The document summarizes key aspects of human fertilization and early embryonic development: 1) Fertilization occurs internally when a sperm meets and fuses with an ovum in the fallopian tubes. 2) A zygote then undergoes cell division to become a blastocyst, which implants in the uterine wall, leading to pregnancy. 3) Twins can occur if more than one egg is fertilized, or if a single zygote splits into two embryos. An ectopic pregnancy occurs when an embryo implants outside the uterus, often in a fallopian tube.

1. By- AYUSH GUPTA
Fig 1

2. LEARNING OUTCOMES
By the end of this topic , You will be able to
i. Explain internal fertilization in Human.
ii. Describe intervening stages of Zygote development during Implantation.
iii. Give reason for conception of twins and ectopic pregnancy.
BY-A.K GUPTA

3.  Monospermy
 Acrosome
 Ootid
 Polar body
 Secondary oocyte
 Blastocyst
 Blastomere
 Morula
 Implantation
 Fertilisation
 Fraternal twin
 Identical twin
 Ectopic pregnancy
BY-A.K GUPTA
KEYWORDS

4.  The process of fusion of a sperm and ovum is called
fertilization.
 During copulation (coitus) semen is released by the penis
into the vagina is called insemination.
 The motile sperm swim rapidly, pass through cervix,
uterus and finally reach the junction of isthmus and
ampulla (ammpullary-isthmic junction).
 The ovum released from the ovary also transported to
ampullary isthmic junction where fertilization takes place.
 Fertilization only takes place if both sperm and ovum
reach ampullary – isthmic junction simultaneously.
BY-A.K GUPTA
INTRODUCTION
Fig 2 : Insemination

5.  Acrosome of sperm secretes enzymes helps in
penetration into the ovum. This structure is
derived from the Golgi apparatus at anterior
head of Sperm. The acrosome contains
digestive enzymes (including hyaluronidase
and acrosin).
 Once a sperm comes contact with the zona
pellucida of ovum and induces the changes in
the membrane that blocks the entry of
additional sperms. This ensures monospermy
and prevents polyspermy. Only one sperm
fertilizes with one ovum.
BY-A.K GUPTA
Fig NCERT pg
51
Fig 3: Ovum surrounded by few
Sperms ( NCERT; pg 51

6.  Entry of sperm into the ovum induces the
ovum to complete its second meiotic
division of Secondary oocyte.
 Meiosis-II is also unequal cytokinesis
resulting production of one large ovum
(ootid) and one small second polar
body.
 Haploid nucleus of sperm(n) fused with
the haploid nucleus of ovum (n) to form a
diploid zygote (2n).
BY-A.K GUPTA Fig 4 : Fertilisation of Gametes (Human )

7.  Chromosomal sex is determined at the
time of fertilization.
 The presence of X or Y chromosome in
the sperm determines the sex of the
embryo.
BY-A.K GUPTA
Fig 5 : Mechanism of Sex
detrmination

8.  Repeated mitotic division of the zygote without growth resulting a
multicellular ball like embryo is called cleavage.
 Cleavage starts soon after fertilization.
 Daughter cells produced during cleavage are called blastomeres.
 The product of cleavage is called Morula, which is 8 to 16 celled.
 The morula continues to divide, grow and get transformed into
blastocyst.
BY-A.K GUPTA

9.  The blastomeres in blastocyst arranged into an
outer layer called trophoblast and an inner
mass of cells attached to trophoblast called
inner cell mass.
 Trophoblast cells attached to the endometrium
helps development of placenta.
 Inner cell mass gets differentiated into the
embryo.
 After attachment the uterine cells divide
rapidly and cover the blastocyst.
 Blastocyst completely embedded in the uterine
endometrium. This is called implantation.
BY-A.K GUPTA Fig 6 : Mammalian
Blastula

10. BY-A.K GUPTA
Fig 7a : Transport of ovum, fertilisation and passage of growing embryo through fallopian
tube.(NCERT; pg 52 ) and Fig 7b: Implantation
Fig 7a
Fig: 7b

11. BY-A.K GUPTA Fig 8: Implantation

12. 1.if more than one egg is produced, you can have more than one embryo
a) FRATERNAL TWINS result when 2 eggs are fertilized by 2 different sperm cels
b) IDENTICAL TWINS result with 1 egg gets fertilized by 1 sperm; during
cleavage, the zygote breaks off into two completely separate cells.
2. ECTOPIC PREGNANCY: An ectopic pregnancy occurs when a fertilized egg
implants and grows outside the main cavity of the uterus. An ectopic
pregnancy most often occurs in a fallopian tube. This type of ectopic pregnancy is
also called a tubal pregnancy.
BY-A.K GUPTA

13. RECAPITULATION
1. After coitus, sperms are transported to the junction of the isthmus and
ampulla, where the sperm fertilises the ovum leading to formation of a
diploid zygote.
2. Fertilisation is always internal and only one sperm fertilizes with one
ovum.
3. The presence of X or Y chromosome in the sperm determines the sex of
the embryo.
4. The zygote undergoes repeated mitotic division to form a blastocyst,
which is implanted in the uterus resulting in pregnancy.
BY-A.K GUPTA

14. 1.NCERT (2006). Biology: Textbook for class XII. National Council for Educational Research and
Training, New Delhi. India.
2. E- sources of GIF Images;
i) Fig1-http://members.chello.nl/r.kuijt/en_fertilization.htm
ii) Fig 2 : Insemination:https://www.s-cool.co.uk/gcse/biology/fertilisation/revise-it/fertilisation
iii) Fig 4 : Fertilisation of Gametes -
https://embryology.med.unsw.edu.au/embryology/index.php/Human_Fertilization_Movie
iv) Fig 5: Mechanism of Sex determination- https://www.topperlearning.com/answer/how-is-sex-
determined-in-human-beings/vpi853gg
v) Fig 6 : mammalian Blastula;--https://courses.lumenlearning.com/wm-biology2/chapter/cleavage-
and-blastula-stage/
vi) Fig 7b: implantation-
http://www.goldiesroom.org/Note%20Packets/17%20Human%20Reproduction/00%20Human%20Re
production--WHOLE.htm
vii) Fig 8:Implantation GIF: https://www.best-hospital-infertility.com/advanced-treatments/blastocyst-
culture-technique
BY-A.K GUPTA
REFERENCES