Cleavage is a series of mitotic divisions that divides the egg cytoplasm into smaller cells called blastomeres, starting after fertilization and ending when the embryo balances nucleus and cytoplasm. The type of cleavage (holoblastic or meroblastic) depends on the yolk content, affecting cell division patterns. Additionally, cleavages can be indeterminate, allowing for flexibility in blastomere fate, or determinate, where blastomeres are predetermined to develop into specific embryo parts.

1. • Meaning
• Cleavage, a series of mitotic divisions where by the enormous
volume of egg cytoplasm is divided into numerous smaller,
nucleated cells.
• Cleavage begins soon after fertilization and ends shortly after the
stage when the embryo achieves a new balance between nucleus
and cytoplasm.
• These cleavage-stage cells are called blastomeres.
• Rate of cell division is totally under control of the proteins and
mRNAS stored in the oocyte by the mother.
Cosmos Studies 1
CLEAVAGE
2.1

2. Cosmos Studies 2
 Depending mostly on the amount of Yolk in the egg , the cleavage can
be HOLOBLASTIC (total or entire cleavage) or MEROBLASTIC
(partial cleavage).
 Yolk is the deposited in the eggs of animals that will provide the energy
and building blocks required for development and growth.
CLEAVAGE
2.1

3. Cosmos Studies 3
CLEAVAGE

4. Cosmos Studies 4
Cleavage
HOLOBLASTIC MEROBLASTIC
Isolecithal Mesolecithal Telolecithal Centralecithal
Radial
Spiral
Bilateral
Routational
Displaced Radial
Bilateral
Discoidal
Superficial
-Echino..
-Annelid ,Molluscs,
-Flatworms
-Tunicates
-Mammals
-Nematoda
-Amphibians
-Cephelopod
,Molluscs
Fish , reptiles,
birds
-Insects
(Uniformed Yolk Distributed)
Eg-Sea Urchin
(Moderate Yolk)
Eg.-Frog
(Large Yolk Near One end)
Eg.-Birds, Reptiles, Fish
( Yolk in center)
Eg.-Insects
Complete InComplete
A A
B B
CLEAVAGE
2.1

5. Vegetal and Animal poles
• Many eggs are polarized with a yolk rich pole, termed the
vegetal pole and a yolk poor pole termed the animal pole, eg.
frog. Zygotes with relatively little yolk (Isolecithal And
Mesolecithal) cleave HOLOBLASTICALLY.
• The cleavage furrow extends all the way through the egg.
Cosmos Studies 5
CLEAVAGE
2.1

6. • While telolecithal and centrolecithal zygotes undergo
MEROBLASTIC cleavage where the cleavage plane extends
only to the accumulated yolk.
• In Centrolecithal eggs (many insect eggs) cleavage is
meroblastic and Superficial, while in telolecithal eggs (birds
and fish) cleavage is discoidal.
Cosmos Studies 6
CLEAVAGE
2.1

7. Cosmos Studies 7
CLEAVAGE

8. Cosmos Studies 8
CLEAVAGE

9. • Indeterminate cleavge
• Determinate cleavge
• Indeterminate cleavage - In indeterminate cleavage, the fate blastomeres is
not so rigidly sealed, but the blastomeres exhibit plasticity.
• For examples, each of the 2 blastomeres a zygote, if separated after the first
cleavage, can produce one complete embryo. This type of cleavage ,in
which future of blastomeres is not predetermined, is knows as
Indeterminate cleavage.
• Examples - vertebrates
Cosmos Studies 9
On the basis of potentiality of blastomere
CLEAVAGE

10. • Determinate Cleavage- In some animals (Ascaris), the
blastomeres have a predetermined future, i.e definite blastomeres
are determined to give rise to specific parts of the embryo.
• This type of cleavage is known as Determinate cleavage. As a
result of determinate cleavage, a mosaic type of blastula is
formed. This type of development is called MOSAIC
DEVELOPMENT.
• Example- Ascaris egg. Cosmos Studies 10
On the basis of potentiality of blastomere
CLEAVAGE