1. 4.3 – E AR LY
D E VE LO P M E N T O F TH E
H U M AN ZYG O TE

2. LEARNING
OUTCOMES
 To describe what fertilisation
is,
 Todescribe in simple terms the
early development of a zygote,
 Toname the two main
developmental stages of a
zygote in preparation for
implantation,

3. Development of Human

Zygote
4 m illions s p e rm s are re le as e d d u ring c op u lation
 only s m all nu m b e r w ill re ac h th e ovid u c t &
only one s p e rm w ill fu s e w ith th e ovu m  fril aio .
etist n
 Z yg ote  a fe rtilis e d e g g
 A fte r form ation of z yg ote , a p rote c tive laye r is
form arou nd its e lf to p re ve nt th e e ntry of oth e r
s p e rm s .

4.  4 days after fertilisation
 zygote divides
repeatedly by mitosis to
form a solid ball of cells
 morula
 Morula develop into a
hollow ball of cells
 blastocyst/ blastula (each cell called
blastomere).
 Blastocyst has a localised thickening called the
inner cell mass. The remainder of the
blastocyst is made up of cells called
trophoblast.

5.  Theblastocyst will be implanted at the
endometrium of the uterus after 7
days of fertilisation.
 Morula & blastocyst are the two main
stages in the development of a zygote
in preparation for implantation.

6. IMPLANTATION
 After 7 days, the blastocyst begins to embed
itself in the endometrium of the uterus 
implantation.
 The trophoblast forms villi which are in
intimate contact with the endometrial tissue.
 Enzymes are secreted by the embryonic cells
which digest & break down maternal
capillaries in the endometrium.
 The villi are bathed by a pool of maternal
blood, enabling exchange of materials
between the mother & the growing embryo.

9. LEARNING
OUTCOMES
 To describe the formation of twins,
 To compare identical twins with
fraternal twins,
 to state the functions of the placenta
in the foetal development,
 The explain the advantages of the
foetus having a separate circulatory
system from that of the mother.

10. IDENTICAL TWINS
FRATERNAL TWINS
SIAMESE TWINS

11. ID E N T IC A L
T W IN S

12. ID E N T IC A L
T W IN S
 Involves only one sperm & one ovum.
Involves only one sperm & one ovum.
 One zygote is form  splits into two
halves by mitosis  each half develops
into a foetus.
 Share same placenta

13. IDENTICAL TWINS :
SIMILARITIES :
- S AME HAI R TEXTURE
- S AME COL OUR OF EYES
- S AME S EX
DIFFERENCES:
F I NGER PRI NT

14. FRATERNAL TWINS

15. F R A TE R N A L
T W IN S
 Involve two ova & two sperms.
 Two ova are released at the same time by
the ovaries  each fertilised by a sperm
 two zygotes will be formed  develops
into an independent foetus.
 Have two separate placentas.

16. FRATERNAL TWINS
TH E Y M AY H AVE :
1 . S AM E O R D IF E R E N T
H AIR C O LO U R O R H AIR
TE X TU R E
2. S AM E O R D IF F E R E N T
E YE C O LO U R
3. S AM E O R D IF F E R E N T
S EXES
4. D O N O T S H AR E S AM E
P LAC E N TA

17. SIAMESE
TWINS

18. S IA M E S E
T W IN S

19. S IA M E S E T W IN S
 In the course of formation of identical twins,
if the splitting of the zygote stops halfway,
while the embryo continues to develop, then
Siamese twins will be formed.
 Identical twins jointed at certain parts of
their bodies/ certain organs.
 Possible to separate by surgery if they do
not share vital organs.

20. SIAMESE
TWINS
1.SAME SEXES
2.SAME GENETIC
PARTICULAR
3.SAME APPEARANCE

21. S IA M E S E T W IN S
USUALLY THE BODIES WILL JOIN AT THE:
1.HEAD
2.CHEST- SHARING HEART
3.FRONT ABDOMEN – SHARING LIVER AND
DIGESTIVE SYSTEM
4. HANDS
5. CAN SEPARATE DEPEND DEGREE OF
SHARING OF INTERNAL ORGAN.

22. WORK SHEET
2. DRAW FLOW CHARTS FOR THE FORMATION OF IDENTICAL
TWINS,FRATERNAL TWINS AND SIAMESE TWINS.

23. CONCLUSION
TWINS

25. P L A C E N TA IN
F O E TA L
D E VE L O P M E N T

26. LEARNING OUTCOMES
 To state the functions of the placenta
in foetal development.
 To explain the advantages of foetus
having a separate circulatory
system from that of the mother.

27.  The organs in a foetus
PLACENTA are not fully
developed yet 
depends on the
mother for food &
oxygen, & to get rid
of its waste products.
 Functions in the
exchange of
substances between
the foetus & the
mother.

28.  The exchange occurs via trophoblastic
villi (TV).
 TV  human chorionic gonadotrophin
(HCG)  stimulates the Corpus
Luteum to increase in size & continue
to produce oestrogen & progesteron
 maintain the structure of
endometrium & inhibit FSH
production.

29.  The foetus is
connected to the
placenta by the
umbilical cord
(umbilical arteries &
umbilical vein).
 In the placenta, the
umbilical arteries
branch into a network
of blood capillaries in
the trophoblastic villi
of the placenta.

30.  Surrounding the
villi is a space
filled with
maternal blood.
 Inthe placenta,
foetal blood &
maternal blood do
not mix. They are
separated by two
layers of cells
which facilitates
diffusion.

31.  Oxygen, food & some
antibodies diffuse
from the maternal
blood into the foetal
blood.
 Carbon dioxide, &
nitrogenous waste
products diffuse from
the foetal blood into
the maternal blood &
is excrete by the
mother.

32.  Foetal blood containing oxygen & food
is carried back to the heart of the
foetus by the umbilical vein.
 Any molecules which is small enough
will be able to cross from the maternal
blood to the foetal blood.
 Harmful substances  alcohol,
nicotine, other drugs, lead particles,
viruses (rubella & HIV) are able to cross
the placenta.

33. Why i t ’ s s o i mp o r t ant f o r t he
f o e t us t o hav e a s e p ar at e
c i r c ul at o r y s y s t e m f r o m t he
mo t he r ’ s ?
 Ifthe foetus has a blood group which is
not compatible with the mother’s, the
mixing of their blood could lead to
agglutination & death.
 Blood pressure of maternal circulation is
much higher than the foetal circulation
 prevents from damaging the delicate
tissues & organs of the developing
foetus.