CLASS 12 Biology CHAPTER 4 REPRODUCTIVE HEALTH (NCERT Notes)

CLASS 12 Biology                                                                                                    CHAPTER 4                                                                                                      REPRODUCTIVE HEALTH (NCERT  Notes)

Reproductive health refers to healthy reproductive organs with normal functions. It has a broader perspective and includes the emotional and social aspects of reproduction.

According to the World Health Organisation (WHO), reproductive health means a total well-being in all aspects of reproduction, i.e., physical ,emotional , behavioural and social .Therefore ,a society with people having physically and functionally normal reproductive organs and normal emotional and behavioural interactions among them in all sex-related aspects might be called reproductively healthy.

REPRODUCTIVE HEALTH–PROBLEMS AND STRATEGIES                                                        India was amongst the first countries in the world to initiate action plans and programmes at a national level to attain total reproductive health as a social goal. These programmes called ‘family planning’ were initiated in 1951 and were periodically assessed over the past decades.                                            

Improved programmes covering wider reproduction-related areas are currently in operation under the popular name ‘Reproductive and Child Health Care (RCH) programmes’ . Creating awareness among people about various reproduction related aspects and providing facilities and support for building up a reproductively healthy society are the major tasks under these programmes.                        

With the help of audio -visual and the print-media governmental and non-governmental agencies have taken various steps to create awareness among the people about reproduction-related aspects. Parents, other close relatives, teachers and friends, also have a major role in the dissemination of the above information. 

                                                                                                                                     

Introduction of sex education in schools should also be encouraged to provide right information to the young so as to discourage children from believing in myths and having misconceptions about sex-related aspects.                                                                                                                                              

Proper information about reproductive organs, adolescence and related changes ,safe and hygienic sexual practices, sexually transmitted diseases (STD), AIDS , etc ., would help people ,especially those in the adolescent age group to lead a reproductively healthy life.                                                        

Educating people, especially fertile couples and those in marriageable age group, about available birth control options, care of pregnant mothers, post -natal care of the mother and child, importance of breast feeding ,equal opportunities for the male and the female child, etc. ,would address the importance of bringing up socially conscious healthy families of desired size.                                                     

Awareness of problems due to uncontrolled population growth, social evils like sex-abuse and sex-related crimes, etc., need to be created to enable people to think and take up necessary steps to prevent them and thereby build up a socially responsible and healthy society.

Amniocentesis                                                                                                                                        The government has ban on amniocentesis for sex-determination to legally check increasing menace of female foeticides , massive child immunisation, etc., are some programmes that merit mention in this connection. In aminocentesis some of the amniotic fluid of the developing foetus is taken to analyse the fetal cells and dissolved substances. This procedure is used to test for the presence of certain genetic disorders such as, down syndrome, haemoplilia, sickle-cell anemia, etc. , determine the survivability of the foetus .                                                                                                                                                      
Research on various reproduction- related areas are encouraged and supported by governmental and non-governmental agencies to find out new methods and/or to improve upon the existing ones.        Saheli’–a new oral contraceptive for the females–was developed by scientists at Central Drug Research Institute (CDRI) in Lucknow, India.                                                                                  

Better awareness about sex related matters ,increased number of medically assisted deliveries and better post-natal care leading to decreased maternal and infant mortality rates, increased number of couples with small families, better detection and cure of STDs and overall increased medical facilities for all sex- related problems, etc. all indicate improved reproductive health of the society.                      

POPULATION STABILISATION AND  BIRTH  CONTROL                                                              Increased health facilities along with better living conditions had an explosive impact on the growth of population. The world population which was around 2 billion (2000 million) in 1900 rocketed to about 6 billion  by 2000 and 7.2 billion in 2011. A similar trend was observed in India too. Our population which was approximately 350 million at the time of our independence reached close to the billion mark by 2000 and crossed 1.2 billion in May 2011.                                                                                                                                                            
A rapid decline in death rate , maternal mortality rate (MMR) and infant mortality rate (IMR) as well as an increase in number of people in reproducible age are probable reasons for this. Through our Reproductive Child Health (RCH) programme, though we could bring down the population growth rate, it was only marginal.                                                                                                                            

According to the 2011 census report, the population growth rate was less than 2 per cent, i.e., 20/1000/ year, a rate at which our population could increase rapidly. Such an alarming growth rate could lead to an absolute scarcity of even the basic requirements, i.e., food, shelter and clothing, inspite of significant progress made in those areas. Therefore ,the government was forced to take up serious measures to check this population growth rate.     

The most important step to overcome this problem is to motivate smaller families by using various contraceptive methods.                                                                                                                  Advertisements in the media as well as posters/ bills,etc .,showing a happy couple with two children with a slogan Hum Do Hamare Do (we two, our two).                                                                              Many couples, mostly the young, urban, working ones have even adopted an‘ one child norm’. Statutory raising of marriageable age of the female to 18 years and that of males to 21years ,and incentives given to couples with small families are two of the other measures taken to tackle this problem.  

Some of the commonly used contraceptive methods ,which help prevent unwanted pregnancies.  
An ideal contraceptive should be user-friendly, easily available ,effective and reversible with no or least side-effects. It also should in no way interfere with the sexual drive, desire and/or the sexual act of the user.

Wide range of contraceptive methods are presently available which could be broadly grouped into the following categories :                                                                                                              

Natural methods work on the principle of avoiding chances of ovum and sperms meeting.        

Periodic abstinence is one such method in which the couples avoid or abstain from coitus from day10 to17 of the menstrual cycle when ovulation could be expected. As chances of fertilisation are very high during this period, it is called the fertile period. Therefore, by abstaining from coitus during this period, conception could be prevented.                                                                                                                    

Withdrawal or coitus interruptus is another method in which the male partner withdraws his penis from the vagina just before ejaculation so as to avoid insemination.                                                          

Lactational amenorrhea (absence of menstruation) method is based on the fact that ovulation and therefore the cycle do not occur during the period of intense lactation following parturition. Therefore, as long as the mother breast-feeds the child fully, chances of conception are almost nil. However, this method has been reported to be effective only up to a maximum period of six months following parturition. As no medicines or devices are used in these methods, side effects are almost nil. Chances of failure ,though, of this method are also high.                                                                                          

In barrier methods, ovum and sperms are prevented from physically meeting with the help of barriers. Such methods are available for both males and females.                                                                          

Condoms are barriers made of thin rubber/ latex sheath that are used to cover the penis in the male or vagina and cervix in the female, just before coitus so that the ejaculated semen would not enter into the female reproductive tract. This can prevent conception.‘Nirodh’ is a popular brand of condom for the male. Use of condoms has increased in recent years due to its additional benefit of protecting the user from contracting STIs and AIDS. Both the male and the female condoms are disposable, can be self-inserted and there by gives privacy to the user.                                                                                            

Diaphragms, cervical caps and vaults are also barriers made of rubber that are inserted into the female reproductive tract to cover the cervix during coitus. They prevent conception by blocking the entry of sperms through the cervix. They are reusable. Spermicidal creams, jellies and foams are usually used along with these barriers to increase their contraceptive efficiency.                                

Another effective and popular method is the use of Intra Uterine Devices (IUDs). These devices are inserted by doctors or expert nurses in the uterus through vagina. These Intra Uterine Devices are presently available as the non-medicated IUDs(e.g. , Lippes loop), copper releasing IUDs (Cu T, Cu7, Multi load 375) and the hormone releasing IUDs ( Progestasert LNG-20).                          

IUDs increase phagocytosis of sperms within the uterus and the Cu ions released suppress sperm motility and the fertilising capacity of sperms.                                                                                          

The hormone releasing IUDs ,in addition, make the uterus unsuitable for implantation and the cervix hostile to the sperms. IUDs are ideal contraceptives for the females who want to delay pregnancy and /or space children. It is one of most widely accepted methods of contraception.                                         
Oral administration of small doses of either progestogens or progestogen–estrogen combinations is another contraceptive method used by the females. They are used in the form of tablets and hence are popularly called the pills. Pills have to be taken daily for a period of 21 days starting preferably within the first five days of menstrual cycle. After a gap of 7 days (during which menstruation occurs) it has to be repeated in the same pattern till the female desires to prevent conception. They inhibit ovulation and implantation as well as alter the quality of cervical mucus to prevent /retard entry of sperms.            

Pills are very effective with lesser side effects and are well accepted by the females. Saheli–the new oral contraceptive for the females contains a non-steroidal preparation. It is a ‘once a week’ pill with very few side effects and high contraceptive value.                                                                                     

Progestogens alone or incombination with estrogen can also be used by females as injections or implants under the skin . Their mode of action is similar to that of pills and their effective periods are much longer. Administration of progestogens or progestogen- estrogen combinations or IUDs within 72 hours of coitus have been found to be very effective as emergency contraceptives as they could be used to avoid possible pregnancy due to rape or casual unprotected intercourse.                                                              

Surgical methods, also called sterilisation ,are generally advised for the male/female partner as a terminal method to prevent any more pregnancies Surgical intervention blocks gamete transport and there by prevent conception .                                                                                                      

Sterilisation procedure in the male is called‘ vasectomy ’and that in the female, ‘tubectomy’ .              In vasectomy ,a small part of the vas deferens is removed or tied up through a small incision on the scrotum where as in tubectomy, a small part of the fallopian tube is removed or tied up through a small incision in the abdomen or through vagina. These techniques are highly effective but their reversibility is very poor. 

The selection of a suitable contraceptive method and its use should always be undertaken in consultation with qualified medical professionals. One must also remember that contraceptives are not regular requirements for the maintenance of reproductive health. In fact, they are practiced against a natural reproductive event, i. e., conception/ pregnancy. One is forced to use these methods either to prevent pregnancy or to delay or space pregnancy due to personal reasons. Nodoubt, the wides pread use of these methods have a significant role in checking uncontrolled growth of population . However ,their possible ill-effects like nausea, abdominal pain, break through bleeding ,irregular menstrual bleeding or even breast cancer.

MEDICAL TERMINATION OF PREGNANCY (MTP)                                                                    Medical termination of pregnancy (MTP) or induced abortion. Nearly 45 to 50 million MTPs are performed in a year all over the world which accounts to1/5th of the total number of conceived pregnancies in a year. Whether to accept/legalise MTP or not is being debated upon in many countries due to emotional, ethical, religious and social issues involved in it. 

Government of India legalised MTP in1971 with some strict conditions to avoid its misuse .Such restrictions are all the more important to check indiscriminate and illegal female foeticides which are reported to be high in India.                        
To get rid of unwanted pregnancies either due to casual unprotected inter course or failure of the contraceptive used during coitus or rapes. MTPs are also essential in certain cases where continuation of the pregnancy could be harmful or even fatal either to the mother or to the foetus or both.                  MTPs are considered relatively safe during the first trimester, i.e., up to 12 weeks of pregnancy. 

Second trimester abortions are much more riskier . One disturbing trend observed is that a majority of the MTPs are performed illegally by unqualified quacks which are not only unsafe but could be fatal too. Another dangerous trend is the misuse of amniocentesis to determine the sex of the unborn child. Frequently ,if the foetus is found to be female, it is followed by MTP -this is totally against what is legal. Such practices should be avoided because these are dangerous both for the young mother and the foetus.  Effective counselling on the need to avoid unprotected coitus and the risk factors involved in illegal abortions as well as providing more health care facilities could reverse the mentioned un healthy trend.

The Medical Termination of Pregnancy (Amendment)Act,2017 was enacted by the government of India with the intension of reducing the incidence of illegal abortion and consequent maternal mortality and morbidity .According to this Act, a pregnancy may be terminated on certain considered grounds within the first 12 weeks of pregnancy on the opinion of one registered medical practitioner. If the pregnancy has lasted more than 12 weeks, but fewer than 24 weeks, two registered medical practitioners must be of the opinion, formed in good faith, that the required ground exist. The grounds for such termination of pregnancies are:                                                                                                      (i)The continuation of the pregnancy would involve a risk to the life of the pregnant woman or of grave injury physical or mental health;                                                                                                                  (ii) There is a substantial risk that of the child were born, it would suffer from such physical or mental abnormalities as to be seriously handicapped.

SEXUALLY TRANSMITTED INFECTIONS (STIs)                                                                         Infections or diseases which are transmitted through sexual intercourse are collectively called    sexually transmitted infections(STI) or venereal diseases(VD) or reproductive tract infections (RTI). Gonorrhoea , syphilis ,genital herpes, chlamydiasis, genital warts, trichomoniasis, hepatitis-B and of course, the most discussed infection in the recent years, HIV leading to AIDS are some of the common STIs. Among these, HIV infection is most dangerous.                                            

Infections like hepatitis–B and HIV can also be transmitted by sharing of injection needles, surgical instruments, etc. , with infected persons, transfusion of blood ,or from an infected mother to the foetus too. Except for hepatitis-B, genital herpes and HIV infections ,other diseases are completely curable if detected early and treated properly.                                                                                                              

Early symptoms of most of these are minor and include itching ,fluid discharge ,slight pain ,swellings, etc. ,in the genital region .Infected females may often be a symptomatic and hence, may remain undetected for long. Absence or less significant symptoms in the early stages of infection and the social stigma attached to the STIs, deter the infected persons from going for timely detection and proper treatment. This could lead to complications later, which include pelvic inflammatory diseases (PID), abortions, stillbirths ,ectopic pregnancies, infertility or even cancer of the reproductive tract. STIs are a major threat to a healthy society. Therefore, prevention or early detection and cure of these diseases are given prime consideration under the reproductive health-care programmes.

One could be free of these infections by following the simple principles given below:                       (i) Avoid sex with unknown partners /multiple partners.                                                                         

(ii) Always try to use condoms during coitus.                                                                                              (iii) In case of doubt, one should go to a qualified doctor for early detection and get complete treatment if diagnosed with infection.

INFERTILITY                                                                                                                                         

A large number of couples all over the world including India are infertile ,i .e. ,they are unable to produce children in spite of unprotected sexual co-habitation. The reasons for this could be many–physical, congenital, diseases, drugs, immunological or even psychological.                                              

In India, often the female is blamed for the couple being childless, but more often than not, the problem lies in the male partner. Specialised health care units (infertility clinics, etc.) could help in diagnosis and corrective treatment of some of these disorders and enable these couples to have children. However, where such corrections are not possible ,the couples could be assisted to have children through certain special techniques commonly known as assisted reproductive technologies(ART).                                                               
In vitro fertilisation (IVF– fertilisation outside the body in almost similar conditions as that in the body) followed by embryo transfer(ET)is one of such methods.                                                          

In this method, popularly known as test tube baby programme ,ova from the wife /donor (female) and sperms from the husband/ donor(male) are collected and are induced to form zygote under simulated conditions in the laboratory.                                                                                                                         

The zygote orearly embryos(with up to 8 blastomeres) could then be transferred into the fallopian tube (ZIFT–zygote intra fallopian transfer)                                                                                              

And embryos with more than 8 blastomeres ,into the uterus (IUT– intra uterine transfer), to complete its further development .                                                                                                                              

Embryos formed by in- vivo fertilisation (fusion of gametes within the female) also could be used for such transfer to assist those females who cannot conceive.                                                                        

Transfer of an ovum collected from a donor into the fallopian tube (GIFT–gamete intra fallopian transfer) of another female who cannot produce one, but can provide suitable environment for fertilisation and further development is another method attempted.                                                           

Intra cytoplasmic sperm injection (ICSI) is another specialised procedure to form an embryo in the laboratory in which a sperm is directly injected into the ovum .                                                                  

Infertility cases either due to inability of the male partner to inseminate the female or due to very low sperm counts in the ejaculates, could be corrected by artificial insemination (AI) technique. In this technique, the semen collected either from the husband or a healthy donor is artificially introduced either into the vagina or into the uterus (IUI – intra-uterine insemination) of the female.            

CLASS 12 Biology CHAPTER 1 REPRODUCTION IN ORGANISMS (NCERT Notes)

CLASS 12 Biology                                                                                                  CHAPTER 1                                                                                              REPRODUCTION IN ORGANISMS (NCERT Notes)

The period from birth to the natural death of an organism is called lifespan. Life spans of organisms are not necessarily correlated with their sizes; the sizes of crows and parrots are not very different yet their life spans show a wide difference. Similarly ,a mango tree has a much shorter life span as compared to a peepal tree. No individual is immortal, except single-celled organisms.

Reproduction is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself. The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle of birth ,growth and death. Reproduction enables the continuity of the species, generation after generation.                                                                                                                                          

The organism’s habitat, its internal physiology and several other factors are collectively responsible for how it reproduces.  Based on whether there is participation of one organism or two in the process of reproduction, it is of two types. When offspring is produced by a single parent with or without the involvement of gamete formation, the reproduction is a sexual. When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual reproduction.

ASEXUAL REPRODUCTION                                                                                                  

Single individual (parent) is capable of producing offspring. The offspring that are produced are not only identical to one another but are also exact copies of their parent. Clone is used to describe  morphologically and genetically similar individuals.                     

Asexual reproduction is common among single-celled organisms, and in plants and animals with relatively simple organisations. In Protists and Monerans, the organism or the parent cell divides into two to give rise to new individuals. In these organisms cell division is itself a mode of reproduction. Many single-celled organisms reproduce by binary fission ,where a cell divides into two halves and each rapidly grows into an adult (e.g., Amoeba, Paramecium).                                                                

In yeast, the division is unequal and small buds are produced that remain attached initially to the parent cell which ,eventually  gets separated and mature into new yeast organisms (cells).                                  
Under unfavourable condition the Amoeba withdraws its pseudopodia and secretes a three-layered hard covering or cyst around itself. This phenomenon is termed as encystation. When favourable conditions return, the encysted Amoeba divides by multiple fission and produces many minute amoeba or pseudopodiospores; the cyst wall bursts out, and the spores are liberated in the surrounding medium to grow up into many amoebae. This phenomenon is known as sporulation.                                  

Members of the Kingdom Fungi and simple plants such as algae reproduce through special asexual reproductive structures .The most common of these structures are zoospores that usually are microscopic motile structures.  Other common asexual reproductive structures are conidia (Penicillium), buds (Hydra) and gemmules (sponge). Adventitious buds arise from the notches present at margins of leaves of Bryophyllum.

In plants the term vegetative reproduction is used. The units of vegetative propagation such as runner (mint), rhizome (ginger, banana), sucker(banana) ,tuber (potato), offset( water hycinth) , bulb (onion) are all capable of giving rise to new offspring. These structures are called vegetative propagules.  Since the formation of these structures does not involve two parents, the process involved is asexual.                                                                                                                                                    

Organisms in which the body breaks into distinct pieces (fragments) each fragment grows into an adult capable of producing offspring (e.g., Hydra). This is also a mode of asexual reproduction called fragmentation.

The scourge of the water bodies or about the ‘terror of Bengal’. This is an aquatic plant ‘water hyacinth’ which is one of the most invasive weeds found growing wherever there is standing water. It drains oxygen from the water, which leads to death of fishes.

SEXUAL REPRODUCTION                                                                                                               Sexual reproduction involves formation of the male and female gametes, either by the same individual or by different individuals of the opposite sex. These gametes fuse to form the zygote which develops to form the new organism. Because of the fusion of male and female gametes sexual reproduction results in offspring that are not identical to the parents or amongst them selves. 
All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. That period of growth is called the juvenile phase. It is known as vegetative phase in plants. 

Few plants exhibit unusual flowering phenomenon such as bamboo species flower only once in their lifetime ,generally after 50-100 years, produce large number of fruits and die. Another plant , Strobilanthuskunthiana (neelakuranji), flowers once in 12 years. Plant flowered during September-October 2006. Its mass flowering transformed large tracks of hilly areas in Kerala, Karnataka and Tamil Nadu into blue stretches and attracted a large number of tourists.                                                                
In animals the juvenile phase is followed by morphological and physiological changes prior to active reproductive behaviour. Birds in captivity (as in poultry farms) can be made to lay eggs through out the year. Laying eggs is not related to reproduction but is a commercial exploitation for human welfare.
The females of placental mammals exhibit cyclical changes in the activities of ovaries and accessory ducts as well as hormones during the reproductive phase. And devided into two groups :-                      In non-primate mammals like cows, sheep ,rats, deers ,dogs ,tiger ,etc., such cyclical changes during reproduction are called oestrus cycle where as in primates(monkeys, apes ,and humans) it is called menstrual cycle.                                                                                                                              

Mammals those breeders only during favourable seasons in their reproductive phase are called  seasonal breeders. Example frog , snakes ,lizards.                                                                          Mammals those  are reproductively active throughout their reproductive phase are called continuous breeders. Example monkry , apes , human beings.                                                                                   
In both plants and animals, hormones are responsible for the transitions between the three phases. Interaction between hormones and certain environmental factors regulate the reproductive processes and the associated behavioural expressions of organisms.

Events in sexual reproduction :                                                                                                        Sexual reproduction is characterised by the fusion (or fertilisation ) of the male and female gametes and  the formation of zygote and embryogenesis.                                                                                  Sequential events may be grouped into three distinct stages :                                                              

Pre-fertilisation                                                                                                                                      Fertilisation                                                                                                                                              Post-fertilisation 

The two main pre-fertilisation events are gametogenesis and gamete transfer.          Gametogenes is refers to the process of formation of the two types of gametes – male and female. Gametes are haploid cells. In some algae the two gametes are so similar in appearance that it is not possible to categorise them into male and female gametes are called homogametes (isogametes).          In sexually reproducing organisms the gametes produced are of two morphologically distinct types  are called heterogametes. In such organisms the male gamete is called the antherozoid or sperm and the female gamete is called the egg or ovum.                                                                                  

Plants  having both male and female reproductive structures in the same plant are called bisexual plant. Plants having both male and female reproductive structures on different plants are called unisexual plant . In fungi and plants terms homothallic and monoecious are used to denote the bisexual condition and heterothallic and dioecious are used to describe unisexual condition.                  

In flowering plants the unisexual male flower is staminate bearing stamens while the female is pistillate or bearing pistils.                                                                                                                      
Flowering plants in which both male and female flowers may be present on the same individual are called monoecious plant. Some examples of monoecious plants are cucurbits and coconuts.                Flowering plants in which both male and female flowers may be present on separate individuals  are called dioecious. Some examples of dioecious plants are papaya and date palm.                              

Animals in which individuals of a species is either male or female are called unisexual . Cockroach is an example of a unisexual species. Animals in which individuals of a  species which possess both the reproductive organs are called bisexual. Earthworms, sponge, tapeworm and leech, typical examples of bisexual animals that possess both male and female reproductive organs, are  hermaphrodites. 

Cell division during gamete formation:                                                                                        Gametes in all heterogametic species are of two types namely ,male and female. Gametes are haploid though the parent plant body from which they arise may bee it her haploid or diploid. A haploid parent produces gametes by mitotic division. Organisms belonging to monera, fungi, algae and bryophytes have haploid plant body. But organisms belonging to pteridophytes ,gymnosperms, angiosperms and most of the animals including human beings, the parental body is diploid.                                             

In meiosis the reduction division has to be occur if a diploid body has to produce haploid gametes. In diploid organisms specialised cells called meiocytes (gamete mother cell) undergo meiosis. At the end of meiosis, only one set of chromosomes gets incorporated in to each gamete.

Gamete Transfer                                                                                                                                 
After their formation, male and female gametes must be physically brought together to facilitate fusion (fertilisation). In majority of organisms male gamete is motile and the female gamete is stationary. Exceptions are a few fungi and algae in which both types of gametes are motile .There is a need for a medium through which the male gametes move. Plants like algae, bryophytes and pteridophytes, water is the medium through which this gamete transfer takes place. Large number of the male gametes however, fail to reach the female gametes. To compensate this loss of male gametes during transport, the number of male gametes produced is several thousand times the number of female gametes produced.                                                                                                                                                      
In seed plants pollen grains are the carriers of male gametes and ovule have the egg. Pollen grains produced in anthers have to be transferred to the stigma before it can lead to fertilisation.                        

In bisexual self-fertilising plants e. g., peas transfer of pollen grains to the stigma is relatively easy as anthers and stigma are located close to each other; pollen grains soon after they are shed, come in contact with the stigma. In cross pollinating plants (including dioecious plants), a specialised event called pollination facilitates transfer of pollen grains to the stigma. Pollen grains germinate on the stigma and the pollen tubes carrying the male gametes reach the ovule and discharge male gametes near the egg.  In dioecious plant male and female gametes are formed in different individuals.

Fertilisation                                                                                                                                                  The most vital event of sexual reproduction is the fusion of gametes. This process is called as syngamy (fertilisation) results in the formation of a diploid zygote. 

Some organisms like rotifers ,honeybees and even some lizards and birds (turkey), the female gamete undergoes development to form new organisms without fertilisation. This phenomenon is called parthenogenesis.                                    

In most aquatic organisms  majority of algae and fishes as well as amphibians, syngamy occurs in the external medium (water) ,i.e. ,outside the body of the organism. This type of gametic fusion is called external fertilisation.                                                                                                                                   
In the bony fishes and frogs where a large number of offspring are produced. A major disadvantage is that the offspring are extremely vulnerable to predators threatening their survival up to adulthood.        Terrestrial organisms  such as fungi, higher animals  reptiles like birds, mammals and  majority of plants (bryophytes ,pteridophytes, gymnosperms and angiosperms) ,syngamy occurs inside the body of the organism this process is called internal fertilisation. In all these organisms, egg is formed inside the female body where they fuse with the male gamete. Organisms exhibiting internal fertilisation, the male gamete is motile and has to reach the egg in order to fuse with it. In these even though the number of sperms produced is very large, there is a significant reduction in the number of eggs produced. In seed plants, however, the non-motile male gametes are carried to female gamete by pollen tubes.

Post-fertilisation Events                                                                                                                  Formation of the diploid zygote is universal in all sexually reproducing organisms. Organisms with external fertilisation zygote is formed in the external medium (usually water),where as in those exhibiting internal fertilisation, zygote is formed inside the body of the organism.                            

Development of the zygote depends on the type of life cycle the organism has and the environment it is exposed to. In organisms belonging to fungi and algae ,zygote develops a thick wall that is resistant to dessication and damage. It undergoes a period of rest before germination.  In organisms with haplontic life cycle  zygote divides by meiosis to form haploid spores that grow into haploid individuals.   

Zygote is the vital link that ensures continuity of species between organisms of one generation and the next.                                                                                                                                          

Embryogenesis refers to the process of development of embryo from the zygote. During embryogenesis zygote undergoes cell division (mitosis) and cell differentiation. While cell divisions increase the number of cells in the developing embryo; cell differentiation helps groups of cells to undergo certain modifications to form specialised tissues and organs to form an organism.          

Animals are categorised into oviparous and viviparous based on whether the development of the zygote take place outside the body of the female parent or inside, i. e .,whether they lay fertilised/ unfertilised eggs or give birth to young ones.                                                                                            
In oviparous animals like reptiles and birds, the fertilised eggs covered by hard calcareous shell are laid in a safe place in the environment; after a period of incubation young ones hatch out. In viviparous animals (majority of mammals including human beings), the zygote develops into a young one inside the body of the female organism. After attaining a certain stage of growth, the young ones are delivered out of the body of the female organism. Because of proper embryonic care and protection, the chances of survival of young ones is greater in viviparous organisms.

In flowering plants, the zygote is formed inside the ovule. After fertilisation the sepals, petals and stamens of the flower wither and fall off. Can you name a plant in which the sepals remain attached? The pistil however, remains attached to the plant.  The zygote develops  into the embryo and the ovules develop into the seed. The ovary develops into the fruit which develops a thick wall called pericarp that is protective in function.