HOW DO ORGANISMS REPRODUCE

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1 HOW DO ORGANISMS REPRODUCE Reproduction It is the formation of new similar young living organisms by the existing or grown up organisms of a species or race. In simple words reproduction is the creation of new living organisms. Reproduction is essential for the survival of a species because all living organisms grow old with time and ultimately die after surviving for a limited period of time. So, living organisms produce more organisms of their kind so as to maintain the life of their species on this earth. If however, some species of the living organisms cannot reproduce due to certain reasons, then the organisms of those species will gradually die out and disappear from this earth one day. Do Organisms Create Exact Copies Of Themselves? A basic event in reproduction is the creation of DNA copy. Cells use chemical reactions to build copies of their DNA. This creates two copies of the DNA in a reproducing cell and they will need to be separated from each other. DNA copying is also accompanied by the creation of additional cellular apparatus and then the DNA copies separate, each with its own cellular apparatus. No biochemical reaction is absolutely reliable; therefore the process of copying of DNA will have some variations each time. Thus the exact copies will not be created. This inbuilt tendency for variation during reproduction is the basis for evolution. Note Variation is useful for the survival of species over time. That is why existence of variations is very important. Types Of Reproduction Methods of reproduction are mainly divided in two groups: - 1. Asexual Reproduction 2. Sexual Reproduction 1. Asexual Reproduction In asexual reproduction the new organism is produced from single parent without the formation and fusion of gametes. In asexual reproduction method certain body cells of the parent organism undergo repeated mitotic cell divisions to form two or more new organisms of the same kind. 1. Fission It is a type of asexual reproduction in which the parent organism splits to form two or more new organisms. Types Of Fission (i) Binary Fission In binary fission the parent organism splits to form two new organisms. When this happens, the parent organism ceases (stops) to exist and two new organisms come into existence. Example The unicellular organisms like planaria (flatworm), amoeba, euglena, paramecium, bacteria etc. A fully grown paramecium divides its body into two parts to form two smaller paramecia. First the nucleus of the parent paramecium divides followed by the division of cytoplasm and two small paramecia are produced. Then these two grow to their full size and split again to produce four new paramecia. And this process of reproduction is repeated again and again giving rise to their population. Binary Fission In Amoeba Binary Fission In Euglena Characteristics Of Asexual Reproduction 1. Cell division through mitosis only. 2. Genetic similarity among parent and young ones. 3. Rapidity and absence of haploid, diploid alteration. Different Methods Of Asexual Reproduction 1. Fission (i) Binary Fission (ii) Multiple Fission 2. Budding 3. Regeneration 4. Fragmentation 5. Spore Formation 6. Vegetative Propagation (ii) Multiple Fission In multiple fission, parent organism splits to form many new organisms at the same time. During multiple fission nucleus of parent cell divides into many nuclei. Each nucleus gets surrounded by a bit of cytoplasm and forms daughter cells. As many daughter cells are formed from a single parent cell as the number of daughter nuclei produced by the division of parent nucleus. When the favourable conditions arrive, then the cyst breaks open and many daughter cells present in it are released, each forming a new organism.

2 Example Plasmodium reproduces by multiple fission. 2. Budding In budding a small part of the body of the parent organism grows out as a bud, which then detaches and becomes a new organism. Budding is observed in hydra and yeast. Budding may occur in multicellular as well as unicellular organisms. Example In yeast a little cytoplasm gets accumulated at one end of the cell and forms a bud, on the outside of the cell wall. The nucleus of parent cell then divides into two parts and one part enters into the bud. Ultimately the bud separates off from the parent yeast cell and forms a new yeast cell. In yeast, bud formation is very fast thus in short time a chain of yeast cells is produced. After some time all the yeast cells of the chain separate from one another and form individual yeast plants. 4. Fragmentation (Special Case Of Regeneration) The natural breaking up of a multicellular organism into two or more parts each of which grows to form a new organism is called fragmentation. Fragmentation is a special case of regeneration in which a parent multicellular organism on maturing breaks up naturally to produce two or more daughter organisms. For example, spirogyra and alga plant breaks up into two or more fragments on maturity and each fragment grows into new spirogyra. Budding In Hydra 3. Regeneration Regeneration is the ability of the organisms to replace their lost parts or we can say that the process of getting back full organism from its body parts is called regeneration. If the individual is somehow cut or broken into many pieces, many of these grow into separate individuals. In some plants as well as animals small cut parts of their body can grow to form whole new organism complete in all the respects. Regeneration is carried out by specialized cells. These cells multiply and make large number of cells. From this mass of cells, different cells undergo changes to become various cell types and tissues. Difference Between Fission And Fragmentation In fission a unicellular organism breaks up to form two or more daughter organisms whereas in fragmentation a multicellular organisms breaks to form multiple new daughter organisms. 5. Spore Formation Formation of new organisms from the tiny spores produced by parent plant in hundreds of number is called spore formation. Spores are tiny spherical unicellular structures protected by a thick wall, are formed under unfavourable conditions. They grow into a new plant when conditions are favourable. Example Bread mould (rhizopus), mucor (pin mould), penicillium (blue green worm) and bacteria reproduce by the method of spore formation. Example Animals like hydra, planaria (flatworm), spirogyra and sponges show regeneration. It happens as follows: - During the growth of a fungus plant, tiny round, blob like structures called sporangia develop at the top of the erect hyphae, which are called sporangiophores. In the sporangium

3 nucleus divides several times and each part of nucleus along with a bit of cytoplasm develops into a spore. When the sporangium bursts then the spores spread into air. When these air borne spores land on food under favourable conditions like moisture and warmth, they germinate to form new fungus plants. At first they look like a white cottony mass covering which later on turns black. The tiny spores of rhizopus are present in air around us. 6. Vegetative Propagation In vegetative propagation new plants are obtained from the parts of old plants like stems, roots and leaves without the help of any reproductive organs. It involves the growth and development of one or more buds present on the old part of plant to form a new plant. Theses buds are in inactive state in the old part of plant. In suitable conditions these buds grow to form new plants. There are two ways of vegetative propagation or vegetative reproduction. (i) Natural Vegetative Propagation (ii) Artificial Vegetation Propagation (i) Natural Vegetation Propagation In this method various plant structures take part like roots, stem, leaves and bud. By Roots Some plants like reddish, sweet potato, danlia, carrot etc reproduce by roots. By Stems Some plants reproduce by means of stems. They may be aerial like runners, suckers or underground like ginger, potato and onion. By Leaves Some plants have ability to reproduce from the leaves. The buds are formed on the leaf margin and give rise to new plants like that in Bryophyllum. The new plants remain attached to the leaf for some time and then are detached and grow into an independent plant. 6. It is easier, cheaper and quicker method. Tissue Culture (Micro propagation) The production of new plants from isolated plant cells or small pieces of plant tissue in a synthetic medium called culture solution is called micro propagation or tissue culture. The medium used for growing plant cells is very important in this process because it contains various plant nutrients and plant hormones which are necessary for growth of plant. 2. Sexual Reproduction Sexual Reproduction involves the existence of two sexes, each contributing a gamete (mature sex cell) formed after meiosis, for the formation of the new individual after fertilization. Meiosis reduces the number of chromosomes in the gametes by half. Thus the fusion of male and female gametes results in a diploid new individual. First diploid cell formed after fusion is known as zygote which later divides mitotically to form complete new individual. During meiotic division, genetic material contained in the chromosomes gets mixed between homologous chromosomes. This results in new combinations of genetic material and accounts for the uniqueness of individual. Sexual Reproduction In Plants Sexual Reproduction in plants takes place through flowers. Like humans plants also have male and female sex organs, though they are different from humans. In most plants the same flower contains the male as well as female organs. The majority of plants are bisexual having the male and female reproduction organs in the same plant or same flower (hermaphrodites). The function of the flower is to make male and female gametes and to ensure that fertilization will take place to make new seeds for the reproduction of plants. Structure Of A Flower Advantages Of Artificial Vegetative Propagation 1. The new plants produced by artificial vegetative propagation will be exactly like the parent plants. Any desirable feature of the parent plant will be replicated in the new plants. 2. The fruit trees grown from seeds may take many years before they start to bear fruit but the fruit trees grown from cuttings or by grafting start to bear fruits much earlier. 3. The plants grown by vegetative propagation usually need less attention in their early years than the plants grown from seeds. 4. Disease free plants can be produced. 5. It is the only method of reproduction for those plants which do not produce seeds. Pedicel The stalk of flower is known as pedicel. Receptacle The broadened base of flower which lies at the tip of pedicel is called thalamus. At this point all parts of a flower are attached. It is also called receptacle. Sepals The green leaf like parts in the outer most circle of a flower is called sepal. In some plants it may be colored to attract insects. It protects the flower especially in bud conditions. Calyx All sepals taken together are called calyx. It is the outermost series of green non essential floral organs. Petals The colourful parts of a flower are called petals. They attract

4 various insects and animals for pollination. Perianth If both sepals and petals are coloured and cannot be distinguished from each other then their whorl (circle) is known as perianth and its constituents are known as tepals. Stamen (Androecium) Stamen is the male reproductive organs in plant. These are little stalks with swollen tops just inside the ring of petals in a flower. The stamen is made up of two parts: - 1. Filament Stalk of stamen. 2. Anther Upper sac like structure is known as anther. Anther is made up of two anther lobes which further contain two pollen sacs. Pollen sacs contain pollen grains. The pollen grains appear to be yellow powder like substance to us. Pollen grains contain the male gametes of the plant. Each pollen grain contains two male gametes of the plants. There are many stamens in the flowers. Carpel (Pistil, Gynoecium) Carpel is the female reproductive organ of flower. A carpel is made up of three parts: - 1. Stigma The top part of carpel is called stigma. Stigma is for receiving the pollen grains from anther of stamen during pollination. 2. Style The middle part of carpel is called style. Style is a tube which connects stigma to the ovary. 3. Ovary The swollen part at the bottom of a carpel is called ovary. The ovary makes ovules and stores them. Ovules contain female gametes of the plant. There are many ovules in the ovary. Each ovule contains only one female gamete of the plant. The female gamete present inside ovule is called ovum or egg. Formation Of Seeds A seed of plant is formed when the male gamete present in the pollen grain fuses with female gamete present in ovule. Formation of seeds occurs in two steps: - 1. Pollination 2. Fertilization Pollination The transfer of pollen grains from the anther of a stamen to the stigma of a carpel is called pollination. Types Of pollination 1. Self Pollination Pollination between same flower or flowers of same plant is called self pollination. (i) Autogamy It is the pollination in the same flower. (ii) Geitonogamy It is the pollination between different flowers of the same plant. 2. Cross Pollination (Xenogamy) It is the pollination between the flowers of different plants. Transfer of pollen grains can take place by various ways: - (i) Wind Pollination (ii) Insect Pollination (iii) Water Pollination (iv) Bird Pollination (v) Bat Pollination As geitonogamy resembles cross pollination in the requirement of pollen transfer or pollinating agency, both xenogamy and geitonogamy are included under allogamy though this term is most commonly used for cross pollination. Pollen Grain (Microspore) A pollen grain is rounded in outline. Internally it has: - 1. Tube or vegetative cells with degenerating nucleus. 2. Vacuolated cytoplasm rich in starch and unsaturated oils. 3. One generative cell or two male gametes derived from it. Embryo Sac Each ovule inside the ovary of a flower consists of a large oval shaped cell called embryo sac. It is the female gametophyte. The embryo sac is surrounded by two protective membranes. The embryo sac grows and its nucleus undergoes repeated mitotic divisions until eight nuclei are produced. Thus a mature embryo contains eight nuclei out of which one is female gamete called ovule. The eight nuclei of embryo sac are present in groups of four nuclei at the opposite poles of embryo sac. The embryo sac has a small opening at its one end, which is called micropyle. Through micropyle the male gametes through pollen tubes enter into embryo sac. An embryo sac is seven celled and 8 nucleate structures. Three micropylar cells contain three micropylar nuclei. Three chalazal cells contain 3 chalazal nuclei. One central cell contains two polar nuclei. 2. Fertilization When a pollen grain falls on the stigma of the carpel. It grows a pollen tube downward in the style. This pollen tube contains two male gametes from which one male gamete reaches the ovary where ovules are present. The male gametes enter into embryo sac of ovule through micropyle. Now one male gamete fuses with one female gamete (egg) present in upper part of embryo sac to form zygote. This fusion whose product is a zygote is called syngamy. In seed plants fertilization is also called siphonogamy. It is also called generative fertilization. The other male gamete fuses with two polar nuclei one from each end of embryo sac. This fusion forms an endosperm nucleus.

5 Triple Fusion The fusion of one male gamete and two polar nuclei is called triple fusion. It is also called vegetative fertilization. During triple fusion endosperm nucleus is formed which acts as the food for the developing zygote. Zygote The cell which is formed by the fusion of gametes (male gamete and female gamete) is called a zygote. Zygote is actually a fertilized egg. Double Fertilization In the embryo sac of the ovule two fusions, syngamy and triple fusion takes place. This process of two fusions of gametes taking place in embryo sac together is called double fertilization. In other words it is the fusion of two male gametes with two different structures in the same female gametophyte (embryo sac) to produce two different structures for future development of plant is called double fertilization. After the completion of this process the remaining nuclei present in the embryo sac disintegrate and disappear. Zygote develops into an embryo and endosperm nucleus develops into an endosperm, the food storage tissue of seed. Endosperm is meant for nourishing the embryo. The embryo and endosperm together make the seed. After fertilization ovule forms seed of the plant and ovary forms the fruit with seeds inside it. Seed A seed is the reproductive unit of a plant. It contains embryo and its food. The plant present in seed is in inactive state. When suitable conditions are provided the new plants grow out of the seeds. Germination The seed contains a future plant or embryo which develops into an seedling under appropriate conditions. This process is known as germination. Sexual Reproduction In Animals Male And Female An animal having male sex cells called sperms in its body is called male. An animal having female sex cells called ova or eggs in its body is called female. Gametes The cells involved in sexual reproduction are called gametes. Ovum Or Egg Female gamete in animals is called Ovum (plural Ova) or Egg. Sperm The male gamete in animal is called sperm. Sperm and ova are extremely small cells, which can be seen only with the help of a high power microscope. Fertilization The process of fusion of male gamete (sperm) with a female gamete (egg) to form a zygote during the sexual reproduction is called fertilization. Embryo The stage of development between the zygote and the newly formed baby is called embryo. Types Of Fertilization 1. Internal Fertilization 2. External Fertilization 1. Internal Fertilization The fertilization, which occurs inside the female body, is called internal fertilization. In mammals, birds and reptiles internal fertilization takes place. In internal fertilization the male animal puts his sperms into the female animal s body. This transfer of sperms occurs at the time of copulation (mating). These sperms fertilize the egg inside the female animal s body. 2. External Fertilization The fertilization, which occurs outside the female s body, is called external fertilization. In amphibians and fish external fertilization takes place. In external fertilization male and female release their sperms and eggs in water, fertilization takes place by collisions between sperms and eggs. Types Of Organism Depending upon the presence of reproducing organs the organisms can be divided into two types: 1. Unisexual Organisms 2. Bisexual Organisms 1. Unisexual Organisms An organism which has only one kind of reproductive organs (male or female) in its body is called unisexual organisms. For example, humans, frogs, birds, reptiles etc are all unisexual organisms. In humans the males have only one reproductive

6 organ called testes and human female has only female reproductive organs called ovaries in their bodies. 2. Bisexual Organisms (Hermaphrodites) An organism, which has both male and female reproductive organs in its body, is called a bisexual organism. The animals like earthworms, tapeworms, leeches and starfish and most flowering plants are bisexual organisms. hormone called testosterone. Testes also make the male gametes (sperms) from puberty onwards throughout the whole life. Advantage Of Sexual Reproduction 1. Sexual reproduction leads to greater variety or population. 2. All the offsprings have genetic variation. 3. Sexual reproduction promotes diversity of characters in the offsprings. 4. Sexual reproduction plays an important role in the origin of new species having different characteristics. 5. Sexual reproduction leads to the continuous evolution of various species to form better and still better organisms. Human Reproductive System Human males and females have different reproductive systems. The reproductive systems in human beings become functional at a definite age called puberty. Puberty The age at which the sex hormones or gametes begin to produce and the boy/girl becomes sexually mature i.e., become able to reproduce is called puberty. Males attain puberty at the age of 13 to 14 years. Females reach puberty at an age of 10 to 12 years. In addition to producing gametes male and female gonads (testes and ovaries) also produce and secrete sex hormones with the start of puberty. Testes produce male sex hormones called testosterone and ovaries produce two female sex hormones called estrogen and progesterone. Changes Taking Place After Puberty Some changes are common to both boys and girls which are as follows: - (i) Thick hairs growing between armpits and the genital area between thighs. (ii) Hairs appear on legs and arms as well as on the face. (iii) The skin becomes oily and pimples start appearing. Functions Of Sex Hormones 1. The sex hormones control the process of gametogenesis i.e., production of gametes. 2. The sex hormones maintain the structure and functions of accessory sex organs. 3. The sex hormones develop the secondary sex characteristics. Male Reproductive System The human male reproductive system consists of the following organs: 1. Testes (Primary Reproductive organ in male) 2. Scrotum 3. Epididymis 4. Vas Deferens 5. Seminal Vesicles 6. Prostate Gland 7. Penis 1. Testes Testes are the oval shaped organs, which lie outside the abdominal cavity of a man. A man has two testes (singular testis). The function of testes is to make the male sex 2. Scrotum It is a small muscular pouch outside the abdominal cavity in which the testes lie. Being outside the abdominal cavity the temperature of scrotum is 1 to 3 degrees lower than the inside body temperature. In this way the testes provide an optimal temperature for the formation of sperms. 3. Epididymis It is a coiled tube in which the sperms get stored temporarily after their formation. 4. Vas Deferens (Sperm Duct) It is a long tube through which sperms are carried into an organ called seminal vesicle. 5. Seminal Vesicles And Prostate Gland Seminal vesicle provides a fluid medium to the sperms called semen and prostate gland provides nourishment to sperms. 6. Penis It is an organ, which opens outside body. Sperms are carried by urethra to penis in liquid called semen. Note In males there is only one opening for the urine and sperms to pass out from body. Female Reproductive System The human female reproductive system consists of the following organs 1. Ovaries (Primary Reproductive Organ) 2. Fallopian Tubes (Oviduct) 3. Uterus 4. Cervix 5. Vagina 1. Ovaries Ovaries are the oval shaped organs, which lie inside the abdominal cavity of woman near the kidneys. A woman has two ovaries. The function of the ovaries is to make female gametes called eggs or ova and also to make two female sex hormones estrogen and progesterone. Each ovary consists of several thousand ovarian follicles (immature eggs). At puberty

7 these follicles mature to form the ripe eggs which are required for fertilization 2. Fallopian Tubes These are tubes, which lie just above the ovaries. The fallopian tubes are not directly connected to ovaries but have a funnel shaped opening covering the ovary. The ovum released by an ovary goes into the fallopian tube through funnel shaped opening. The fertilization of egg by a sperm takes place in fallopian tubes. fertilization. Thus fertilization of the ovum takes place in the fallopian tube. 4. Then ovum divides rapidly by mitosis as it moves down slowly in the fallopian tube and forms a hollow ball of hundreds of cells. Implantation The hollow ball of cells now called an embryo sinks into the soft and thick lining of the uterus and gets embedded in it. The embedding of embryo in the thick lining of the uterus is called implantation. After implantation, a special tissue develops between the uterus walls and the embryo called placenta. It is through placenta that all the requirements of a developing embryo (foetus) like nutrition, respiration and excretion etc. are met from the mother s body. The umbilical cord joins the embryo to the placenta. 3. Uterus (Womb) It is a bag like organ, which is connected by two fallopian tubes. The growth and development of a fertilized ovum into a baby takes place in uterus. 4. Cervix It is a narrow opening to which uterus is connected. It is connected to another tubular structure called vagina, which opens outside the body. It is meant to hold the developing zygote in the uterus. 5. Vagina Vagina receives the penis for putting sperms into woman s body. It is also called birth canal because it is through this passage the baby is born after the completion of development inside the uterus of mother. Note In woman, the urinary opening and the vaginal openings are separate. Complete Process Of Reproduction In Humans Reproduction in humans takes place in the in the following steps. Ovulation Release of an ovum or egg from an ovary is called ovulation. The ovulation takes place in the mid of the sexual cycle. Fertilization In human beings internal fertilization takes place. The process occurs as follows: 1. The sperms made in testes of male are introduced in the vagina of female through penis during copulation. In this way millions of sperms are released into vagina at one time. The sperms are highly active and moving. 2. The sperms then move up through cervix into the uterus. 3. From uterus the sperms pass into the fallopian tube. One of the fallopian tubes contains an ovum released by an ovary during ovulation. Only one sperm fuses with the ovum in the fallopian tube to form a zygote. This is called Gestation The time period from the fertilization upto the birth of baby is called gestation. The average gestation period in humans is about 280 days or 3 months 10 days or 40 weeks. Parturition During the gestation period the foetus grows to become a baby. Birth begins when strong muscles in the walls of the uterus start to contract. The contraction of uterus muscles gradually pushes the baby out of the mother s body through vagina or birth canal. This process is called parturition commonly called labour. The average weight of newborn baby is about 3.5 kg. Sexual Cycle In Females: Menstruation The sexual cycle in females is called menstruation cycle or menstruation. It occurs as follows: - 1. When a girl reaches puberty at the age of about 12 years, the sex hormones released into her blood cause some of the ova in her ovaries to become mature. 2. Usually one mature ovum is released from the ovary into the fallopian tube once every 28 days. 3. Before ovulation the inner lining of the uterus becomes thick and spongy and full of tiny blood vessels and prepares itself to receive the fertilized egg. 4. If the ovum does not get fertilized due to the nonavailability of sperms in the female body then the thick and soft inner lining of uterus is no longer needed and hence it breaks. So, the thick and soft inner lining of uterus along with the blood vessels and dead ovum comes out of vagina in the form of bleeding called menstruation. 5. Menstruation usually occurs 14 days after ovulation and usually lasts for about 2 to 5 days. 6. This cycle of menstruation is repeated again and again in a female after 28 days and is controlled by hormones till menopause.

8 Menarche The beginning of menstruation at puberty is called menarche. Menopause The age at which the menstruation stops and a female loses her ability to bear children is called menopause. Note Menstruation also stops temporarily when a female gets pregnant. Population Control Contraception For the prevention of pregnancy in women as population is increasing rapidly it is necessary to take birth control measures, family planning measures or population control measures. All birth control measures can be broadly divided into three categories: - 1. Barrier Method 2. Chemical Method 3. Surgical Method 1. Barrier Method In the barrier methods of preventing pregnancy, the physical devices such as condoms and diaphragms (cervical caps) are used. Condoms are used by males and females use diaphragms. These devices prevent the sperms from meeting the ovum by acting as a barrier between them. 2. Chemical Method In the chemical methods of preventing pregnancy the females use two types of pills. 1. Oral Pills 2. Vaginal Pills (i) Oral Pills It contains hormones, which stop the ovaries from releasing eggs into the fallopian tubes. These are also called oral contraceptives (OC) (ii) Vaginal Pills It contains chemicals called spermicidal which kill sperms. Sexually Transmitted Diseases There are mainly four sexually transmitted diseases, which are: - 1. Syphilis 2. Gonorrohoea 3. Trichomoniases 4. AIDS 1. Syphilis Bacterial Disease Bacterium Treponema Allidium Symptoms 1. Lesions in mucus membrane of urinogenital tract and oral cavity. 2. Incubation period about 34 weeks. It can be controlled by antibiotics. 2. Gonorrohoea Bacterial Disease Bacterium Neisseria Gonorrhoea Symptoms 1. Inflammation of mucus membrane of urinogenital tract 2. Burning sensation during urination 3. Can lead to female sterility or arthritis. 3. Trichiomoniases Protozoan Disease Symptoms 1. Burning sensation during urination. 2. Passing of urethral discharge containing pus 3. Sores in the genitals. All the above diseases are curable by antibiotics. 4. AIDS (Viral Disease - Virus HIV) Symptoms 1. Damages Immune System of Body. 2. A person becomes weak. 3. Body is not protected against diseases. It is an incurable disease. 3. Surgical Methods Surgical methods of birth control are available for both male and female. (i) Surgical Method For Males In males a small portion of sperm duct is removed by surgical operation and both the cut ends are tied (legated) properly. This prevents the sperms from coming out. This is called vasectomy. Another method is called castration in which testes are removed by surgical methods. (ii) Surgical Method For Female In females a small portion of the fallopian tubes is removed by surgical operation and the cut ends are tied (legated). This prevents the ovum from entering into the fallopian tube. This is called tubectomy. Another method is called ovariectomy in which ovaries of female are removed by surgical methods. Intra Uterine Contraceptive Device (IUCD) An intra uterine device copper T is placed inside the uterus prevents the implantation of fertilized egg in the uterus.