Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
VERTEBRATES Vertebrates are members of the larger phylum Chordata, and show all of the major chordate features at some point in their life cycles: notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. But what makes a vertebrate a vertebrate, is the presence of the vertebral column, or backbone, which surrounds and protects the main nerve cord. Vertebrates belong to the phylum chordate. Vertebrates are the most complex and successful life forms on earth. During the embryonic development of all chordates, they pass through a stage called the pharyngula, having these features: 1. A dorsal, tubular nerve cord, running from anterior to posterior. At its anterior end, it becomes enlarged to form the brain. 2. A flexible, rodlike notochord that runs dorsal to the digestive tract and provides internal support. In vertebrate chordates, it is replaced by a vertebral column or backbone long before maturity. 3. Pairs of gill pouches. These lateral outpocketings of the pharynx are matched on the exterior by paired grooves. In aquatic chordates, one or more pairs of gill pouches break through to the exterior grooves, forming gill slits. These provide an exit for water taken in through the mouth and passed over the gills. 4. A tail that extends behind the anus. Seven classes of vertebrates are recognized: Class Agnatha: These are jawless, fish-like animals with poorly developed fins. Lampreys and hagfishes are the only jawless vertebrates to survive today. They both have a round mouth and for this reason are often referred to as cyclostomes. They are the most primitive of the vertebrates. Besides lacking jaws, their notochord persists for life, never being completely replaced by a backbone even in the lampreys. They have no scales. Class Chondrichthyes: This comprises 800 living species of sharks, rays, skates and rat fishes, all of which have a cartilaginous skeleton, true jaws, and a number of other distinctive characters. They got their names from the fact that their skeletons are made of cartilage, not bone. With their gills exposed to sea water, all marine fishes are faced with the problem of conserving body water in a strongly hypertonic environment. Sea water is about 3.5% salt, over 3 times that of vertebrate blood. The cartilaginous fishes solve the problem by maintaining such a high concentration of urea in their blood (2.5% — far higher than the 0.02% of other vertebrates). Class Osteichthyes: includes some 20,000 species of true fishes, with a bony skeleton, a sutured skull, teeth fused to the jaws, lobed or rayed fins, and a number of other distinguishing features. Bony Fishes (Osteichthyes). They have paired fins. As their name indicates, the skeleton in this group is made of bone. The group is subdivided into the ray-finned fishes (Actinopterygii) and lobe-finned fishes (Sarcopterygii). Class Amphibia: This includes some 3,500 living species of frogs, toads, salamanders, newts, and caecilians, all of which have four limbs (making them tetrapods), a moist glandular skin, external fertilization, and a complex life cycle. Some have poison glands. Amphibians are vertebrates that spend part of their lives under water (breathing with gills) and the remainder on land (breathing with lungs). Amphibians are cold-blooded; their body temperature depends on the temperature of their environment. All amphibians have moist skin (their skin must be moist to absorb oxygen, even though they breathe with their lungs). Most amphibians lay shell-less, jelly-covered eggs, usually in the water. There are 3 groups (orders) of living amphibians: frogs and toads (Anurans); newts and salamanders (Urodeles); caecilians (the worm-like Gymnophiones ). Class Reptilia: These are four-legged, tailed animals, with dermal scales, internal fertilization and direct development. Living reptiles include about 6,200 species of crocodiles, turtles, lizards, snakes, and tuataras. Important extinct groups of reptiles include the dinosaurs, hadrosaurs, ichthyosaurs, pterosaurs and plesiosaurs. Reptiles (meaning "to creep") are a group of animals that have (or modified scales), they breathe through lungs and usually lay eggs. They are cold-blooded vertebrates. They all have scales (or plates, in the case of turtles) that prevent them from losing moisture through their skin and protects against injury. Scales are made up of keratin, the same material found in fingernails. Most lay eggs. There are four main groups of reptiles: turtles and tortoises; lizards and snakes; crocodiles and alligators; and lastly tuatara. Class Aves: the birds: Birds are animals that have wings, feathers, a beak, strong hollow bones and powerful flight muscles. They are warm-blooded (or homoiothermic) tetrapods whose forelimbs are specialized for flight (although some species are secondarily flightless or even flightless). Birds have a characteristic covering of feathers and reproduce by laying eggs. Not all flying animals are birds; and not all birds can fly. The ability to fly has developed independently many times throughout the history of the Earth. Bats (flying mammals), pterosaurs (flying reptiles from the time of the dinosaurs), and flying insects are not birds. The fastest running bird is the ostrich, but it cannot fly. The ostrich is also the fastest two-legged runner of all the animals on Earth. The ostrich is the largest bird. Bird is quite varied; most can fly, some can run very well, some swim, and some do combinations of these. They all have feathers. All birds hatch from eggs, and most birds build nests for the eggs. Some birds deposit their eggs in other birds’ nests (like the cowbird). Bird bones are mostly hollow, thus very light (yet strong). Flightless birds include kiwi, ostrich and emu. The only bird that can fly backwards is the humming bird. Class Mammalia: This includes more than 4,000 species of homoiothermic tetrapods, with epidermal hair and female mammary glands for suckling the young. All give birth to young, although a very few, primitive species reproduce by laying eggs. Characteristics They possess mammary glands – produce milk to feed young They have sweat glands – only mammals perspire (in order to maintain body temperature) They are warm-blooded/homoiothermic (use food energy to maintain certain temperature) They have two pairs of limbs All have hair at some time in their life, and most have an outer hair (or fur) covering, though some are practically hairless (whale, elephant). Breathing is by lungs, diaphragm assists with this (mammals are the only animal that have diaphragm) They have well developed brains. As with most animal groups, there are more herbivores (plant-eaters) than there are carnivores (meat-eaters). Types of Mammals Monotremes: These are egg-laying mammals. Monotremes are very rare and unusual! Marsupials: They give birth to their young in a very immature state; most female marsupials have pouches. Their babies live in the pouches until they are big enough to survive on their own. Placental Mammals: Their young are born at a more advanced stage than the young of other mammals. Before birth, the young are fed through a placenta. Most mammals are placental mammals. Vertebrate Skeletal System Fig I: The Human Skeleton The skeletal system consists of the bones, ligaments and cartilages joined together to form a strong, flexible framework for the body. The vertebrate skeleton is divided into 2 namely, axial skeleton and appendicular skeleton. It is composed of 206 bones (Axial skeleton: longitudinal axis, 80 bones; Appendicular skeleton: limbs, 126 bones). Axial Skeleton The axial skeleton consists of the skull, vertebral column, and rib cage and the sternum. The Skull The skull encloses and protects the brain and its related structures. It has a number of individual bones tightly fitted together by immovable joints. At birth many of these joints are not completely sutured together as bone, leading to a number of "soft spots" or fontanels, which do not completely join until the age of 14-18 months. The cranium is made up of 8 bones and facial skeleton of 14 bones. With the exception of the mandible, all are immovable and joined by sutures. Vertebral Column The vertebral column supports the head and encloses the spinal cord. There are 33 bones or vertebrae in the spine, i.e., 7 cervical vertebrae in the neck, 12 thoracic vertebrae in the upper back corresponding to each pair of ribs, 5 lumbar vertebrae in the lower back, 5 sacral vertebrae are fused together to form 1 bone called the sacrum and 4 coccygeal vertebrae that are also fused to form the coccyx or tailbone. We refer to the vertebrae by their names and numbers so that the cervical vertebrae are C1, C2, C3………..C7 where “C” stands for “cervical” and the number is the position of the vertebrae counting down from the head. Therefore the thoracic vertebrae become T1 –T12. The lumbar are L1 – L5, and the sacrum and coccyx do not have numbers and each is thought of as one bone. Its functions include: 1) Support weight – transmits weight to pelvis and lower limbs 2) Houses and protects spinal cord - spinal nerves leave cord between vertebrae 3) Permits movements 4) Provides for muscle attachments - muscles of back; also muscles of head, neck, upper extremity, thorax The Rib This is a curved, slightly twisted strip of bone, making up part of the skeleton of the thorax. In man there are 12 pairs of ribs. The head of each rib fits into one of the 12 thoracic vertebrae of the backbone. The first seven pairs are termed true ribs, connected directly to the sternum by their costal cartilages. The next 3 pairs are called false ribs, because each is connected to the rib above it, while the last 2 pairs are called floating ribs because they are not connected in front to the sternum. The rib (ribcage) houses and protects the heart and lungs. Sternum This is also called the breastbone. It is usually 15- 20 cm long, extending from the base of the neck to just below the diaphragm and forming the front part of the skeleton of the thorax. The Appendicular Skeleton The appendicular skeleton consists of the limbs and their girdles. The upper limbs are attached to the pectoral girdle while the lower limbs are attached to the pelvic girdle. The Pectoral Girdle The Pectoral Girdle is composed of two scapulae and two clavicles, one on each side of the body in order to form two shoulders. The purpose of the pectoral girdle is to provide a point where the upper extremity, the arms, attach to the axial skeleton, and it also provides attachment points for many muscles that move these limbs. The clavicle helps to hold the scapula (and arm) in position, but also allows the scapula to move, resulting in increased mobility of the shoulder and arm. The scapula is a triangular-shaped bone that is mobile, serves as an attachment site for the arm bone (humerus) to the trunk, muscles moving the arm and muscles holding or moving the scapula. The Acromion process is the part of the scapula joining the clavicle. The Glenoid cavity (fossa) is the part of the scapula joining to the arm bone (humerus). This ball-and-socket joint allows a wide range of arm movements. The Pelvic Girdle The pelvic girdle consists of the right and left coxae (hip bones) and the sacrum. Its functions include: 1.It supports the weight of the head, upper limbs and trunk. 2. The hip joint between the pelvic girdle and lower limbs allows movement of the lower limbs. 3. Muscles that move the lower limbs, trunk, and arms attach to the pelvic girdle. 4. The pelvic girdle supports and protects abdominopelvic organs. The pelvis consists of the two coxae, the sacrum and the coccyx. Coxa (pl. coxae) consists of the fused ilium, ischium, and pubis bones. The sacroiliac joint connects the sacrum and ilium. The pubic symphysis (symphysis pubis) is the anterior connection of the two coxae (at the pubis bones). The Hip joint is the connection between the head of the femur and the acetabulum of the coxa; it transfers weight from the coxa to the femur while allowing a wide range of movement. The acetabulum is the depression in the coxa that holds the head of the femur, but allows it to move. The obturator foramen decreases the weight of the coxa and is mostly closed by a connective tissue membrane. The superior entrance to the pelvic girdle is the pelvic inlet and the inferior exit is the pelvic outlet. It is a passageway for the fetus during delivery. Female vs. Male Pelvis The female pelvis has larger pelvic inlet and outlet, the pelvic inlet is oval, and subpubic angle is greater than 90 degrees. Contrariwise, the male pelvis has a smaller pelvic inlet and outlet, the pelvic inlet is heart-shaped, and subpubic angle is less than 90 degrees. Appendages Human limbs are built on a pentadactyl pattern, i.e., they end in five digits. The biggest bone in the fore limb is the humerus. The head of the humerus fits into the glenoid cavity and articulates interiorly with the scapula and posteriorly with the other smaller bones, i.e, the ulna and the radius, preceded by the metacarpals and the phalanges (fingers) in that order. A similar arrangement is seen in the hind limb. The biggest bone in the hind limb is the femur. Following the femur posteriorly are 2 smaller bones called tibia and fibula, which are followed by the tarsals (ankle bones), which are then preceded by the metatarsals and the phalanges (the toes). The Joints A joint is a place where two or more bones meet. Most joints are movable. The opposing surfaces of the two bones are lined with cartilaginous, fibrous or soft (synovial) tissue, this permits the joints to move smoothly without damaging the bones. There are 3 main classes of joints, namely: diarthrosis (freely moveable) examples include, gliding joint, hinge joint (ginglymus) and pivot joint (trochoid joint) and ball-and-socket joint; amphiarthrosis (slightly moveable), e.g., the joints between the ribs and the sternum and synarthrosis (immovable) e.g., fixed joint. • Hinge Joints - A hinge joint can only move in one direction – up or down. The knee and elbow are hinge joints • Pivot Joints - The pivot joint allows rotation. Your wrist and the attachment of the head is called the spinal chord are pivot joints. This is why you can turn your head. • Ball-and-Socket Joints - These joints allow you to turn your arms and legs in a complete circle. There is a ball or knob at the end of one bone. At the end of another is a hole or socket. A joint is formed where the ball fits into the socket. Shoulder joints and hip joints are ball-andsocket joints. • Gliding Joints – A gliding joint allows two bones to move one over another. These occur between the vertebrae in the backbone and also in the carpus and tarsus. The amount of movement is small. • Fixed joints – These are joints where the bones are tightly joined together and cannot move. The skull bones are fixed joints. Fig II: Joints of the Human Skeleton Muscles A muscle is a special kind of tissue that brings about movement. The kind of movement that muscles carry out depends on where the muscles are located. Bones and joints will be useless without muscles. Moving, breathing and swallowing are all functions of the muscles. When muscles contract, they pull the body parts to which they are attached. This action usually causes some movement. Sometimes, muscular contraction resists motion, as when they help the body parts in postural positions. Muscles are also responsible for the movement of the body fluids such as blood and urine, and they function in heat production that helps maintain body temperature. There are 600 muscles in the human body. Most muscles are attached to bones and extended from one bone to another. Each end of the muscles is attached to a bone by a strong connective tissue, a tendon. Bones of the muscles move only when the muscles attached to them pull. Muscles do not push; they only pull. This is accounts for the fact why most muscles come in pairs. The muscles of the arm are good examples of how muscles work as partners. The two muscles of the arms are called biceps and triceps. The biceps is a flexor muscle. It produces a flexing or bending action by pulling the lower arm bone at the joint towards the body. The triceps is an extensor muscle. It produces an extending or straightening action by returning the bone to the relaxed position. Not all muscles are connected to the bones. For example, heart muscles work by squeezing. Some muscles are connected to the skin. This is how you are able to smile by moving 14 muscles. It takes 75 muscles to frown. Fig III: Muscles of the Upper Arms Types of Muscles There are three kinds of muscles: (1) Skeletal muscle, also known as striated or voluntary muscles. It makes up about half of an individual’s body weight. (2) Smooth or involuntary muscles, which forms the muscle layers of the digestive tract and other internal organs; and (3) cardiac or heart muscles. The skeletal muscle consists of a mass of muscle fibers grouped together and are attached to bones. This muscle has a striation when seen under a microscope. Striped or striated muscles are used to make fast, precise movement. The movement of striped muscles of striped muscles can be controlled. A muscle that can be controlled is called a voluntary muscle. Fig IV: A Skeletal Muscle The smooth muscles are made of long, thin cells that form layers. Their movements cannot be controlled. The smooth muscle is a type of involuntary muscle. Smooth muscles control many of the internal organs of the body. Examples of smooth muscles are the muscles of the stomach, arteries and uterus. Fig V: A Smooth Muscle Cardiac muscle is found only in the heart. It works like a smooth involuntary muscle. The cardiac muscle looks like bundles of fibers twisted into a rope. Fibers from one bundle branch into the next bundle. Thus, the bundles of cardiac muscles are all connected and can work together to keep the heart pumping. Fig VI: A Cardiac Muscle. References Czura, A.W. (2004). Skeletal System: Axial and Appendicular Skeleton Lecture Materials. SCCC BIO130 Chapters Lecture Notes, pp 7- 8. Daniel, L. (1994). Life science. Westerville, OH: Merill Publishing Co., Mcmillan/McGrawHill. Grabowski, T. (2003). Principles of Anatomy and Physiology. N.Y.: John Wiley and Sons, Inc. Holo, W. (1984). Human Anatomy and Physiology. 3rd ed. Iowa: W. C. Brown Publishers. Dubuque. Hopson, J.L. and Wessells, N.K. (1990). Essentials of Biology. USA: McGraw-Hill Publishing Company. Kaskel, A., Hummer, P.J. & Daniel, L. (1981). Biology on Everyday Experience. USA: Merill Publishing Co. Marieb, E. N. (2004). Human Anatomy & Physiology 6th ed. San Francisco: Pearson Benjamin Cummings, Mariele, E.N. (1998). Essentials of Human Anatomy and Physiology. 3rd ed. New York, USA: Addison-Wesley Longman, Inc. Martini, F. H. (2004). Fundamentals of Anatomy & Physiology 6th ed. San Francisco: Pearson Benjamin Cummings, Payne, H. (1995). Understanding Your Health. St Louis, Missouiri: Mosby Publishing Company. Pikering, W.H. (2000). Complete Biology. New York: Oxford University Press Wong. Reproduction Reproduction is the process by which a living organism is able to produce more of its own kind. The continuity of life on earth, from its origin to the present day, has been possible only because of reproduction. Living organisms reproduce in two ways—asexual and sexual reproduction. In consequence, the scope of this work encompasses the modes of reproduction in plants and animals especially humans, population growth and its control and sexually transmitted diseases. Asexual Reproduction: This is the reproduction which requires only one parent. Simpler forms of life such as bacteria, amoeba and yeasts, reproduce asexually. In all types of asexual reproduction the offspring are exactly like the parent because they have the same DNA as the parent. Asexual reproduction involves the production of an offspring from body parts other than reproductive organs. It is a common process of reproduction in lower plants and animals. Basic features of asexual reproduction i) It involves only one organism i.e. different sexes are not involved. ii) New individuals produced are genetically identical to the single parent. iii) It is a fast mode of multiplication. Sexual reproduction Sexual reproduction is the formation of a new individual following the union of two gametes. In humans and the majority of other eukaryotes, plants as well as animals, the two gametes differ in structure and are contributed by different parents Basic features of sexual reproduction i) It is the production of offspring by the fusion of egg and sperm, which are the sex cells or gametes. ii) Upon fertilization, the male and female gametes unite to form a zygote, which develops into a mature organism. Parthenogenesis This is a type of reproduction in which an organism develops from an unfertilized ovum. It is common in plants and occurs in some lower animals e.g., sap-sucking insects such as aphids. Female aphids are able to produce young ones without having their eggs fertilized by a male. This permits them to increase in numbers very rapidly when environmental conditions are beneficial. Moreover, few animals that are capable of parthenogenesis also have a sexual stage in their life cycles, which produces genetic variety in their young. Asexual Reproduction in Organisms 1. Fission Fission is of two types: Binary fission and multiple fission. Binary fission: In binary fission, two individuals are formed from a single parent. This type of reproduction is found in organisms like bacteria, yeast and Amoeba FIG1: Binary Fission in Amoeba Multiple fission: In multiple fission, many individuals are formed from a single parent. This type of reproduction by multiple fission occurs during unfavourable conditions. In this type of reproduction, the unicellular organism develops a protective covering called cyst over the cell. The nucleus of the cell divides repeatedly producing many nuclei. Many daughter cells are produced within the cyst. The cyst breaks and small offsprings are liberated. This type of reproduction is seen in many algae and in some protozoans, such as the malaria parasite (Plasmodium). Fig 2: Multiple Fission in Plasmodium Budding: In this type of reproduction, a bulb-like projection or outgrowth arises from the parent body known as bud, which detaches and forms a new organism. For example, Hydra reproduces by budding (Fig.3). A small protruberance arises from one side of its body, which grows, develops tentacles and gets detached to lead an independent life. Yeasts also reproduce by this method. Fig 3: Budding in Hydra Regeneration or Fragmentation In this type of reproduction, the body of an individual breaks up into two or more parts and each part develops into a complete individual. Example: Planaria. 4. Spore formation In lower forms of life like the alga, Chlamydomonas and even fungi the protoplast of the cell divides to form 4–8 spores. These being motile are termed as zoospores. When spores are released in the surrounding medium they develop into new plants. In fungi motile and non motile spores are formed which are designed for aerial dispersion. REPRODUCTION IN PLANTS Like animals, plants also reproduce both asexually and sexually. Asexual reproduction in plants is either by vegetative propagation or vegetative reproduction of plant parts. Sexual reproduction is by fusion of male and female gametes and it occurs in flowering plants. Vegetative propagation or vegetative reproduction in plants Vegetative reproduction (or vegetative propagation) is a form of asexual reproduction in plants in which a bud grows and develops into a new plant. In this type of reproduction, any vegetative part of the plant body like leaf, stem or root develops into a complete new plant. Vegetative reproduction can take place by two methods—natural and artificial. Vegetative reproduction by natural methods This type of vegetative reproduction can involve roots, stem or leaves. Some common modes of vegetative reproduction are given below: i) By roots The roots of sweet potato produce new plants when planted in the soil. ii) By stem When the stem of some plants are cut off, they will form roots and grow into new plants when placed in the soil e.g. geranium. Underground horizontal stems, or rhizomes, are also important reproductive structures, particularly in grasses and sedges. Rhizomes invade areas near the parent plant, and each node can give rise to a new flowering shoot. The noxious character of many weeds results from this type of growth pattern. Corms, bulbs, and tubers are also stems specialized for storage and reproduction. In many plants the stem develops buds on it. The part of the stem that bears buds serves as an organ for vegetative multiplication, e.g. the modified parts of stem, such as runners of grass, suckers of mint and Chrysanthemum, bulbs of onion and tulip, rhizomes of ginger, corms of Colocasia (cocyam) and tubers of potato, etc. iii) By leaves In some plants, e.g. in Bryophyllum and Bigonia, adventitious buds are developed in the margins of their leaves. When the leaf falls on moist soil, these buds develop into small plantlets, which can be separated and grown into independent plants iv) By Runners. Some plants reproduce by means of runners— long, slender stems that grow along the surface of the soil. In the cultivated strawberry, for example, leaves, flowers, and roots are produced at every other node on the runner. Just beyond each second node, the tip of the runner turns up and becomes thickened. This thickened portion first produces adventitious roots and then a new shoot that continues the runner. v) By Suckers The roots of some plants—for example, cherry, apple, raspberry, and blackberry—produce “suckers,” or sprouts, which give rise to new plants. Commercial varieties of banana do not produce seeds and are propagated by suckers that develop from buds on underground stems. B) Vegetative propagation by artificial methods Some plants can be propagated artificially. The methods of artificial propagation include grafting, layering, cutting and tissue culture. i) Grafting: It is the method of obtaining a superior quality plant from two different plants, taking the root system of one plant and the shoot system of another plant. The plant whose root system is taken is called stock. The plant whose shoot system is taken is called scion. The ends to be grafted, of the stock and the scion, are cut obliquely and placed face to face and are bound firmly with tape (Fig. 4). The stock supplies all the desired nutrients to the scion. This technique has been used in raising superior quality plants of mango, apples, roses, rubber and citrus. Fig 4: Vegetative propagation by grafting ii) Cutting: In some plants like rose and sugarcane, this method is used quite frequently. Stem cuttings with nodes and internodes are placed in moist soil which give rise to adventitious roots, and grow into new plants. iii) Layering: Layering is the development of roots on a stem while it is still attached to the parent plant. The stem or the branch that develops adventitious roots while still attached to the parent plant is called a layer. It is a means of reproduction in black raspberries, jasmine (Jasminum), blackberries, etc. iv) Tissue culture: This is a modern technique of vegetative propagation. In this technique, a small piece of tissue is cut from a plant and is transferred to a container with nutrient medium under aseptic conditions. The tissue utilizes nutrients from the medium, divides and re-divides, and forms a callus. Small portions of this callus are transferred to another medium which induces differentiation and plantlets are produced. These plantlets are transplanted in soil to form an adult plant. Orchids, Chrysanthemum, Asparagus and many other plants are now being grown by using plant tissue culture technique. Sexual reproduction in plants In flowering plants, flower is the reproductive part of a plant. Most flowers have both male and female reproductive organs. A typical flower has four whorls, namely: calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (carpels) (Fig. 5). The androecium and gynoecium are directly concerned with sexual reproduction. The androecium is the male part of the flower. It consists of stamens. Each stamen has anther and a filament. Each anther possesses many pollen grains, which are the male gametes in pollen sacs. Gynoecium is the female reproductive part of a flower. The female part contained in this whorl is called pistil. Each pistil consists of three parts—an upper flat stigma, a medial, long, cylindrical style, and a lower, swollen ovary. • The stigma receives pollen grains during pollination. • The style bears the stigma at a suitable position to receive the pollen grains. • The ovary contains ovules that are found attached to the placenta. Ovules are the structures in which embryo sacs develop, and mature into seeds after fertilization. The arrangement of ovules in the ovary is called placentation. Fig 5: Parts of a Flower. Pollination Pollination is the process of transfer of pollen grains from the anther to the stigma of a flower. It is of two types: i) Self-pollination: If the pollen grains from the anther of a flower are transferred to the stigma of the same flower, it is termed as self-pollination or autogamy (auto: self; gamy: marriage) e.g. pea and china rose. ii) Cross pollination: If the pollen grains from anther of one plant reach the stigma of a flower on another plant of the same species, then this is called as cross pollination or allogamy (allos: other; gamy: marriage). Cross pollination has the advantage of increasing the chances of variations. Fertilization • After pollination, the pollen grains germinate on the stigma to produce a pollen tube. • This tube grows down through the style and finally reaches the ovule. • The ovule contains the egg cell inside the embryo sac. • The tip of the pollen tube ruptures in the ovule and discharges two male gametes into it. • One of the male gametes fuses with the egg to form the zygote. This fusion is called fertilization. • The other male gamete fuses with the diploid secondary nucleus and forms the endosperm nucleus. • The zygote that is formed as a result of fertilization divides several times and gives rise to an embryo. The endosperm nucleus grows to form the endosperm of the seed. Following fertilization, the sepals, petals, style and stigma degenerate and usually fall off. The ovary wall ripens and forms the pericarp of the fruit. Each ovule develops into a seed. The seed contains a potential plant or embryo. The whole ovary after fertilization changes into a fruit Fig 6: Zygote Formation REPRODUCTION IN HUMANS Humans reproduce sexually. Reproductive organs in humans are described below. Male reproductive system The male reproductive system in humans consists of the following organs—a pair of testes, a pair of epididymis, a pair of vasa deferentia, an ejaculatory duct, a urethra, penis and accessory glands (Fig. 7). The testes produce, sperms, the male gametes. The process of formation of sperms in the testes of an organism is called spermatogenesis. Each testis contains certain coiled tubes called seminiferous tubules that are actually responsible for the production of sperms. These sperms are released from the testes and stored in the epididymis until mating. At the time of mating, the sperms are passed from the epididymis through the vas deferens to the ejaculatory duct. The ejaculatory duct opens into the urethra. In human males, the urethra is a common passage for sperms and urine. The urethra passes through an organ called penis, which is the copulatory organ (organ for transfer of sperms during mating or copulation) in humans. During their passage from the epididymis to the urethra, the sperms are mixed with certain secretions from the accessory glands. The sperms along with the secretions form the semen. During copulation, semen is discharged. The process of discharging of semen is called ejaculation. In one ejaculation about 200,000,000 (2 × 108) sperms are discharged. Fig 7: The Male Reproductive System in Humans Female reproductive system The female reproductive system consists of a pair of ovaries, a pair of oviducts (or fallopian tubes), uterus and vagina (Fig. 8). A pair of ovaries lies in the lower part of the abdominal cavity, one on each side of the body. Ovaries produce ova and secrete female sex hormones, oestrogen and progesterone. The process of formation of egg in the ovary is known as oogenesis. There is a pair of oviducts or fallopian tubes in the human female reproductive system. One end of each oviduct is funnel-shapped. It collects the eggs released by the ovary. Both fallopian tubes open into the uterus. The uterus is a pear-shaped, muscular, thick-walled organ. The lower end of the uterus opens into the vagina that opens to the outside by a genital opening. Vagina is the organ where the penis is inserted during coitus for the discharge of semen. It serves as the birth canal during childbirth. In a human female, the urethra and the genital duct have separate openings. Fig 8: The Female Reproductive System in Humans Puberty The reproductive organs in human beings become functional at an age of 13–14 years in males and 12–13 years in case of females. This age is known as puberty. During sexual maturity, hormonal changes take place in males and females, and under the influence of some hormones secondary sexual characteristics are developed. • Development of secondary sexual characteristics in males include: deepening of voice, widening of shoulders, appearance of beard and moustaches, and growth of axillary and pubic hair. • Development of secondary sexual characteristics in females include: growth of axillary and pubic hair, widening of pelvis and hip, enlargement of breasts and initiation of the menstrual cycle. SEXUAL CYCLE IN HUMAN FEMALES (MENSTRUAL CYCLE) The period of life during which a female has the capacity to produce young ones is called the fertility period. In human females, it extends from about 12–13 years (puberty) up to 45–50 years (menopause). Between puberty and menopause, the female reproductive system passes through a regular monthly sequence of events called the menstrual cycle. The events of menstrual cycle are given here. • During each menstrual cycle, an ovum matures and is released once every 28 days. • The menstrual cycle starts with menstrual flow, during which cellular lining of the uterus is shed off along with blood flow. This process continues for 3–5 days. • From the 5th up to the 13th day of the onset of menstrual cycle, growth and maturation of the graafian follicle takes place. It consists of an ovum and a mass of cells surrounding it. • The graafian follicle produces the female hormone, oestrogen. • The cells lining the uterus grow rapidly and develop a dense network of blood vessels. • The release of the ovum from the ovary is called ovulation. Ovulation takes place 12–13 days after the onset of menstruation. The graafian follicle ruptures to release the ovum. • The cells of the ruptured follicle form the corpus luteum, which secretes the hormone, progesterone. • The ovum reaches the uterus via the fallopian tube on the 13th or 14th day and remains there up to the 16th day (for 48–72 hours). • If the ovum does not receive any sperm during this period it starts degenerating. At the end of the 28th day this ovum is rejected along with the uterine lining. • This marks the start of a slow disintegration of the thickened lining of the uterus and the next menstrual cycle. However, if the ovum receives sperm, it is in the fallopian tube, the two unite to form a zygote. This is called fertilization. Fertilization occurs in the fallopian tube. The zygote immediately begins to divide and forms a mass of cells called morula, which passes down to the uterus and fixes itself to the wall of the uterus (known as implantation). Menstruation does not occur and the female is said to be pregnant. The developing young one or the foetus is attached to the uterus by a tissue called placenta. Placenta supplies oxygen and nourishment from the maternal blood to the foetus. It also transports carbon dioxide and excretory waste from the foetal blood to the maternal blood. Placenta also produces two hormones— progesterone and oestrogen. Under the influence of these hormones neither ovulation nor menstruation takes place till pregnancy continues. The Umbilical cord is a tough structure that serves as the blood vascular connection between the foetus and uterine wall. From the first few weeks of development, the embryo is enclosed in a sac called amnion, which is filled with amniotic fluid. This fluid acts as a shockabsorber and helps to protect the embryo from damage. Test tube babies In some women, the fallopian tube gets blocked, which prevents the ova from being fertilized. This problem can be overcome by the test tube baby technique. In this technique, one or more mature ova are sucked from a woman’s ovaries using a special syringe. The sperms are taken from her husband. These sperms and ova are kept together in a container for a few hours for fertilization to take place. When a sperm fertilizes an ovum a zygote is formed, which divides repeatedly to form an embryo. This embryo is them inserted into the woman’s uterus where it gets implanted and develops into a baby. Fertilization of an egg by a sperm outside the body of the female is called in-vitro fertilization (IVF). Twins In every reproductive cycle, usually, an ovary releases only one ovum. But, sometimes more than one egg may be released and fertilized by more than one sperm or an ovum may divide into two or more cells after fertilization which separate and develop as different individuals. This is how twins, triplets and quadruplets etc. are produced. a) Identical twins When a fertilized egg divides into two independent sets of cells, both of which continue to divide, two identical embryos are produced from the same egg. The twins thus produced are identical twins. b) Non-identical or fraternal twins When two eggs are produced at the same time and a different sperm fertilizes each egg, nonidentical or fraternal twins are produced. POPULATION CONTROL Reproduction serves to replace the older generation. It is also a phenomenon that leads to increase in the number of individuals of a species to ensure that at least some will survive in the struggle for existence. In humans, due to a variety of reasons more and more children have been surviving. This has currently led to what we call the population explosion. The increase in population has created many problems like problems of food, shelter, clothing, etc. So it has become very essential to limit the human population. There are various ways to prevent fertilization and hence to check the increase in population. Let us study about few such methods. Education Imparting education to the people about various ways of fertility control is the most effective method of population control. Education helps to make people aware of the advantages of a small family and the disadvantages of a large family. Preventive Methods These methods prevent the fusion of the egg and the sperm. Two important preventive methods are discussed below. (i) Vasectomy This is a method of sterilization in males. In this method, each vas deferens is cut and tied at both cut ends by a thread (ligature). (ii) Tubectomy This is a method of sterilization in females. In this method, the fallopian tube is cut and the two ends are tied to prevent passage of ova down the fallopian tubes. Thus, in this method, the eggs continue to be released but do not reach the fallopian tube and no fertilization takes place. Contraceptive Methods These methods involve prevention of fertilization and conception. The main methods of contraception are given below: i) Natural Methods of Contraception • Rhythm method of contraception: In this method copulation is avoided for those days when the ovum is available for fertilisation. Thus, fertilization can be avoided. • Coitus interruptus: In this method, the penis is withdrawn from the vagina prior to ejaculation. ii) Mechanical Methods of Contraception In this approach, various mechanical methods are used to prevent the passage of semen to the fallopian tube or to prevent implantation. • Condoms: It is a thin rubber tube worn over the penis before sexual intercourse. The ejaculate gets collected in this tube and is not discharged into the vagina. • Diaphragm: It is fitted over the cervix in a woman’s body by a doctor to prevent the entrance of sperms into the cervical canal. • Intra uterine device (IUD): IUD or loop is made of plastic or stainless steel. It is inserted in the uterus, which releases certain secretions that prevent the implantation of embryo in the uterine wall. iii) Chemical Methods of Contraception •Spermicides: Strong spermicidal (sperm-killing) creams, jellies, etc. are applied in the vagina before copulation, which kill the sperms and prevent fertilization. •Oral Contraceptives or Pills: The oral contraceptives or pills are taken daily, which prevent ovulation in females. These pills prevent ovulation but allow monthly shedding of the uterine lining through menstrual bleeding. iv) By Medical Termination of Pregnancy (MTP) MTP methods are also known as contraceptive methods. They are applied if conception has taken place. Abortion and aspiration are two corrective methods in which pregnancy can be terminated by either mechanical method or by using hormones. SEXUALLY TRANSMITTED DISEASES Diseases which spread through sexual contact are called sexually transmitted diseases (STD). Sometimes microorganisms may infect areas around reproductive parts. During the act of sexual intercourse, these microorganisms may be easily transmitted from one person to another. There are so many STIs and most are of viral origin, moreover, a good number of them are incurable. Three important sexually transmitted diseases are: i) Syphilis ii) Gonorrhoea iii) Acquired Immuno Deficiency Syndrome (AIDS) Syphilis and Gonorrhoea Causative organism Both these diseases are caused by bacteria. Syphilis: Treponema pallidum and Gonorrhoea: Neisseria gonorrhoeae Modes of spread Sexual contact with the infected person Incubation period Symptoms of gonorrhoea disease occur in about 2-5 days and that of syphilis are seen in 10-90 days. Symptoms The common symptoms of these bacterial diseases are given below: i) Fever, followed by sores appearing on the skin, in the throat and urinogenital areas especially vagina or penis, anus, rectum and mouth. ii) Break out of rashes on hands, feet and palms. iii) White patches in the mouth. iv) Acne-like warts in the groin area. v) Hairfall occurs in patches from infected areas. Prevention and cure Abstinence is the best method to avoid them, but any of the underlisted can be helpful: i) Having sexual intimacy with only one person ii) Avoiding prostitution and homosexuality iii) Taking appropriate medical treatment Acquired Immuno Deficiency Syndrome (AIDS) AIDS is caused by the Human immunodeficiency virus (HIV). Upon entry into the body, the virus lives and grows in the body fluids and blood cells of the infected person. Mode of transmission HIV may be transmitted in the following ways: (i) Sexual contact with the affected person i.e. through semen or vaginal fluid. (ii) Exposure to infected blood and blood products. This may occur by using the same sharp object, e.g., needle and syringe, clippers etc with an infected person, and by use of infected blood during blood transfusion. (iii) Organ transplant from an affected person to a healthy person. (iv) During pregnancy, from an infected mother’s blood to her baby’s blood. Although Prevention of Mother to Child Transmission Therapy (PMTCT), is now possible. Incubation period The average incubation period of HIV virus is about 28 months (range 15–57 months). Symptoms i) The person feels fatigued or tired, suffers from loss of weight and fever, and sweats profusely. ii) Persistent dry cough, oral rash and shortness of breath may be observed. iii) Headache, visual disturbance and vomiting are also witnessed. iv) Gastro-intestinal problems like mild diarrhoea may occur. v) Skin blotches, eczema, fungal infection and sometimes skin cancer may be observed. vi) Nervous system may be affected, the brain may be badly damaged leading to a loss of memory, and ability to speak and to think. vii) A completely infected AIDS patient may die within three years of infection. Prevention and control Although there is no cure for AIDS, the HIV infection can be prevented by i) Avoiding multiple sex partners ii) Using a condom or other method of contraception iii) Avoiding prostitution and homosexuality iv) Screening of blood before transfusion v) Treatment of all blood and other products used in transfusion to destroy the HIV vi) Avoiding sharing of injection needles vii) Avoiding pregnancy if the mother is HIV positive viii) Educating people. References Anonymous, (2009). Birth Control. Thagard Student Health Center, Florida State University, 1: 1- 17 Anonymous, (2005). Reproduction. Reproduction in Living Things, 1: 238- 257. Bernstein, H., Hopf, F.A. and Michod, R.E. (1989). The Evolution of Sex: DNA Repair Hypothesis, The Sociobiology of Sexual and Reproductive Strategies, Rasa, C. and Voland, E. (Eds), Chapman and Hall, London, p. 4. Salem, M. R. and Vidya, S. (2006). Key Facts About the Menstrual Cycle.” INFO Reports. No. 7. Baltimore, Johns Hopkins Bloomberg School of Public Health, The INFO Project. Available online at: http://www.infoforhealth.org/inforeports/. Accessed March 15, 2011. Schecter, J. (1984). How Did Sex Come About? Bioscience 34: 680 T autz, S. (2004). Unsafe Abortion: A Review and Discussion Paper. In: Health and Population Supraregional Project: Promotion of Reproductive Health. A. Gabriel and C. Neckermann (eds). Sector project Reproductive Health/evaplan gmbh, Germany, pp. 1- 41 Thompson, B. and Harrub, B. (2004). Evolutionary Theories on Gender and Sexual Reproduction. The Reproduction Journal, 18 (1): 97- 104. Crow, J.F. (1988). The Importance of Recombination, The Evolution of Sex: An Examination of Current Ideas, Michod and Levin, Sinauer Associates, Sunderland, p. 35.