Link https://bichep.com/ppt-biology-ncert-based-for-class-11-and-12/ Fertilization is the fusion of haploid gametes, egg and sperm, to form the diploid zygote. Note though there can be subtle differences in the fertilization process which occurs naturally within the body or through reproductive technologies outside the body, the overall product in both cases is a diplod zygote. Parthenogenesis is a natural form of asexual reproduction in which growth and development of embryos occur without fertilization. In animals, parthenogenesis means development of an embryo from an unfertilized egg cell. In plants parthenogenesis is a component process of apomixis.

1. Fertilization

2. The process in which union of male and female gametes (formed by
gametogenesis) and fusion of pronuclei of sperm and ovum takes place
thus diploid zygote is formed, is called fertilization.
Fertilization
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3. Fertilization has following processes
The union of male and female gametes is
called Syngamy, where as intermixing of
their cytoplasm is called plasmogamy. The
fusion of pronuclei of sperm and ovum is
called karyogamy. The intermingling of their
chromosomes is called amphimixis.
Due to fertilization, a diploid zygote is
formed, by the union of two different types of
gametes.
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4. Site of fertilization
(a) Internal fertilization –
Fertilization in the body (i.e., genital organs of animal)
is called internal fertilization. In this type of fertilization,
sperms are discharged by male directly into the genital
tract of female after coitus.
- Whole process of fertilization takes place within the
body of female. This is the most common adaptation in
terrestrial animals.
Examples :- Aschelminthes, reptiles, birds and
mammals.
(b) External Fertilization –
External fertilization takes place outside the body of
females i.e, in water.
Examples :- In most of the invertebrates, some
protochordates, amphibia and most of the fishes.
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5. Types of fertilization
(a)Self fertilization –
This process takes place in the body of single animal i.e,
fusion of male and female gametes produced by male and
female organs of the same animal. This is called self-
fertilization. This is possible only in bisexual or
hermaphrodite animals.
Examples :- Taenia solium
(b)Cross-Fertilization –
Fertilization takes place between two (male & female)
different animals of same species. This is called cross
fertilization.
-This process is found in all unisexual animals. These
animals are also called dioecious animals
-Cross- fertilization is also found in most of the bisexual or
hermaphrodite animals because in these animals male
genital organs develop first. This condition is called
protandrous condition. In some of the species female
organs develop first, this condition is called protogynous
condition e.g. sponges. BICHEP.COM

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7. Mechanism - We can understand the process of
fertilization in following steps.
1. Movement of sperms towards ovum.
2. Entrance of sperms in the ovum.
3. Activation of ovum.
Approach of sperm towards the egg
It is a chance factor, so sperms perform random
(directionless) movement. To increase the chances of
approach of sperm towards egg there are mainly two
adaptation
(a) Number of sperms is very high :- e.g. In man 20
to 120 million sperms are present per cubic mm of
semen.
(b) Egg is 1000 times larger than sperm
- Some special proteins are found on the surface of
egg and sperm to help in fertilization.
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8. According to Lillie, chemicals named as
"fertilizins" are found on the surface of egg.
Fertilizins are glycoproteins or acid
mucopolysaccharides.
According to Ballinsky, an acidic protein named
as Antifertilizin is present on the surface of
sperms. "Fertilin" proteins are also present on
sperm surface.
Both the proteins are specific for a
particular species. Antifertilizin present on sperm
of a particular species will react with fertilizin
present on egg of the same species of animals.
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9. If we place some eggs of sea-urchin in sea-water, this
sea water becomes viscous, this is called egg-water.
When some sperms come in contact with this egg
water, sperms adhere with each other. It is called
agglutination. Here the reaction of fertilizin (dissolved
in water from egg) and antifertilizin of sperm is
observed clearly.
Fertilizins behave like lock and antifertilizins behave
like keys.
Fertilization is always intraspecific.
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10. - According to Washerman and Sailing
(1989) a specific pair of protein
molecules is found on the surface of
mammalian sperm, which can recognise
specific carbohydrates and proteins in
ZP3 region of zona pellucida. The bindin
protein of sperm reacts with these
molecules to initiate the changes in
acrosome. A specific sugar galactose
remains attached with ZP3 glycoprotein.
The sperm fails to recognise the ovum of
its own species, if this sugar is removed
from zona pellucida.
- In addition to these
glycoproteins, there are some hormones
also, which help in fertilization.
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11. • The hormones present at the surface of sperm are
called androgamones. These are of two types.
Androgamone first & androgamone second.
• Androgamones I help in the energy conservation
of sperms.
• Androgamones II dissolve the gelatinous covering
present all over the egg.
• Hormones present at the surface of egg are called
gyanogamones these are of 2 types.
• (a) Gyanogamones I - this hormone neutralizes
Androgamone I and activates sperm to move
• (b) Gyanogamones II - It makes sperm head
sticky.
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13. • Enzyme of acrosome (Hyaluronidase and sperm lysins) dissolve the egg membrane. This is called acrosomal reaction.
• As the result sperm head make the contact with the plasma membrane of egg, now inner membrane of acrosome evaginates
outside and form rigid tube is called acrosomal filament.
• Acrosomal filament provide stimulus to plasma membrane of egg and due to stimulus of sperm, egg is induce for fertlization.
• Mammalian sperms do not form this type of filament becouse mammalian sperms are highly active and provide stimulus to
plasma-membrane of egg without any filament. Mammalian sperms acquire activity at two places. First-epididymis and second-
vagina. Vaginal secretion make the sperm highly active and sperm acquire capacity of fertilization is called capacitation.
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14. Activation of egg : Due to stimulus of
sperm an enzyme is induced in
plasma-membrane of egg it is called
adenyl cyclase enzyme and function of
this enzyme is to catalyze c-amp in
egg cytoplasm.
c-AMP is the second messanger.
Cyclic AMP receives stimulus from
plasma membrane of egg and
transfers it in egg cytoplasm and
induces all the response of egg for
sperm.
All the response of egg for
sperm are collectively called
gyanogenesis.
.
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15. Due to stimulus of sperm, permeability of plasma
menbrane of egg increases specially for k+ and
Ca2+ ions. Function of Ca2+ ions is to inactivate
the cytostatic factors in egg. as a result egg is
now ready for cleavage (In egg cytoplasm
special type of protein called cytostatic factor are
present these factors prevent the cleavage in
unfertilized egg)
Due to stimulus of sperm, H+–Na+
pump activates and induces the plasma-
membrane of egg.
Function of this pump is to continuously influx H+
ions and outflux Na+ions. As a result
concentration of H+ion increases in egg
cytoplasm and develops an acidic medium. In
acidic medium, proteolytic enzyme become
active and liberate the m-RNA from
informosome. These m-RNA become active and
rapidly synthesize different types of protein and
enzymes . Due to more availability of protein and
enzymes metabolic activity of egg increases.
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16. Response of egg:
(1) Due to stimulus of sperm, meiosis-II is induced in human egg by excluding second polar body becoming
mature ovum
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17. • (2) At the point of contact with sperm and plasma-
membrane of egg a cone-like structure is formed
called reception cone.
• After some time reception cone sinks in egg
cytoplasm along with sperm (entry of sperm is a type
of phagocytosis).
• With the entry of sperm all the cortical granules burst
and secrete a membrane around the egg is called
fertilization membrane (cortical reaction).
• It is secreted on inner surface of primary egg
membrane and perivitelline space become more
wide and amount of perivitelline fluid is also
increase.
• Function of perivitelline fluid and fertilization
membrane is to prevent the entry of sperm in egg.
• So normally only one sperm enter inside the egg
(monospermy).
• Sometimes more than one sperm enter inside
the egg (polyspermy).
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18. Two types of polyspermy are found in nature.
(1) Pathological polyspermy: In it the nuclei of all
the sperms fuse with egg nucleus. In such type
of condition embryo development does not
occur. (Due to polyploidy condition)
(2) Physiological polyspermy : In physiological
polyspermy nucleus of only one sperm fuses with
egg nucleus and rest of the sperm die in egg
cytoplasm. Dead sperm are called merocytes. In
physiological polyspermy normal embryo
development occurs.
Polyspermy is absent in human beings.
Polyspermy mostly occurs in megalecithal eggs.
Fertilization membrane and cortical granules are
absent in egg of rat & guinea pig.
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19. • In majority of animals, only head and
middle piece enter inside the egg and
tail is left outside.
• In mammals, whole sperm enters in the
egg.
• In some animals, only head of sperm
enters in the egg tail and middle piece
remain outside e.g. Hydra, Neries etc.
• After entering inside the egg, sperm
rotates by 1800. All the structures of
sperm dissolve in egg cytoplasm except
sperm nucleus and proximal centriole.
• The centriole of egg itself degenerates
at the time of second maturation
division.
• So proximal centriole of sperm starts
division, it divides into 2 daughter
centrioles, which migrate towards
opposite pole and start forming spindles. BICHEP.COM

20. Fate of sperm nucleus
The nucleus of sperm absorbs water from egg
cytoplasm and becomes enlarged. Now it is called
male pronucleus.
After meiosis - II egg nucleus occur in the form of
scattered vesicles then it is called as karyomeres
and after some time all the karyomeres assembled
to form complete nucleus is called female
pronucleus.
Male pronucleus and female pronucleus migrate
through definite routes and come close to each
other. These routes are called fertilization path.(It
has following parts)
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21. (1)Sperm penetration path - Male pronucleus for
some distance, moves at the equator of egg.This is
called sperm penetration path.
(2)Sperm copulation path :- Male pronucleus starts
migrating towards female pronucleus.
(3)Egg copulation path :- Female pronucleus
migrates towards male pronucleus. Both the
pronuclei come close to each other.
(4)Cleavage path - Both the pronuclei move together
to their final position which is somewhere in animal
pole.
At this final position nuclear membrane of both the
pronuclei degenerate and chromosomes of male and
female pronuclei form pairs. The mixture of male and
female chromosomes is called amphimixis.

22. 2. Newport was of all first observed the entry of sperm into the egg
1. Amphimixis was discovered by O.Hertwig in the eggs of sea - urchin.

23. 1. Oocyte completes its second maturation division on coming in
contact with the sperm.
2. Amphimixis process leads to the formation of a diploid zygote to
restore the normal diploid number of the chromosomes.
3. The centriole of sperm after entering into egg induces the egg to
undergo cleavage.
4. The paternal and maternal characters are transmitted to the
offsprings through the process of fertilization.
5. The peripheral changes occurring in the egg prevent the further
entry of sperm into the ovum, thus checking polyspermy.

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25. The development of embryo without fertilization is called
parthenogenesis. The animals which are formed by
unfertilized eggs are called parthenotes.
The discovery of parthenogenesis was done by Charles
Bonet in the eggs of sea-urchins.
Parthenogenesis is of two types -
(a) Natural
(b) Artificial
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26. (a) Natural parthenogenesis -
Some animals show parthenogenesis by nature e.g.
Honey bees, wasps, ants, grass-hoppers, ticks, mites
and sea-urchins.
Natural parthenogenesis is of 2 types
(i) Haploid parthenogenesis or Arrhenotoky :- In this
case eggs are formed by meiosis. Eggs are haploid,
they have the power of fertilization sometimes male
animals are developed by unfertilized eggs. In
Honeybees, unfertilized eggs develop into males
(drones), and fertilized eggs develop into queen and
soldiers. Thus male honey bees are always haploid
and queen along with soldiers are always diploid.
(ii) Diploid parthenogenesis or Thelytoky :- In this
case, eggs are formed without meiosis division. Eggs
are diploid they do not have the power of fertilization.
Diploid eggs give rise to female generation only. Male
members are absent in these species.
Examples :- Lacerta sexicola
armenica (lizard), Caresius aratus gibelio(Fish). BICHEP.COM

27. Diploid parthenogenesis may also be divided into
two types -
(A) Ameiotic Thelytoky- In this type of
parthenogenesis, during oogenesis first meiotic division
does not take place but second meiotic division occurs
as usual. In this situation the ovum still remains diploid.
These ova, when reproduce parthenogenetically give
rise to diploid offsprings. For example, Trichoniscus,
Daphnia pulex etc.
(B) Meiotic thelytoky- If the eggs are formed by
normal oogenesis process, but by one or other reasons
the eggs retain their diploid chromosomal number, then
the parthenogenesis is called meiotic thelytoky. It may
happen because of autofertilization . Some species of
order - Lepidoptera exhibit this type of parthenogenesis.
-In some animals parthenogenesis alternates with
normal sexual reproductive cycle. This is called cyclic
parthenogenesis.e.g., Honey bee.
- In some animals, development of animals is always by
parthenogenesis and sexual reproduction is absent in
these species. This is called complete parthenogenesis.
Males are absent.
Example - Lacerta sexicola armenica (lizard) BICHEP.COM

28. b) Artificial parthenogenesis :-
This type of parthenogenesis is done by
artificial methods. Artificial parthenogenesis
is done by putting eggs in different
atmospheres or by giving special stimulus
to the eggs. Different artificial methods used
for this purpose are as follows –
(1)If we place eggs in brine or salt solution,
KCl solution. Then eggs show
parthenogenesis e.g. eggs of sea-urchins.
(2)By short exposure of radiations on eggs
or exposure of silk insect egg to sunlight.
(3)If eggs are given shocks of temperature.
(4)If eggs are pierced by needle dipped in
the blood of some animal. The eggs of frog
show parthenogenesis by this method.
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