Reproduction in Animals PDF

REPRODUCTION IN ANIMALS D o you recall the processes of Table 6.1 digestion, circulation and respiration which you have S. No. Animal Young one studied in your previous classes? These 1. Human Baby pr...

REPRODUCTION IN ANIMALS D o you recall the processes of Table 6.1 digestion, circulation and respiration which you have S. No. Animal Young one studied in your previous classes? These 1. Human Baby processes are essential for the survival of every individual. You have also learnt 2. Cat about the process of reproduction in 3. Dog plants. Reproduction is essential for the continuation of a species. Imagine what 4. Butterfly would have happened if organisms had 5. Hen Chick not reproduced. You will realise that reproduction is very important as it 6. Cow ensures the continuation of similar 7. Frog kinds of individuals, generation after generation. Just as in plants, there are two modes You have already learnt in your by which animals reproduce. These are: previous class about reproduction in (i) Sexual reproduction, and plants. In this chapter, we shall learn (ii) Asexual reproduction. how reproduction takes place in animals. 6.2 Sexual Reproduction 6.1 Modes of Reproduction Try to recall reproduction in plants which you studied in Class VII. You will Have you seen the young ones remember that plants that reproduce of different animals? Try to name some sexually have male and female of the young ones by completing reproductive parts. Can you name these Table 6.1 shown in examples at S. No. parts? In animals also, males and 1 and 5. females have different reproductive parts You must have seen the young or organs. Like plants, the reproductive ones of various animals being born. parts in animals also produce gametes Can you tell how chicks and that fuse to form a zygote. It is the zygote caterpillars are born? How are kittens which develops into a new individual. and puppies born? Do you think that This type of reproduction beginning these young ones looked the same from the fusion of male and female before they were born as they do now? gametes is called sexual reproduction. Let us find out. Let us find out the reproductive parts 2024-25 in humans and study the process of sperm is a single cell with all the usual reproduction in them. cell components. Male Reproductive Organs Head The male reproductive organs include a pair of testes (singular, testis), two sperm ducts and a penis (Fig. 6.1). The testes Middle piece produce the male gametes called sperms. Millions of sperms are produced by the testes. Look at Fig. 6.2 which shows the picture of a sperm. Though sperms are very small in size, each has Tail a head, a middle piece and a tail. Does it appear to be a single cell? Indeed, each Fig. 6.2 : Human sperm What purpose does the tail in a sperm serve? Sperm duct Female Reproductive Organs The female reproductive organs are a pair of ovaries, oviducts (fallopian tubes) and the uterus (Fig. 6.3). The ovary produces Penis Oviduct Ovary Uterus Testis Fig. 6.1: Male reproductive organs in humans Fig. 6.3 : Female reproductive organs in humans REPRODUCTION IN ANIMALS 67 2024-25 female gametes called ova (eggs) (Fig. 6.4). In human beings, a single Sperms 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. Nucleus Ovum Fig. 6.5 : Fertilisation you know that the zygote is the beginning of a new individual? Fusing Fig. 6.4 : Human Ovum nuclei Boojho recalls that the size of eggs in animals varies. The egg may be very small as in humans, much larger as in hens. Ostrich egg is Fig. 6.6 : Zygote the largest! The process of fertilisation is the meeting of an egg cell from the mother Fertilisation and a sperm cell from the father. So, The first step in the process of the new individual inherits some reproduction is the fusion of a sperm characteristics from the mother and and an ovum. When sperms come in some from the father. Look at your contact with an egg, one of the sperms brother or sister. See if you can recognise may fuse with the egg. Such fusion of some characters in them similar to those the egg and the sperm is called of your mother or your father. fertilisation (Fig. 6.5). During Fertilisation which takes place inside fertilisation, the nuclei of the sperm and the female body is called internal the egg fuse to form a single nucleus. fertilisation. Internal fertilisation This results in the formation of a occurs in many animals including fertilised egg or zygote (Fig. 6.6). Did humans, cows, dogs and hens. 68 SCIENCE 2024-25  Have you heard of female lays hundreds of eggs. Unlike test tube babies? hen’s egg, frog’s egg is not covered by a shell and it is comparatively very Boojho and Paheli’s teacher once told delicate. A layer of jelly holds the eggs them in the class that in some women oviducts are blocked. These women together and provides protection to the are unable to bear babies because eggs (Fig. 6.7). 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. 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 Fig. 6.7 : Eggs of frog grow in test tubes. As the eggs are laid, the male You will be surprised to know that deposits sperms over them. Each in many animals fertilisation takes sperm swims randomly in water with place outside the body of the female. the help of its long tail. The sperms In these animals, fertilisation takes come in contact with the eggs. This place in water. Let us find out how results in fertilisation. This type of this happens. fertilisation in which the fusion of a male and a female gamete takes place Activity 6.1 outside the body of the female is called Visit some ponds or slow-flowing external fertilisation. It is very streams during spring or rainy common in aquatic animals such as season. Look out for clusters of fish, starfish, etc. f r og’s eggs floating in water. Write down the colour and size of the eggs. Why do fish and During spring or rainy season, frogs frogs lay eggs in hundreds whereas a and toads move to ponds and slow- hen lays only one flowing streams. When the male and egg at a time? female come together in water, the REPRODUCTION IN ANIMALS 69 2024-25  How could a single cell Though these animals lay become such a big hundreds of eggs and release individual? millions of sperms, all the eggs do not get fertilised and develop Development of Embryo into new individuals. Fertilisation results in the formation of This is because the eggs and zygote which begins to develop into an sperms get exposed to water embryo [Fig. 6.8(a)]. The zygote divides movement, wind and rainfall. repeatedly to give rise to a ball of cells Also, there are other animals in [Fig. 6.8(b)]. The cells then begin to form the pond which may feed groups that develop into different tissues on eggs. Thus, production and organs of the body. This developing of large number of eggs and structure is termed an embryo. The sperms is necessary to embryo gets embedded in the wall of the ensure fertilisation uterus for further development of at least [Fig. 6.8(c)]. a few of them. The embryo continues to develop in the uterus. It gradually develops body Uterus Ovary Zygote (b) Embedding embryo Uterus Ovulation wall (a) Developing embryo (c) Fig. 6.8 : (a) Zygote formation and development of an embryo from the zygote; (b) Ball of cells (enlarged); (c) Embedding of the embryo in the uterus (enlarged) 70 SCIENCE 2024-25 parts such as hands, legs, head, eyes, place inside the egg shell during this ears etc. The stage of the embryo in period? After the chick is completely which all the body parts can be developed it bursts open the egg shell. identified is called a foetus (Fig. 6.9). In animals which undergo external When the development of the foetus is fertilisation, development of the embryo complete, the mother gives birth to takes place outside the female body. The the baby. embryos continue to grow within their egg coverings. After the embryos develop, the eggs hatch. You must have seen Uterus numerous tadpoles swimming in ponds and streams. 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 Fig. 6.9 : Foetus in the uterus between viviparous and oviparous animals. Internal fertilisation takes place in hens also. But, do hens give birth to Activity 6.2 babies like human beings and cows? You Try to observe eggs of the following know that they do not. Then, how are organisms – frog, lizard, butterfly chicks born? Let us find out. or moth, hen and crow or any other Soon after fertilisation, the zygote bird. Were you able to observe eggs divides repeatedly and travels down the of all of them? Make drawings of the oviduct. As it travels down, many eggs that you have observed. protective layers are formed around it. The hard shell that you see in a hen’s The eggs of a few animals are easy egg is one such protective layer. to observe because their mothers lay After the hard shell is formed around them outside their bodies. These are the developing embryo, the hen finally examples of oviparous animals. But you lays the egg. The embryo takes about 3 would not be able to collect the eggs of weeks to develop into a chick. You must a dog, cow or cat. This is because they have seen the hen sitting on the eggs to do not lay eggs. The mother gives birth provide sufficient warmth. Did you know to the young ones. These are examples that development of the chick takes of viviparous animals. REPRODUCTION IN ANIMALS 71 2024-25  Can you now give some more The tadpoles transform into adults examples of viviparous and oviparous capable of jumping and swimming. The animals? transformation of the larva into an adult through drastic changes is called Young Ones to Adults metamorphosis. What about the The new individuals which are born or changes that we observe in our body as hatched from the eggs continue to grow we grow? Do you think we too undergo till they become adults. In some metamorphosis? In human beings, animals, the young ones may look very body parts similar to those present in different from the adults. The life cycle the adults are present from the time of frog is shown in Fig. 6.10. of birth. Observe the different stages of frog starting from the egg to the adult stage. 6.3 Asexual Reproduction We find that there are three distinct So far, we have learnt about reproduction stages, that is, egg → tadpole (larva) → in some familiar animals. But what adult. Don’t the tadpoles look so about very small animals like hydra and different from the adults? Can you microscopic organisms like amoeba? Do imagine that these tadpoles would some you know how they reproduce? Let us day become frogs? find out. (b) Early tadpole (a) Eggs (c) Late tadpole (d) Adult frog Fig. 6.10 : Life cycle of frog 72 SCIENCE 2024-25 Activity 6.3 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 Fig. 6.11 : Budding in Hydra the Fig. 6.11. 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 (a) 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 Dividing nucleus hydra, this type of asexual reproduction (b) 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 (c) amoeba is a single-celled organism [Fig. 6.12(a)]. It begins the process of reproduction by the division of its Daughter amoebae nucleus into two nuclei [Fig. 6.12(b)]. This is followed by division of its body into two, each part receiving a nucleus [Fig. 6.12(c)]. Finally, two amoebae are produced from one parent amoeba (d) [Fig. 6.12(d)]. This type of asexual Fig. 6.12 : Binary fission in Amoeba REPRODUCTION IN ANIMALS 73 2024-25 reproduction in which an animal there are other methods by which a reproduces by dividing into two single parent reproduces young ones. individuals is called binary fission. You will study about these in your Apart from budding and binary fission, higher classes. 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 [Fig. 6.13 (c)]. Dolly was born on 5th July 1996 and was the first mammal to be cloned. (a) Finn Dorsett sheep (b) Scottish blackface ewe (c) Dolly Fig. 6.13 During the process of cloning Dolly, a cell was collected from the mammary gland of a female Finn Dorsett sheep [Fig. 6.13 (a)]. Simultaneously, an egg was obtained from a Scottish blackface ewe [Fig. 6.13 (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 14th February 2003 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. 74 SCIENCE 2024-25 KEYWORDS WHAT YOU HAVE LEARNT ASEXUAL Ü There are two modes by which animals repro- REPRODUCTION duce. These are: (i) Sexual reproduction, and (ii) Asexual reproduction. BINARY FISSION Ü Reproduction resulting from the fusion of male and female gametes is called sexual BUDDING reproduction. Ü The reproductive organs in the female include EGGS ovaries, oviducts and uterus. Ü The reproductive organs in male include EMBRYO testes, sperm ducts and penis. Ü The ovary produces female gametes called ova EXTERNAL and the testes produce male gametes called FERTILISATION sperms. Ü The fusion of ovum and sperm is called fertilisation. FERTILISATION The fertilised egg is called a zygote. Ü Fertilisation that takes place inside the female FOETUS body is called internal fertilisation. This is observed in human beings and other animals INTERNAL such as hens, cows and dogs. FERTILISATION Ü Fertilisation that takes place outside the female body is called external fertilisation. This METAMORPHOSIS is observed in frogs, fish, starfish, etc. Ü The zygote divides repeatedly to give rise to an OVIPAROUS ANIMALS embryo. Ü The embryo gets embedded in the wall of the SEXUAL uterus for further development. REPRODUCTION Ü The stage of the embryo in which all the body parts are identifiable is called foetus. SPERMS Ü Animals such as human beings, cows and dogs which give birth to young ones are called VIVIPAROUS ANIMALS viviparous animals. ZYGOTE Ü Animals such as hen, frog, lizard and butterfly which lay eggs are called oviparous animals. Ü The transformation of the larva into adult through drastic changes is called metamorphosis. Ü The type of reproduction in which only a single parent is involved is called asexual reproduction. Ü In hydra, new individuals develop from buds. This method of asexual reproduction is called budding. Ü Amoeba reproduces by dividing itself into two. This type of asexual reproduction is called binary fission. REPRODUCTION IN ANIMALS 75 2024-25  Exercises 1. Explain the importance of reproduction in organisms. 2. Describe the process of fertilisation in human beings. 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 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. ( ) 5. Give two differences between a zygote and a foetus. 6. Define asexual reproduction. Describe two methods of asexual reproduction in animals. 7. In which female reproductive organ does the embryo get embedded? 8. What is metamorphosis? Give examples. 9. Differentiate between internal fertilisation and external fertilisation. 76 SCIENCE 2024-25  10. Complete the crossword puzzle using the hints given below. EXERCISES 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. Extended Learning — Activities and Projects 1. Visit a poultry farm. Talk to the manager of the farm and try to find out the answers to the following. (a) What are layers and broilers in a poultry farm? (b) Do hens lay unfertilised eggs? (c) How can you obtain fertilised and unfertilised eggs? (d) Are the eggs that we get in the stores fertilised or unfertilised? (e) Can you consume fertilised eggs? (f) Is there any difference in the nutritional value of fertilised and unfertilised eggs? REPRODUCTION IN ANIMALS 77 2024-25  2. 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. 3. 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 36 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. 4. 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. For more information on animal reproduction, you can visit : l www.saburchill.com l www.teenshealth.org/teen/sexual-health Did You Know? 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 35°C to incubate the eggs. 78 SCIENCE 2024-25