called sperms.
• Millions of sperms are produced by the testes.

• Each sperm consists of three parts: Head, middle piece and tail.

Female reproductive system

• It consists of ovaries, oviduct and uterus.
• Ovaries produce ova or eggs.
• A single matured egg is released from ovary into oviduct every month.
• Baby develops in the uterus.
• Egg is also single celled like sperm.

Fertilization

• The process of fusion of male and female gametes (egg and sperm) to form zygote is known as fertilization.
• It is of two types:
  (i) Internal fertilization
  (ii) External fertilization

Internal fertilization

• In this, the fusion of sperm and egg takes place inside the female's body.
• It occurs in cows, dogs and humans.

External fertilization

• In this, the fusion of sperm and egg takes place outside the female's body in a surrounding medium, generally water.
• It occurs in frogs, fishes, starfish, etc.

Test tube baby

• A baby conceived by fertilization that occurs outside the mother’s body is called test tube baby.
• Development of the embryo
• The zygote repeatedly divides to form a ball of cells.
• The ball of cells then starts differentiating into tissues and organs. At this stage, it is called embryo.
• Embryo gets attached to the wall of the uterus and develops various body parts such as hands and legs.
• Foetus is a stage of embryo that shows main recognizable feature of mature organism.
• Foetus develops for nine months inside the mother’s womb and is finally delivered.

Fertilization in Humans

• Fusion of the nucleus of the sperm with the ovum to form a zygote. It occurs in the fallopian tube of females.

• Zygote divides to form an embryo.

• Embryo is implanted in the uterus.

• Foetus develops inside the mother’s body for nine months (gestation period).

Asexual reproduction

• The type of reproduction which involves only a single parent and the new individuals are formed without the fusion of gametes is known as asexual reproduction.
• Three common methods of asexual reproduction are:
  (i) Budding
  (ii) Fission
  (iii) Cloning

Budding

• It involves the formation of new individual from the bulging of the parent body.

• This phenomenon is very common in plants, fungi and animals such as Hydra and yeast.

Fission

• Binary fission is the type of asexual reproduction that occurs in Amoeba.
• It is a type of asexual reproduction in which a single cell divides into two halves.

Cloning

• Cloning is the process used to create an exact copy of a cell, tissue or an organism.
• Dolly, a sheep was the first mammal to be cloned. It was cloned by Ian Wilmut and his colleagues in 1996.

Oviparous animals

• The animals that lay eggs are called oviparous animals.

• The examples include all kinds of birds, lizards, snakes, and frogs.

Viviparous animals

• The animals that give birth to young ones are called viviparous animals.
• The examples include cows, dogs, and humans.

Metamorphosis

• The biological process of transformation of larva into an adult is known as metamorphosis.

• The life cycle of frog consists of the following stages:
  Egg → Tadpole (larva) → Adult
• Hormones controlling metamorphosis in frogs
• Thyroxin (produced by the thyroid gland) initiates the process of a tadpole’s metamorphosis into an adult frog.
• In the absence of thyroxin, the tadpole does not transform into an adult and remains in the tadpole stage.

Life cycle of silkworm

• Silkworm grows on mulberry trees and feeds on its leaves.
• During a stage in its life cycle, silkworm spins a cocoon around itself.
• Silk is obtained from this cocoon.

Hormone responsible for metamorphosis in insects

• In insects, metamorphosis is controlled by the insect hormones. Some of the insect hormones are:
  (i) Prothoracicotropic Hormone (PTTH)
  (ii) Ecdysone
  (iii) Juvenile Hormone (JH)