Notes-Part-2-Class-12-Biology-Chapter-2-Reproduction in Lower & Higher Animals-Maharashtra Board

Reproduction in Lower & Higher Animals

Maharashtra Board-Class-12th-Biology-Chapter-2


Topics to be Learn : Part-1

  • Reproduction
  • Asexual Reproduction in animals
  • Sexual Reproduction in animals
  • Menstrual cycle (Ovarian cycle)
  • Gametogenesis

Topics to be Learn : Part-2

  • Fertilization / Syngamy
  • Embryonic development
  • Pregnancyl Gestation
  • Placenta
  • Parturition
  • Lactation
  • Reproductive Health
  • Birth control
  • Sexually Transmitted Diseases (STDs)
  • Infertility

Fertilization / Syngamy

Fertilization : Process of fusion of the haploid male and female gametes into diploid zygote is called fertilization.

Fertilization is internal and in the ampulla of the fallopian tube the gametes meet.

The sequence of events of fertilization :

The sequence of events of fertilization :

  • Capacitation of sperms : Movement of sperms towards the secondary oocyte.
  • Release of acrosomal enzymes : Enzymes are secreted by sperm head when it touches secondary oocyte.
  • Separation of cells of corona radiata : This happens due to acrosomal enzymes like hyaluronidase and corona penetrating enzymes.
  • Compatibility reaction/ Fertilizin—antifertilizin interaction : Fertilizin of egg binds with anti fertilizin of sperm. The zona pellucidaof egg has fertilizin receptor proteins (ZP3, ZP2). Also acrosome ruptures releasing enzyme acrosin / zona lysin.

  • Acrosome reaction : Acrosin dissolves zona pellucida and vitelline membrane at the point of contact of sperm head.
  • Entry of sperm nucleus in ovum : Nucleus and centriole enter the ovum.
  • Formation of fertilization membrane/ Cortical reaction : Vitelline membrane of ovum is converted into a fertilization membrane. This prevents polyspermy.
  • Activation of ovum : Completion of meiosis-ll of secondary oocyte takes place. When ovum receives centriole from sperm, it completes meiosis-II. This releases second polar body.
  • Syngamy/ Karyogamy : Fusion of male pronucleus and female pronucleus. Synkaryon is formed after fusion of male and female nucleus.
  • Formation of diploid zygote conclude the process of fertilization


Significance of fertilization :

Significance of fertilization :

  • Fertilization forms the zygote which eventually produces new offspring.
  • Fertilization restores diploid number of chromosomes in the zygote as two haploid gametes come together in a zygote.
  • During fertilization, centrioles are passed on to the ovum, due to this secondary Oocyte can complete meiosis-II. The fertilization thus concludes the process of oogenesis.
  • By fertilization the genetic characters of two parents are mixed. This leads to variation and has significance in evolution.
  • Due to fertilization the sex of young one is determined.


Embryonic development :

  • After syngamy the zygote that is formed undergoes divisions to form embryo. These mitotic divisions are called cleavages.
  • When the zygote passes through the fallopian tube, the cleavages start.
  • Till morula stage, zona pellucida layer is retained as it prevents the implantation of the blastocyst at an abnormal site.

Cleavage :

  • Cleavages are rapid mitotic divisions of zygote to form a hollow spherical multieellular blastula. Cleavage converts the zygote first into a mass of cells called morula.
  • Cleavage occurs during its passage through the fallopian tube to the uterus.
  • In human beings, cleavage is holoblastic, equal and indeterminate.
  • Cleavage divisions are rapid with short interphase.
  • There is no time for cells to grow in size. Thus, cells become progressively smaller. The resulting daughter cells are called blastomeres.
  • Cleavage shows faster synthesis of DNA.

Cleavage Stages :

Cleavage Direction Time Results
First Vertical along animal pole — vegetal pole 32 hours after fertilization Two blastomeres
Second Vertical at the right angles to the first one 60 hrs after fertilization Four blastomeres
Third Horizontal / latitudinal division and at right angles to the first two cleavages 32 hrs after fertilization Eight celled stage
Successive Division Reaches uterus about 4 — 6 days after fertilization. Morula— 16 celled having outer layer of smaller clearer cells and an inner mass of larger cells


Blastulation :

  • During blastulation hollow and multicellular blastocyst is formed from 16-32 celled stage of morula.
  • Blastocyst now reaches uterus and starts absorbing glycogen rich uterine milk.
  • The outer layer of cells forms trophoblast layer Whereas inner large cells form inner cell mass or embryoblast.
  • Size of blastocyst doubles from 0.15 mm to 0.30 mm.
  • Due to the absorption of glycogen rich uterine milk, the trophoblast cells become flat and a cavity called blastocyst cavity is formed.
  • Cells of rauber : These are the cells of trophoblast which are in contact with the embryonal knob.
  • Blastocyst becomes fully developed by the end of the 7th day.

Implantation :

  • Implantation is the process of embedding the blastocyst in the uterine endometrium for further gestation.
  • On 7th to 10th day after fertilization, implantation takes place. Embryo is completely buried inside the endometrium.
  • Trophoblast divides into two layers, viz. cytotrophoblast and syncytiotrophoblast. With processes of synctiotrophoblast the blastocyst is buried in the endometrium layer of uterus.

Gastrulation :

Formation of gastrula from the blastocyst is called gastrulation. It starts at about 8 days after fertilization.

Two important events during gastrulation are :

  • [a] Differentiation of blastomeres : Three germinal layers are formed by rearrangement of blastomeres.
  • [b] Morphogenetic movements : Movements of cells to reach their destined area of differentiation is called morphogenetic movements.

Gastrulation and implantation of blastocyst takes place simultaneously.

Gastrulation sequential changes :

Gastrulation involves the following sequential changes :

  • Formation of endoderm
  • Formation of embryonic disc
  • Formation of amniotic cavity
  • Formation of ectoderm
  • Formation of mesoderm ‘
  • Formation of extra-embryonic coelom
  • Formation of chorion and amnion


Organogenesis : Process of forming organs after the process of gastrulation is called organogenesis.

Fate of germinal layers : At the end of gastrulation, embryo develops into three germinal layers viz., ectoderm, mesoderm and endoderm. Different tissues and organs are formed from germinal layers. This process is called histogenensis.

Three germinal layers :

Fate of ectoderm : Following tissues, structures and organs develop from the ectoderm :

  • Epidermis of the skin, epidermal derivatives such as hair and nails
  • sweat glands
  • conjunctiva
  • cornea
  • lens
  • retina
  • internal and external ear
  • enamel of teeth
  • nasal cavity
  • adrenal medulla
  • Swmodaeum and proctodaeum
  • Neurohypophysis and entire nervous system.

Fate of mesoderm : The mesoderm forms the following derivatives :

  • All types of muscles
  • connective tissue
  • dermis of skin
  • adrenal cortex
  • kidney
  • circulatory system
  • heart
  • blood vessels
  • blood
  • lymphatic vessels
  • middle ear and dentine of teeth.

Fate of endoderm : The following organs develop from the endoderm :

  • Epithelium of gut from pharynx to colon
  • glands of stomach and intestine
  • tongue and tonsils
  • lungs, trachea, bronchi, larynx, etc.
  • urinary bladder, vestibule and vagina
  • liver and pancreas
  • adetwhypophysig
  • thymus, thyroid and parathyroid
  • eustachian tube
  • epithelium of urethra and associated glands.


Pregnancy / Gestation :

Gestation or pregnancy is the condition of developing foetus in the uterus. It may be one or two asin twins.

In human beings, gestation period is about 280 days. It is divided into three trimesters of three months each.

Three trimesters :

First Trimester : (From fertilization to 12th week)

During first trimester there are radical changes in the body of mother as well as in the embryo.

  • The embryo receives nutrients in the first 2~4 weeks directly from the endometrium.
  • It is the main period of organogenesis and the development of body organs.
  • By the end of eight weeks, the major structures found in the adult are formed in the embryo in a rudimentary form. It is now called foetus and is about 3 cm long.
  • Arms, hands, fingers, feet, ‘toes, CNS, excretory and circulatory system including heart are formed and begins to work.
  • Progesterone level becomes high and menstrual cycle is suspended till the end of pregnancy. ‘
  • At the end of first trimester foetus is about 7—10 cm long.
  • The maternal part of placenta grows, the uterus becomes larger. In this period, the mother experiences morning sickness. (nausea, vomiting, mood swings, etc.)

Second Trimester : (From 13th to 26th week)

  • The foetus is very active and grows to about 30 cm.
  • The uterus grows enough for the pregnancy to become obvious.
  • Hormone levels stabilize as hCG declines, the corpus luteum deteriorates and the placenta completely takes over the production of progesterone which maintain the pregnancy.
  • Head has hair, eyebrows and eyelashes appear, pinnae are distinct. Baby’s movement can be easily felt by the mother.
  • The baby reaches half the size of a new born.

Third Trimester : (From 27th week till the parturition]

  • Foetus grows to about 50 cm in length and about 3-4 kg in weight.
  • As the foetus grows, the uterus expands around it, the mother’s abdominal organs become compressed and displaced, leading to frequent urination, digestive blockages and strain in the back muscles.
  • At the end of third trimester the foetus becomes fully developed and ready for parturition.


Placenta : Placenta is a flattened, discoidal organ attached to the wall of the uterus and to the baby’s umbilical cord.

  • It facilitates the supply of oxygen and nutrients and also for removal of carbon dioxide and excretory products produced by the foetus.
  • Placenta is the only organ, which is formed of tissues from two different individuals the mother and the foetus.
  • Foetal placenta is the choronic villi while maternal placenta is the highly vascularized uterine wall. So human placenta is called haemochorial.
  • The placenta also acts as an endocrine tissue and produces hormones like hCG. progesterone, estrogen and relaxin.

Note :

  • Cells of trophoblast do not form any part of the embryo proper.
  • They form ectoderm of the chorion (extra embryonic membrane).
  • They play important role in formation of placenta.

 Parturition : Parturition is the birth process which is accompanied with labour pains.

  • It is a neuro-endocrine mechanism which involves rise in estrogen : progesterone ratio and increase in oxytocin receptors in myometrium of uterine wall.
  • The fully developed foetus gives signals for the uterine contractions by secreting Adrenocorticotropic hormone (ACTH) from pituitary and corticosteroids from adrenal gland.
  • This triggers release of oxytocin from mother’s pituitary gland, which acts on uterine muscles of mother and causes vigorous uterine contractions.
  • This leads to expulsion of the baby from the uterus.
  • Parturition involves three stages, viz. dilation stage, expulsion stage and after birth or placental stage.
Steps of parturition :

Steps of parturition :

Dilation stage :

  • Dilation stage means dilating the birth canal or passage through which baby is pushed out.
  • In the beginning uterine contractions start from top and baby is moved to cervix. Due to compression of blood vessels and movements of flexible joints in pelvic girdle, mother experiences labour pains.
  • Oxytocin is secreted later in more amount causing severe uterine contractions. This pushes baby in a head down position and closer to cervix.
  • Cervix and vagina both are dilated. This stage lasts for about 12 hours.
  • At the end, amniotic sac ruptures and amniotic fluid is passed out.

Expulsion stage :

  • During second stage of about 20 to 60 minutes, the uterine and abdominal contractions become stronger.
  • Foetus moves out with head down position through cervix and vagina.
  • The umbilical cord which connects the baby to placenta is tied and cut off close to the baby’s navel.

After birth or placental stage :

  • In the last stage of 10 to 45 minutes, once the baby is out then the placenta is also separated from uterine wall and is expelled out as “after birth". This is accompanied by severe contractions of the uterus.

Lactation : After parturition the new born is given nourishment through milk. The process of secretion of milk is called lactation.

  • In which mammary glands of mother become functional.
  • The first milk is called colostrum which is rich in proteins, lactose and mother’s antibodies e.g. IgA.
  • Lactation is also neuroendocrine process, in which almost all endocrine glands of mother are involved.


Distinguish between Asexual reproduction and Sexual reproduction :

Distinguish between Asexual reproduction and Sexual reproduction :

Asexual reproduction Sexual reproduction
1-Asexual reproduction requires single parent.

2-Meiosis does not take place in asexual reproduction. Only mitosis takes place.

3-Gamete formation, fertilization and zygote formation does not take place.

4-Progeny and parent are identical genetically

5-Large number of progeny is developed by asexual reproduction. E.g. Spore formation, gemmule formation, budding, regeneration are the types of a sexual reproduction.

1-Sexual reproduction needs two different parents.

2-Sexual reproduction involves meiosis and mitosis.

3-Gamete formation, fertilization and zygote formation are important processes in sexual reproduction.

4-Progeny and parents are genetically dissimilar.

5-Limited number of progeny is developed by sexual reproduction. E.g. Sexual reproduction is only by a single method.


Reproductive health : Total wellbeing of a person's emotional, behavioural, physical and social aspects involving reproduction is called reproductive health.

  • In India, Reproductive and Child Health Care (RCH) programmes are undertaken to improve reproductive health.
  • India was amongst the first few countries in the world to initiate action plans and programmes at a national level to improve reproductive health.
  • One of the objectives of this programme is to control the population growth of India.

Goals of RCH Programmes :

Goals of RCH Programmes:

  • To create awareness among people about various aspects related to reproduction.
  • To provide the facilities to people to understand and build up reproductive health.
  • To provide support for building up a reproductively healthy society.
  • To bring about a change mainly in three critical health indicators i.e. reducing total infertility rate, infant mortality rate and maternal mortality rate.

The goals of RCH can be achieved by the following ways:

  • Sex education in schools to be introduced. Proper information about safe and hygienic sexual practices, sexually transmitted diseases (STD, AIDS), problems related to adolescence and proper information about reproductive organs.
  • With the help of audio- visual and the print media, government and non-government organisations should take various steps to create awareness about various aspects related to reproduction.
  • Younger generation should be educated about birth control measures, pre-natal care of pregnant woman and post-natal care of the mother and child, importance of breast feeding.
  • Developing awareness about problems arising due to uncontrolled population growth, social evils like sex abuse and sex related crimes and take up necessary steps to prevent them.
  • Creating awareness about statutory ban on amniocentesis for sex determination.
  • Creating awareness about child immunization programmes.


Birth control : For controlling the family size, birth control measures are taken which are called contraceptive measures.

Contraceptive methods are of two main types, i.e. temporary and permanent.

Temporary methods :

Temporary methods :

  • Natural method/Safe period/Rhythm method : In the natural method, the principle of avoiding chances of fertilization is used. A week before and a week after menstrual bleeding is considered the safe period for sexual intercourse. It is based on the fact that ovulation occurs on the 14th day of menstrual cycle.
  • Coitus Interruptus or withdrawal : In this method, the male partner withdraws his penis from the vagina before ejaculation, so as to avoid insemination. This method also has some drawbacks, as the pre-ejaculation fluid may contain sperms and this can cause fertilization.
  • Lactational amenorrhoea (absence of menstruation) : This method is based on the fact that ovulation does not occur during the period of intense lactation following parturition so chances of conception are almost negligible. However, this method also has high chances of failure.
  • Chemical means (spermicides): In this method chemicals like foam, tablets, jellies and creams are introduced into the vagina before sexual intercourse, they adhere to the mucous membrane, immobilize and kill the sperms.
  • Mechanical means/Barrier methods :
  • Condom : It is a thin rubber sheath that is used to cover the penis of the male. Condom should be used before starting coital activity. It also prevents STDs and AIDS.
  • Diaphragm, cervical caps and vaults : These devices made of rubber are inserted into the female reproductive tract to cover the cervix during copulation. They prevent conception by blocking the entry of sperms through the cervix.
  • Intra-uterine devices (IUDs): These are plastic or metal objects placed in the uterus by a doctor. These include Lippes loop, copper releasing IUDs (Cu-T, Cu 7, multiload 375) and hormone releasing IUDs (LNG-20, progestasert). They prevent fertilization of the egg or implantation of the embryo.
  • Physiological (Oral) Devices : Birth control pills (oral contraceptive pills) check ovulation as they inhibit the secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) that are necessary for ovulation. The pill ‘Saheli’ is taken weekly.
  • Other contraceptives: The birth control implant is similar to that of pills in their mode of action. It is implanted under the skin of the upper arm of the female.


Permanent methods :

Permanent methods surgical operations : In men surgical operation is called vasectomy and in women it is called tubectomy.

These are surgical methods, also called sterilization. In vasectomy a small part of the vas deferens is tied and cut where as in tubectomy, a small part of the fallopian tube is tied and cut. This blocks, gamete transport and prevent pregnancy.


Medical termination of pregnancy (MTP) : An intentional or voluntary termination of pregnancy before full term is called Medical termination of Pregnancy.

  • MTP is induced abortion.
  • MTP can indirectly control population.
  • MTP is legalized in India since 1971.
  • MTP is performed if unwanted pregnancy has to be discontinued or if there are defects in the growing foetus.
  • MTP to abort healthy female embryo is illegal.
  • MTP can be safely done only during the first trimester of pregnancy.

Amniocentesis :

  • Process in which amniotic fluid containing foetal cells is collected using a hollow needle inserted into the uterus under ultrasound guidance.
  • This is done for studying the chromosomes to check any possible abnormality in the developing foetus.
  • Sex determination by amniocentesis is legally banned in India.


PC-PNDT Act : The Pre-conception and Pre-natal Diagnostic Techniques (Prohibition of Sex Selection) Act or PC-PNDT Act 2003.

Its mandates as follows :

  • To provide for the prohibition of sex selection, before or after conception.
  • For regulation of pre-natal diagnostic techniques for the purposes of detecting abnormalities or metabolic disorders or chromosomal abnormalities or certain congenital malformations or sex-linked disorders.
  • The prevention of their misuse for sex determination leading to female foeticide and for matters connected therewith or incidental thereto.

The main purpose of enacting the Act is to ban the use of sex selection techniques after conception and prevent the misuse of pre-natal technique for sex selective abortions. This Act is enacted to stop female foeticides and thereby reduce the declining sex ratio in India. Hence this mandate is essential to prevent foetal sex determination and sex selective abortion. I


Sexually Transmited Diseases (STDs) : Disease or infections which are transmitted through sexual intercourse are collectively called Sexually Transmited Diseases (STDs) or Venereal Diseases (VDs) or Reproductive Tract Infections (RTI).

The major venereal diseases are syphilis and gonorrhoea.

Syphilis :

A. Syphilis : Syphilis is a sexually transmitted veneral disease caused by a Spirochaete bacterium Treponema pallidum.

The site of infection is the mucous membrane in genital, rectal and oral region.

Modes of transmission :

  • Infection through sexual intercourse, kissing and close body contact.
  • Incubation period 3—4 weeks.
  • Mothers transmit disease to newborn through placenta. (Congenital syphilis).

Symptoms of syphilis :

  • Primary lesion known as chancre at the site of infection.
  • They are seen on the external genitalia in males and inside the vagina in females.
  • Skin rashes accompanied by fever, inflammed joints and loss of hair.
  • Paralysis
  • Degenerative changes in the heart and brain.

Preventive measures :

  • Education about sex practices, sex hygiene, avoiding sex with unknown partner or multipartners, using condom during coitus.

Treatment : Prompt treatment with Antibiotic-Penicillin


Gonorrhoea :

B. Gonorrhoea : Gonorrhoea is a sexually transmitted veneral disease caused by Diplococcus bacterium, Neisseria gonorrhoeae.

Modes of transmission :

  • Sexual contacts and infested cloths.
  • The incubation period is 2 to 14 days in males and 7 to 21 days in females.
  • Infection sites are mucous membrane of urino-genital tract, rectum, throat and eye.

Symptoms in Males : Partial blockage of urethra and reproductive ducts, pus from penis, pain and burning sensation during urination, arthritis, etc.

Symptoms in female : Pelvic inflammation of urinary tract, sterility, arthritis. The children born to affected mother suffer from gonococcal ophthalmia.

In girl-child, there is occurrence of gonococcal vulvovaginitis before puberty.

Preventive measures :

  • Sexual hygiene
  • Use of condom during coitus.
  • Sex with unknown partner or multiple partners should be avoided.

Treatment : Treated with Cefixime which is antibiotic.


Infertility : Infertility is defined as the inability to conceive naturally after (one year of) regular unprotected intercourse.

Today infertile couples have many options to have a child such as fertility drugs, test tube babies, artificial insemination, IUI, surrogate motherhood, etc.

The couple could be assisted to have child / children through certain special techniques commonly known as Assisted Reproductive Technologies (ART).

Various Assisted Reproductive Technologies (ART) :

  • In Vitro Fertilization (IVF) : When fertilization process is carried out of the body and the embryo is transferred back into the mother’s body, then it is called IVF technique. (Commonly known as test-tube baby.)
  • Zygote Intra Fallopian Transfer [ZIFT] : The embryo is transferred in the fallopian tubes by ZIFT (Zygote Intra Fallopian Transfer) technique.
  • Gamete Intra Fallopian Transfer (GIFT) : Transferring the ovum collected from the donor into the fallopian tube of another female who can act as a surrogate mother (a female with suitable environment for fertilization and development) is called GIFT.
  • ICSI (Intra Cytoplasmic Sperm Injection) : Single sperm cell is injected directly into cytoplasm of an ovum in the laboratory in this technique.
  • Artificial insemination (AI) : The collected sperms are artificially introduced into the cervix of female, for the purpose of achieving a pregnancy through in vivo fertilization.
  • IUI (Intra Uterine Insemination) : Similar to artificial insemination, but in this technique the sperms are introduced into the uterine cavity instead of cervix.
  • Sperm bank / Semen bank: Sperms are collected from donors and stored in a sperm bank or semen bank. These are stored by cryopreservation method and given to needy couples.
  • Adoption : A couple or a single parent can legally adopt a child. They also get legal rights, privileges and responsibilities for raising up adopted child.
  • Surrogate mother : Some women have problem in implantation of embryo in uterus. In this, embryo is implanted in surrogate mother, who is not the biological mother.


Note: Tobacco, marijuana and other drugs smoking may cause infertility in both men and women.

Nicotine blocks the production of sperm and decreases the size of testicles.

Alcoholism by men interferes with the synthesis of testosterone and has an impact on sperm count.

Use of cocaine or marijuana may temporarily reduce the number and quality of sperm.

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