• Robert Hooke was the first scientist who observed thin slices of cork (obtained from the bark of a tree) through his self-designed microscope , in 1665.
• He observed that they had honey-comb like structures consisting of little compartments (in Latin, 'cell' means 'a little room').
• It was later explained that these 'compartments' were actually 'dead cells, bound by a 'cell wall'. 
• The cells have the same basic structure, but they are different, with respect to their number, shape and size, in different living organisms.

Cell Number

• The shapes of cells differ not only in different organisms but also in different organs of the same organism.
• They may be oval, spherical, cuboidal, fibre-like or polygonal.
• These differences in shapes are due to their location and function in the tissue. For instance, a nerve cell has to transmit nerve impulses to organs located in different parts of the body. Hence, they possess a long fibre-like structure. 
• The smallest cell PPLO (Pleuro pneumonia-like organism), also called mycoplasma, is about 0.1 micron (denoted as 'p') in diameter (1 p = 10-6m).
• The ostrich egg, considered to be the largest cell, is (nearly) 170 mm in diameter. The hen's egg also represents a single cell; it is big enough to be seen with the unaided eye.

• Take a hen's egg.
• Gently break its shell and transfer the contents to a flat plate.
• You will observe two clear portions.
• The central yellow mass is the yolk. It is surrounded by a transparent white jelly-like fluid, called albumen.
• Albumen and yolk represent the reserve food material in the cytoplasm.
• Hen's egg is a single cell. Its different parts have been labelled in the diagram given here.
  
  

Activity on Cheek Cell

• To observe animal cells make a temporary mount of cheek cells.
• Take a clean toothpick.
• Scratch it gently on the inner side of your cheek.
• Some frothy material appears on the toothpick.
• Rub it in the centre of a clean glass slide.
• Put a drop of methylene blue.
• Let it stain for a minute.
• Put a cover slip and observe it under the microscope.

• Follow the instructions given below to make a slide of onion peel. (Onion peel is the thin membrane-like layer present around fleshy scale leaves of onion.)
• Put a drop of water on a glass slide.
• Place a small piece of neatly cut onion peel on it.
• Put a drop, or two, of saffranin.
• Stain for a minute. 
• Put a cover slip and observe it under the microscope.

• All living cells are bound by a membrane called the plasma membrane, or the cell membrane.
• It surrounds its inner gel-like material called protoplasm.
• The plasma membrane controls the entry and exit of substances as per the requirements of the cell.
• Cell wall: The cells of plants, fungi and bacteria have an additional outer covering called the cell wall.
  The cell wall is an important covering in plant cells; it provides rigidity and protection to the cell against variations in the environment. It also gives a definite shape, size and support to the cell. 

• It is the most important part of the cell. It generally lies in the centre of the cell, however, in some cases, it may also occupy peripheral positions.
• It controls all the activities of the cell.
• The nucleus is a dense structure bound by a nuclear membrane.
• The protoplasm of the nucleus is called nucleoplasm.
• Chromatin: It has a thread-like network called chromatin.
• Chromosomes: When the cell is ready to divide, this chromatin condenses to form thicker, thread-like structures, called chromosomes.
• These chromosomes are the structures responsible for the characters (genes) inherited by one generation from the earlier generations. Many small living structures are present in the cell.
• These are equivalent to the organs of the body. Hence, they are named as 'cell organelles'.

1. Plastids
2. Mitochondria
3. Endoplasmic Reticulum
4. Golgi Complex
5. Vacuole
6. Ribosomes
7. Cilia and Flagella

• These are large cell organelles, characteristic of plant cells.
• These may contain pigments that provide colour to the cell.
• The green-coloured plastids are called chloroplasts.
• They manufacture food for green plants by the process of photosynthesis.
• The plastids, associated with the different coloured parts of the plants (like fruits, vegetables and flowers) are called chromoplasts.
• They are responsible for imparting colour (other than green) to the different parts of the plant.
• Plants also contain some colourless plastids called leucoplasts; these provide space to store starch, proteins, oils, etc.

• These are rod-shaped or spherical structures.
• They are present in large numbers in cells engaged in different physiological activities.
• They are responsible for cellular respiration and for generation of energy for different activities of life. 
• Hence, they are also called the powerhouse of the cell.

• It is a network of membranes.
• It provides channels for transport of materials in a cell.
• ER is of two types:
  (i) Rough ER : This type of ER has a rough appearance as it is studded with ribosomes. It plays a vital role in synthesis of proteins.
  (ii) Smooth ER: This type of ER does not have ribosomes attached to it; it, therefore, has a smooth appearance. It helps in the synthesis of fats.

• They are sac-like structures stacked one above the other.
• They are involved in the processing and packaging of materials produced by the cell.

• It appears as an empty space in the cytoplasm.
• It is generally large in plant cells.
• It stores excess of water and waste products.
• In Amoeba, food materials are held in its food vacuoles for digestion.

• These are tiny granules present in the cytoplasm and on the rough ER.
• They help in protein synthesis.

• Some cells have these small extensions on their cell membrane.
• They help in locomotion and collection of food.
• Unicellular organisms, like Paramoecium, have numerous cilia while Euglena has a single flagellum.

• In unicellular organisms, like Amoeba, a single cell performs all the necessary functions.
• It captures and digests food, respires, excretes, grows and reproduces.
• Multicellular organisms have cells that are specialised to perform specific functions.
• A group of cells, performing a specialised function, forms a tissue (for example, nervous tissue).
• A group of tissues, performing a specific function, forms an organ (for example, kidney).
• A number of such organs work together to form an organ system (for example, digestive system).

1. Digestive
2. Respiratory
3. Circulatory
4. Excretory
5. Skeletal
6. Muscular
7. Nervous
8. Reproductive
9. Endocrine
10. Integumentary

• All the cells have some common features.
• They can appear different in different parts of the organism. For example, the blood and liver cells (in animals), the root or leave cells (in plants) have different appearances.
• The plant and animal cells, however, have some major differences between them.

• Plant cells generally have a definite shape due to a rigid cell wall around them. In comparison, animal cells have a cell membrane as their outer cover. This provides flexibility to animal cells; hence they can show a large variation in their shapes.
• Plant cells have plastids; these are absent in animal cells.
• Plant cells generally have large vacuoles; animal cells, on the other hand, either lack vacuoles, or have very small vacuoles.

1. cell: basic structural and functional unit of life. 
2. cell membrane: a thin membrane that surrounds the protoplasm of every cell. 
3. cell organelles: a specialised sub-unit, within a cell, that has a specific function. 
4. chromosomes: thread-like structures found in the nucleus; responsible for the inheritance of characters. 
5. cytoplasm: portion of protoplasm, lying between the cell membrane and the nuclear membrane. 
6. cilia and flagella: extensions on the cell membrane, these help in locomotion and procurement of food in organisms like Amoeba and Paramoecium. 
7. endoplasmic reticulum: network of membranes which provides channels for transport of materials in the cell and helps in synthesis of proteins.
8. genes: unit of inheritance which gets transferred from one generation to the next.
9. golgi complex: sac-like structures; these help in processing and packaging of materials produced by the cell. 
10. mitochondria: rod-shaped structures inside a cell; these help in cellular respiration and production of energy. 
11. nucleus: a specialised structure in the cells, bound by the nuclear membrane; responsible for controlling all cellular activities. 
12. plastids: cell organelles found in plant cells. These may contain pigments which help in photosynthesis and are responsible for imparting colour to fruits, vegetables and flowers.
13. protoplasm: gel-like living matter present inside the cell membrane.
14. ribosomes: tiny granular structures found in the cytoplasm and on the endoplasmic reticulum; they help in protein synthesis.
15. tissue: group of cells performing a specialised function.
16. vacuole: sac-like membrane bound structures in cells; used for storing various materials. 

Types of Microorganisms

Viruses

Where do Microorganisms Live?

Role of Microorganisms in Our Life

Microbial Population in the Human Body

• Under normal conditions, our bodies house a large population of microorganisms.
• Microorganisms, however, kept in balance and are usually harmless.
• These microorganisms are important for our body; they form an essential system that helps our body.

• Curd and cheese formation
  Lactobacillus is a bacterium that helps in the formation of curd. At favourable temperatures, it multiplies in milk and converts it into curd. Some bacteria and fungi are also involved in the making of cheese. 
• Fermentation process in bakery
  Fungi, like yeast, reproduce rapidly and produce carbon dioxide. This gas, when trapped in dough, or batter (used for idlies, dosas), causes it to increase in volume and makes it fluffy and soft. This is known as fermentation.
• Alcoholic beverages
  Many microorganisms are used in the manufacture of alcohol, wine and acetic acid. Fungi, like yeast, convert natural sugars, present in cereals and fruits, into alcohol; this alcohol is then used to make alcoholic beverages. Acetic acid, commonly known as vinegar, is also produced by a similar process. 

1. Many microorganisms, present in the alimentary canal of some animals (like cows), help in digestion and absorption of food. The bacteria, present in our large intestine, help in bowel movement.
2. Microorganisms are also used in production of antibiotics. Antibiotics are chemicals that inhibit the growth of (other) harmful microorganisms by affecting their life processes.
   For example, penicillin is an antibiotic obtained from a fungus, Penicillium notatum. Streptomycin, tetracycline and erythromycin are some antibiotics obtained from fungi and bacteria.
   • Antibiotics are extremely effective in treatment of various microbial infections/diseases, like, tuberculosis, cholera, etc.
   • However, antibiotics should be taken only on the advice of a qualified doctor and that too only in the prescribed dosage and for the prescribed duration.
   • Not completing the prescribed course may make them ineffective when used in future. If they are taken when not really required, they may kill some of the useful bacteria in the body. 
3. When microorganisms, like bacteria or viruses, enter our body, they are recognised by special kind of blood cells.
   • These cells get stimulated to produce antibodies. Antibodies identify and destroy such disease causing organisms.
   • During this process the body 'remembers' the type of microorganisms; if the same microorganism enters the body again, it gets recognised and destroyed much faster. This is called 'immunity'.
   • Immunity is, therefore, the natural ability of an organism to have an inbuilt mechanism to resist, and destroy, the infection that some microorganism may cause.

• Some bacteria, and blue-green algae (cyanobacteria), are able to 'fix' (for use by plants) atmospheric nitrogen into usable forms of salts of nitrogen. They are called biological nitrogen fixers.
• Rhizobium lives in symbiotic association in the root nodules of leguminous plants and enriches the soil with nitrogen compounds. Some cyanobacteria do the same in rice fields and in association with the roots of Cycas plant.

• Many microorganisms produce ethanol by fermentation of sugars and produce methane in the biogas reactors. Both ethanol and methane are used as fuel for production of energy. 

• When a plant or animal dies, it leaves behind nutrients and energy in the organic material that formed its body structure.
• Decomposers eventually convert all such organic matter into carbon dioxide and nutrients. These nutrients (like nitrogen, phosphorous, magnesium, etc.) become a part of the soil.
• This process eventually replenishes nutrients back to the ecosystem, thereby, allowing the plants to grow. Bacteria and fungi are some of the common decomposers.

• Edward Jenner was an English doctor who pioneered the vaccination process. Jenner's discovery in 1796 — inoculation with cowpox gave immunity to smallpox — was an immense medical breakthrough and has saved countless lives. Smallpox has now been eradicated from the world. 
• Probiotics (dietary supplements of live bacteria or yeasts) can help prevent, and treat diseases through a number of mechanisms. One way is by interacting directly with the disease-causing microbes making it harder for them to cause disease. An example of this is the ingestion of probiotic bacteria to prevent, or to treat, diarrhoea. These organisms help reinforce the natural bacterial barrier that exists on the lining of the digestive tract; they thus, provide additional protection against pathogenic organisms that can cause diarrhoea. 

• Bacteria also grow on food particles that may get lodged in between our teeth. Acids produced by these bacteria, corrode the tooth enamel and cause cavities.
• Many microorganisms grow in water bodies and decrease their oxygen level. This is harmful to the other organisms living in these water bodies.

• During our lifetime, we come in contact with many microorganisms. Some of these microorganisms leave undesirable effects on our body.
• They invade our body and multiply inside it.
• They release some harmful materials, called toxins, in our body.
• These toxins adversely affect our body and can make us suffer from diseases.

• Diseases, which can spread from an infected person to a healthy person, are called communicable diseases.
• Communicable diseases spread from one person to another through air, water, food, physical contact or insects.
• In diseases, like tuberculosis and pneumonia, when a patient sneezes or coughs, small droplets carrying germs, are released in the air. These germs, if inhaled, can infect a healthy person.
• The consumption of contaminated food can result in diseases like cholera, typhoid or hepatitis.
• Different types of insects can also spread various diseases. For example, malaria is spread by the female Anopheles mosquito, dengue by Aedes mosquito and plague by rat flea.
  
• Diseases, like common cold and conjunctivitis, can spread through direct, or indirect, contact with a patient.

Modes of Transmission of Pathogens

• These plants may have been affected by a pathogen. Most plant diseases are caused by fungi, bacteria and viruses.
• These microorganisms may reduce crop yield and, at times, cause total destruction of the crops.
• The Irish Famine in 1740-41 was caused because of complete destruction of the potato crop due to the disease, 'late blight', caused by a fungus.

• Food poisoning is a disease that results from the consumption of contaminated food.
• Symptoms of food poisoning are vomitting, nausea, severe pain in the abdominal region, diarrhoea and dehydration.
• It may also cause weakness, fatigue and damage to the nervous system. These symptoms arise due to the presence of bacteria, or other microbes, in food.
• They may also be due to ingestion of toxins contained in food (including those produced by bacteria). Some bacteria, like Clostridium and Staphylococci, and fungi, like Aspergillus, cause food poisoning. 
• Food must be properly prepared and stored to prevent food poisoning.
• Food poisoning can occur when food is left unrefrigerated for long periods of time.
• This often happens at picnics and large parties.
• Sometimes, mishandling also causes food poisoning. Persons, who handle or prepare food, should wash their hands to prevent contamination of food. 

• Food preservation is the process of treating and handling food so as to stop, or greatly slow down, spoilage (loss of quality, edibility or nutritive value) caused, or accelerated, by microorganisms.
• Preservation usually involves preventing the growth of bacteria, fungi and other microorganisms, as well as retarding the oxidation of fats which cause rancidity.
• Rancidity means having disagreeable odour, or taste due to decomposition of oils or fats.
• Some preservation methods require the food to be sealed after treatment to prevent recontamination with microbes.
• Other methods, such as drying, allow food to be stored without any special containment for long periods.

• This method reduces water content sufficiently and, thereby, prevents, or delays, bacterial growth.
• Drying also reduces weight, making food more portable.
• Some common food stuffs that are preserved by drying are: apples, pears, bananas, mangoes, papaya, apricot and coconut. Drying is also the normal means of preservation for cereals (grains), such as wheat, maize, oats, barley, rice, millet and rye.
  

• Sugar and salt reduce the water content and make it unavailable for the growth of microorganisms. As a result, the food gets preserved.
• Meat and fish are covered with salt to check bacterial growth.
• Some pickles are also preserved by the addition of specific quantities of salt.
• Jams and jellies are usually preserved by addition of sugar.

• Acetic acid, in the form of vinegar, is used in the manufacture of several pickled products.
• Vinegar also stops the growth of microorganisms.
• Citric acid and phosphoric acid are also used, in carbonated beverages and fruit drinks, for both flavouring and preservation.

• Chemical preservatives, like sodium benzoate and potassium metabisulfite are used to preserve jams, jellies and pickles. These chemicals inhibit the growth of microorganisms.

• Some food items, like milk, are usually boiled, before their use, or storage.
• Boiling kills many microorganisms.
• Several food stuffs are also stored in the refrigerator; the low temperature inhibits the growth of microorganisms.

• Pasteurisation is 'pressurised heating' for a short time; it may be considered as a mild form of heat treatment. The temperature, used during pasteurisation, is below 212°F (100°C).
• Milk is pasteurised to destroy microorganisms. However, there are many more heat-resistant organisms in it that only get reduced in number. Hence, pasteurised milk needs to be stored under refrigeration to keep bacterial growth in check.
• In addition to destroying some microorganisms, pasteurisation also inactivates some enzymes; that, at times, can be a disadvantage.

• Vacuum packing stores food in a vacuum environment; usually an air-tight bag or bottle.
• The vacuum environment deprives bacteria of the oxygen needed for their survival; it, therefore, slows 'spoiling'.
• Vacuum packing is commonly used for storing nuts; it helps to reduce their loss of flavour caused by their oxidation.
  

• It involves cooking food and sealing it in sterile cans or jars; this is followed by boiling the containers to kill or weaken any remaining bacteria (a form of sterilisation).
• However, food preserved by canning, or bottling, is at immediate risk of spoilage once the can or bottle has been opened.
• At times, the cans or jars get damaged and show puffing or enlargement. This indicates spoilage of food; the food, in such puffed or enlarged containers, should never be consumed.

• When foods are kept at below freezing temperatures, most chemical changes take place at such a reduced rate that only minor changes are noticeable even after long periods of storage.
• However, microorganisms are generally more resistant to cold than to heat. Although some are killed by freezing, most bacterial spores and a large number of organisms survive and get revitalised when the food is thawed. 

'The Thief's Story' is written by Ruskin Bond. It is about a 15 year old boy, Hari Singh, whose life changes when he meets Anil, a 25 year old writer. Anil's unspoken words and kind gestures leave very positive imprints on Hari Singh who is an artful thief.

Chapter 3- The Midnight Visitor by Robert Arthur

Ausable is a secret agent who is expecting to get a very important report. Another secret agent, Max, threatens him with a pistol, demanding the report. This story tells how Ausable outwits Max by making up convincing stories.

Chapter 4- A Question of Trust by Victor Canning

Horace Danby collects rare books, and finances his hobby by breaking safes. This year, however, he is caught by a pretty young lady claiming to be the wife of the house owner. She tricks him into opening the safe and handing her the jewels. Horace is later arrested for the jewel robbery.

Chapter 5- Footprints without Feet by H.G. Wells

This story is about a scientist who discovered how to make himself invisible. As he was afraid of being discovered in London, where he had committed some crimes, he ran away to a remote village. However, he ran out of money and resorted to robbery, using his advantage of invisibility.

Chapter 6- The Making of a Scientist by Robert W. Peterson

The lesson is about the great scientist Richard H. Ebright. He had a habit of collecting butterflies, rock fossils and coins since he was a kid. He started scientific research on the study of butterflies. His curiosity and will to win for the right reasons made him a successful scientist.

Chapter 7- The Necklace by Guy de Maupassant

'The Necklace' is a story of a middle class lady named Matilda. She was born in a clerk family and was married to a clerk named M Loisel. She was very pretty but was dissatisfied in life because of her 'poverty'. Once she borrowed a necklace from her friend Madame Forestier and lost it in the party. To replace the necklace, she had to live a very miserable life. The tale is about how her desire to have more, lead to her ruin.

Chapter 8- The Hack Driver by Sinclair Lewis

The story 'The Hack Driver' is about a man named Oliver Lutkins. He was a cunning man who was a witness in a case, but he ignored all the summons for appearing. A young lawyer was given the duty to find this man in New Mullion. There he met a helpful hack driver. The driver took him around the village in search of Lutkins but was unable to find him. Later on the lawyer came to know that the hack driver was Oliver Lutkins himself. Hence the young lawyer became the laughing stock of all.

Chapter 9- Bholi by K.A. Abbas

'Bholi' is the story of Sulekha. She is a young girl neglected by all. Education transforms het It inspires courage and confidence in her. She raises her voice for her rights.

Chapter 10- The Book that Saved the Earth by Claire Boiko

This play is set in the 25th century at the museum of Ancient History. A historian tells the students how the Earth had been invaded by Martians in the 21st century, but was saved by a book of nursery rhymes.

Related Readings

Students can also take help of these below provided study materials for class 10. We have provided the other related learnings for the students studying in class 10. They can take help from these resources for getting good marks in their board examination.

• Summary for Class 10 English Footprints without Feet
• NCERT Solutions for Class 10 English First Flight
• NCERT Solutions for Class 10
• NCERT Solutions for Class 10 Maths

This supplementary reader has a total of ten chapters and all of them are prose. Each piece begins with questions under the head ‘Read and Find Out’ which is divided into two to three sections, each section prefaced with relevant questions. Word glosses are also there to encourage inferences about meaning from sentential and discourse contexts. The questions under the head ‘Think About It’  are meant to take the learner beyond factual comprehension to contemplate on the issues that the texts raise. These questions are open-ended and thought provoking. The questions given under ‘Talk About It’ are intended to encourage the learners to express their own ideas in a creative and coherent way.

• NCERT Solutions for Class 10 Civics
• NCERT Solutions for Class 10 Geography
• NCERT Solutions for Class 10 History
• NCERT Solutions for Class 10 Social Science
• NCERT Solutions for Class 10
• NCERT Solutions for Class 10 Maths

CBSE Class 10 Social Science Chapter Wise Marking Scheme and Weightage Distributions

Questions wise Marking Scheme for Social Science

Word Limit for different questions:

•  3 marks questions – about 80 words.
•  5 marks questions – about 100 words.

For Map-Based Questions:

•  One outline political map of India will be provided.
•  After the completion of the answer, attach the map inside the answer sheet.

Details of Chapters for Class 10 History

1. Asexual reproduction
2. Sexual reproduction

• The production of young ones by a single parent, without the formation and fusion of gametes is called asexual reproduction (Uniparental reproduction).
• New individuals, so formed, are genetically identical to the parent. It is a primitive method of reproduction that is simpler than sexual reproduction.

1. Binary fission
2. Multiple fission
3. Budding

• It is usually observed in unicellular organisms.
• It is common in organisms like bacteria and Amoeba.
• In this process, a parent cell divides into two daughter cells and each cell, thus formed, grows to form an individual. 

• It is the repeated division of a parent cell into a number of small individuals.
• For example, the Plasmodium (malarial parasite) parent cell undergoes repeated division.
  

• The formation of a new individual, from a small protuberance, (called the bud), arising on the parent body, is called budding.
• For example, budding takes place in organisms like Hydra and yeast.
• The offspring remains attached to the parent during its growth.
• It separates at an appropriate time to live as an independent organism.
  

• Production of young ones by the formation and fusion of special cells (called gametes), contributed generally by two parents, (male and female) is termed as sexual reproduction (Biparental reproduction).
• Unlike asexual reproduction, the young ones are not genetically identical to the parent.
• In multicellular organisms the needed special cells, or gametes, are produced by the reproductive organs. 
• The male gametes, called the sperms, are produced by the male reproductive part and the female gametes, called the eggs or ova (singular-ovum), are produced by the female reproductive part. 
• In most cases, these two types of gametes are produced by two different individuals. 
• However, in some cases, like that of the earthworm, the same individual produces both types of gametes. The earthworm is called a hermaphrodite animal. 
• When the two types of gametes, i.e. sperm and ovum unite, they form a single cell, called the fertilised egg, or zygote. This process is known as fertilisation. The zygote then develops into an offspring.

1. External Fertilisation and External Development
2. Internal Fertilisation and External Development
3. Internal Fertilisation and Internal Development

• Parents release sperms and eggs into the surrounding water where fertilisation occurs and zygotes develop into offsprings.
• For this pattern to succeed, the male and the female must shed their gametes in large numbers at the same time and place. Frogs belong to this type of animals.

• Sperms are passed from the male into the female body. The fertilisation takes place within the female body.
• The zygote, after its formation, is surrounded by some secretions along with some reserve food and a shell. These help in the development and protection of the growing embryo.
• For example, in animals, like the hen, the development of the chick takes place after the egg has been laid.

• Both fertilisation and development take place in the female body.
• This means that the formation of the zygote and the development of the young one takes place inside the body of the female.
• Human beings, the elephant, the cow and many other animals belong to this category.

• If nothing stopped them, Amoebae would multiply so fast that they would cover up the entire planet. But Amoebae have predators; they also face competition for food and space.
• Parthenogenesis is a form of asexual reproduction in which females produce eggs that develop without fertilisation. Parthenogenesis occurs naturally in aphids, daphnia, rotifers, some invertebrates and in many plants.
• In many social insects, such as the honeybee and the ant, the unfertilised eggs give rise to the male drones and the fertilised eggs to the female workers and a queen. This has also been observed in some snakes, fish and monitor lizard.
• A rabbit (which becomes an adult in 8 months) can produce 10-30 offsprings per year and a fruit fly (which becomes an adult in 10-14 days) can produce about 900 offsprings per year. Animals with few offsprings, devote more resources to the nurturing and protection of each individual offspring. This reduces the need for many offsprings for such animals. 

1. Male Reproductive System
2. Female Reproductive System

• It includes a pair of testes, a scrotum, two sperm ducts, urethra and penis.
• Testes are oval organs (two in number) (singular-testis) present in a sac called scrotum.
• The sperms develop in the testes. Sperms pass from the testes through the sperm ducts to the urethra.
• As the sperms move, they collect fluids from the glands. This fluid, having sperms in it, is called the semen.
• The last part of the urethra is surrounded by a muscular structure called the penis.
• Penis conducts urine as well as semen; however, the two cannot pass through it at the same time. 

• A sperm is smaller in size than the ovum and can only be seen through a microscope. Each sperm cell has three parts: a head, a middle piece and a tail.
• A structure, at the tip of the head, produces enzymes that help the sperm to penetrate the female ovum (egg).

• The female reproductive system consists of a pair of ovaries, a pair of oviducts, uterus and vagina.
• The main functions of the female reproductive system include production of eggs, receiving the sperms and providing the site for fertilisation.
• The growing embryo gets attached to the uterus and the foetus develops there. 
• Ovaries produce eggs. An egg reaches the oviduct; when a sperm fuses with the egg, a zygote is formed.
• It is this zygote that divides and develops to form the young one.
• Ovaries also produce hormones that control the production of ova (eggs). These hormones also help in the maintenance of pregnancy.

• Fertilisation takes place in the initial part of the oviduct.
• When the sperm and ovum (male and female gametes) fuse, their fusion (fusion of the sperm and egg) is called fertilisation.
• Since it takes place inside the body of the female, it is an internal fertilisation.
• During this process, the nuclei of the sperm and egg fuse to form a single nucleus. The resultant single cell, formed after fertilisation, is called a zygote.
• The, zygote contains the nuclear part of sperm (from the father) and of the egg (from the mother).
• It is for this reason that a child has some characteristics of the father and some characteristics of the mother. 

• The zygote immediately divides into two cells; these cells then divide again and again, over the next couple of days, and the resulting cluster of cells makes its way to the uterus (womb). This cluster of cells is called the embryo.
• The embryo plants itself in the lining of the uterus and continues dividing its cells to make billions of new cells.
• It gradually develops the different body parts, such as the head, eyes, hands, legs, etc.
• When the embryo reaches the stage where all its body parts can be identified, it is called the foetus.
• The foetus, when born, is called an infant.

• In hens, fertilisation is internal as it takes place inside the body of the female.
• But unlike humans, the development of their young ones takes place outside the body of the female bird. This is, therefore, an example of internal fertilisation and external development.
• After fertilisation, the zygote starts dividing and moves down the oviduct.
• As it moves down, a number of protective layers get formed around it. The egg shell is one of these protective layers. It is deposited around the egg in the lower part of the oviduct of the hen, just before the egg is laid.
• The egg shell is made of calcite, a crystalline form of calcium carbonate.
• The embryo develops inside the egg for (around) 21 days (the incubation period), after which the chick gets fully developed.
• During the incubation period, the eggs require a specific temperature; this is provided by the hen by sitting on the eggs. (Sometimes, this is done artificially by keeping the eggs in an incubator).
• Finally, the egg shell breaks down and the chick comes out. This process is called hatching. 

• Viviparous animals (vivus = alive) are those animals whose females give birth to the young ones.
• The embryo develops inside the body of the mother; the mother gives birth to the young one. This characteristic distinguishes mammals from other groups.

• Oviparous animals (ova = egg) are animals that lay eggs; the eggs have little, or no other embryonic development, within the mother.
• The development of the egg takes place in the relevant external surroundings. It is the reproductive method of (most) fish, amphibians, reptiles, birds and insects.

• In many animals the young one, at birth, is a miniature (smaller) version of the adult form. The development of such young ones (into adults), is known as direct development.
• In some animals, the young ones, initially, are very different from their adults.
• For example, a tadpole looks very different from an adult frog.
• Similarly, a caterpillar, or the pupa of a silk moth, or a butterfly, looks very different from its parents.

• A marked, and more or less abrupt development change in the structure and habits of an animal (as in case of a butterfly or a frog), occurring during its life cycle, is termed as metamorphosis.
• During the life cycle of an amphibian, eggs are laid in water.
• A tadpole then emerges from the egg and swims freely in water. The tadpole has gills, a tail and a small circular mouth.
• The tadpole grows until it begins to undergo metamorphosis.
• Metamorphosis begins with the development of the hind legs followed by the front legs; then the lungs develop.
• The tadpole then begins to swim to the surface of the water to breathe.
• The intestine shortens to adapt the animal to a carnivorous diet.
• In case of frogs, the tail gets absorbed by the body during the last stages of metamorphosis. A tailed tadpole, thereby, changes into a tail-less frog.