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Fertilisation and Implantation

BiologyHuman ReproductionFor NEET aspirants

Fertilisation and implantation link the gametes to pregnancy: a sperm fuses with the ovum to form a diploid zygote, and the embryo that develops from it embeds in the uterine wall. This page covers insemination, the meeting of gametes, entry of a single sperm, sex determination, cleavage, the morula and blastocyst, and implantation, as in the NCERT Class 12 chapter Human Reproduction. In NEET, fertilisation and implantation questions often ask the site of fertilisation, the morula and blastocyst, and how the father's sperm decides the sex of the baby.

On this page1Insemination2Fertilisation3Sex determination4Cleavage5Implantation6Exam essentials7Quick revision8Solved examples9Practice
Key Points at a Glance
  1. Insemination: release of semen by the penis into the vagina during coitus.
  2. ★ Must learn Fertilisation takes place in the ampullary region of the fallopian tube.
  3. Fertilisation happens only if the ovum and sperms reach the ampullary region at the same time.
  4. ★ Must learn Sperm contact with the zona pellucida changes the membrane and blocks other sperms: only one sperm fertilises an ovum.
  5. Acrosome secretions help the sperm enter through the zona pellucida and plasma membrane.
  6. ★ Must learn Sperm entry completes meiosis II of the secondary oocyte: second polar body + haploid ovum (ootid).
  7. Sperm and ovum nuclei fuse to form a diploid zygote.
  8. ★ Must learn All ova carry X; sperms carry X or Y (50:50). XX = female, XY = male: the father decides the sex.
  9. Cleavage: 2, 4, 8, 16 blastomeres; 8-16 blastomeres = morula; then the blastocyst.
  10. ★ Must learn Blastocyst: outer trophoblast (attaches to the endometrium) and inner cell mass (becomes the embryo).
  11. Implantation: the blastocyst becomes embedded in the endometrium; it leads to pregnancy.

1. Insemination and Transport of Gametes

  • Insemination: during copulation (coitus), semen is released by the penis into the vagina.
  • The motile sperms swim rapidly, pass through the cervix and enter the uterus.
  • ★ Exam imp They finally reach the ampullary region of the fallopian tube.
  • The ovum released by the ovary is also transported to the ampullary region.
  • Fertilisation takes place in the ampullary region.
  • Fertilisation can occur only if the ovum and the sperms are transported to the ampullary region simultaneously.
  • This is why not all copulations lead to fertilisation and pregnancy.
Memory Trick Path of sperms: Very Clever Users Arrive = Vagina, Cervix, Uterus, Ampullary region of the fallopian tube.
★ Very important The site of fertilisation in humans is the ampullary region of the fallopian tube, not the uterus. The ovum and the sperms must reach it at the same time.
Key idea
Sperms travel vagina → cervix → uterus → ampulla; the ovum must arrive there at the same time.

2. Fertilisation

  • Fertilisation: the process of fusion of a sperm with an ovum.
  • Around the ovum lie the perivitelline space, the zona pellucida and the cells of the corona radiata (Figure 1).
An ovum with its coverings, surrounded by a few sperms A round ovum at the centre with a large nucleus near one side, small granules along its edge and one sperm already inside it. A thin clear perivitelline space separates the ovum from a thick zona pellucida. Outside the zona pellucida lie many irregular cells of the corona radiata. Five more sperms approach from the right: one swims free, one lies among the corona cells and three have reached the zona pellucida. Ovum Sperm Cells of thecorona radiata Zonapellucida Perivitellinespace
Figure 1: Ovum surrounded by a few sperms. A sperm must pass the cells of the corona radiata and the zona pellucida before it can enter the ovum.

Events of fertilisation:

  1. A sperm comes in contact with the zona pellucida layer of the ovum.
  2. It induces changes in the membrane that block the entry of additional sperms, so only one sperm fertilises an ovum.
  3. The secretions of the acrosome help the sperm enter the cytoplasm of the ovum through the zona pellucida and the plasma membrane.
  4. Sperm entry induces the completion of the meiotic division of the secondary oocyte.
  5. The second meiotic division is also unequal: it forms a second polar body and a haploid ovum (ootid).
  6. The haploid nucleus of the sperm and that of the ovum fuse to form a diploid zygote with 46 chromosomes.
Memory Trick Events of fertilisation in order: Contact with the zona pellucida, Block to other sperms, Entry helped by the acrosome, Finishing of meiosis II, Fusion of nuclei = Cats Begin Eating Fresh Fish.
★ Very important Changes induced at the zona pellucida block other sperms, so only one sperm fertilises an ovum. Sperm entry then triggers the second meiotic division, which gives the ootid and the second polar body.
Tips and Tricks Two different jobs, two different structures. The zona pellucida changes stop extra sperms; the acrosome secretions help the chosen sperm get in. Ootid is simply the name of the haploid ovum after the second meiotic division.
NEET Focus The secondary oocyte released at ovulation finishes meiosis only after a sperm enters. So the second polar body forms at fertilisation, not at ovulation. Statements that place the second meiotic division inside the Graafian follicle are wrong.
Quick Recall: tap to check
Where does fertilisation take place in humans?
In the ampullary region of the fallopian tube.
What ensures that only one sperm fertilises an ovum?
The sperm's contact with the zona pellucida induces membrane changes that block additional sperms.
What is an ootid?
The haploid ovum formed by the second (unequal) meiotic division, completed after sperm entry.
How many chromosomes does the zygote have?
46. It is diploid, formed by fusion of two haploid nuclei with 23 each.
Key idea
One sperm enters; the oocyte finishes meiosis; two haploid nuclei fuse into a diploid zygote.

3. Sex Determination at Fertilisation

  • ★ Exam imp The sex of the baby is decided at fertilisation itself.
  • The chromosome pattern is XX in the human female and XY in the male.
  • All the haploid gametes (ova) produced by the female carry the sex chromosome X.
  • ★ Exam imp Sperms carry either X or Y: 50 per cent carry X and the other 50 per cent carry Y.
SpermOvumZygoteBaby
X-bearingXXXFemale
Y-bearingXXYMale
★ Very important Because every ovum carries X, the sex of the baby is determined by the father and not by the mother: it depends on whether an X-bearing or a Y-bearing sperm fertilises the ovum.
Memory Trick Y comes only from the father. The mother always gives X, so the sperm decides: X from the father gives a girl, Y gives a boy.
Key idea
Ovum always X; sperm X or Y; XX = girl, XY = boy, so the father's sperm decides the sex.

4. Cleavage, Morula and Blastocyst

  1. Cleavage: the mitotic division of the zygote, which starts as it moves through the isthmus of the oviduct towards the uterus.
  2. Cleavage forms 2, 4, 8 and 16 daughter cells called blastomeres.
  3. Morula: the embryo with 8 to 16 blastomeres.
  4. The morula continues to divide and transforms into the blastocyst as it moves further into the uterus.
  5. In the blastocyst, the blastomeres are arranged into an outer layer, the trophoblast, and an inner group of cells attached to it, the inner cell mass.
Trophoblast
  • Outer layer of the blastocyst
  • Gets attached to the endometrium
Inner cell mass
  • Inner group attached to the trophoblast
  • Differentiates as the embryo
Memory Trick Order: Zygote → Blastomeres → Morula → Blastocyst. Roles: Trophoblast Touches the uterus; Inner cell mass becomes the Infant (embryo).
Key idea
Cleavage in the oviduct gives blastomeres, then a morula (8-16 cells), then a blastocyst with trophoblast and inner cell mass.

5. Implantation

  1. The trophoblast layer gets attached to the endometrium.
  2. The inner cell mass differentiates as the embryo.
  3. After attachment, the uterine cells divide rapidly and cover the blastocyst.
  4. The blastocyst becomes embedded in the endometrium of the uterus.
  • ★ Exam imp Implantation: the embedding of the blastocyst in the endometrium; it leads to pregnancy.
Transport of the ovum, fertilisation and passage of the growing embryo through the fallopian tube An ovary with its follicles lies beside the curved fallopian tube, which leads into the upper part of the uterus. Numbered stages 1 to 10 follow the path: the released ovum near the funnel-shaped end of the tube, the ovum surrounded by sperms, the zygote and its first division, two-, four- and eight-celled stages moving along the tube, the morula entering the uterus, the blastocyst in the uterine cavity and the blastocyst implanting into the uterine wall. Below, enlarged circles numbered 1 to 10 show each stage in detail, from the single ovum with its outer coat to the hollow blastocyst and the implanting embryo. 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 Ovum(secondaryoocyte) Fertilisation Zygote Firstcleavage 2-cell stage 4-cell stage 8-cell stage Morula Blastocyst Implantation Fallopian tube Ovary Uterus
Figure 2: Transport of the ovum, fertilisation and passage of the growing embryo through the fallopian tube. The ovum is fertilised in the ampulla, cleavage stages and the morula move along the tube, and the blastocyst implants in the uterine wall.
NEET Focus Sequence questions on this topic follow one fixed order: insemination → fertilisation (ampulla) → cleavage (isthmus) → morula → blastocyst → implantation. Remember that the trophoblast, not the inner cell mass, attaches to the endometrium.
Quick Recall: tap to check
What is a morula?
The embryo with 8 to 16 blastomeres.
Which layer of the blastocyst attaches to the endometrium?
The trophoblast.
Which part of the blastocyst forms the embryo?
The inner cell mass.
Key idea
Trophoblast attaches, uterine cells cover the blastocyst, and implantation in the endometrium begins pregnancy.

6. Exam Essentials

Pairs to Match

TermMeaning or role
InseminationRelease of semen into the vagina during coitus
Ampullary regionSite of fertilisation
Zona pellucidaSperm contact here induces changes that block extra sperms
Cells of the corona radiataLie outside the zona pellucida
Perivitelline spaceLies between the ovum and the zona pellucida
Acrosome secretionsHelp sperm entry through the zona pellucida and plasma membrane
OotidHaploid ovum after the second meiotic division
ZygoteDiploid cell from the fusion of sperm and ovum nuclei
XY zygoteMale baby
CleavageMitotic division of the zygote in the oviduct
BlastomeresDaughter cells formed by cleavage
MorulaEmbryo with 8 to 16 blastomeres
TrophoblastOuter layer; attaches to the endometrium
Inner cell massDifferentiates as the embryo
ImplantationBlastocyst embedded in the endometrium; leads to pregnancy
Exceptions
  • Fertilisation takes place in the ampullary region of the fallopian tube, not in the uterus.
  • Not all copulations lead to fertilisation; the gametes must reach the ampulla together.
  • Only one sperm fertilises an ovum.
  • The second meiotic division of the oocyte happens only after sperm entry, and it is unequal.
  • All ova carry X; only sperms can carry Y.
  • The sex of the baby is determined by the father, not the mother.
  • The trophoblast does not form the embryo; the inner cell mass does.

Numbers to Remember

  • Sperms that fertilise one ovum: 1.
  • Sperms carrying X : Y = 50 : 50 per cent.
  • Chromosomes: each gamete 23; zygote 46.
  • Cleavage: 2, 4, 8, 16 blastomeres; morula: 8-16 blastomeres.
  • Sex chromosome patterns: female XX; male XY.

7. Quick Revision

  • Insemination: semen released into the vagina during coitus.
  • Sperms: vagina → cervix → uterus → ampullary region of the fallopian tube.
  • Fertilisation in the ampullary region, only if ovum and sperms arrive together.
  • Ovum surrounded by the perivitelline space, zona pellucida and corona radiata cells.
  • Sperm contact with the zona pellucida blocks extra sperms: one sperm per ovum.
  • Acrosome secretions help entry through the zona pellucida and plasma membrane.
  • Sperm entry completes meiosis II: second polar body + ootid (haploid ovum).
  • Sperm and ovum nuclei fuse: diploid zygote (46).
  • Ova all X; sperms 50% X, 50% Y; XX female, XY male; the father decides the sex.
  • Cleavage begins in the isthmus: 2, 4, 8, 16 blastomeres.
  • Morula: 8-16 blastomeres; it becomes the blastocyst in the uterus.
  • Blastocyst: trophoblast (outer) + inner cell mass (embryo).
  • Trophoblast attaches to the endometrium; uterine cells cover the blastocyst: implantation → pregnancy.

8. Solved Examples

Solved Example 1
Match List I with List II.
List I: A. Ampullary region, B. Zona pellucida, C. Trophoblast, D. Inner cell mass
List II: I. Differentiates as the embryo, II. Site of fertilisation, III. Attaches to the endometrium, IV. Changes here block additional sperms
Choose the correct answer:
(A) A-II, B-IV, C-I, D-III
(B) A-IV, B-II, C-III, D-I
(C) A-II, B-III, C-IV, D-I
(D) A-II, B-IV, C-III, D-I
Solution:

Answer: (D). Fertilisation occurs in the ampullary region (II); zona pellucida changes block extra sperms (IV); the trophoblast attaches to the endometrium (III); the inner cell mass forms the embryo (I).

Solved Example 2
Read the statements.
A. All ova produced by a woman carry the X chromosome.
B. Half of the sperms carry the Y chromosome.
C. The sex of the baby is determined by the mother.
D. A zygote carrying XY develops into a male baby.
E. The sex of the baby is decided only after implantation.
Choose the correct answer:
(A) A, B and D only
(B) A, C and D only
(C) B, D and E only
(D) A, B, D and E only
Solution:

Answer: (A). C is wrong: the father decides the sex. E is wrong: sex is decided at fertilisation itself.

Solved Example 3
Arrange the events in the correct order.
A. Formation of the morula
B. Fusion of sperm and ovum nuclei
C. Implantation
D. Insemination
E. Formation of the blastocyst
Choose the correct answer:
(A) D, B, E, A, C
(B) B, D, A, E, C
(C) D, B, A, E, C
(D) D, A, B, E, C
Solution:

Answer: (C). Insemination (D) → fertilisation (B) → morula (A) → blastocyst (E) → implantation (C).

Solved Example 4
Which of the following is NOT correct about fertilisation in humans?
(A) It occurs in the ampullary region
(B) The acrosome secretions help the sperm enter the ovum
(C) Several sperms fuse with one ovum
(D) Sperm entry completes the meiotic division of the secondary oocyte
Solution:

Answer: (C). Changes induced at the zona pellucida block additional sperms, so only one sperm fertilises an ovum.

Solved Example 5
Statement I: Cleavage of the zygote begins as it moves through the isthmus of the oviduct towards the uterus.
Statement II: The embryo with 8 to 16 blastomeres is called a blastocyst.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrect
Solution:

Answer: (B). The embryo with 8 to 16 blastomeres is the morula; the morula later transforms into the blastocyst.

Solved Example 6
The haploid ovum formed after the second meiotic division is called:
(A) Oogonium
(B) Primary oocyte
(C) Ootid
(D) Blastomere
Solution:

Answer: (C). The second meiotic division, completed after sperm entry, gives a second polar body and the haploid ovum (ootid).

9. Practice Questions

Practice Questions
  1. Match List I with List II.
    List I: A. Insemination, B. Cleavage, C. Morula, D. Implantation
    List II: I. 8-16 blastomeres, II. Blastocyst embedded in the endometrium, III. Semen released into the vagina, IV. Mitotic divisions of the zygote
    Choose the correct answer:
    (A) A-III, B-IV, C-I, D-II
    (B) A-IV, B-III, C-I, D-II
    (C) A-III, B-I, C-IV, D-II
    (D) A-III, B-IV, C-II, D-IAnswer: (A). Insemination-III, cleavage-IV, morula-I, implantation-II.
  2. Read the statements.
    A. Fertilisation occurs only if the ovum and sperms reach the ampullary region simultaneously.
    B. Every copulation leads to pregnancy.
    C. Acrosome secretions help the sperm enter through the zona pellucida.
    D. The second meiotic division of the oocyte is equal.
    E. The nuclei of sperm and ovum fuse to form a diploid zygote.
    Choose the correct answer:
    (A) A, B and C only
    (B) A, C and E only
    (C) C, D and E only
    (D) A, C, D and E onlyAnswer: (B). Not all copulations lead to fertilisation (B wrong); the second meiotic division is unequal (D wrong).
  3. Arrange the path of sperms after insemination: A. Uterus, B. Ampullary region, C. Vagina, D. Cervix.
    (A) C, A, D, B
    (B) C, D, A, B
    (C) D, C, A, B
    (D) C, D, B, AAnswer: (B). Vagina → cervix → uterus → ampullary region of the fallopian tube.
  4. Which of the following is NOT a feature of the blastocyst?
    (A) An outer trophoblast
    (B) An inner cell mass attached to the trophoblast
    (C) It becomes embedded in the endometrium
    (D) It is formed in the ampulla before fertilisationAnswer: (D). The blastocyst forms from the morula as the embryo moves into the uterus, long after fertilisation.
  5. Fill in the blanks: (a) Fertilisation is ____ in humans (external/internal). (b) The zygote is ____ (diploid/haploid). (c) The fusion of male and female gametes is called ____. (d) Fertilisation takes place in the ____. (e) The zygote divides to form the ____, which is implanted in the uterus.Answer: (a) Internal. (b) Diploid. (c) Fertilisation. (d) Ampullary region of the fallopian tube. (e) Blastocyst.
  6. In our society women are often blamed for giving birth to daughters. Explain why this is not correct.Answer: All ova carry X. Sperms carry X or Y in equal proportion. A daughter (XX) results when an X-bearing sperm fertilises the ovum, so the sex of the baby is determined by the father's sperm, not by the mother.
  7. How many eggs are released by a human ovary in a month? How many eggs would have been released if a mother gave birth to identical twins? Would the answer change for fraternal twins?Answer: One ovum is released in each menstrual cycle. Identical twins come from one egg fertilised by one sperm, so one egg. Fraternal twins come from two eggs fertilised by two sperms, so two eggs.
  8. How many eggs were released by the ovary of a female dog that gave birth to 6 puppies?Answer: At least 6 eggs, one for each puppy, assuming no identical twins among them.

Common Mistakes to Avoid

Watch out
  • Writing that fertilisation occurs in the uterus. Correct: it occurs in the ampullary region of the fallopian tube.
  • Saying the sex of the baby is decided by the mother. Correct: all ova carry X, so the sperm (X or Y) from the father decides.
  • Calling the embryo with 8-16 blastomeres a blastocyst. Correct: it is a morula; the blastocyst forms later.
  • Saying the inner cell mass attaches to the endometrium. Correct: the trophoblast attaches; the inner cell mass forms the embryo.
  • Placing the second meiotic division of the oocyte at ovulation. Correct: it is completed only after sperm entry.
  • Saying the second meiotic division of the oocyte is equal. Correct: it is unequal, giving the ootid and a second polar body.
  • Thinking several sperms can enter one ovum. Correct: changes at the zona pellucida block additional sperms.
  • Calling cleavage a meiotic division. Correct: cleavage is mitotic division of the zygote.

Frequently Asked Questions

Where does fertilisation take place in humans?

Fertilisation takes place in the ampullary region of the fallopian tube. After insemination, sperms swim through the cervix and uterus to the ampulla, and the ovum released by the ovary is also carried there. Fertilisation can occur only when both reach the ampullary region at the same time.

How is the entry of more than one sperm prevented?

When a sperm comes in contact with the zona pellucida of the ovum, it induces changes in the membrane that block the entry of additional sperms. This ensures that only one sperm fertilises an ovum. The acrosome secretions of that sperm help it pass through the zona pellucida and plasma membrane.

What happens to the secondary oocyte when a sperm enters it?

Sperm entry induces the completion of the meiotic division of the secondary oocyte. This second meiotic division is unequal and forms a small second polar body and a haploid ovum, the ootid. The haploid nuclei of the sperm and the ovum then fuse to form a diploid zygote.

Why is the sex of a baby determined by the father?

Human females are XX, so every ovum carries an X chromosome. Males are XY, so half the sperms carry X and half carry Y. If an X-bearing sperm fertilises the ovum, the zygote is XX and becomes a girl; a Y-bearing sperm gives XY, a boy.

What is the difference between a morula and a blastocyst?

Cleavage of the zygote forms blastomeres. The embryo with 8 to 16 blastomeres is the morula. As it moves further into the uterus, the morula continues to divide and becomes the blastocyst, whose cells are arranged into an outer trophoblast and an inner cell mass.

What are the trophoblast and the inner cell mass?

They are the two cell groups of the blastocyst. The trophoblast is the outer layer, which gets attached to the endometrium during implantation. The inner cell mass is a group of cells attached to the trophoblast on the inside, and it differentiates as the embryo.

What is implantation?

Implantation is the embedding of the blastocyst in the endometrium of the uterus. The trophoblast attaches to the endometrium, the uterine cells then divide rapidly and cover the blastocyst, and it becomes embedded in the uterine wall. Implantation leads to pregnancy.

Which points on fertilisation and implantation are most asked in NEET?

Learn the ampullary region as the site of fertilisation, the block to additional sperms, completion of meiosis II after sperm entry, the ootid, sex determination by the father, the morula with 8 to 16 blastomeres, the roles of the trophoblast and inner cell mass, and implantation.

Previous year questions on Fertilisation and Implantation

4 questions from past papers, each with a step-by-step solution.

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