Welcome dear students! Today we are going to learn about Reproduction in Animals from Class 8 Science. Do you recall the processes of digestion, circulation and respiration which you have studied in your previous classes? These processes are essential for the survival of every individual. You have also learnt about the process of reproduction in plants. Reproduction is essential for the continuation of a species. Imagine what would have happened if organisms had not reproduced. You will realise that reproduction is very important as it ensures the continuation of similar kinds of individuals, generation after generation. You have already learnt in your previous class about reproduction in plants. In this chapter, we shall learn how reproduction takes place in animals.
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Let us begin with section six point one, Modes of Reproduction. Have you seen the young ones of different animals? Try to name some of the young ones by completing Table six point one. 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? Let us find out. In Table six point one, the first entry is Human, and the young one is Baby. The second is Cat, and the young one is Kitten. The third is Dog, and the young one is Puppy. The fourth is Butterfly, and the young one is Caterpillar. The fifth is Hen, and the young one is Chick. The sixth is Cow, and the young one is Calf. The seventh is Frog, and the young one is Tadpole. Just as in plants, there are two modes by which animals reproduce. These are sexual reproduction, and asexual reproduction.
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Moving on to section six point two, Sexual Reproduction. Try to recall reproduction in plants which you studied in Class seven. You will remember that plants that reproduce sexually have male and female reproductive parts. In animals also, males and females have different reproductive parts or organs. Like plants, the reproductive parts in animals also produce gametes that fuse to form a zygote. It is the zygote which develops into a new individual. This type of reproduction beginning from the fusion of male and female gametes is called sexual reproduction. Let us find out the reproductive parts in humans and study the process of reproduction in them. The male reproductive organs include a pair of testes, two sperm ducts and a penis. As shown in Figure six point one, these organs are arranged in the lower pelvic region. The testes produce the male gametes called sperms. Millions of sperms are produced by the testes. Look at Figure six point two which shows the picture of a sperm. 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. The tail helps the sperm swim towards the egg.
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Now let us look at the Female Reproductive Organs. The female reproductive organs are a pair of ovaries, oviducts, also known as fallopian tubes, and the uterus, as shown in Figure six point three. The ovary produces female gametes called ova or eggs, shown in Figure six point four. In human beings, a single matured egg is released into the oviduct by one of the ovaries every month. The uterus is the part where development of the baby takes place. Like the sperm, an egg is also a single cell. The size of eggs in animals varies. The egg may be very small as in humans, much larger as in hens. The ostrich egg is the largest. 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, as shown in Figure six point five. 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 or zygote, shown in Figure six point six. Did you know that the zygote is the beginning of a new individual?
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The process of fertilisation is the meeting of an egg cell from the mother and a sperm cell from the father. 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 recognise some characters in them similar to those of your mother or your father. Fertilisation which takes place inside the female body is called internal fertilisation. Internal fertilisation occurs in many animals including humans, cows, dogs and hens. Have you heard of test tube babies? 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 in vitro fertilisation, which means 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. 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.
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You will be surprised to know that in many animals fertilisation takes place outside the body of the female. In these animals, fertilisation takes place in water. Let us find out how this happens through Activity six point one. Visit some ponds or slow flowing streams during spring or rainy season. Look out for clusters of frog’s eggs floating in water. Write down the colour and size of the eggs. During spring or rainy season, frogs and toads move to ponds and slow flowing streams. When the male and female come together in water, the female lays hundreds of eggs. Unlike hen’s egg, frog’s egg is not covered by a shell and it is comparatively very delicate. A layer of jelly holds the eggs together and provides protection to the eggs, as shown in Figure six point seven. As the eggs are laid, the male deposits sperms over them. Each sperm swims randomly in water with the help of its long tail. The sperms come in contact with the eggs. This results in fertilisation. This type of fertilisation in which the fusion of a male and a female gamete takes place outside the body of the female is called external fertilisation. It is very common in aquatic animals such as fish and starfish. Why do fish and frogs lay eggs in hundreds whereas a hen lays only one egg at a time? Though these animals lay hundreds of eggs and release millions of sperms, all the eggs do not get fertilised and develop into new individuals. This is because the eggs and sperms get exposed to water movement, wind and rainfall. Also, there are other animals in the pond which may feed on eggs. Thus, production of large number of eggs and sperms is necessary to ensure fertilisation of at least a few of them.
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How could a single cell become such a big individual? Let us study the Development of Embryo. Fertilisation results in the formation of zygote which begins to develop into an embryo, as shown in Figure six point eight part a. The zygote divides repeatedly to give rise to a ball of cells, shown in part b. 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, shown in part c. The embryo continues to develop in the uterus. It gradually develops body parts such as hands, legs, head, eyes, ears. The stage of the embryo in which all the body parts can be identified is called a foetus, as shown in Figure six point nine. When the development of the foetus is complete, the mother gives birth to the baby. Internal fertilisation takes place in hens also. But, do hens give birth to babies like human beings and cows? You know that they do not. Then, how are chicks born? Soon after fertilisation, the zygote divides repeatedly and travels down the oviduct. As it travels down, many protective layers are formed around it. The hard shell that you see 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 three weeks to develop into a chick. You must have seen the hen sitting on the eggs to provide sufficient warmth. Did you know that development of the chick takes place inside the egg shell during this period? After the chick is completely developed it bursts open the egg shell. In animals which undergo external fertilisation, development of the embryo takes place outside the female body. The embryos continue to grow within their egg coverings. After the embryos develop, the eggs hatch. You must have seen numerous tadpoles swimming in ponds and streams.
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Let us learn about Viviparous and Oviparous Animals. We have learnt that 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. The following activity will help you understand better and differentiate between viviparous and oviparous animals. Activity six point two. Try to observe eggs of the following organisms: frog, lizard, butterfly or moth, hen and crow or any other bird. Were you able to observe eggs of all of them? Make drawings of the eggs that you have observed. The eggs of a few animals are easy to observe because their mothers lay them outside their bodies. These are examples of oviparous animals. But you would not be able to collect the eggs of a dog, cow or cat. This is because they do not lay eggs. The mother gives birth to the young ones. These are examples of viviparous animals. Can you now give some more examples of viviparous and oviparous animals? 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. The life cycle of frog is shown in Figure six point ten. Observe the different stages of frog starting from the egg to the adult stage. We find that there are three distinct stages, that is, egg, tadpole or larva, and adult. Don’t the tadpoles look so different from the adults? Can you imagine that these tadpoles would some day become frogs? The tadpoles transform into adults capable of jumping and swimming. The transformation of the larva into an adult through drastic changes is called metamorphosis. What about the changes that we observe in our body as we grow? Do you think we too undergo metamorphosis? In human beings, body parts similar to those present in the adults are present from the time of birth.
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Now we move to section six point three, Asexual Reproduction. So far, we have learnt about reproduction in some familiar animals. But what about very small animals like hydra and microscopic organisms like amoeba? Do you know how they reproduce? Let us find out. Activity six point three. Get permanent slides of hydra. Observe them using hand lens or a microscope. Look out for any bulges from the parent body. Count the number of bulges that you see in different slides. Also, note the size of the bulges. Draw the diagram of hydra, as you see it. Compare it with Figure six point eleven, which shows budding in hydra. In each hydra, there may be one or more bulges. These bulges are the developing new individuals and they are called buds. Recall the presence of buds in yeast. In hydra too the new individuals develop as outgrowths from a single parent. This type of reproduction in which only a single parent is involved is called asexual reproduction. Since new individuals develop from the buds in hydra, this type of asexual reproduction is called budding. Another method of asexual reproduction is observed in the microscopic organism, amoeba. Let us see how this happens. You have already learnt about the structure of amoeba. You will recall that amoeba is a single celled organism, shown in Figure six point twelve part a. It begins the process of reproduction by the division of its nucleus into two nuclei, shown in part b. This is followed by division of its body into two, each part receiving a nucleus, shown in part c. Finally, two amoebae are produced from one parent amoeba, shown in part d. 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, there are other methods by which a single parent reproduces young ones. You will study about these in your higher classes.
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Let us read the Story of Dolly, the Clone. Cloning is the 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, shown in Figure six point thirteen part c. Dolly was born on fifth July nineteen ninety six and was the first mammal to be cloned. During the process of cloning Dolly, a cell was collected from the mammary gland of a female Finn Dorsett sheep, shown in part a. Simultaneously, an egg was obtained from a Scottish blackface ewe, shown in part b. The nucleus was removed from the egg. Then, 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. The egg thus produced was implanted into the Scottish blackface ewe. Development of this egg followed normally and finally Dolly was born. Though Dolly was given birth by the Scottish blackface ewe, it 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. Dolly was a healthy clone of the Finn Dorsett sheep and produced several offspring of her own through normal sexual means. Unfortunately, Dolly died on fourteenth February two thousand three due to a certain lung disease. Since Dolly, several attempts have been made to produce cloned mammals. However, many die before birth or die soon after birth. The cloned animals are many a times found to be born with severe abnormalities.
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Before we proceed to the exercises, let us review the keywords and what you have learnt. There are two modes by which animals reproduce: sexual reproduction and 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 male include testes, sperm ducts and penis. The ovary produces female gametes called ova and the testes produce male gametes called sperms. The fusion of ovum and sperm is called fertilisation. The fertilised egg is called a zygote. Fertilisation that takes place inside the female body is called internal fertilisation. This is observed in human beings and other animals such as hens, cows and dogs. Fertilisation that takes place outside the female body is called external fertilisation. This is observed in frogs, fish, and starfish. The zygote divides repeatedly to give rise to an embryo. The embryo gets embedded in the wall of the uterus for further development. The stage of the embryo in which all the body parts are identifiable is called foetus. Animals such as human beings, cows and dogs which give birth to young ones are called viviparous animals. Animals such as hen, frog, lizard and butterfly which lay eggs are called oviparous animals. The transformation of the larva into adult through drastic changes is called metamorphosis. The type of reproduction in which only a single parent is involved is called asexual reproduction. In hydra, new individuals 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.
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Now let us solve the Exercises step by step. Question one: Explain the importance of reproduction in organisms. Answer: Reproduction is essential for the continuation of a species. It ensures the continuation of similar kinds of individuals, generation after generation. Without reproduction, organisms would not be able to survive as a species over time. Question two: Describe the process of fertilisation in human beings. Answer: In human beings, fertilisation begins when millions of sperms produced by the testes reach the female reproductive tract. When sperms come in contact with an egg released by the ovary, one of the sperms fuses with the egg. This fusion of the egg and the sperm is called fertilisation. During fertilisation, the nuclei of the sperm and the egg fuse to form a single nucleus, resulting in the formation of a fertilised egg or zygote. This process takes place inside the female body, so it is called internal fertilisation. Question three: Choose the most appropriate answer. Part a: Internal fertilisation occurs. The correct answer is one, in female body. Part b: A tadpole develops into an adult frog by the process of. The correct answer is two, metamorphosis. Part c: The number of nuclei present in a zygote is. The correct answer is two, one.
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Question four: Indicate whether the following statements are True or False. Part a: Oviparous animals give birth to young ones. False. Part b: Each sperm is a single cell. True. Part c: External fertilisation takes place in frog. True. Part d: A new human individual develops from a cell called gamete. False. It develops from a zygote. Part e: Egg laid after fertilisation is made up of a single cell. True. Part f: Amoeba reproduces by budding. False. It reproduces by binary fission. Part g: Fertilisation is necessary even in asexual reproduction. False. Part h: Binary fission is a method of asexual reproduction. True. Part i: A zygote is formed as a result of fertilisation. True. Part j: An embryo is made up of a single cell. False. It is made up of a ball of many cells. Question five: Give two differences between a zygote and a foetus. Answer: First, a zygote is a single cell formed immediately after the fusion of sperm and egg, whereas a foetus is a later stage of development where all body parts are identifiable. Second, a zygote undergoes repeated cell division to form an embryo, while a foetus develops from the embryo and grows in the uterus until birth. Question six: Define asexual reproduction. Describe two methods of asexual reproduction in animals. Answer: Asexual reproduction is the type of reproduction in which only a single parent is involved. Two methods are budding and binary fission. In budding, new individuals develop as outgrowths or buds from a single parent, as seen in hydra. In binary fission, an animal reproduces by dividing its nucleus and then its body into two individuals, as seen in amoeba.
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Question seven: In which female reproductive organ does the embryo get embedded? Answer: The embryo gets embedded in the wall of the uterus for further development. Question eight: What is metamorphosis? Give examples. Answer: The transformation of the larva into an adult through drastic changes is called metamorphosis. Examples include the life cycle of a frog, where the egg hatches into a tadpole which then transforms into an adult frog, and the life cycle of a butterfly, where a caterpillar transforms into a butterfly. Question nine: Differentiate between internal fertilisation and external fertilisation. Answer: Internal fertilisation takes place inside the female body, as seen in humans, cows, dogs, and hens. It usually involves fewer eggs and higher survival rates. External fertilisation takes place outside the female body, usually in water, as seen in frogs, fish, and starfish. It involves the release of hundreds of eggs and millions of sperms into the environment, with lower survival rates due to environmental hazards and predators. Question ten: Complete the crossword puzzle using the hints given below. Across one: The process of the fusion of the gametes. Answer: FERTILISATION. Across six: The type of fertilisation in hen. Answer: INTERNAL. Across seven: The term used for bulges observed on the sides of the body of hydra. Answer: BUDS. Across eight: Eggs are produced here. Answer: OVARY. Down two: Sperms are produced in these male reproductive organs. Answer: TESTES. Down three: Another term for in vitro fertilisation. Answer: TESTTUBE. Down four: These animals lay eggs. Answer: OVIPAROUS. Down five: A type of fission in amoeba. Answer: BINARY.
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Now let us explore the Extended Learning Activities and Projects. Activity one: Visit a poultry farm. Talk to the manager of the farm and try to find out the answers to the following. Part a: What are layers and broilers in a poultry farm? Layers are hens bred specifically for laying eggs, while broilers are chickens bred for meat production. Part b: Do hens lay unfertilised eggs? Yes, hens lay unfertilised eggs regularly even without mating with a rooster. Part c: How can you obtain fertilised and unfertilised eggs? Fertilised eggs are obtained when hens are kept with roosters, allowing mating. Unfertilised eggs are obtained from hens kept separately from roosters. Part d: Are the eggs that we get in the stores fertilised or unfertilised? The eggs sold in commercial stores are generally unfertilised. Part e: Can you consume fertilised eggs? Yes, fertilised eggs are safe to consume and are nutritionally similar to unfertilised eggs. Part f: Is there any difference in the nutritional value of fertilised and unfertilised eggs? There is no significant difference in nutritional value between fertilised and unfertilised eggs. Activity two: Observe live hydra yourself and learn how they reproduce by doing the following activity. During the summer months collect water weeds from ponds or ditches along with the pond water and put them in a glass jar. After a day or so you may see several hydra clinging to the sides of the jar. Hydra is transparent, jelly like and with tentacles. It clings to the jar with the base of its body. If the jar is shaken, the hydra will contract instantly into a small blob, at the same time drawing its tentacles in. Now take out few hydras from the jar and put them on a watch glass. Using a hand lens or a binocular or dissection microscope, observe the changes that are taking place in their body. Note down your observations.
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Activity three: The eggs we get from the market are generally the unfertilised ones. In case you wish to observe a developing chick embryo, get a fertilised egg from the poultry or hatchery which has been incubated for thirty six hours or more. You may then be able to see a white disc like structure on the yolk. This is the developing embryo. Sometimes if the heart and blood vessels have developed you may even see a red spot. Activity four: Talk to a doctor. Find out how twinning occurs. Look for any twins in your neighbourhood, or among your friends. Find out if the twins are identical or non identical. Also find out why identical twins are always of the same sex? If you know of any story about twins, write it in your own words. Identical twins occur when a single fertilised egg splits into two separate embryos during early development. Because they originate from the same zygote, they share identical genetic material, which is why they are always of the same sex. Non identical twins occur when two separate eggs are fertilised by two different sperms. For more information on animal reproduction, you can visit the websites www dot saburchill dot com and www dot teenshealth dot org slash teen slash sexual health.
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Finally, let us read the Did You Know section. An interesting organisation is observed in a honey bee hive, a colony of several thousand bees. Only one bee in the colony lays eggs. This bee is called the queen bee. All other female bees are worker bees. Their main job is to build the hive, look after the young and feed the queen bee adequately to keep her healthy so that she can lay eggs. A queen bee lays thousands of eggs. The fertilised eggs hatch into females, while the unfertilised eggs give rise to males, called drones. It is the job of the worker bees to maintain the temperature of the hive at around thirty five degrees Celsius to incubate the eggs.
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Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]