Cleavage to Three Germ Layers

1. CLEAVAGE
TO
3 GERM LAYERS

2. CLEAVAGE
It is the repeated amitotic divisions of
zygote forming blastomeres.
It occurs in fallopian tube in human.
 It is holoblastic e.g. zygote
completely divides into blastomeres
or daughter cells.
During early cleavages, embryo
slowly moves towards uterus and
reaches at end of 4th day.

3. CHARACTERS OF CLEAVAGE
It is similar to mitosis division but
resulting blastomeres divide without
growth phase, so called as amitosis.
In early stages, all blastomeres divide
simultaneously and later some slower.
Blastomeres remain attached.
Size of blastomeres decrease in each
successive cleavage.
Rate of DNA synthesis and oxygen
consumption increase in cleavage.

4. TYPES OF CLEAVAGE
 BASED ON PLANES OF CLEAVAGE
 MERIDIONAL – Cleavage along animal –
vegetal axis, passing through centre of
zygote.
 VERTICAL – Cleavage along animal –
vegetal axis, not passing through centre
of zygote.
 EQUITORIAL - Cleavage along a plane
perpendicular to animal – vegetal axis,
passing through centre of zygote.

5. TYPES OF CLEAVAGE
 LATITUDINAL – Cleavage along a plane
perpendicular to animal – vegetal axis,
not passing through centre of zygote.
 BASED ON ARRANGEMENT OF BLASTOMERES
 RADIAL – Blastomeres arranged radially
along animal vegetal axis. e. g. Sponge.
 SPIRAL – Blastomeres arranged spirally
along animal vegetal axis. e. g. Ascaris.
 BILATERAL – Blastomeres arranged
bilaterally along animal vegetal axis. e.
g. Mammals, Amphioxis.

6. TYPES OF CLEAVAGE
 BASED ON AMOUNT & DISTRIBUTION OF YOLK
 HOLOBLASTIC – In alecithal,
microlecithal and mesolecithal eggs,
cleavage is rapid and completely divide
zygote into blastomeres. e. g. Human.
 MEROBLASTIC – In megalecithal eggs,
cleavage is incompletely divide part of
zygote into blastomeres. e. g. Birds.

7. HUMAN CLEAVAGE
Cleavage
No.
Direction
No. Of
Blastomeres
Occur after
fertilisation
1st Longitudinal 02 30 hours
2nd Transverse 04
Within 40
hours
3rd Longitudinal
double
08 72 hours

9. SIGNIFICANCE OF CLEAVAGE
 Unicellular zygote become
multicellular morula.
 Protoplasmic mobility increases
to facilitate morphogenetic
movement.

10. MORULA TO 3 GERM LAYERS
MORULA – It is a solid ball of 32 cells
with equal size to the zygote. It is still
surrounded by zona pellucida that
not allows its implantation.
BLASTULA – Morula undergo
cavitation by absorbing outer fluid. It
now has a central cavity called
blastocoels, an outer layer of cells …

11. called Trophoblasts and an inner cell
mass. Inner mass is as a knob at one
pole and form embryo proper and
trophoblast form extra embryonic
membranes for placenta. Now zona
pellucida become thin and
disappears.
IMPLANTATION – It is attachment of
blastula to inner wall of uterus about

12. one week after fertilisation. At point
of contact, trophoblast digests
uterine endometrium. It completely
dissolve blastula in endometrium.
GASTRULATION – Cells of inner mass
of blastula show morphogenetic
movement and form three germ
layers of gastrula. Inner endoderm,
outer ectoderm and marginal ….

13. mesoderm around blastopore.
Blastopore is opening of new cavity
of gastrula called archenteron.
FATE OF THREE GERM LAYERS –
Ectoderm forms :- Epidermis of skin,
nervous system, eye, internal ear,
nasal epithelium, enamel, epithelium
of fore gut and hind gut and some
glands ( sweat gland, salivary gland,

14. mammary gland, lacrimal gland and
oil glands ). Mesoderm forms :-
Dermis of skin, muscles, connective
tissue, kidney, gonads, notochord etc.
Endoderm forms :- Inner lining of gut,
thyroid, thymus, trachea, bronchi,
lungs etc.