Class: 8 SCIENCE Chapter – 9 Reproduction In Animals

GENERAL INSTRUCTIONS: 1. Read the text thoroughly. 2. YouTube links for the explanation of the chapter are as follows: https://www.youtube.com/watch?v=5EvnCzOpy2o https://www.youtube.com/watch?v=WFMgtBKrSbc

INTRODUCTION:  There are two modes by which animals reproduce. These are: (i) Sexual reproduction, and (ii) Asexual reproduction.  Reproduction resulting from the fusion of male and female gametes is called Sexual reproduction.  The reproductive organs in the female include ovaries, oviducts and uterus.  The reproductive organs in the male include testes, sperm ducts and penis.  The ovary produces female gametes called ova and the testes produces male gametes called sperms.  The fusion of ovum and sperm is called fertilisation. The fertilised egg is called zygote.  The type of reproduction in which only a single parent is involved is called asexual reproduction.  In hydra, new individual develop from buds. This method of asexual reproduction is called budding.  Amoeba reproduces by dividing itself into two. This type of asexual reproduction is called binary fission.

MIND MAP

EXPLANATION

Reproduction: Reproduction is the process by which living organisms produce more living organisms of its own kind. It is very important as it ensures the continuation of similar kinds of individuals, generation after generation.

Modes of Reproduction: There are two main types of reproduction in living organisms.  Sexual Reproduction: The type of reproduction beginning from the fusion of male and female gamete is known as sexual reproduction. E.g. Human, tiger. In animals, males and females have different reproductive parts or organs. The reproductive parts in animals produce gametes that fuse to form a zygote. It is the zygote which develops into a new individual.  Asexual Reproduction: The type of reproduction in which new individuals are produced from a single parent. E.g. Amoeba, Hydra.

Reproductive Organs in Humans: Male Reproductive Organs: 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 male gametes (sperms) are produced by the testes. Though sperms are very small in size, each has a head, a middle piece and a tail. Sperm is a single cell with all the usual cell components.

Female Reproductive Organs: The female reproductive organs include a pair of ovaries, oviducts (Fallopian tubes) and the uterus. Ovary produces female gametes called ova (eggs). In human beings, a single matured egg is released by one of the ovaries, into the oviduct every month. Uterus is the part where development of the baby takes place. An egg is also a single cell.

Fertilisation: The fusion of a male gamete (Sperm) with a female gamete (Ovum) 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.

Types of Fertilisation: There are two types of fertilisation in animals:  Internal Fertilisation: When fertilisation takes place inside the female’s body, it is called internal fertilisation. Internal fertilisation occurs in many animals including humans, cows, dogs and hens.  External Fertilisation: In this type of fertilisation, the fusion of a male and a female gamete takes place outside the female’s body. It is very common in aquatic animals such as fish, starfish, etc. Example: During spring or rainy season, frogs and toads move to ponds and river. When the male and female come together in water, the female lays the eggs, the male deposits sperms over them. Each sperm swims randomly in water with the help of its long tail. The sperms then come in contact with the eggs.

Development of Embryo: Development of embryo takes place in the female reproductive system through following steps:  Every month, a single egg is released by one of the ovary and reaches the fallopian tube of female reproductive system.  When the sperms reach the fallopian tube, it fertilises the egg. This results in formation of the zygote.

 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. It gradually develops the 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.

Formation of Egg Shell in Hens: Internal fertilisation takes place in hens. After fertilisation, the zygote divides continuously and moves to the oviduct. As it travels down, many protective layers are formed around it. The hard shell in a hen’s egg is one such protective layer. After the hard shell is formed around the developing embryo, the hen finally lays the egg. The embryo takes about 3 weeks to develop into a chick. The hen sits on the egg to provide sufficient warmth. After the chick is completely developed, it bursts open the egg shell.

Viviparous and Oviparous Animals:  The animals which give birth to young ones are called viviparous animals. Examples: Human being, dog, cow.  The animals which lay eggs are called oviparous animals. Examples: hen, frog etc.

IVF (In Vitro Fertilisation):  Some women are unable to bear babies because their oviducts are blocked. So, 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).  If fertilisation occurs, the zygote is allowed to develop for about a week and then it is transferred in the mother’s uterus. 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.

Young Ones to Adults: In some animals, the young ones may look very different from the adults. E.g. frog, butterfly, silk moth, etc. In case of frog, fertilised egg (zygote) develops into tadpole (larva) which later develops into an adult frog. 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.

Metamorphosis:  The transformation of the larva into an adult through drastic changes is called metamorphosis.  Larva of butterfly undergoes metamorphosis to become a butterfly. A tadpole undergoes metamorphosis to become a frog.

Asexual Reproduction: The process of reproduction in which new individuals are produced from a single parent is called asexual reproduction. E.g. Budding, Binary Fission  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 gets separated from the parent hydra and leads an independent life. Another, example of asexual reproduction by budding is seen in yeast.

 Binary Fission: This method of reproduction is seen in unicellular organisms, example: amoeba. 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.

Cloning: Cloning is the process of production of an exact copy of a cell, any other living part, or a complete organism. Cloning of an animal was successfully performed for the first time by Ian Wilmut and his colleagues at the Roslin Institute in Edinburgh, Scotland. They successfully cloned a sheep named Dolly.

During the process of cloning sheep named Dolly, a cell was collected from the mammary gland of a female Finn Dorsett sheep. Simultaneously, an egg was collected from a Scottish blackface ewe. The nucleus was removed from the egg. After that, the nucleus of the mammary gland cell from the Finn Dorsett sheep was inserted into the egg of the Scottish blackface ewe whose nucleus had been removed. Then this egg was implanted into the Scottish blackface ewe. Development of this egg followed normally and then finally Dolly was born. Dolly was found to be absolutely identical to the Finn Dorsett sheep from which the nucleus was taken. Since the nucleus from the egg of the Scottish blackface ewe was removed, Dolly did not show any character of the Scottish blackface ewe.

Note: Do the following work in Science Notebook

QUESTION-ANSWERS:

Q1. Explain the importance of reproduction in organisms. A1. The importance of reproduction in organisms is: (i) It is essential for the continuation of a species. (ii) It ensures the continuation of similar kinds of individuals, generation after generation.

Q2. Describe the process of fertilisation in human beings. A2. The fusion of a male gamete (Sperm) with a female gamete (Ovum) is called fertilisation. Every month, a single egg is released by one of the ovary and reaches the fallopian tube of female reproductive system. During fertilisation, when the sperms reach the fallopian tube, 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. In human being, the fertilisation takes place inside the female’s body so it is known as internal fertilisation.

Q3. Give two differences between a zygote and a foetus. A3. The differences between a zygote and a foetus are:

Zygote Foetus

It is unicellular. It is multi-cellular.

It does not have well defined It has well defined limbs and body parts. other body parts.

Q4. Define asexual reproduction. Describe two methods of asexual reproduction in animals. A4. Asexual reproduction is a mode of reproduction in which only one parent is involved to reproduce offspring. Two methods of asexual reproduction in animals are as follows:

 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 gets separated from the parent hydra and leads an independent life.

 Binary Fission: This method of reproduction is seen in unicellular organisms, example- amoeba. 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.

Q5. In which female reproductive organ does the embryo get embedded? A5. The female reproductive organ in which the embryo gets embedded is in the walls of the uterus.

Q6. What is metamorphosis? Give examples. A6. The transformation of the larva into an adult through drastic changes is called metamorphosis. Example: In case of frog, fertilised egg (zygote) develops into tadpole (larva) which later develops into an adult frog. 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. Similarly, larva of butterfly undergoes metamorphosis to become a butterfly.

Q7. Differentiate between internal fertilisation and external fertilisation. A7. The differences between internal and external fertilisation are: Internal Fertilisation External Fertilisation

Fusion of male and female gametes Fusion of male and female gametes takes place inside the female’s body. takes place outside the female’s body. High chances of survival of offspring. Low chances of survival of offspring.

Less numbers of eggs are produced. Large numbers of eggs are produced.

Example: Cows, Hens, Human beings, Example: Fish, Frog, etc. etc.