Reproduction 8th Grade PDF

## Reproduction Reproduction is the process of producing organisms of one's own kind. It is necessary for continuity of life on Earth. Without this process, humans or other organisms will cease to exist, as there will be no one younger to replace the old and dying. In living things, there are two...

## Reproduction Reproduction is the process of producing organisms of one's own kind. It is necessary for continuity of life on Earth. Without this process, humans or other organisms will cease to exist, as there will be no one younger to replace the old and dying. In living things, there are two modes of reproduction: sexual reproduction and asexual reproduction. ### Sexual Reproduction Sexual reproduction involves two organisms of the same species to produce a new one, whereas in asexual reproduction, only a single parent is involved in producing its young one. In the previous class, you learnt how plants reproduce. Some reproduce asexually by: - Budding - Fragmentation - Spore formation - Vegetative propagation Others reproduce sexually through seeds and flowers. ### Reproduction In Animals #### Sexual Reproduction Sexual reproduction, as said above, involves two individuals of the same species, usually a male and a female. Most plants and animals reproduce through sexual reproduction. Males and females of a species have different reproductive organs. Each organism or parent produces reproductive cells, called gametes. The male organism produces sperm and the female gamete is known as egg or ovum. Sperm and ovum come together and fuse with each other to form a new cell, called a zygote. This process of fusion of sperm and ovum is called fertilisation. Generally, in animal species such as humans, birds, reptiles, and fishes, male and female reproductive organs are present in different individuals. The male carries the sperm and the female carries the eggs or ova. Such animals that possess only one type of sex organs and can produce only one type of reproductive cells are known as **unisexual animals**. However, in some species, such as the earthworm, both male and female reproductive cells are produced inside the same organism. Such organisms are called **hermaphrodites**. #### Fertilisation Fertilisation is the process of fusion of male and female gametes. In other words, when a sperm and an ovum fuse together and form a zygote, the process is known as fertilisation. There are two types of fertilisation: external fertilisation and internal fertilisation. ##### External Fertilisation In some animals, eggs and sperm meet outside the body of the organism. For example, during spring or rainy season, female frogs and toads lay eggs (also called frogspawn) and the males come to deposit sperms over them. Eggs and sperm present in water meet by chance, resulting 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 in a medium such as water is called external fertilisation. It is very common in aquatic animals such as fish, starfish, toads, sponges, jellyfish, and salamanders. Fish, frogs, and such other animals lay eggs and sperm in large numbers, though all the eggs do not get fertilised and develop into new individuals. This happens because the eggs and the sperm get exposed to water movement, wind, and rainfall, and other animals in the water. Most of the eggs are destroyed or eaten up. Therefore, a large number of eggs and sperm gives them better chances of fertilisation of at least a few of them. ##### Internal Fertilisation In animals such as hen, lizard and snake, females produce eggs inside their bodies and the males deposit the sperm inside the female body. Thus, the eggs get fertilised inside the body of the female. Such fertilisation is called internal fertilisation. Humans, reptiles and birds reproduce through internal fertilisation. In hermaphrodites also, such as earthworms and leeches, internal fertilisation takes place. Although they are hermaphrodites, the same individuals do not fertilise their own eggs. Two earthworms come together and exchange their sperms with each other for fertilisation. After fertilisation, the egg, now the zygote, comes out of the body of the female. When it comes out, it is covered with a hard shell made of calcium carbonate. This shell protects the embryo till it develops into a baby. ### Reproduction In Humans Almost every growing person has wondered where babies come from. Let us learn about this as you are a grown-up adolescent now. Learning about it will help you to be responsible in your social behaviour and activities. In humans, as in sexual reproduction, two individuals are involved: a male and a female. Human males and females have different reproductive systems. Due to internal fertilisation, the complete development of the embryo into the young one takes place inside the body of the female. A fully developed young one is delivered by the female after it envelops inside her for nine months. #### The Male Reproductive System The human male reproductive system includes the following organs: 1. **Testes** (singular: testis): Testes are present in a pair. These are held in place by the scrotum (scrotal sacs). The scrotum is a bag of tough skin. The testes produce millions of male cells called sperm and the male reproductive hormone called testosterone. 2. **Prostate Gland**: The job of the prostate gland is to secrete prostate fluid, one of the components of semen. The mixture of sperm and fluids is called semen or seminal fluid. This fluid passes through the urethra to move out of the body during ejaculation. 3. **Sperm Ducts**: The sperm ducts transfer sperm from testes to urethra. They also collect secretions from glands such as prostate. These secretions along with sperm form the semen. These fluids help the sperm in locomotion and their nourishment. 4. **Urethra**: Urethra is a tube inside the penis. It carries urine and semen at different times. It is the continuing part of the excretory system that you learnt about in the previous class. The penis is a muscular organ present in males through which the urethra passes. Since it is muscular, it can expand and contract. During sexual activity, it expands and becomes erect. It helps in transferring semen from the male's body to the female's reproductive organs. The role of the penis is to pass urine out of the body and to help transferring sperm into the female reproductive organ. #### The Female Reproductive System The human female reproductive system is made up of ovaries, fallopian tubes, uterus, cervix, and vagina. 1. **Ovaries**: Each human female possesses two fist-sized ovaries in their abdomen. Females are born with hundreds of undeveloped female egg cells or ova (singular: ovum). These eggs are stored in the ovaries and after puberty, one of them is released by the ovaries every month. Unused eggs disintegrate and pass out during menstruation. 2. **Fallopian Tubes (Oviduct)**: The fallopian tube or oviduct is the narrow tube through which an egg cell or female gamete travels to the uterus. Each ovary is connected to the uterus by an oviduct. The sperm entering the reproductive system of females swim up to the ovum or egg in this tube. 3. **Uterus**: The uterus is an inverted pear-shaped muscular bag with very soft inner lining. The lining secretes some fluid and is supplied with lots of blood vessels, bringing nutrients and gases through blood. The uterus holds the fertilised egg and nurtures it till it is a fully developed baby. The developing baby makes connection with the uterus walls and the blood vessels so that it receives its nutrition. 4. **Cervix**: The cervix is a ring of muscle located at the lower third portion of the uterus. It forms a barrier between the uterus and the vagina. Until birth, the baby is held in place by the cervix. During birth, the cervix expands and the baby passes through it. 5. **The Vagina**: This is an elastic, long muscular canal that connects the cervix to the outside of the body. It is the tube that receives the penis that ejaculates semen during mating. During menstruation, it is the same tube through which the blood flows out. Along with cervix, it makes the birth canal for the baby. ##### Fertilisation As we now know that fertilisation takes place in the fallopian tube. The sperm cells make their way through to the top of the vagina, cervix and into the uterus. But they do not stay there. They continue their journey into the fallopian tubes and travel upward. If there is any ovum (egg cell) released from one of the ovaries, a sperm cell enters the fallopian tube of that side. The chances of fertilisation at that time are very high. The sperm cell that manages to reach the egg cell first, fertilises it. Afterwards a barrier forms around the fertilised egg and other sperm cannot fertilise it any more. During fertilisation, the nuclei of the sperm and the egg fuse to form a single nucleus. Such fusion of the egg and the sperm that results in the formation of a fertilised egg or zygote is called fertilisation. The zygote (fertilised egg) at this stage possesses half the genes (characteristics) of each of its two parents. If the sperm and egg do not meet, both of them just disintegrate and get washed away. In this process, along with the sperm and the egg, some blood as well as uterine muscles are also expelled from the body of the female. This process is called menstruation. Mammals such as humans, cows, and elephants usually produce one mature egg at one time and thus only a single baby is produced. Other animals such as dogs and cats produce multiple eggs and can give birth to multiple babies at a time. #### Development of Embryo After fertilisation, the zygote divides itself to form a very tiny ball of cells called an embryo. The embryo continues to move downwards from the fallopian tube, and finds a comfortable place in the uterus. There, the embryo attaches itself to the lining (walls) of the uterus through a tissue called placenta. The female is said to be pregnant now. Placenta helps to receive nutrition, oxygen, and other materials required by the developing embryo. It also helps the embryo to send out wastes to the mother's body. The cells of the embryo begin to form groups that develop into different tissues and organs of the body. 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 (FEE-TUHS). After nine months, the baby is fully developed and ready to be born. The baby at this stage weighs between 6 and 8 pounds. #### Sex Determination Whether a zygote will develop into a boy or a girl is decided at fertilisation. The gender is decided by the “sex chromosome,” Sex chromosomes are a pair of chromosomes that determines the gender in a baby. There are a total of 46 chromosomes (23 pairs) in each cell of a human being. In each pair, one chromosome comes from each parent. Out of the total 23 pairs, one pair forms the sex chromosomes. There are two different forms of these chromosomes: 1. Males have one large (X-shaped) and one smaller (Y-shaped) sex chromosomes. So, males are said to possess XY sex chromosomes. 2. Females have two identical (X-shaped) sex chromosomes. So females are said to possess XX sex chromosomes. Each gamete (egg cell or sperm cell) carries one type of sex chromosome (either X or Y). Females produce only one type of egg cell containing an X-chromosome. Total sperm cells produced by a male may be of two types: - 50 per cent carry the X-chromosomes - 50 per cent carry the Y-chromosomes There is an equal probability of fertilisation of the ovum with the sperm carrying either an X or a Y chromosome. In case the ovum fertilises with a sperm carrying an X-chromosome, the zygote develops into a female (XX). If the fertilisation of ovum occurs with a Y-chromosome carrying sperm, the result is a male offspring. From the above, we can see that the gender of the child is decided by the male partner, although he has no control on what kind of sperm cell to release. It is also evident that in each pregnancy, there is always 50 per cent probability of either a male or a female child. It is unfortunate that in our society, women are blamed for producing female children and have been ill-treated because of this false notion. Now, you know that the sex chromosomes of the father determine the sex of an unborn baby! The belief that the mother is responsible for the sex of her baby is completely wrong and to blame her for it totally unjustified. ### Viviparous and Oviparous Animals We have learnt that some animals give birth to young ones, whereas some animals first lay eggs which later develop into young ones. The animals that give birth to young ones are called **viviparous animals**. Examples are cat, dog, rat, whale, monkey, cattle, horse, elephant, and human. Animals that lay eggs are called **oviparous animals**. Examples are insects, beetles, spiders, snails, fish, frog, and reptiles. The eggs of an oviparous animal are easy to collect because the female lays them outside their bodies. Further development of animal takes place outside the body of the parents. Viviparous animals do not lay their eggs, but allow fertilisation to occur within the female's body. The zygote formation and its development into a young animal takes place while taking nutrition from the mother. The mother gives birth to the young ones. #### Development of Embryo In Oviparous and Viviparous Animals The animals such as hen generally look after the eggs after laying them. You must have seen a hen sitting over her eggs, providing them warmth. It is called incubation. The warmth helps the embryos to develop into chicks inside the eggs properly. An embryo takes about 3 weeks (21 days) to develop into a chick. After the chick is completely developed, it breaks open the egg shell. This is termed as hatching of eggs. In animals with external fertilisation, development of the embryo takes place outside the female body. The embryo continues to grow within its egg coverings. **Young ones to adults**: If you are a regular visitor to the place, you may find their tadpoles, changing their body slowly to little froglets with their tails shortening, and body becoming larger finally turning into a big frog. Recall the life cycle of the silkworm (egg larva or caterpillar pupa adult). 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 transformation of the larva into an adult through drastic changes is called metamorphosis. In other words, the change from one stage to another by changing body form is called metamorphosis. If the baby animal and adult have the same body form, it is termed as direct development. In human beings, body parts similar to those present in the adults are present from the time of the birth. Metamorphosis in insects is controlled by insect hormones. in a frog, it is controlled by thyroxin, the hormone produced by thyroid gland. Thyroxin production requires the presence of iodine in water. If the water in which the tadpoles are growing does not contain sufficient iodine, the tadpoles cannot become adults. ### Asexual Reproduction When an organism produces more individuals of its own type without the involvement of gametes and their fusion, it is termed as asexual reproduction. In the previous class, you learnt about some asexual ways of reproduction in plants such as spore formation, fragmentation, and vegetative propagation. Now, let us study about two ways of asexual reproduction in animals. #### Binary Fission In amoeba, cell division (binary fission) leads to the creation of new individuals. An amoeba simply splits into two equal halves in any plane during cell division. Binary fission is also seen in Leishmania (which causes kala-azar), Trypanosoma (which causes sleeping sickness) and Paramecium. Other single-celled organisms, such as the malarial parasite, Plasmodium, divide into many daughter cells simultaneously by multiple fission. #### Budding Organisms such as hydra reproduce through the process of budding. In this process, a bud develops as an outgrowth at one specific site of the organism. The bud develops into a tiny individual. When fully mature, it detaches from the parent body and becomes a new independent individual. There are two types of twins: identical twins and non-identical twins. In non-identical twins, two different egg cells are released, one in each fallopian tube. Two sperm cells fertilise them and produce two zygotes. Because they are from two different eggs, they tend to have clear differences among them when these twins are born. They may have different genders as well. The scientific name for this kind of twins is dizygotic (formed from two zygotes) and are also called fraternal twins. In identical twins, there is only one egg cell and one sperm cell involved in fertilisation. After fertilisation, the zygote starts dividing to increase the number of cells but instead of making one ball, it splits into two balls of cells. This means they grow into two very similar foetuses and identical babies. The scientific name for such twins is monozygotic. This process does not happen often. In fact, identical twins make about one-third of all the twins born. They are either both boys or girls.